New 4X1 X 4X1's Rig - Problem with audio spikes - Questions - Pictures

[K1JJ]

Brett,

Yep, wish I had a smaller SS amp to use, but it's all I have right now.  I'm moving the amp across the room right now. The good news is I can now unkey the rig and it is not affecting the SS amp across the room. However, it is not hooked to the rig yet, just has leads dangling from the 8 ohms output. Before, the SS amp wud act up when in this config.  We'll see.

Thanks for the redrawing effort, Bruce. I'll bet others will be interested too, considering it's a direct coupled driver and will use 2 volts drive. And, easy to get 807's.

Shane, yes I did try several bypassing schemes with the amp in the old position on top of the rig. I think it was simply magnetic coupling thru the steel cabinets into sensitive solid state circuitry.  This amp always worked FB when placed remote from the rigs. I shuda remembered that from before, but didn't.

Bruce, yes, using a low impedance 8 ohm run is better - I just was concerned about the amps 8 ohms sensitivity from the last test. But thinking more, of course the 8 ohm run is superior to the high impedance grids...

But all in all, this episode has caused me to beef up a lot of things that wud have not shown themselves without the arcing. And many of us have probably learned a few things too...



T

[K5UJ]

tnx Bruce; T if you run shielded audio cables (which you should do) you might try grounding the shield at only one end of the run; leave it open at the other end.  I assume the s.s. amp is not built for broadcast tx site; couple things Orban did on my 422A compressor is put small disc ceramic .01 mfd caps to ground on the audio leads inside the cabinet right at the terminal strips on the back panel and small ferrite beads on the audio leads.   I think I read somewhere above the low level audio is driving a transformer so that should give isolation to the rig; if the s.s. amp has a three wire balanced output it would good to use shielded balanced cable to the transformer--ground the shield at the s.s. amp and connect the audio + and - to the primary of the transformer.

73

Rob
K5UJ

[K1JJ]

OK, Rob, will keep that in mind when I put in the new SS amp.

New?  Yes new.  I moved this one to the new location and wired it up. It appears all the problems were gone and the rig was very stable. Then being tired, I finalized the wiring and put the wrong cables on the SS amp's output - dead short.  It didn't do anything except the fan came on after 20 seconds. Then the audio power output  slowly dropped off and never came back. Smoked after all this effort. Right at the end of the job too. Son ofa bitch.

So, tmw I'll hit a few pawn shops and find a used 100 watter to replace it. I'm pretty confident it will work FB now. The old one was just too close to the RF field.  I wonder what type I shud look for to avoid the coupling capacitor problems again?

I will use the new one and maybe later build up that 807 driver.

Happy days.

T

[K1JJ]

Opcom, you were axing about the NFB details via PM. Here's the schematic below. The NFB circuit is from the WA3KLR cathode driver.  I use the resistive ladder of one modulator plate cap tapped at the bottom as shown to derive the NFB. The audio and NFB are mixed into an IC and this output is fed to the audio amp at about 1 volt.  Works FB.

T

[K5UJ]

Tom that SUCKS reminds me of the time I worked all tksgvg weekend putting up an antenna and on Sunday evening finished, was so tired and excited that when I went to put the swr analyzer on the feed in the basement I dropped it on the concrete floor.
I bet we have all had something like this happen when tired, excited and eager to see results.

R.

[N2DTS]

I bought a samson 2x120 watt amp from some on-line music place, the same place I got the berringer box from, it was about $120.00 I think, and was also one rack space high.
It worked fine, but the symetrics had a bridge switch which dualed up both inputs allowing me to split the line level signal to both amps but with level control on each amp, so I kept that and sold the samson amp (cheap).

The combo of a berringer (one rack space, and a power amp (one rack space) makes for a small, compact setup that can sound very good, sit it on the operating desk and it takes up very little space, and you have the adjustments at your fingertips, and can watch all those purdy lights flash.
Stick it in a nice cabinet with the mod monitor and it makes things nice.

In my shack, I have the berringer and amp in a cabinet above the homebrew receiver, the mod monitor and marantz audio amp are in another cabinet.
I got the cabinets (3 matching) from mouser, they were called econo cabinets and were not expensive.

Brett



 

[AB2EZ]

Tom

I have a Samson Servo 120 Amp (puts out 120 watts into 8 ohms when operated in the bridged mode) that is surplus to my needs. I use it to externally plate modulate (via a backward connected Hammond output transformer) my Johnson Ranger and other rigs as well. I will make you an offer that you can't refuse. Contact me here: sdp2@verizon.net.

[K1JJ]

Hi Stu,

I sent you an email, thanks.

