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THE AM BULLETIN BOARD => Technical Forum => Topic started by: K1JJ on October 30, 2005, 07:16:58 PM



Title: Simple Audio Driver for 833A modulator
Post by: K1JJ on October 30, 2005, 07:16:58 PM
I wonder if anyone has given thought to simple modulator audio drivers. Most guys use solid state amps driving backwards audio output transformers. Some use a line up of tubes using direct coupling or transformers.

Some guys have used the IJY/QIX mosfet board too, though that requires a lot of power supply voltages, heat sinks, complexity.

I'm designing up a single 833A final, plate modulated by a pair. I want a silent rig with no blowers. I was wondering if a single Mosfet in the cathode would drive each 833A modulator? If the tube needed a positive bias on the grid to come to life, that could be easily done to give it the ability to draw grid current.

A p-p fet driver into the two cathode Mosfets sounds like a real simple method.

Will this work?  Any other suggestions for simplicity?

[Frank/GFZ , I'm doing metal work on the 4X1  X 4X1 PDM rig now.]

T



Title: Re: Simple Audio Driver for 833A modulator
Post by: w3jn on October 30, 2005, 07:24:04 PM
Quote
I wonder if anyone has given thought to simple modulator audio drivers

Probably.  Not me, though.


Title: Re: Simple Audio Driver for 833A modulator
Post by: Bacon, WA3WDR on October 30, 2005, 10:50:55 PM
Might be better to use zero-bias tubes like 3-500Z for the modulators.  Then you could avoid the dual-supply issue, because the filaments only need to be equal to, or negative, with respect to the grids.

Grounded grid, and series fets to a single negative supply could then work if the filament transformer capacitance is low enough.  Maybe a 200 KHz filament supply?  Otherwise a totem-pole of fets is required, but that's not too bad if only a single driver supply is needed.  But then with a totem-pole, and a grounded filament and a single positive driver supply, you could drive the grids with much less current.  In any event, it's much simpler with zero-bias tubes, I think.  Darn, can the 3-500Z take the heat without a blower?  Maybe with a quiet blower.  Push-pull parallel 805's instead? Multiples of them?  Grasping at straws here.


Title: Re: Simple Audio Driver for 833A modulator
Post by: W2VW on October 30, 2005, 11:28:08 PM
You need to get back on the air Bacon.
3-500Z's are really not zero bias. No big deal though all you need is a Zener in the cathode return.
Same thing would work for an 833 but you would need a lot more Zener Voltage.
Cathode drive could be done with a pair of discreet fil xfmrs. Drive the center taps. Shunt capacitance might not be as much of an issue as the impedance is most likely less than 100 Ohms.
Grounded grid 833's cathode driven. I like It Vu. 833's stink in cathode driven service at R.F. Anything other than high Mu triodes will not fly for cathode driven service up at radio freq. Audio might be another story. I like the whole cathode driven idea as the degenarative feedback cleens up the output.
You are mastal innolvatol.


Title: Bacon Beacon
Post by: WA3VJB on October 31, 2005, 09:00:53 AM
Quote
You need to get back on the air Bacon.

Hey Bacon !


Title: Re: Simple Audio Driver for 833A modulator
Post by: Bacon, WA3WDR on October 31, 2005, 09:12:19 AM
Arrr, I've been monitoring... but all I hear is my computer.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on October 31, 2005, 09:38:11 AM
Tom,
I've often thought about driving modulators with FETs in the cathode.

What is the cut off voltage of the 833 and how far positive does the
grid need to swing? This should be easy and the 900 volt fets used in the class E rigs should work fine with a good sized heat sink. i'm thinking 3 FETs one for each tube and a third as a phase splitter.  3 FETs, 6 resistors and 2 caps. 
also how much cathode current per 833? Also need 2 fil transformers.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on October 31, 2005, 10:09:37 AM
Tom,
I've often thought about driving modulators with FETs in the cathode.

What is the cut off voltage of the 833 and how far positive does the
grid need to swing? This should be easy and the 900 volt fets used in the class E rigs should work fine with a good sized heat sink. i'm thinking 3 FETs one for each tube and a third as a phase splitter.  3 FETs, 6 resistors and 2 caps. 
also how much cathode current per 833? Also need 2 fil transformers.

Interesting and looking do-able... :o

Check out these 833A curves on page 6 to determine viabilty for cathode driven service:

http://www.tubebuilder.com/images/tubedata/833a.pdf


Dave, you brought up a good point that I didn't think of... built in negative feedback of the GG config!  I wonder if that wud be enuff or will it still need a loop around the fet driver line up to help that area out with neg feedback too?

Frank, I checked the 833A curves and it shows that at 3kv, the tube draws 900ma at zero grid volts. And it gets cut off at about -70V. So that's 70V of monkey swing from the fet. That's kinda ideal, I think, cuz a fet can turn on in the cathode and generate enuff current without having to add a positive bias on the grid. So true grounded grid simplicity.  BTW, my supply is 0-5kv HV at about 1.2A, but I expect to run it around 3kv so that I need just a silent muffin fan.

I have two matched and  separate 10A/10V fil transformers.  I wonder if it wud best best to mount the fil transformer above the chassis  on fiberglass to minimize audio coupling/capacitance to ground??  The fil bypass caps wud be about .1 ufd or something else?

Frank, if you think it will work, please detail out that fet driver you described. I will build it up. I have line level audio to drive it with.

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on October 31, 2005, 10:31:17 AM
Tom,
I think this is very doable if 0 volts on the grid gives you enough current and 70 volts cuts the tube off. Line level you will need about a gain of 50 or so let me think about it while i'm working. I have something later.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on October 31, 2005, 11:35:26 AM
Tom,
Setting up for a gain of 50 in the driver 50 ohm in the drain and 1 ohm in the source would be a problem. This will put about 25 volts of bias at .5 amps.
A higher voltage audio source would make it easier so there is less positive bias on the cathode. You can't avoid a bit of voltage on the cathode because it is an analog signal. Need to keep this voltage under 10 volts or you will a negative bias on the driver or a bit of positive on the grid. A stage of gain is possible ahead of the phase splitter. That would take another FET.


Title: Re: Simple Audio Driver for 833A modulator
Post by: Steve - WB3HUZ on October 31, 2005, 05:32:53 PM
I don't know how well this circuit may work, but it's interesting. Take a look.

http://www.qsl.net/wa5bxo/driver1.html


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on October 31, 2005, 06:22:27 PM
OK, Frank.

We'll have to see what you come up with.  So far, how many fets are you looking at counting the one in the cathode?   Pre splitter, splitter, 2 drivers and 2 cathode fets?

Thanks for the info on the driver, Steve.  Well, that one uses a transfomer [ :o] to drive the fets, a 600V? supply and drives the grids of the tubes.   I was hoping for a low level supply to power the fets maybe < 50V, no transformers and cathode driven.  Simple if it can be done.  If it gets too complex, I'll just PDM the 833A's like the 4X1 rig. But was hoping to have a plate modulated rig again with 12"X 12" viewing windows for both the final and modulators..  for your viewing pleasure, caw mawn.

T



Title: Re: Simple Audio Driver for 833A modulator
Post by: kc2ifr on October 31, 2005, 06:29:16 PM
Tom...looks like there is no free lunch.....


Title: Re: Simple Audio Driver for 833A modulator
Post by: Steve - WB3HUZ on October 31, 2005, 06:40:16 PM
It has a transformer only because the builder/author chose to use one. I think the circuit could be modified to be transformerless (is that a word?).


Title: Re: Simple Audio Driver for 833A modulator
Post by: W2VW on October 31, 2005, 06:43:43 PM
Stick with the cathode drive if you can. It eliminates the problem of dead FETs when the tube arcs internally. Sooner or later they will burp.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on October 31, 2005, 07:38:30 PM
Stick with the cathode drive if you can. It eliminates the problem of dead FETs when the tube arcs internally. Sooner or later they will burp.

Yep, no free lunches for sure, Bill. It's amazing how it's all a compromise, otherwise everyone would be doing the same technique and the rest wud be forgotten... ;D

Dave: Talk about internal tube shorts... I remember the time when Gary/INR had the IJY fet board driving the grids of his 833A modulators with a 4X1 final. G had sold the full rig to some African govt for a hefty price and was gonna ship it asap. Tron was visiting the shack and let out one last atomic yallo into the mike. A loud buzz rang out as an arc ripped it's way across the fet board, destroying every solid state device.  ;D ;D ;D   

BTW, I may have scored a nice 1 KW broadcash mod transformer. It was originally from a pair of 4-400's modulating a pair.  Now I GOTTA do the plate modulation 833A scheme.  I wonder how it will match up to a pair modulating a single 833A? I don't want to do TWO finals cuz this is supposed to be the "medium" power rig.  I don't know the xfmr ratio or impedances yet, but was just wondering. 

From what I know, a 1:1 ratio is good for one final modulated by a similar pair. And a 2:1 step down is good for two modulated by a similar two. Any truth to that?

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on October 31, 2005, 08:43:42 PM
Tom,
3 FETs 2 go in the cathode circuit 1 per tube. third fet a phase splitter.
Cap the couple the phase splitter to each gate of the two drivers.
Say you did a gain of 10 with 10 ohms in the driver drains and 1 ohm in each source
At 1/2 amp peak there would be positive 5 volts on the cathode. The closer to gain of 1 the closer to zero volts you can get. A resistor is required in the drain and source for it to be linear. the ratio of the resistors sets the gain. There has to be some voltage in the cathode circuit to be linear. wattage of each resistor IXIXR=W
a fourth fet could be added as a voltage gain stage ahead of the phase splitter.
The phase splitter should only need low voltage 12 or 24 volts to operate.
then a gain stage ahead of it the same voltage if AC coupled
The bias point of each tube would be set with a pair of pots setting the resting gate voltage on each driver fet.
Steve's find does not eliminate transformers or driver tubes just adds followers to lower drive z.
I wonder what kind of plate current you would get with 5 volts on the cathode.
I'll try to draw a schematic if I get a chance.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on October 31, 2005, 09:12:10 PM
It sounds reasonably simple, Frank.  I especially like the single 12V or 24V for the pre-splitter and splitter.  That's the only supply for the whole driver - wow.

You axed about 5V on the cathode. Did you look at the 833A curves posted back a few above?  It would seem to me that it would draw about 800ma with +5v on the cathode. [equiv to -5V on the grid]   800ma is PLENTY of current at 3kv, in fact way too much overhead... ;D    The mod transformer is good for 1KW.  Sounds good, OM.

