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Author Topic: Screen grid modulation questions?  (Read 63821 times)
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VE3GZB
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« on: November 21, 2009, 02:02:33 PM »

I recently went on a parts expedition down to Toronto, and snagged some 6146s! So I've started building something.

My homebrew power supply gives 300 and 500V out (it has other outputs too but I'm only using these two for this Xmitr).

I don't have much for modulation purposes...currently I'm using a pair of 6SK7s feeding a single 6L6 running from the 300V line. 6L6 cathode current is about 40mA (about 19 volts across the 470 Ohm Cathode resistor). Plate load for the 6L6 is a 30H 50mA choke. I have no audio Xformers and there's no place I can get any locally.

Plate B+ on the 6146 is 500V, in case anyone needs to know that. To monitor plate current, I have a 24V 5W lamp in series with the plate feed, prior to the plate RFC. From that I can monitor plate dip.

The plate of the 6L6 is connected to the screen grid of the 6146. But not directly connected because that would put 300V on the screen of the 6146! Instead I have a 10k resistor to drop the screen to about 170V and I have that resistor shunted with a 10uF 350V cap.

Right at the 6146's socket I have 3 X 150pF grounding caps to the screen and an RF choke in series with the DC feed to the screen.

When I'm trying to make a contact and I'm modulating (have my scope connected to the Xmitr's output to monitor the RF feeding the antenna) the modulation is really affecting only on the negative peaks. I'm not seeing positive peaks happening in the carrier.

It really sounds awful on my receiver (but I'm wondering if I'm overloading the receiver's front end merely due to close proximity).

When I scope the screen grid voltage on the 6146 I'm seeing my audio up to 150Vp-p so the audio isn't being cropped at the plate of the 6L6.

I previously tried using a Cathode follower (I have a few 6AS7 dual triodes) to drive the screen of the 6146 directly and had the same results in modulation - positive peaks didn't happen, only negative peaks came about.

I've tried running the 6146 at lower screen voltages (50V, 80V, 100V, 150V) - but the same modulation is observed on the carrier - negative peaks rule.

Has anyone ever seen anything like this?

Thanks and 73s
geo
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K3ZS
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« Reply #1 on: November 21, 2009, 02:27:02 PM »

My experience with screen modulation is that you have to increase the transmitter loading control past the point of maximum power output to get positive modulation.   That is something to try anyway.
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N2DTS
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« Reply #2 on: November 21, 2009, 09:55:13 PM »

I never did screen modulation, but besides being very inefficient, it seems like an interesting way of getting some hifi audio onto a carrier.

I think the screen has to be adjusted so the rf output is 1/4 full power with no audio, so when you modulate the screen, power can go up and down.

If you have a pair of 6146 tubes, they might do 100 watts of carrier, but in screen modulation, you might only get 25 watts, since 100% modulation would be 100 watts pep.

Unlike plate modulation where you ADD modulator power to the carrier, screen modulation takes power away at no audio but allows 100% power to pass on full modulation.

Most screen modulated rigs I have heard of were very low power.

Brett


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VE3GZB
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« Reply #3 on: November 22, 2009, 12:03:36 AM »

I did some research and I've tried a variation on "Controlled Carrier" modulation:

http://www.noding.com/la8ak/f97.htm

I don't have the 6DE7 shown in the schematic, I do have other dual Triodes so I adapted the circuit to fit what I've got.

The circuit as shown modulates quite well but with a lot of distortion with the tubes I have.

K3ZS's advice was good, trying to keep loading up, that helped with the waveforms.
I have just a single 6146 running at 500V being driven by a 6AG7 oscillator.

Not sure if the 6146 is the best choice, maybe I need to switch to a 6BG6 or something?

The antenna is an inverted V cut for 1/2 wave 40m, hung from the top of the TV tower approx. 25 ft. maximum height, the ends tied to two small trees. My friend VE3AWA set it up and with his Kenwood he made quick contacts down to Indiana and West Virginia with this antenna.

I haven't made any contacts yet, but I'll keep trying.