Aw, that big old 300w+ amp was so darn heavy and needed retirement anyway.. :-) 

The arcing did uncover a lot of weaknesses in the rig. I blew out the Heising cap the first hour. The neutralizing cap was arcing over. The Heising cap mounting was arcing. I added needed spark gaps. And there was more. But finally, the rig handled the arcing FB. That was a big acid test. Now that it's calmed down, it will loaf along nicely.

That's the similar way I test out my gear anyway. After it's working, I usually load the rig up to max and beat on it. This shakes out the weak components. As long as the mod iron is not at risk, it is a way to eliminate years of future dicking with failed parts. I hate babying a rig until it has proven itself. Then it can have a cushy life.

I can at least use the rig on CW in the meantime...

T

[WA1GFZ]

Tom,
I have some 4 uf mylars at 400 volts somewhere in the cave if you need any.

[K1JJ]

Tom,
I have some 4 uf mylars at 400 volts somewhere in the cave if you need any.

 Not sure what I'd use them for, but tnx for the offer anyway, Frank...


Stu/AB2EZ  just gave me the best deal I've had in years on his Samson SS amp...and is shipping it out today! I'll mention more when we get on the air. No better friend than that. Thanks again, Stu.    So I'll be all set in a few days for Round II.  Stay tuned.

Again, thanks for all the help, guys!  One way or another the problem was identified and solved. Great group and resource for help here - a homebrewers delight for sure.

T

[flintstone mop]

Just looking at a few posts earlier and maybe hard cut-off of the modulator pubes would be the answer. I had that arrangement in a HB outboard 810 modulator for the T-368.

Fred

[KC2IFR]

Good work Tom.
Just a quick note about my amp. Its a QSC RMX 850. I use it to modulate my Valiant and Ranger using backward connected Hammond output transformers. To drive grids its way to big but I believe in headroom! To big is much better than to small.
As I said, this amp is bullet proof.
http://www.qscaudio.com/products/amps/rmx/rmx.htm  

[K1JJ]

Yep, nice looking 850w unit, Bill.  I can't believe they have a 2400 watt version that's packs into only 2 rack spaces. I can imagine making a mistake with that thing - driver transformer - oh the humanity!

Stu's amp is on its way and I think 60w per or 120w bridged will keep me outa trouble. The specs these days are unreal and put the older tube amps to shame. Things like 3hz - 50khz with .05 % THD... unreal.

But I'm still interested in giving that 807 driver a good look at when the schematic gets posted.

Right now I'm making some more mods to the 4X1 rig to insure the safety of the new amp. Can't wait to get on wid it soon.  ("Calling the 4X1 net")


T

[Opcom]

I don't have a schematic of the SS amp, but will have to open it up.  If it does have that cap, do I just take it out or reduce its size?

I have the mod transformer primary gaps close, but think I'll add some gaps on the mod xfmr secondary too.

I would just disconnect it if there is one and see what happens.

Ding, ding, ding, ding... the spark gap normally goes on the mod transformer secondary not the primary.

My RCA KW iron has factory gaps on both sides. Maybe having only one gap is an engineering philosophy or a cost saving function.

[steve_qix]

I'm not sure if you solved the original problem or not...

How much peak to peak grid voltage is required for your modulator tubes?

If it's less than about 700 volts p-p, you could easily use source-follower MOSFETs to drive the grids - eliminating all of the driver phase shift (because no transformers would be involved), and providing the absolute lowest driving impedance possible (less than an ohm).

The circuit would be fairly straight-forward.

Also didn't get whether you are switching the high voltage on and off, or are you leaving it on all the time.  I tend to kill the HV supply at the primary and allow it to fall to 0V (or a very low voltage), and use a step-start on transmit.

Anyway, just a suggestion     Geesh, a pair of 4X1s modulated by a pair... guess it's time to start building that 100 pill rig 

[N2DTS]

I wonder what blew in the original 300 watt audio amp.
I would have to tear it apart and find out.

I also do a step start on my rigs, although I know its better to leave the voltage on all the time.
When built, most of my designs came out of the Bill Orr books...

It works, big diodes, choke input, even with a lot of energy storage...

Brett
 

[Opcom]

Opcom, you were axing about the NFB details via PM. Here's the schematic below. The NFB circuit is from the WA3KLR cathode driver.  I use the resistive ladder of one modulator plate cap tapped at the bottom as shown to derive the NFB. The audio and NFB are mixed into an IC and this output is fed to the audio amp at about 1 volt.  Works FB.

T

I see the entire unit is to drive class B audio grids, Indeed, the 1 volt to feed the commercial SS amp could be picked off early. Thanks!

[K1JJ]

Hi Steve,

There's a lot to read here, but look at the pics and you'll see it's a SINGLE 4X1 X mod'd by a pair.  With only 3500v on the mess, the modulators can only handle one final. But that's plenty...  

The HV supply idles on all the time, but I key a HV relay to route the HV to the particular amp I'm using - mainly for safety reasons to keep the HV off the rigs most of the time.