If this works out, I'm betting many guys will try this circuit to eliminate transformers, tubes and other devices to drive their modulators.  A good audio driver has always been a pain for many of us, especially with very hard to drive triode tubes like 833A's etc. Dave[APE] mentioned that there can be transformer ringing when driving low impedance grids. The GG, cathode driven fet solution wud be quite FB - simple with  built in negative feedback and bulletproof to internal tube arcs.

I'm looking to see what you come up with for a working schematic. Specify actual fet devices, if you can, that we can order from the suppliers.

Tnx again, OM!

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 01, 2005, 08:16:12 AM
Tom,
I'm pretty busy today but will try to email you something. The class e fets will be perfect for this. 900 volt parts in a 100V circuit 11A parts in a 1 amp circuit.
I figured 5 volts of bias would be OK. fc


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 01, 2005, 09:33:07 AM
Tom just mailed you a schematic have someone simulate it The 2 k pullup on the input fet may need to be 1 k for maximum voltage swing of the phase splitter.
Gain=10. BMP file


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 01, 2005, 10:10:19 AM
Thanks, Frank .

Looks so simple, it's amazing!

I tried to upload it here, but it's too big of a .BMP to handle.  So, I'll have to see if anyone can simulate it and I will send it email.  What load should someone use for the driver output?

Could someone here simulate this driver circuit for us? It's three fets and 18 parts. I can email it directly to you.

Or, Frank, do you have a website to access it via URL, or put it in a format that is less than 300K to post here?  It's presently a 1 meg file.

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 01, 2005, 11:54:12 AM
Sorry Tom no site.  Also I would put a 15 volt transorb Gate- Source of each FET. 12 Volt bypass at each pot and bypass the wipers to keep RF out. All the normal stuff. I would drive the CT of each Fil transformer with good film caps each side to CT. Fets will need to be mounted on a heat sink since biased class A. You want to keep the fets cool so RDS/ gain are stable. first stage 2 K pullup on gate will need to be a little lower if you need more voltage swing.
lets hope someone can simulate it for you...


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 01, 2005, 12:18:47 PM
Hey Guys,
Interesting discussion, this sounds similar to a circuit I had worked up for a pair of 6L6's long ago.
The zero bias problem is a little different but a similar fet-in-the-cathode should work. this is essentialy similar to biasing up a shunt regulator to work like a series cathode or screen modulator. For audio you need two of these, one for each tube and driven out of phase. You could probably bias the grid up at +200volts and put the shunt fet circuit in the cathode. Gain would be approx 200/4=50.
Would have to check gain sensitivity vs temp to estimate idling current drift.

I'll try to attach a simple schematic.
Regards,
BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 01, 2005, 12:48:12 PM
OK Frank -

Good on the transorbs and the RF bypasses.  What value film caps for the fils?

Would the fil transformer sec winding have capacitance to the 110ac primary problems for audio? 

And, how about case to ground stray? Would mounting it above chassis and inch help there?

I tried uploading the schematic to my home page, but it will not take .BMP files.

So, maybe I can email it to someone here and they can post it. Anyone?? A group effort will speed things up to get a finished product.

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 01, 2005, 01:22:59 PM
what kind of file do you need i can convert


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 01, 2005, 01:30:51 PM
what kind of file do you need i can convert

Bill/IFR said he can post it from his home website. I just sent it to him. We should be in business soon.  Thanks, Bill.

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 01, 2005, 02:24:26 PM
Tom A good transformer should have good common mode rejection. The cap valuse would depend on how much you have. Also the CT balance would effect leakage. I suspect you can ground the transformer and if you are lucky it has a faraday shield. I didn't include any frequency roll off that can be added later.
As BobbyT said a pair of shunt regulators driven by a phase splitter. Two pots set the regulation point Keep the temperature stable and it will be fine. K.I.S.S.


Title: Re: Simple Audio Driver for 833A modulator
Post by: kc2ifr on November 01, 2005, 02:49:35 PM
Hi Tom,
Did not receive it as yet. Its 2:48PM....what time did u send it???
Just in case...my email addy
wgr@adelphia.net


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 01, 2005, 03:50:30 PM
Bill,
I just sent jpeg and bmp to you. fc


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 01, 2005, 03:54:52 PM
I see Bobbyt has a bypass to ground at the cathode. Good idea to keep the tube from oscillating.


Title: Re: Simple Audio Driver for 833A modulator
Post by: kc2ifr on November 01, 2005, 03:59:58 PM
Here is the pic.....I changed it to a jpg file...
www.kc2ifr.com/photogallery/833a_driverbmp.jpg (http://www.kc2ifr.com/photogallery/833A_driverbmp.jpg)


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 01, 2005, 04:15:18 PM
Thanks again, Bill. Came out FB.  I originaly sent the .bmp back to you a few minutes after your offer. O'well, lost in cyberspace.

Frank: Reply to your comments:

Tom A good transformer should have good common mode rejection. The cap valuse would depend on how much you have.

*****  How much I have of what... capacitance value?  Name a value. ***

Also the CT balance would effect leakage. I suspect you can ground the transformer and if you are lucky it has a faraday shield.

**** OK, so you're saying to mount the fil transformers directly to the chassis. ***


I didn't include any frequency roll off that can be added later.

*** Freq roll off for what reason... stuff above 10kc generated by the fet driver stages?  ***


As BobbyT said a pair of shunt regulators driven by a phase splitter. Two pots set the regulation point Keep the temperature stable and it will be fine. K.I.S.S.

** I've gotten a number of emails from guys wanting to try this. I'm thinking it would have been a common technique back in the 50's *IF* they had Fets then. For some reason, it didn't catch on once high voltage MosFets were developed..***

*** BTW, I plan to get on with Mike/ZE later tonight after 7PM on 75M, if the band is good, for an 833A discussion. You might join us if you're around Frank. Mike is keenly interested in doing this driver for his 833A rig too.***

T



Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 01, 2005, 04:20:50 PM
It would be a good idea for someone to simulate it. switchcad should work.
This way there are no surprises. Roll off would be easy cap gate source.
As you know these fets work well into HF so some roll off might be a good idea.
Caps across the fil transformer .22 for starters. My guess. Radio shack sells diffent film caps.  fc


Title: Re: Simple Audio Driver for 833A modulator
Post by: Bacon, WA3WDR on November 01, 2005, 09:32:21 PM
I'm liking it... yes, I think you're right, the 833A in AB1 is a good way to go, and it looks like it doesn't even need a driver supply.  I hope the cathode capacitance (rf bypass and filament transformer capacitance to ground) doesn't kill the top end.  Precompensation and negative feedback could probably fix that, if it's even an issue.

This basic approach should be possible at lower power levels with 812s, tetrodes, etc.  Hybrid classic.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 02, 2005, 08:10:11 AM
The fil bypass would go between each side and CT. An RF bypass of low value to ground. The only C would be the transformer but this is a low Z circuit so who knows if it will have an effect. I'm thinking lower resistor values would be better at each gate to keep it RF proof. Noninductive resistors for the 1 and 10 ohm so as not to set up a tank.
someone should simulate it.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 02, 2005, 08:54:23 AM
Hey Guys... Just as a reality check note.

Ballpark 833A 500W design from a quick look at the tube curves:

Po = Ipeak*(Ep-Vpmin)/2 = 0.7*(3000-300)/2 = 945 W
This assumes some mod tranny loss and a bit extra for pos peaks or compression.

Eff = 78.5(1-Vpmin/Ep)=78.5*.9 = 70.5%
RL = 4*(Ep-Vpmin)/Ipeak = 4*2700/0.7 = 15.4k
Popeak = Ipeak*(Ep-Vpmin) = 0.7*2700 = 1890 W
Pavg (check) = Popeak/2 = 945 W
DC supply = Po/Eff = 945/0.705 = 1350 W = 3000V at 0.45A
Pdiss = 1350 -945 = 405W = 203W per tube

Pdrv-peak = Vgpk*Igpk =125*0.2 =25W
Pdrv-avg =Pdrv-pk/2 =12.5W

The above drive power is for classic grid drive not for cathode drive.
Our discussion here is more like a grounded grid Class B Linyear.

As such, we have to support both grid and cathode current.
therefore Ipk grid + cathode = 0.2 + 0.7 = 0.9A.
Looks like the mod driver has to support approx 60Wavg and 120Wpeak.
of which 12.5W is for the grid and the rest is added to the Pout.

Sounds like a 15W solid state block with reversed audio xfmr
might really be the way to go after all.

(You can do the basic audio block with a 40 year old design using
a 25.2v fil xfmr dc supply and about 5 real cheap xstrs plus audio xfmr)

Of course, I may also just be having a senior moment.
Isn't this fun or what!

Regards,
BobbyT   


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 02, 2005, 09:16:40 AM
Tom Vu wants no transformers and grounded grid so that was where we started. Still good idea to keep the other method in brain.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 02, 2005, 09:55:04 AM
Hey Guys... Just as a reality check note.
As such, we have to support both grid and cathode current.
therefore Ipk grid + cathode = 0.2 + 0.7 = 0.9A.
Looks like the mod driver has to support approx 60Wavg and 120Wpeak.
of which 12.5W is for the grid and the rest is added to the Pout.
BobbyT   

Thanks for the schematic you sent email, Bobby!

I have a question about the data above.... since we're using 900V, 10A fets in the cathode, why is it a concern that it is switching 120W? Are you talking about power efficiency of 15w to the grid vs: 120 watt losses associated with the analog fet in the cathode or something else?   I suppose if things get warm we can use a bigger heat sink or simply parallel another fet.  Though, the lack of a bias supply with bias voltages on the fets, and inherent negative feedback of cathode drive with just one 12V supply attracts me more.

BTW, I received another great schematic for some improvements upon Frank's original design from Tom, WA3KLR. It uses low distortion op amps as Fet drivers, negative feedback to these drivers and a regulator chip/ fet to prevent 833A bias drift. This has great potential, I think. A little more complex, but appears well worth it. He said it probably needs tweaking.

I am posting here as a pdf file. Click on the the WA3KLR .pdf file below....  Check it out and please critique. Tom desires a simulation of it. Let's improve upon it.

73,
Tom, K1JJ


Title: Re: Simple Audio Driver for 833A modulator
Post by: 2ZE on November 02, 2005, 10:23:27 AM
I like Tom 'KLR's design alot. I like the bias regulation. What OP amp does he use?
TLO74 or maybe a 5532 would work.
Frank's is so simple however.
As soon as someone sim's it, I have the parts ready to go (from my defunct E-PDM project). Could have it on as early as this weekend.