73s
geo
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WU2D
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« Reply #4 on: November 22, 2009, 12:09:38 AM »

GZB George,

I have never had much success with Screen Modulation but I do know that you need to drive the screen with a low impedance. Do you have a small step down transformer like a 5K to 600 Ohm military output transformer?

You also have to drive the screen negative by about 20 volts to get full modulation. Having the cathode well off ground can cause issues with the filament so be prepared to use a small isolated 6V transformer just for the 6L6.

The ARRL handbook always treated screen modulation very simply - it is not so simple to get clean distortion-free AM with this approach.

There was an interesting schematic in the old Editors and Engineers Handbook that claimed to solve the problems of non-linearity by using a cathode follower biased negatively at the cathode so it can swing below ground. By luck it uses a 6L6 and it would work on the 6146. I adapted the schematic and tied it to a representative (overly simple) RF section to show you how to hook it up.

Here is how they said to adjust it:

The loading of the RF stage should initially be adjusted normally. You will be adjusting the input power to the 6146 is 50% greater than the plate dissipation of the tube when we are done. The thing to remember is that if the carrier level screen voltage is correct for linear modulation of the stage, the loading will have to be somewhat greater than that amount of loading which gives maximum output from the stage.

R1 is Carrier level. R2 is Negative Peak Level. Now we have some controls to perfect the modulation envelope.

Initially set R2 for -50VDC at the pot wiper.

R1 should be adjusted until the carrier level screen voltage on the 6146 is about 1/2 of the rated screen voltage specified for Class-C CW operation. The current in the screen under carrier conditions will be about 1/4 of the normal current for CW conditions. You should try putting your choke in the grid to replace the 220K resistor at some point to see if that works better.

R2 should be adjusted so that negative modulation peaks, even when voltage which can cause overmodulation occurs, does not cause carrier cutoff.

The antenna coupling should now be increased while the carrier level is backed down by R1 until you get a distortion free modulated waveform in the scope.

Mike WU2D




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VE3GZB
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« Reply #5 on: November 22, 2009, 12:29:06 AM »

The schematic is rather similar to what I've constructed this past week (but I didn't have the privilege of the Editors and Engineers Handbook).

Thanks for the input, I'll keep at it (while I'm still waiting on the 811s for the big transmitter).

73s
geo
VE3GZB
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VE3GZB
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« Reply #6 on: November 22, 2009, 12:42:29 AM »

GZB George,

I have never had much success with Screen Modulation but I do know that you need to drive the screen with a low impedance. Do you have a small step down transformer like a 5K to 600 Ohm military output transformer?

Sorry, no Xformers, but I like the cathode modulator - it's a nice low impedance solution.

If you notice the controlled carrier also uses a cathode modulator at the output.

Thanks and 73s
geo
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KX5JT
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« Reply #7 on: November 22, 2009, 01:39:41 AM »

Geo and others, at el....

I am trying to understand how my DX-60 controlled carrier works.  Mine has the WC3K audio mods too.

See http://www.amwindow.org/tech/htm/wc3kmods.htm

Geo,  the resting carrier control in that mod really does help a lot, especially if you ever use it to drive an amplifier.  In general it helps to tailor the modulation quite a bit.  Looking at W2UD's dual control is very intriguing.  Maybe it could be implemented in the DX-60.  The WC3K mod includes "clamping circuit" that flashes an LED (now installed on my front panel) when you overmodulate.  I can tell you it is VERY accurate.  Bright red flashes occur with every flat lining of the envelope on the scope.  

I had a qso with WB4BFS one morning about converting the DX-60 to pure SGM.  John are you following this thread?  I'd love for you to refresh my memory on what it would take to do that.  Is the RC in parallel on the cathode responsible for the controlled carrier?  It couldn't be as simple as removing them could it?