The mosfet grid driver - I've already done that years ago with the old K2IJY board - too many supply voltages needed and certainly takes some heat and power to run. The WA3KLR MosFet cathode driven circuit is a much better approach.  _+ 12V and a 5v supply to power 5 watts to drive a pair of 833A's in the cathodes.   I still have the KLR finished board here and might even try it wid the 4X1 modulators, but am leaning towards building the BC 807's direct coupled tube driver we've been discussing here.  For now, using a SS amp wid backwards 8 ohm scheme.

I posted Tom/KLR's circuit below if you want to refresh. I'm presently using the NFB circuit in it for this rig.

I figgered you'd have that 100 piller finished by now!

T

[K1JJ]

I see the entire unit is to drive class B audio grids, Indeed, the 1 volt to feed the commercial SS amp could be picked off early. Thanks!

Close.... the board drives the cathodes of 833A's or whatever tubes you pick - I used it with triode connected 813's and the audio was flawless. Best rig I ever saw plate modulated.  It takes about 5 watts! of total power to drive the 833A's. I'm surprised more guys have not picked up on this clever design to drive their modulators. The only drawback I can see is it takes TWO separate fil transformers for the modulators.

T

[Opcom]

I really admire the fine job you did on the transmitter!

BTW this whole thing encourages me to do something really scary with loose parts around here.
Four 3-1000's
Gates by Dahl BC-5B hypersil HD mod xfmr
UTC CG-309 plate unit, 3KVDC/1A CCS (heh maybe not enough)
I just do not have the time, I'm afraid. I swore to fix what I have first. Parts is parts.

But thanks for the encouragement!

[steve_qix]

The mosfet grid driver - I've already done that years ago with the old K2IJY board - too many supply voltages needed and certainly takes some heat and power to run.

Hi Tom,

That old K2IJY board was/is a very bad (and unauthorized) implementation.  It came about as the result of a telephone conversation discussing the concept of a driver, and not an actual circuit.  No one would do such an implementation in the practical world.  I have a real-world MOSFET source follower driver circuit that I may post here to show a proper implementation, and something that will work correctly, and with proper arc-back protection and other niceities.

What you use will surely depend on how much grid swing you need.  What is this value?  I don't remember if you are using class AB or class B; triode or tetrode connection.  This will make a huge difference in the power output and the drive requirments.  I generally assume a high grid-to-grid swing is needed, particularly in triode connection - and also assume the grids are driven positively with respect to the cathode, necessitating a low impedance driver.

The cathode driver schematic you posted appears to be capable of 100 to 150V G-G (ultimately).  The DC voltage applied to the tube GRIDS and the MOSFET device ratings will be the major determining factors.  With 0Vdc at the grids, you will be unable to drive the grids positively - which may be OK, not knowing all of your operational parameters 

However, if you need a high grid swing, I do have a circuit which will deliver the goods.  There is a 450TH modulated by a pair of 833A's getting ready to come on line (no, it's not mine!!), and this transmitter will use this driver.

  Pills - I've never made it too far.  The 100 pill transmitter has been in my head for years, but I've been too complacent - need a "mark" so to speak    to spur on the implementation effort.  A pair of 4x1s would definitely have done it!

Talk later and Regards,

Steve

[WA1GFZ]

Steve,
Pretty easy to change the cathode swing with a higher pull up voltage and positive bias on the grids. (and enough heat sink).
I know what you mean about motivation.....get the big rig back on or put down beautiful white oak flooring at GFZ South.

[steve_qix]

Steve,
Pretty easy to change the cathode swing with a higher pull up voltage and positive bias on the grids. (and enough heat sink).
I know what you mean about motivation.....get the big rig back on or put down beautiful white oak flooring at GFZ South.

Exactly !!  I like the cathode drive method within certain parameters.  One has to watch out for capacitance to ground issues with the heater transformers, but it shouldn't be too bad.  Applying a positive voltage to the grid will allow the grid to be driven positively with respect to the cathode.  

The major down side is the fact that quite a bit more power is dissipated in the driver than would be otherwise be dissipated using grid drive.  This is due to the fact that the cathode current flows through the driver MOSFETs all the time, and as the max grid-to-grid voltage is increased, the driver losses increase as the square of the voltage.

Hey, it's winter - time for HAM RADIO PROJECTS !!  Get that rig on the air  

Regards,

Steve

[WA1GFZ]

Agree, but it is a nice place to pick off NFB at a low voltage using the source resistors.

I put down 3 X 41 foot length of flooring. Looks so much nicer than green plywood. Afterburner is on the bench and drilling blasting is going on to add vacuum variable caps.

[W3RSW]

Wow,
-wish I had a 3 x 41 ft. shack! 

Guess all your stuff is 'hairpin' matched too.