Gotta love the the brain power on this board!

Mike
W2ZE


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 02, 2005, 10:33:41 AM
Hola Mickey -
What a great time on the air last night, huh?  True ham radio, exchanging "vital" information. I was disappointed that no one even laughed at my Limbo song. All shop talk... ;D  We gotta do it again, soon.

Anyway, Tom speced using Dual TL 072BCP precision op amps. I think that means ALL the chips including the ones labeled "Fet driver"?

The other three big fets can be the standard 10A, 900 V variety for $250 each. [two cathode drivers and regulator]

He mentioned that he thought there wud be distortion WITHOUT the bias regulation, so added it. Can't hurt.  I'm pretty sure this can be pulled off without any bias on the 833A grids, just grounded. Imagine, just one 12V supply and the whole driver works.  How about that neg feedback loop around the op amps?  I think that with the cathode inherent feedback and this loop, that's all we will need for a super clean total modulator system.

Now we need someone to simulate it.

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: 2ZE on November 02, 2005, 10:48:28 AM
I guess I'm being a dunce here ( woudn't be the first time). How are the tubes getting biased? Are they being biased from the cathode? and how would it be adjustable?
I would like to have control over the idling current, and I just am not seeing it.

Also, how big do the 1 ohm resistors need to be?
also standard gate, drian and source busses need to be used( heavy copper connections)

I actually need to place a DIGI-KEY order for work today, and I will just piggy back my parts onto it.

the only thing different I am going to do is have another OP-amp stage right at the audio input so it is balanced in.

Please advise??

PS - I think everyone was a little "poker faced" with tech talk last night. Thats what happens sometimes. However, it was better to have everyone on the same page discussing one common topic, lowers the risk of QSO implosion ten fold.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 02, 2005, 10:56:37 AM
Tom Vu,
Resting cathode bias is set with current flowing through FET to provide a bit of positive resting voltage on the cathode. Remember the tube would be saturated if the fet was full on....like my pdm driver. The fet needs to be in the linear mode so it has monkey swingage. When the fet is in linear mode it is acting like a variable resistor.
It is also making some heat. Say you need to bias at 50 volts at .5A then the fet is making 25 watts of heat at resting carrier. It swings between full on and full off.
Resting carrier is probably worse case dissipation.  fc


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 02, 2005, 11:03:15 AM
I guess I'm being a dunce here ( woudn't be the first time). How are the tubes getting biased? Are they being biased from the cathode? and how would it be adjustable?
I would like to have control over the idling current, and I just am not seeing it.

Please advise??

Please advise, wilco, over and out...
So solly glashoppa, there IS no bias adjustment. You run them balls to the walls in class AAA until they puke.   ;D ;D

Actually, the tubes are DC biased with cathode bias, controlled as described below.  The dynamic audio voltage [fet] swings from ~ +70V to zero and this equates to zero to -70V referenced to the grid.  -70V = cutoff and 0V = 900ma or so. Mucho monkey swing. All depends upon input audio level, of course.

Mike, your'e looking at the new schematic, right? Take a look at the 78L05 regulator chip at the far right. R5 should really be an adjustable pot. This controls Fet M3 and IC  U1 that controls the gate DC bias of each cathode driver fet. C1 and C3 accepts the audio voltage riding on top.

Needs simulation before you build it. I so vely vely glad you be the first man on Earth to build it on a working set of 833A's. Get the bugs out for Mr. Vu.

Tom/KLR just sent me an email saying he intends to make more improvements on it and make more comments. No one is to build it yet. He will try to tailor it more to the 833A. All  three fets go on the same heatsink. More later. Stay tuned.

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 02, 2005, 11:44:28 AM
Tom KLR
Change feedback to a resistive divider. The 833 should be off at 100 volts on the
cathode. The input of the op amp needs to be 5 or so volts at carrier. A zener clamp or diode clamp could protect the op amp from over voltage at input. This eliminates the caps and 2 resistors in the input bias circuit. This does add the 100 Volt pull up voltage. My circuit I did the series the series resistor to fix the gain at Rd/Rs. I wondered if this would introduce distortion but could not see how. I have seen it done cascaded fet amplifiers. The upper FET has a grounded gate. We all know a FET and triode are the same thing. KLR circuit controlls the drain voltage with an active loop so more complicated but sexy.
I had a boss once who thought a design with the least number of parts was the best.
Lets see how far we can go with it.


Title: Re: Simple Audio Driver for 833A modulator
Post by: 2ZE on November 02, 2005, 11:51:19 AM
Quote
Actually, the tubes are DC biased with cathode bias, controlled as described below.  The dynamic audio voltage [fet] swings from ~ +70V to zero and this equates to zero to -70V referenced to the grid.  -70V = cutoff and 0V = 900ma or so. Mucho monkey swing. All depends upon input audio level, of course.

AHHH HAA!
I semen.
Didn't study schematic close enough to see that Tom mentions adjustment, but didn't draw the actual ass-wiper on the pot. Sooo Solly mastel KLR.

Getting parts together, and will standby for further updates.

Like a fat kid playing dodgeball....
I'm Out!


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 02, 2005, 03:05:23 PM
I was trying to see the difference between the two circuits .. why Tom/KLR wanted a
regulated  bias adj. Now I see that it's for thermal drift of the cathode Fets. The
regulator fet is on the same heatsink and compensates.  So, has nothing to
do with dynamic distortion.

So, if the fets are thermally stable enuff, and I suspect with a range of
0-70V being so huge and loose, and a big heatsink, we might not need the thermal compensation.

The feedback Frank said he cud add with a resistive divider. The remaining question is if precision op amps will make any difference at all in performance. I suspect you'd never notice the diff, but that's open for debate.

These observations are not fact - I'm just trying to generate discussion and maybe pull the best features into one circuit. Frank's is certainly simpler in parts. We'll have to see in simulation what they do. Frank mentioned he has loaded it into Switch-cad for a simulation later.  We're making progress.

T


T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 02, 2005, 03:50:59 PM
I'm stumped how do I simulate an 833 hanging off the drains.
any suggestions? No triodes in th epackage.
Somewhere in th ehouse I have a circuit of a fet driving a tube. I need to put on the miner's helmet.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 02, 2005, 05:15:29 PM
Frank,
In actuality, large tubes are almost direct physical scaleups of small tubes.
Relative physical spacings-etc.

Some times the low power models in spice are not power limited.

I've used a 6L6 at high voltage to simulate a 4-400 for example.
I just tried the 12AX7 and 12AU7 models and generated a kilowatt
of audio per my previous ballpark design thread.
The bias is not quite right, but close enough for having fun.
I normally use the Circuitmaker program.

Regards,
BobbyT

PS-i did find the old schematic for the 30watt 6L6 amp using FET's in the cathode.
I'll try posting it here as a .wmf file. No guarantees-it's just for fun

Note:The .wmf file didn't attach


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 02, 2005, 05:45:06 PM
Just another question:
Remember that the opposite plate is swinging to at least 6000V.
doesn't that mean you'll need at least 150 to 200v to ensure cutoff.

Maybe bias at +100V with FET pullups to +300.

The pullups probably will be needed, since the tube will appear as a lower value current source near cutoff thus with high frequency 10KC audio and a bit of filament capacity to ground may cause loss of the cutoff peak value and cause the tube extra dissapation.

Regards,
BobbyT




Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 02, 2005, 09:27:23 PM
Good piont,
The transformer will swing the voltage to double so yes more positive bias is required.
Pullup: consider this my GG triode connected 4cx3000A linear plate current dropped to zero with only a 50K series resistor with no pull up required (6KV). The FET near cut off will be a very high resistance so a low current will produce plenty of bias.
maybe the fet can be shunted by a 50 K resistor to limit voltage swing. I also put a fuse in the cathode circuit.
I still can't convince myself that the pullup is required. I've thought up a different configuration part mine part Tom KLR. A op amp driving a phase splitter to the two driver FETs with feedback around the whole modulator. Still this adds complication.
mulit op amps in one package has been a problem with high level signals. We actually measured cross talk across a quad op amp die at work. Head spinning after an hour and a half helping my daughter with Algebra homework. Word problems are a trip.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 03, 2005, 08:30:22 AM
Frank,
I seems possible that if the gate is driven to ground that the tube can
pump enough current through the effectively open ckt (high Z) fet drain,
even if its only 10 mils, such that the fet breakdown voltage will be exceeded.
It's very hard to read the tube curves here.

You mentioned a resistor to ground. The nice thing about the 2 resistor,
1 FET KISS circuit is that along with the local negative feedback to improve the distortion,
stabilized dc gain, (and fixed ac gain) is that you already have a built in resistor to gnd.
Note that if the audio driver to the gate has too low an output impedance, it will negate
the local negative feed back and you will need to wrap neg feedback around the driver.

Regards,
BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 03, 2005, 09:24:38 AM
Hi,
Yup phase splitter Z could be an issue. You want the drive Z low enough to avoid RFI problems but not too low. Still injecting feedback is no big deal. I think someone should try my circuit then if there is problems remove the phase splitter and add KLR op amp drive on a small board. The op amp interface to high C gates may need attention. OP amps don't like high C loads and tend to oscillate. This can be eliminated with a small chance to the feedback configuration. Values will depend on the op amp and load.  A 50K resistor across each driver would keep the voltage from soaring above the rating of the FET. Yes I don't have any clue of tube curves at very low cathode currents. I know from GG linear days the 50 Kohm load across the cathode relay contacts limits the voltage. So what if the tube leaks a couple mills it will be more stable rather than sitting as a hair trigger wanting to oscillate.
I take it you agree we don't need a pullup but a 50 K pull down is good insurance.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 03, 2005, 10:46:47 AM
Frank,
Sounds like we're getting closer to building a prototype. 

Mike/ZE said he wants to build something this weekend for his rig.  So maybe the next move might be to simulate it the best you can and come up with a first schematic to try, based upon the current thought.  Mike's tests will give us a lot of data to work from. Give Mike a list of test parameters you want checked.

If Mickey builds it with some room to spare to add to/ modify later, it might work out FB. I won't be building mine for at least a month. I just built two chassis for the 833A final and modulator last night, so far away yet.

T



Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 03, 2005, 11:00:25 AM
Tom I would start with the schematic I sent you and add 2 50K resistors drain to ground. This will add some protection to the FET as BobbyT mentioned when the tube that is off swings above 6 KV. My bias circuit should work. If it doesn't then KLR circuit will need temperature compensation. Just use regulated power supply for the driver. Use a heat sink for at least 100 watts dissipation. Then leave open space to add a little board later if necessary. Sorry don't have the time this week to play simulation. Work gets in the way. You retired guys can do that.  fc


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 03, 2005, 11:30:35 AM
OK, I made changes on the schematic.

Hopefully Mike will try it.

With the schematic as shown, it appears more gain is needed to go from 1/2V  line level input, right?  Maybe this is where an op amp and some neg feedback will be introduced later on.

The cathode fets are the 900V 10A ones, but what do we use for the phase splitter fet... will one of those work there too?

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 03, 2005, 11:46:34 AM
Tom,
I would also use a high voltage fet for the phase splitter just in case a transient is coupled back. Don't forget transorbs gate- source. coupling cap values will effect low end response. I suspect .1 to 1 UF film will work. Radio shack had a good selection of them. Also put a .001 to ground on each filament lead to keep the tubes from oscillating. A .01 ceramic + 1 to 10 UF on each bias pot wiper would also be a good idea. Remember to have bias pots set to zero when you first key the rig so the tubes don't get red in the face. This circuit could be tested in any rig with 2 filament transformers in the modulator. The circuit will work a lot better if all the resistors are noninductive. but it should function with wire wound. This is only to keep any inductance out of the cathode setting up a tank circuit.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 03, 2005, 11:49:44 AM
BTW you will need more than line level but another stage could be added later.
FET would need 4 resistors and a cap. Same power supply. or an op amp stage would also work.


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 03, 2005, 11:53:52 AM
Some status -
Well, I’ve simulated my 1st-pass design and Frank’s.  By the way, I realized that I left out 47k series resistors ahead of U2 and U6’s (-) inputs.  Both circuits do appear to work.  I simulate the voltage and current load that the 833 may present to the FET drivers by using a 125 Ohm pull-up resistor to +100 Volts.  I am not simulating the full application unfortunately!

I found a triode, tetrode and pentode in the SwitcherCAD library and I used the triode symbol in my first published schematic that came via Tom K1JJ.  But later investigation reveals that they are symbols only with no macro-models.  I am not going to attempt to develop models for the tubes.

Since we don’t have the ability to simulate 833’s, the sine output of the modulator transformer isn’t available.  I can’t analyze the class B waveform, so I have to keep the drive level down to the point where the cathode signal is still class A.  I am seeing about 2.0% THD on my circuit and about 4.6% with Frank’s.  The distortion would increase at higher drive levels that will actually be required.

One problem I see with my circuit is that the bias current levels rising with drive.  Frank’s circuit, the bias point remains at 50 mA. per tube during drive.  I do think that the bias point would be very temperature sensitive with Frank’s circuit.  Perhaps his circuit with my bias circuit is a good mix there.

So I am considering having to do a major re-thinking on my circuit.  Right now, the feedback from the driver output is ac-coupled back.  I think it was Frank that may have suggested adding dc feedback and I also feel that this is the necessary way to proceed on my design.  To get everything needed, I think that my first schematic does basically represent the complexity needed to fulfill all requirements.

By the way, the FETs could probably be a smaller FET, like something in the IRF730 or IRF840 class.

All for now, Tom WA3KLR.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 03, 2005, 12:07:45 PM
Tom-JJ,

Sorry, I forgot to address your question on the extra FET driving power.
The intent was just to indicate that the driver had to support (not supply)
more power than a regular push-pull grid driver.The FET can easily handle
having to sink the total cathode current (grid current,plate current
and pullup current).

Note: I'm still estimating higher voltages than the other guys since we
probably want to accomodate older tubes with lower emission.

The drive circuit power supply for the grid (approx +100 volts) only
supplies the grid driving power.

The pullup power supply (approx +300) only sources power for the
pullups (=20ma each =approx 1/2 the idling current value of 40ma each), and actually sinks

cathode current from the tube during cutoff.

The 3000V supplies only the plate current of course. It's interesting, however,
that with the FET in the cathode, biased up about 180-200 volts, the idling dissipation
of the tube is reduced slightly since its drop is now only 2800v. The remaining dissipation
is taken up by the FET. The full modulating swing of 3000v is still available
since during the peak current swing, the FET drain voltage is only 10-20 volts.

Regards,
BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 03, 2005, 12:10:12 PM
Frank,
To address your question on the need for pullups:

It would seem to be of great advantage if we had a modular design which could be
initially tested and aligned to the expected modulator requirements without actually
connecting to the tube or having to mess with 3KV in the vacinity.

The FET driver and power supply for the grid bias and the pullups can be connected
independently of the tube if the pullups are present. Additionally, we could connect
external test resistors (maybe in series with a diode and using a separate supply) in parallel with the pullups
to draw extra power to somewhat simulate the extra current drawn by the tube.
(This might be the way to check for temperature drift)

You could do the initial adjustment to obtain the required 70 volt differential
between Vg and Vk. This way, when connected to the tube, you are already in the
ballpark for the expected idling current. You can also run all of the initial
audio tests, add stray capacitance, etc. Doing this on the bench might be easier
than doing it in the rack, especially for an old fart like me.

Regards,
BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 03, 2005, 12:17:23 PM
Tom,
I agree on the FETs but the 900 volt units would make it a lot stronger.
I wonder what is the source of distortion? Would it be better to use lower resistor values in the phase splitted drain / source? Feed back should be easy to apply later.
Yes I noticed no input resistors in the op amp. Maybe the driver reisitor values are a bit low. Don't forget simulating the 833 with a pullup resistor will set the gain higher.
That could degrade distortion. The tube is going to be a variable resistor load.
Tom also look at using op amps driving high C loads. Usually you have to put another resistor in the gain loop in series with the op amp output with the feedback resistor outside the resistor. I have sen op amps take off without the resistor. Ive even seen oscillation with op amps driving shielded cable.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 03, 2005, 12:42:41 PM
A thought about bias stability:

From the tube curves, it appears that gm = 700ma/50v=14ma/volt

This implies that to achieve approx 50ma +/- 5 ma stability for
the idling current we require about +/- 0.3 volt stability on the bias.

This normally wouldn't be so bad, but since we are trying for approx 70 +/ 0.3volts
for Vgk, and Vgk now depends on both the FET drain-to-ground stability
and the +Vg supply stability it suggests that the +Vg supply might needs to be regulated
with perhaps a front panel small delta-V control available.

If not regulated, e.g use of basic line supply, maybe a very long time constant
differential tracking gizmo can be devised.

BobbyT-NU2B


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 03, 2005, 01:27:40 PM
The whole driver circuit would run off a 7812 voltage regulator. Temperature compensation could be series diodes with 1 side of the bias pots. The RDS -on will increase with temperature so resting current will go down as heat goes up as long as the tube is stable. It could become a hum monster if the power isn't regulated.


Title: Re: Simple Audio Driver for 833A modulator
Post by: 2ZE on November 03, 2005, 03:17:47 PM
Frank, and All:

OK on all so far. What about a FET driver op amp, that is used to drive a High C input. I think it would be better than an op amp, but the FQA11N90's are pretty low C as fets are concerned.
I plan on using a small 10"x6" chassis and mounting it dirctly to the heatsink. I can mount a circuit on perfboard and mount it there. This will also house the transformer for the +300 vdc supply which I can regulate for any voltage 0-300 vdc. makes a nice neat package that I can mount to my mod. deck and keep the grid leads short.
Also Frank, the cathode reisitors I am using now for degen. feedbak are 12 ohms, so even better.
As soon as a design looks good, I can build. Might have to be next weekend now, but no biggie.

Mike


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 03, 2005, 04:14:01 PM
Mike
I don't think you need 300 volts. You do need 12 volts or 24 volts regulated.
I see no reason for a high voltage pull up. A 50 K resistor across the fet will work as well as the resistor in a linear amplifier to hold a tube in cut off. So you have 12 ohms and I have 10 and 1 ohm. You could throw my circuit in with your present bias and test it low level. They you could slowly work your grid bias to zero and see what happens. Yes a fourth fet amplifier could be added for more voltage gain. 4 resistors and 1 cap and the fet. Same 12 or 24 volt reguulated supply. Take my circuit. Add 15 volt transorbs gate source each fet. Add more bypass caps as I said earlier.
Then add 50 K resistors across the FETs to limit voltage. Sounds like your rig would make a good test bed and the grid bias will keep it safe. Yea. just bolt the heat sink to the chassis and mount the parts inside. The little regulator will be the hardest part.  Feed back is no big deal just have to decide how much and where you want to connect it. We will run everything class A and see where it takes us. fc


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 03, 2005, 04:16:43 PM
Mike you don't need no stinkin grid leads for this baby....grounded grid
The drive goes into the CT of each fil transformer. Then put a couple caps cT to each leg and some RF bypass to gnd.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 03, 2005, 05:59:15 PM
We don't need no stinkin' grid bias anymore!   ;D

Hey Franz, I was wondering about the distortion figures Tom simulated. What is considered a good figure, under 1%?  I also will be curious what caused that. I notice that Tom's,  even with feedback, had over 2%, but maybe it was caused by the bias moving around.  Your bias was solid, but no neg feedback.

I wonder what a pair of 833A's ALONE generate for distortion figures?

Knowing that a -30db IMD linear with a good balanced modulator driver is around 0.1% makes me wonder.

It's really something to see three of the brightest engineers on this BB go at this project.  Thanks for the continuing efforts, guys. When we get a finished product, we'll see to it that  it gets well published so that anyone looking to upgrade their modulator can do so.  2ZEe and I will be initial fodder.

Mike, I'm picking up my 1KW mod tranny tonight at around 7PM. So might be back and on the air by 9PM. If not, then catch ya on tmw night for sure. 75M, of course.

T





Title: Re: Simple Audio Driver for 833A modulator
Post by: WA3VJB on November 03, 2005, 09:04:32 PM
Gary brought my attn to this thread. VERY sharp minds at play.
May I volunteer any aircheck recordings to post as MP3 for the group to hear?

Tom -- earlier in the thread you said --
Quote
Dave: Talk about internal tube shorts... I remember the time when Gary/INR had the IJY fet board driving the grids of his 833A modulators with a 4X1 final. G had sold the full rig to some African govt for a hefty price and was gonna ship it asap. Tron was visiting the shack and let out one last atomic yallo into the mike. A loud buzz rang out as an arc ripped it's way across the fet board, destroying every solid state device.  Grin Grin Grin

The historical record shows the rig in use at the time was the INR 4-400 rig.
The Pair 833A into 4X1 was the next job, and still soldiers on, tribanded at W2KBW.
Worked it and him recently on 40m.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 04, 2005, 10:34:53 AM
Tom,
I'm thinking about the distortion. I'm not sure where wer should put the feedback
The 1 and 10 ohm resistors put degenerative feedback on the tube. Heck Mike is running 12 ohms fixed. I would start with no feedback and see what happens.

Look at a grounded grid RF amp and consider what is happening here. Similar.
Brain working but pesky work is getting in the way today.  fc


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 04, 2005, 11:26:14 AM
And I remember one Saturday afternoon when I got a call from a guy in Africa asking about the rig because it blew up. The IJY board had under sized heat sinks that caused the driver to blow up.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 04, 2005, 11:27:53 AM
Mike ZE can you measure your present distortion to get a real base line distortion number?


Title: Re: Simple Audio Driver for 833A modulator
Post by: 2ZE on November 04, 2005, 05:50:42 PM
its around 2-3%, I know.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 04, 2005, 09:47:55 PM
So mike that is the base line. I'm thinking Tom klr simulation of pull up increased the gain a lot since the drain resistor was increased so high. This may have increased the distortion. one thing a pull up would do would be to put the stage more into class A
so who knows it may help on the turn off of the tube????


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 04, 2005, 10:47:14 PM
Guys,

I'll publish my new version schematic here tomorrow.  It is now a shunt dc voltage regulator configuration; same complexity as before.  The tube bias is regulated voltage, not current regulation.  You have the option of the same biass for both tubes or independent bias adjustment by the way you wire that circuit.

You need +15V, -5 V and +120 V.  The -5V is needed to get the common mode input range and output range required from the op amps.  The +120 V is for cathode pull-ups which I feel is needed.  Current on this supply is 24 mA. max.  Does not have to be regulated or super-pure dc, but stay above 120 V worst case.

FET dissipation (each) is 17 Watts worst case.  Only 2 FETs.

Q to Tom K1JJ - What audio level input do you need?
 Right now, the circuit requires about +12 dBm for max output.  This is probably a little high for most folks.

Sleep tight.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 04, 2005, 11:47:13 PM
OK, Tom -

+12dbm is FB for my set up here. I usually run it at 2 volts out.  +12dbm is 0.9 V, so plenty.  I have several adjustments anyway.

Mike is that OK wid you?

Tom, did you simulate your new circuit, and what did you see for distortion this time?

I'd be curious how you think Frank's circuit performance compares with your new version using more components and the two extra power supplies ?  I don't mind building one more complex if it reduces distortion, etc.

The separate bias controls is a good idea. Most of my tubes are pulls of questionable emission.  Never a matched set... :D

T



Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 05, 2005, 09:26:43 AM
We better specify in terms of peak audio of a sine wave.  You appear to be using RF dbm.  I was referring to audio/telephone levels of voltage. +12dbm is 3.1V rms.

I need a little over 4 Volts peak to get the FET driver to full output.  I need to get this down. 

You say 2 Volts is what you use. Is this the rms value or peak value?


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 05, 2005, 10:54:09 AM
OK, Tom-

I meant about 2V p-p audio.   I can go higher or lower, but 2V p-p is what I have the various rigs set to accept.  Standardized in the shack.

But, I think most guys use line level at 0.5 volts, right?  Might want to put in a pot and more gain to accept that...

T




Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 05, 2005, 11:35:28 AM
I don't know what people are running for audio interface level.  When I build my Class E rig and start to use an external processor I will probably use at least 1.5 V peak (3V p-p).

So I will change the design to take about 0.75V peak.  Fixed level, no pot.  For those who run a higher level interface, they can easily add some attenuation by a resistor to gnd. at the input where I have a series resistor and r.f. bypass cap.

I may be able to design out the -5V supply.


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 06, 2005, 11:27:20 AM
Attached is my completed 833 modulator FET cathode driver schematic .pdf file.

I did have to retain the -5 Volts supply for the op-amps.  3 power supplies total.  But all are low current.

The FET dissipation is 5 Watts worst case each.  The dissipation is 3.5 Watts each at idle.  They can be on separate heat sinks.

The total harmonic distortion at 1 kHz. with 75 volts peak-to-peak output swing
was 0.08 % THD in the simulation.

The frequency response is 65 Hz. – 7.9 kHz. for -1dB.
                                     34 Hz. – 15  kHz. for -3 dB.

C7 at the audio input determines the high frequency roll-off.  The driver circuitry following this input network has a response out to 60 kHz. This is needed to achieve low distortion via feedback in the audio range.

The delay through the phase inverter in channel 2 is estimated to be about 1 degree of phase shift at 10 kHz.  So this approach should be fine.

I've never shipped out a schematic before that I haven't prototyped myself.  But I have done quite a bit of work and simulation on this design.  Tom and Mike, it should go well for you.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 06, 2005, 12:15:33 PM
Outstanding Tom!

Those distortion and response figures look FB. Regulated bias, negative feedback around the driver, what's not to like?

You obviously spent a lot of time on it and we appreciate a working circuit.

Probably Mike will build it next weekend and I will follow through within a month, once my new 833A rig is together.

Maybe a few of the guys here will have suggestions for improvements before I build mine.

Thanks again, OM.

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: W2VW on November 06, 2005, 02:28:23 PM
My suggestion is for Tom Vu to buy lunch for Tom KLR. Not that bait stuff either.


Title: Re: Simple Audio Driver for 833A modulator
Post by: W3SLK on November 06, 2005, 04:20:44 PM
Forgive me for being thick here but what are the advantages of driving the cathodes of your modulator tubes VS the grids?


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 06, 2005, 05:37:30 PM
Hi Mike,

Well, the first reason was for simplicity, though as it turned out, this circuit is as complex as other circuits out there driving the grids - though uses easier to find low level components.

That aside, one advantage is there's built in negative feedback for the 833A's being a grounded grid stage. Just like a GG RF linear.

833A's are a hard tube to drive. There has been known to be transformer grid ringing with these "dumb" low mu triodes. A fet can sink plenty of current and ideally suited to service in the cathode.

Another reason is that the normal -100V [or whatever] regulated grid supply is no longer needed.  The grids get tied directly to ground and the cathode fet sets the bias via a pot for each tube.

Done correctly, an older electronically regulated tube grid supply used a lot of components. Also, when using the old IJY  FET driver board into the grids, normally needed were several HV supplies, like plus and minus 300 and +600 if I remember correctly.

Tom's requires just +5, +15, and  120VDC at 15ma.  Pretty modest considering this powers the complete driver including providing both modulator regulated biases.

Tom's design uses very little power, like 10 watts worst case - check his specs above.
And I like the .08 % THD figure for the driver...hard to beat that.

Another advantage pointed out is that internal tube arcing has been known to wipe out fets driving the grid, wheras a cathode fet is less prone to this.

I think that covers most of them - there's probably other reasons. But, in the end, driving the grids is a FB method too. I think what inspired us was this application was relatively new for most Amers with few guys using it.

Personally, I wouldn't want anything else if I were building a new rig. [and I am] Good conversation topic when it's running on the air. When perfected, we hope to showcase it and encourage others to build it who are presently running iron transformer drivers of various kinds.... or just want a good winter project to improve their rig's fidelity.

73,
T


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 06, 2005, 06:27:42 PM
Wondering about the power supply?  Not to worry.

The total d.c. power required is about 2.3 Watts.  Use one of those 5 – 10 VA international transformers with dual primaries and dual secondaries.  Run the 120 V line into one primary and use the other primary winding for the 120 V supply.

See attached power supply schematic for a suggestion.


Title: Re: Simple Audio Driver for 833A modulator
Post by: W3SLK on November 06, 2005, 06:29:33 PM
I'm contemplating building a single 833 rig modulated by a pair of 810s. Could I apply that same circuit to the cathodes of the 810s and expect to modulate the 833 to at least 100%?


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 06, 2005, 07:22:10 PM
The scheme is to keep the modulator grid grounded and only drive the tube in AB1 as the cathode only gets within a volts of ground at the peak.  The grid could be biased positive in this scheme to allow some positive grid swing.  So there is much less power developed than the maximum the tube can do in the data sheets.  You want to have the plate voltage at the maximum ratings.

We’ll have to see what Mike 2ZE gets with his modulator but it’s probably at most 15 – 30 % of the maximum output in the data sheet.  I am just guessing at this point.

For a pair of 810’s, I think it is looking rather iffy for developing enough power for a legal limit final. 


Title: Re: Simple Audio Driver for 833A modulator
Post by: W2VW on November 06, 2005, 07:26:18 PM
OK you just lost me. 2.3 Watts power consumption? I'd expect the 833 stage itself to have less than 10 db of gain. It doesn't add up. The Fets need to make a lot more soup or I am missing someting??? Lost in Joisy.


Title: Re: Simple Audio Driver for 833A modulator
Post by: W2VW on November 06, 2005, 07:28:14 PM
Oh AB1. That's it. But I still dunno if you can develop the grid cathode Voltage at that power.


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 06, 2005, 07:38:35 PM
The FETs are acting as variable cathode resistors. The current comes from the tube and it's plate supply.  The grid-to-cathode voltage swing is from -110 Volts to -1 Volts.  The FET basically doesn't take any power to control it.


Title: Re: Simple Audio Driver for 833A modulator
Post by: W2VW on November 06, 2005, 07:39:56 PM
Gotcha thanks.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 06, 2005, 09:04:44 PM
The power supply is pretty slick too, using that extra 120V winding. The transformer can even be ordered from Mousier for $9.

I see a typo on the driver notes under "DC Gain Trim". It says to set the bias pots to 1000V. I think you mean 1V, right?

Can't wait to build and try this or whatever version we have a month from now. You're one heck of an engineer, Tomas!


T


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 06, 2005, 10:25:34 PM
Mike,

I’ve been studying the plate current/platevoltage/grid bias charts for the 810 and 833.  I think I can say what the power being developed is for grid swing to 0 Volts. 

For 810’s at 2500 Volts, it looks like you can get about 330 Watts out.

For 833’s at 3000 Volts, it looks like you can get about 770 Watts out.

The rule of thumb, then looks like for AB1 only, 40 – 45 % of the full output in the Class B modulator tube table.

As I mentioned before, with this FET driver you could drive the tubes into AB2.  You put positive bias on the grids and raise the cathode pull-up supply voltage.  The FET driver puts out a greater cathode voltage swing and handles a higher peak cathode current.  The 2 FETs will dissipate more power.


Tom,

The decimal point is in there after the 1.  But it is very small OM.


Title: Re: Simple Audio Driver for 833A modulator
Post by: W3SLK on November 06, 2005, 10:29:36 PM
Thanks Tom. I never even considered driving the cathodes. I'll keep this circuit in mind when the time comes. I think we'll wind up discussing this on 160 one of these nights.


Title: Re: Simple Audio Driver for 833A modulator
Post by: Steve - WB3HUZ on November 07, 2005, 12:35:58 AM
JJ sez:

"833A's are a hard tube to drive. There has been known to be transformer grid ringing with these "dumb" low mu triodes. A fet can sink plenty of current and ideally suited to service in the cathode."

Why would a transformer be in play in the driver if you are running the modulators AB1?


"Another reason is that the normal -100V [or whatever] regulated grid supply is no longer needed.  The grids get tied directly to ground and the cathode fet sets the bias via a pot for each tube. "

Now you have a 120 volt supply. What's the difference? No doubt this is a cool circuit. But if you are running the tubes AB1, I don't see much of an advantage. A simple RC coupled driver will do FB. (Better yet, use tube you can get some power out of in AB1, like 4-400s). If this circuit drives the 833 in B/AB2, that would be a very hot ticket!


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 07, 2005, 09:05:20 AM
Hi Guys,
Looking good but a bit more complicated than I thought. Tom KLR I think C4 and C6 need a series resistor to set the input Z of the op amp U3 pins 2 and 6 negative inputs. Does it also need a feed back resistor to the negative input??? The loop feed back looks much better but what sets the stage gain. You may be able to connect negative input to output and run as gain of 1 follower and still have negative feedback.   Also does the Gate time constant allow high frequency to pass. I suspect yes but there looks like a bit of phase shift. My head in in switch mode so may be ok.
Now here is something to consider. You could have pulled the feedback off the 1 ohm resistors and eliminate some parts. I think we are getting close. The high Z pullup
I still think can be eliminated with a 50 Kohm across the FETs. How many GG amps have a puullup on the cathode. How many cathode modulators have a pull up on the RF final cathode???? It is a feel good though to have it like belts and suspenders.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 07, 2005, 10:50:27 AM
Tom KLR,
Why not eliminate the phase inverter stage and add a simple phase splitter with 1 transistor and 4 resistors. This would eliminate the phase shift on one side.
 Any unbalanced phase shift will increase distortion. Also you eliminate parts.
 I see you are trying for a gain of 100 but still think you need 1 k resistors in series with C4 and C6 to balance the input Z of the op amps.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 07, 2005, 11:30:24 AM
Why would a transformer be in play in the driver if you are running the modulators AB1?

Now you have a 120 volt supply. What's the difference?


Good questions, transformer breath... ;D

Looking at the 833A specs, for AB2 operation they show 20 watts of drive, 70V of bias and 400V p-p to the grids. That means grid current in AB2. This is about 1650W of audio out. I realize that Tom's circuit currently does not put the tubes into grid current, but I was referring to most applications where transformers are required to supply grid current. Maybe adding a positive bias to the grids is all that's needed for the grid current requirement in this circuit. But then that's another regulated supply... sigh.  I agree with Tom that 800 watts of audio from a pair at 3KV is sure babying them. But, grid current with cathode drive would be the goal and show the advantage over transformer drive. [phase distortion, etc]


As for the 120V pull-up supply compared to the usual -70V bias supply....  Well, looking at Tom's circuit, the tube's bias regulation is done entirely with a chip low level in the cathode.  The additional 120V pull-up supply is unregulated, thus simple.  Frank thinks it can be eliminated. The jury's still out. Generally, guys use electronic regulator tube schemes in the grid to pull it off with greater complexity. But it's probably a wash there cuz -70V could be reasonably regulated high level with a zenor.

Even if cathode drive turns out to be a little more complex than grid drive, I am still leaning towards it for my project here.

We'll have to see if Frank's well thought out comments lead to an even simpler circuit.

T


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 07, 2005, 12:07:32 PM
The pull up would hold the regulator stable when in receive. This could be a good idea so the stage isn't slammed full on when the rig first goes into TX. The 120 volts does not to be regulated because Tom's circuit is a shunt voltage regulator with the change to DC feedback. It just needs low ripple.  Today I think the pull up is good insurance for stability on start up.
I would change the front end to two op amps inverting/noninverting or a simple phase splitter to match the phase delay.


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 07, 2005, 02:14:58 PM
Frank,

There are no missing resistors in this version.  With the FET on the output of the driver op amps, the FET gives a voltage inversion.  Since the feedback is from the output of the FET back to the input of the op amp, the input signs of the op amp change assignments.  You can think of the op amp and FET together as one power op amp.  The gain is defined by( R10/R17 ) + 1, since I am driving the non-inverting input.

The simulation shows distortion of 0.06 % at 1 kHz, per channel.  So I can’t have made any major mistakes.  One bad thing in general about the circuit is that I am modulating the non-inverting input which you should generally avoid.  It requires that both differential inputs are being swung and thus calls heavily on the common mode rejection of the circuit.  To avoid this I would have to add 2 more op-amps to the “simple FET driver”.

The distortion at 10 kHz. with a large voltage swing is up to 0.36 %.  The phase delay between the 2 channels at 10 kHz. appears to be about 210 nanoseconds in the simulation.  This is about 0.75 degrees.  I previously estimated that there would be about 1 degree of phase shift.  In view of the fact that the individual driver distortion is 0.36 %, I don’t think that the differential delay in channel 2 is a concern.  Off the top of my head I can’t think of a quick and confident way to combine the 2 push-pull signals to get a valid resulting additional distortion.  Maybe an ideal transformer?  But it’s not worth the time.

The circuit may work without the +120V pull-up supply, but I did think about the start-up of the tubes without it and like you, it appeared to me that the tubes may draw a spike of current at B+ turn-on until they settle in.

Late last night it did occur to me that if I modified the design and grab the feedback from the 1 Ohm FET source resistor, then we are including the 833 in the feedback loop.  This could be the best way to go.  The audio input waveform is compared to the current of the 833.  The present circuit delivers a low-distortion voltage waveform to the 833 input and hopes for a perfectly linear transfer function.  With the 833 only operating AB1, this may not be far off.

Tom and Mike,

Ideally you should measure the distortion in your stock modulators first, at 2 or 3 power levels that would be repeated with the FET drivers.  You would need a low-distortion sine wave generator, audio distortion analyzer, large value dummy load resistors.  I don’t know what r.f. susceptibility the FET driver may have.  So it would be best to run the modulator without an r.f deck running, if possible.

This proposed 833 current feedback version would be a minor modification of my last published circuit.  It would be good to have distortion data of all 3 configurations.  In the end, the modulation transformer may be the component limiting the distortion level.  But these experiments would be a very interesting opportunity to learn what can be achieved.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 07, 2005, 03:35:33 PM
Hey Guys,
Maybe I'm still missing something, but if you look at the load line requirements it seems that in order to get about 0.6 amp peak plate current at say 250 volt minium plate voltage swing you need at least +75 volts on the grid and have capability to draw 0.1 amp of grid current. This means  that you need to bias the grids up to +75 volts and have the initial cathode bias at the -70 volt volts for about 30-40 ma of idle current. this means the cathode needs to sit at  75 -(-70) = +145 volts volts. Then when the FET sucks the cathode to ground the 0.1 amp of grid current can be drawn to support the peak plate current at Vpmin.
The other issue is the pullups. To maintain the sine wave into the feedback loop you need at least 2X145=290volts(minimum) . If you try for less, the loop will try to do the impossible and internally the loop will go nuts and internal limiting and phase shifts will occur.
This is how I came to the original estimate last week of using +100 and +300 volt supplies.
Regards, BobbyT
 



Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 07, 2005, 05:26:05 PM
HI Bobby,

No you're not missing anything.  You are correct.  But the guys have a pair of 833's to modulate with.  Staying in AB1, theywill still have enovhg audio.  We coulddo what you say.  My circuit does hit the rail on the negative swing. but the transient response is good.  By the time the waveform comes up out of the cut-off region, everything is fine.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 07, 2005, 08:27:36 PM
Tom, I know, I was just trying to get the most out.
Yep, my model shows the loop slamming the rail but the TL082 seems
to recover OK. I also incorporated Franks suggested phase splitter
using a 2222A. This allows true line level +/- 1.1 vpk input.
Just split the collecter and emitter loads into 2 resistors each
to get a 6 db drop, then the input handles line level plus 3db peaks.

Also, strap the grids to whatever supply (+15,+24,+75...) you have
to get a little more plate and grid monkey swing and also to move
the bias node up from 0.7v (.85,.94,1.45...) More monkeyswing.
This means that you can handle a little more audio in.

Do you know if TL074, TL084 are true rail-to-rail?
If so, then you might be able to get rid of the -5V supply.
My model starts to crap out when the TL084 gets close to
the rail using 0V rather than -5V supply.

Send me your E-mail and I'll send you the mod on how to turn
a 12AX7 into an 833A. I'll try to post it here when I have more
time. Oh, did you ever get a chance to test drive the Class-E
program I sent?

Regards,
BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 07, 2005, 09:32:57 PM
Bobby,

I’ve made 3 pdfs of simulation results of the situation that you are concerned about. 

Sim1 – Shows both of the cathode driver output waveforms.  The simulation has a load that roughly models the current draw and cathode voltage relationship of the 833s.

Sim2 – I’ve overlayed a matching sine wave to the driver output waveform.  The driver output waveform limits at 109.2 Volts dc.  When the sine wave limits at 109 Volts, the loop will go out of regulation.  At the peak of the input waveform, the driver op amp’s differential inputs separate by 265 mV.  I have circled the region where the loop starts to recover and come back into regulation.

Sim3 – This is a blow-up of the region circled in Sim2.  Use the point where the “test” waveform crosses the output waveform as a reference for recovery time start.  This is 405 uS on the graph’s X axis.  The cathode output waveform snaps back downward with a very well behaved response with only one cycle of ringing.  At 420 uS on the graph, the output waveform has reconverged with the “test” sine wave.  This was 15 uS, to recover.  At this point the voltage is 104 volts dc.  Still in the cut-off region.  Another 15 uS. to go to get to 100 V. (-100V.).

The pull-up power supply voltage could be raised so that the recovery is at a more negative grid-to cathode voltage.  But I don’t see that as being necessary as of yet.  The loop has been compensated for a wide range of FETs.  But it would be best to use one specific type of FET with the lowest capacitance necessary and re-do the compensation study for the best performance with that FET.  But I think that you will agree that the performance is not an impossible situation with terrible outcome.

So I feel that it is not necessary to make the waveform swing any farther “negative” than it already does.  The simulations show that it is not necessary to mimic the whole voltage swing of the old grid driver transformer.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 08, 2005, 08:55:44 AM
hi Guys,
Tube curves show plenty of plate current at 3 KV and grids at zero. This is where Tom Vu and I started. 70 volts on the cathode should cut it off.
Tom klr I still like eliminatinting the first op amp and eliminating the extra delay in the negative phase. Yup a fet or a 2n2222 would do the job. KISS. The only other way around it is use the spare amp as a noninverting the match the phase delay.
Pulling the grids positive would be a last resort to get more current. Save that thought.
074 and 084 are not rail to rail but the fets need about 4 volts to turn on so -5 negative rail should be fine to yank them off.. -12 good also.

Tom KLR extra resistors in input only balance the input Z eliminating possible offset voltages. yes your circuit will play but I'm picking fly crap out of the pepper shaker for best possible performance.
Feedback in the source resistor may have start up transient so you have to rely on your soft start at bias pots. This means the driver needs to be switched with the
rig to avoid mod transformer jumping up going into TX. cool stuff


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 08, 2005, 12:10:25 PM
Tom, as Frank said, Cool Stuff.

Good on the graph details. The transients look good.
I can't wait till somebody puts one together.

Here's the 12AX7 starting model I promised.

*Vacuum Tube Triode (Audio freq.) pkg:VT-9 (A:1,2,3)(B:6,7,8)
.SUBCKT X12AX7 1 3 4
B1 2 4 I=((URAMP((V(2,4)/85)+V(3,4)))^1.5)/580
C1 3 4 1.6E-12
C2 3 1 1.7E-12
C3 1 4 0.46E-12
R1 3 5 50E+3
D1 1 2 DX
D2 4 2 DX2
D3 5 4 DX
.MODEL DX D(IS=1.0E-12 RS=1.0)
.MODEL DX2 D(IS=1.0E-9 RS=1.0)
.ENDS X12AX7

Also,
Here's a first approximation for an 833A Ip vs Vgk and Vp curve.
with cutoff at approx -70V and drawing about 0.9 amp at 3KV.

  Ip= (Vpk/42.9) + Vgk)^1.5)/650

Folks could change the model to reflect this, add the proper tube capacity and
maybe change R1 to about 400 ohms to approximate the grid requirements.
Is this fun or what!

Regards,
BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 08, 2005, 12:25:07 PM
Imagine a 3 by 4 inch board with 2 fets hanging off it with a little transformer supplying power driving a pair of 833s. We are debating resistors. Imagine all the hardware that can go to the scrap heap.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 08, 2005, 12:33:05 PM
The next thing to do is come up with an old buzzard design to go along
with the old buzzard tube. That is, just use solid state triodes like transistors
and FETs along with zeners.

"We don't need no stinkin' opamps and voltage regulator chips!"


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 08, 2005, 12:41:34 PM
Hey Tom JJ,
You can be the first to build this!

Just build 1 channel and convert your CakePan rig to series Cathode Modulation.


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 08, 2005, 03:13:13 PM
Hey Tom JJ,
You can be the first to build this!

Just build 1 channel and convert your CakePan rig to series Cathode Modulation.

Oh, but it's already being done here in the shack here Bobby. My 6AQ5 series modulated by another 6AQ5 sees occassional use on 75M... ;D  I like cathode modulation.  2W output and clean 200% easily.

http://www.amwindow.org/tech/htm/series.htm

So, what's the next step here? Maybe Frank, you and BobbyT can come up with the simplified version based on discussion today. I'm glad I'm not ready to build it now cuz I wouldn't know which to pick.

T



Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 08, 2005, 03:44:53 PM
Frank, you are right that a current feedback version would have start-up problems.  I hadn’t thought about it yet.  The loop will be asking for zero bias to get current.  There probably will be grid current in standby, if there is a plus supply on the grids, equal to the desired 50 mA cathode current the board would be set up for.  I don’t see any grid current or grid dissipation specs in my book but there has to be.  So the current feedback version needs to be a little more complex than the present “voltage output” version.

The voltage output version and 833’s are happy all the time.  The driver board doesn’t need to know what mode the modulator is in.

Not previously stated, the voltage driver board can be used with full AB2 and the positive grid bias with no changes to the board design; that is not precluded.  It’s just that you need to adjust the bias pots, have a higher pull-up voltage, and have larger heat sinks for the increased FET dissipation.  Since the driver has fixed gain, more audio drive would be needed for greater output voltage swing.  For the design as it is, with the cheapo supply I published, you could install a second set of diodes and a big filter cap feeding an additional 7818 for the grid supply.  This would help somebody like Mike W3SLK to get his 810’s from AB1 330 Watts I stated before, to about 415 Watts, over the hump I think, for him.

About the 210 nS delay in channel 2 – It hit me that to combine both of the outputs together for analysis, all I needed to do was grab an ideal op amp and I set it to a gain of 10, duh.  I had to run both channels at a level below clipping; both channels are putting out sine waves.  I ran the transient simulation at 10 kHz. and measured one channel’s distortion and the distortion from the combining op amp’s output.  I measured 0.204 % on the cathode2 output.  Coming out of the combiner op amp I measured 0.144 %.  You have to remember that the push-pull “combining” attenuates the even harmonics.  I think that we are fine with the technically offset timing.  This is just a voice product.

I measure the distortion by asking for the FFT of a waveform.  I then measure how many dB down the second, third, fourth and fifth harmonics are.  I plug those numbers into a THD calculator spreadsheet I made.  It has been a very good tool to use.

So then I put in the 2 resistors ahead of the op amp inputs (100k in parallel with 1k = 990 Ohms in this case) you are talking about Frank, and re-ran the simulation.  I got 0.386 % on the cathode2 output and 0.380 % on the combiner output; a surprising result.  It is worse!

I never put those in anything I did except a thermocouple amplifier, which had much, much more input bias current that these bi-FET input op amps.  The TL072 input bias current at room temperature is 65 pA.  With a 1k series input resistor this is 65 nV.  With a gain of 101, this is 6.6 uV on the output, totally insignificant in this application.  You also need to remember that the input offset voltage spec. on this part is 3 mV.

I also thought about the fact that we could put the unused op amp in channel 1 as a voltage follower.   This is a possibility, but again more complexity for little gained.  We may need it for the current feedback version.

So I think that the circuit still stands as is.



Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 08, 2005, 03:47:39 PM
Tom's circuit will work, go for it. My suggestion of a phase splitter would be not make much difference. I was just being anal. But you have to be anal to get the last couple tenths of a percent.  
Be the first on your frequency to cathode drive your modulator.
Linear that is, I've been driving driving the cathodes of my pdm rig since '83 and never blew a single fet.




Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 08, 2005, 03:57:09 PM
Tom KLR,
Yup I agree. Interesting on the input resistors. Yea input current is so low the offset will be in the mud. Not like the old days of bipolar amps.
I think we have a very good base line, and just need a good heat sink for growth.
It would be cool to keep the grids at ground. Maybe a negative reference below the FETs since audio is ac coupled in. That would be cool to provide a path back to the high voltage supply....hmmm maybe a little positive grid bias is safer.

SOOOOOO who is first I'm going back to my new RA6830 project.


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 08, 2005, 04:08:53 PM
Tom KLR,
One last thing. Rig in RX with high voltage on the modulators. Will there be enough tube leakage to soar the drains of the fets. GG linears always put 50 Kohms across cathode relay contacts. Do we need a high resistor across each FET or will the pull up hold it low enough. Also don't want to back feed the pullup supply...maybe a 1n4007 series diode ? I'm not sure what happens when the cathode is an open circuit.  I would suspect that voltage may get high since there is no leakage path.
The circuit may be off so no current flows in RX. Just a safety thought a SMS guy.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 09, 2005, 09:16:18 AM
Tom, Frank
What if you rebiased the opamp reference nodes higher to eliminate
the negative supply.

For instance:
Use +15V
Ground the old -5V opamp supply terminal.
Bias the (-opinput) to +7.5V with 2 100k res, R1,R2
Insert R3- approx 11k to ground- in series with the existing 1k to (+opinput)
Both opamp inputs are now at +7.5v nominal.
Bypass R3 with about 100uf to ground to restore original ac loop gain
and establish low freq corner.

This should give about 70 volts at the cathode as before.
Make part of R3 the bias adjust pot.

Can use higher than +15V but if higher than +18V use zener to clamp the gate.
This is not soft-start, so AC supply for +15V should be powered from the same
source as the 833A fils. Might also provide a 3KV plate lock out
in the absence of +15 and +150 pullup supply.

If +150V supply has a bit more extra mils, the whole thing can be run from one supply

Also, what if a separate dc bias loop were added to the grid terminal.
This would provide a tracking input to follow the grid supply.
This way if the grids were biased to a higher voltage, the idle current
could be made to track. Just feed back 51mv per volt of grid bias to the
bottom of the 100k (-opamp) using a 10k and 510 ohm res?

What's your opinion on this working?

Regards,BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 09, 2005, 11:51:37 AM
I would think you could raise everything higher as long as the op amp negative rail voltages goes low enough to turn the fets off. I'm not sure it will be as quiet in  the hum department. The op amp negative rail at ground never seems to work as well as a negative supply.  I would leave the grid out of the loop for now as it complicates things. I prefer to see the grid at ground for starters.


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 09, 2005, 12:48:34 PM
That's true Frank,
You loose the inherent opamp CM power supply rejection.
However, the ps bias splitter is still inside the closed loop,
so (for the +15 V single supply situation), you could go to
+15.5V and have the output only change about 2V.
This is common mode for both cathodes such that
the push-pull output is not disturbed.

If you use a 30V PS the effect is halved
(also get full opamp output sink-source performance to drive FET cap at high freqs)
 You could also maybe pump this in as a reference to the other half and cancel the effect.
Maybe not even use a regulator, just a Zener for the 15V bias splitter.
Maybe all of the above?

Regards, BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 09, 2005, 03:01:39 PM
I would prefer to keep it simple. Fixed bias on the grids would be compensated by the cathode voltage. Floating op amps with  offset bias is just drift looking for temperature change. 


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 09, 2005, 04:30:03 PM
"Duh-brainfart"
The bias is definitely not in the loop.
That's what I get for trying to type and think at the
same time.

The other comments were hopefully on target and were an attempt to KISS.
Can always try it both ways.

BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 12, 2005, 11:53:34 AM
Folks,
Here's a better approximation for an 833A Ip vs Vgk and Vp curve.
with 50ma bias at -70V and drawing about 0.896amp at 3KV, cutoff is about -82V.

  Ip= (Vpk/36.6) + Vgk)^1.5)/828

Another thought.
I'm not sure what happens with an open circuit cathode but
maybe Frank's 50kohm pulldown should be used directly at the833 cathode.
This means if the fuse blows, the tube will bias up to about 82v.
This way the grid voltage breakdown would not be exceeded.
If breakdown at 500v was reached does that mean that Vg=Vk=zero bias  and maybe
0.7amps of grid current is drawn? Does anyone know?

Regards,
BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 13, 2005, 08:28:13 PM
Op-amp re-biasing to eliminate the – 5 Volt supply:
This could probably be done with another op amp choice, but I am not going to pursue it.  Rail-to-rail input and output op amps were designed for the battery-run equipment.  I think that we would want one that handles more than 5 Volts.  They probably exist but I am not going to do the research.

What we would need is common mode input to ground and the output would not need to go to either rail.

Yes you would have to bias the feedback divider at R17 to some positive voltage.  I feel quite uncomfortable using a high impedance source at the bottom of R17 though.  The AC and DC gain will be different.  If I were to modify the design, I would use a 78L05 regulator at the bottom of R17.  Then we are eliminating the 79L05 and putting in a 78L05.  So I don’t see an advantage here.  2 diodes and a 7XL0X i.c. are cheap.

I was looking at the 833A curves again today and I plotted the class B load line from the tables and the current output is quite linear until + 50 Volts bias.  If you add a positive voltage regulated grid supply, I still don’t see any  need to do any other bias regulation than setting the bias voltage in the present circuit.  The grid to cathode bias would be defined.

I did an experiment on the bench the other day with a 6J5, grounded grid.  Plate supply was + 360 Volts d.c.  I dug out my high voltage probe that goes with my RCA Senior VoltOhmyst.  The probe is 1100 MegOhms.  I saw 138 Volts on the cathode of the tube.  Scaling up 2 orders of magnitude for the 833, I can’t imagine that even with 1 MegOhm on the cathode that the voltage would climb near the breakdown rating of the FETs.  But this is just a guess.  We will have 10 k to +125 Volts anyway.  So I think that we are o.k. in the situation of the triode as a possible source of high voltage.

The fuse was an idea in case the FET shorted.  The 833 can conduct over 2 Amps.  At first the current would follow the load line.  But when the tube and transformer sit there steady, wouldn’t the tube operating point be vertically straight up the 3000 V line to where it intersects the extrapolated grid bias line?  The fuse rating should cut down some if incorporated.  Looks like the tube would sit at about 0.85 Amps considering the 1 Ohm resistor in the FET source.  Of course, if and when the fuse pops, that might make quite a voltage transient in the mod trranformer; a bad thing.

I don’t know what the grid breakdown rating is so if it is 500 Volts, perhaps a resistor past the fuse is good.   If the cathode was floating, there is no current path for grid current.

Perhaps there should be no fuse except in the mod. plate supply.


A current-feedback version cathode driver:

I have taken a few stints at the current-feedback version driver.  The situation is not easy.  I have a working circuit with about 1 % distortion.  But I am having problems with the linearity at low currents.  The source resistor that was 1 Ohm in the voltage feedback driver had to be cut down to 0.1 Ohm so that it didn’t interfere with the gate voltage.  Now we are dealing with 5 milliVolts for bias current reference.


Bobby,

I ran your CE04 program this weekend.  I like the table showing the component currents and voltages.  I had thought about doing this in my Class E spreadsheet but didn’t want to take the time to develop rules of thumb checked at various impedances and QLs.  I haven’t compared your voltage and current values to my old PA simulations yet.

I like the coil section where the actual Q of the described coil dimensions is estimated.  I have never seen this before.  Have you ever measured any real coils against the calculated Q?


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 14, 2005, 01:03:43 PM
OK Tom,
I was just trying to nail any last minute "gotchas" or domino failure modes.
Was concerned that maybe old gassy 833 pulls with maybe 5 mils of leakage
would not cut off like a hard vacuum tube would. This might pullup to the point either of Vgk breakdown or filament transformer breakdown.
Also was thinking of tube "burps" that APE mentioned.

On the ClassE program:
I don't have a Q-meter so haven't made measurements. The Q estimate should be valid. Wheeler's estimate is too optimistic, Butterworths is closer and seems to match the ITT handbook estimate. Try the help button to get you to the references to the literature.
I put in the component voltage and current so folks could better estimate tuning and loading cap requirements. Maybe just a breadslicer would do rather than a vacuum var, etc.

BobbyT


Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 15, 2005, 12:46:11 PM
Tom,
Your power supply with one transformer that has a dual primary is
a great idea to save space and keep KISS going.

I have a question on how to figure the rating though.
Normally the transformer should be derated to 70% for cap input usage.
If we start with a 10 VA transformer, we're down to 7VA.
Since the transformer was originally wound so that both primaries supported the rating,
does this mean that with only one primary excited, the single primary can only support
3.5 VA without overheating?

Maybe we could model this. See below info from a spreadsheet generated a while back when
I was looking at how to model this kind of stuff.
The values are for full load 40 deg rise. So for room temp divide resistance by 1.2.

BobbyT
---------------------------------------------------------------------------
Simple Iron-Core Transformer Model with winding loss but excluding core-loss.   

Select         Calculated
            
PRI Vrms   120   PRI-VA=   9.53
Freq-Hz        60   PRI-Irms= 0.079
      SEC-VA=   8.10
SEC Vrms     36   Loss watts=1.43
SEC Irms   0.225   Rpri-Reflected-Tot-ohms= 227
      Eff Vin-rms= 102.0
            
%Efficiency  85           Nsec/Npri=     0.353
      Sec-Vrms OpenCkt  42.35

      Rpri=   113.3
      Rsec=   14.19
Lpri-HY           8   Lsec-HY=     .997
            
      CT Rsec=    7.06
      CT Lsec-hy=  0.249
            
      Pri-Reactance   3015.93
      Exciting Current   0.04


Title: Re: Simple Audio Driver for 833A modulator
Post by: Tom WA3KLR on November 15, 2005, 03:33:46 PM
Bobby,

I went back and looked at my power supply simulation.  I have a 20 mA. load for the 78L15 supply,  15 mA. for the 79L05 supply, and 15 mA. for the +127V supply.  The currents are a little over-estimated.  I came up with 864 mW true power for the low voltage secondaries and 1.435 W for the 120 V winding.  This is a total of 2.3 Watts true power. 

The SE Asians are pretty good at pinching each parameter in their transformer designs for lowest cost, but I really don’t know what the transformer’s limiting factor is.  My guess is that each primary winding is good for more than 3VA/0.85 = 3.53 VA and that the core is the limiting factor.  But assuming that each primary is only good for handling 3.53 VA, I may be a hair over the spec. I think that a 6 VA transformer would be fine in reality.  Remember the loads are a little over-estimated and the service is PTT.

But if you feel uncomfortable, go to a 10 VA unit.  Here’s a Mouser 10 VA unit – a good old Fred Hammond p.c.b. mount however, 546-183G36, 10 VA, for $12.15 .

Added later (sometimes I get too concise in an attempt to save time) -

So total secondary power is 2.3 Watts.  Using your 85 % efficiency, 2.3/0.85 = 2.706 Watts at primary.  ASSuming 0.75 Power Factor at the primary ( a very reasonable number) this gives 2.706 W/0.75 PF = 3.6 VA.  The transformer was sized orginally with the idea that only one primary winding would be used and that the total load was a little under half the rating.



Title: Re: Simple Audio Driver for 833A modulator
Post by: nu2b on November 16, 2005, 12:35:07 PM
Tom,
It wasn't so much concern as oldfart conservatism. It seems that when you get
started on an interesting design problem like this, the juices get flowing and
new thoughts and ideas keep popping up such that the problem gets engineered
to death. Always seems that new stuff wants to get added in and the original
design iterates toward a universal solution rather than a point design. I'd keep the power supply
separate from the board design for this reason.

Closing thoughts before this gets beat into oblivion:
 
1) Change from individual bias pots to a master bias pot plus balance pot.

2) Add series res from each 1 ohm source resistor to a 3 position switch to
provide cathode A, cathode B and total modulator current metering.

3) And finally, howsabout adding an old buzzard 6E5 magic eye to the center of the front
panel below the 833 viewing cutout for "just because" reasons only.

Regards,
BobbyT

PS: The transformer spreadsheet didn't print too well, so I'll send the excel
file to you when get a chance.

Also, I've been trying to model assymetric (pos peaker) modulation and came up
with a KISS symmetrical line level compressor which if offset slightly should
give the effect. Maybe could be used as neg peak control also. Needs more
thought. Maybe you have some modeling thoughts on this.

Also again, were you able to create a tube macro/subcircuit in SwitcherCad?
How about a modified depletion mode FET with gorilla characteristics?


Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 16, 2005, 12:39:46 PM
Hey guys ,
The spare op amp would be a great negative peak limiter with the addition of a couple resistors a diode and maybe a level pot. Compression inside the chassis sounds dangerous. fc


Title: Re: Simple Audio Driver for 833A modulator
Post by: K1JJ on November 16, 2005, 02:01:25 PM
Looks like things are progressing along.  Some good additions. I don't mind lots of bells and whistles, as long as they do something worth while. Most seem logical. The neg peak limiter of the unused op amp is a great idea.

I intend on building the version that exists when I get everything else finished on the 833A X 4-1000A rig.  Maybe in a month I'll be at that point.

Right now I'm doing metalwork. Finished building the four big chassis and made a sub chassis air system  for the 4X1 as well as mounted the 833A's with small muffin fans below them to add cooling there.

Question:  Can I mount the 833A fil transformers right on top of the mod transformer without seeing audio feedback problems between the cathode and plate? I expect there will be some big EM fields there.  Or maybe the 833A's are such low mu it doesn't matter.  Room is at a premium on the mod deck with the big iron taking up 3/4's .

Also: I use two 150H @ 300 ma chokes in PARALLEL for the heising reactor. If they sit side by side, do the windings have to be phased correctly due to core EM coupling? I planned on putting the cores at right angles to one another, but still wonder about weird effects from core overlap and proper winding phase as a result, if even needed.

73,
T



Title: Re: Simple Audio Driver for 833A modulator
Post by: WA1GFZ on November 16, 2005, 03:26:12 PM
I don't think the chokes will be a problem because they are in parallel. The Fil transformer not sure. Can you put a steel plate between them?
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