John KX5JT
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Don
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« Reply #8 on: November 22, 2009, 04:22:32 AM »

You cannot completely screen modulate 100% without distortion.  The modulation linearity of the class-C stage goes south somewhere about 85-90% in the negative direction.  It is possible to maintain good linearity on positive peaks that extend above 100%.
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Don, K4KYV                                       AMI#5
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« Reply #9 on: November 22, 2009, 05:43:53 AM »

You cannot completely screen modulate 100% without distortion.  The modulation linearity of the class-C stage goes south somewhere about 85-90% in the negative direction.  It is possible to maintain good linearity on positive peaks that extend above 100%.

I'm a little confused Don.  Are you saying that if the positive peaks extend above 100% "good linearity" can be maintained?  I understand that controlled carrier is intrinsically non-linear, since the carrier is changing in the output how can it be?  Omitting the actual carrier, what about the "audio" wave riding this changing carrier... is it able to be fairly linear in a controlled carrier enviroment... a pure screen grid modulated enviroment? 

I was perusing the links Derb posted in Terry's post on that Continental Broadcast site.  They used screen grid modulation. 

To quote from near the bottom of this page http://www.rossrevenge.co.uk/tx/continental.htm

"An even later version of the 316, the 316F is generally regarded as being the finest sounding AM transmitter ever built, with exceptionally low distortion and noise levels and excellent phase characteristics."


That seems pretty exciting to me!
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« Reply #10 on: November 22, 2009, 06:42:25 AM »

Moderate modifications to a DX-60 produced an almost HIFi signal. Feedback was the key.
http://home.comcast.net/~suptjud/HomemadeHiFi.htm#Heathkit%20DX-60%20Audio%20Mods
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« Reply #11 on: November 22, 2009, 10:16:18 AM »

I agree - screen grid is not as linear as high level plate; the screen grid is square law in nature. But it can be improved to almost perfection as they did in the broadcast rigs.

First let him get it at least as good as a DX-60. Once you have it good...then we can play.

RF Feedback from a linear AM detector sampling the output to a remote cutoff driver stage like a 6AG7 control grid...

Mike WU2D
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VE3GZB
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« Reply #12 on: November 22, 2009, 10:26:27 AM »

Geo and others, at el....

I am trying to understand how my DX-60 controlled carrier works.  Mine has the WC3K audio mods too.

See http://www.amwindow.org/tech/htm/wc3kmods.htm

Geo,  the resting carrier control in that mod really does help a lot, especially if you ever use it to drive an amplifier.  In general it helps to tailor the modulation quite a bit.

Yes, when I was working out what the resting screen grid drive should be, I found this to be true as well.

When arranging my circuit I saw I could leave the screen grid at a very low level and make the plate current swing nicely but then endure tons of negative clipping (bad). Or I could run the screen grid at a moderate level (between 100-200V) and get nice symmetry but not much plate current swing.

I chose the latter. Screen grid runs around 150V now and I'm using a scope to monitor my output so I don't overmodulate. If I get the plate loading just right then the output looks very traditional and proper.

Still, I've been trying to make contacts with it last night, and again this morning on 40m. Nothing.

I know the antenna is good (it seemed to be naturally resonant close to 7.1, that's where the SWR reading is at it's lowest point, below 1.5:1) - VE3AWA tested the antenna for me with his rig and right away he made contacts, one as far as West Virginia!

But me? Nothing, it's like I don't exist. It's so frustrating!
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WA1GFZ
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« Reply #13 on: November 22, 2009, 10:33:18 AM »

Take a power transformer with a center tapped high voltage winding. Build a push pull amp using the winding as a termination to  the modulator plates. 6146s or 6l6s will work. Now heising connect one of the modulator plates to the final. You will need a choke in the B+ lead.
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VE3GZB
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« Reply #14 on: November 22, 2009, 01:22:13 PM »

Take a power transformer with a center tapped high voltage winding. Build a push pull amp using the winding as a termination to  the modulator plates. 6146s or 6l6s will work. Now heising connect one of the modulator plates to the final. You will need a choke in the B+ lead.

I'm running out of parts now, I can't afford any hamfests until tax refund time next year.

So if anyone has an old broken or unwanted PA amp suitable for plate modulation (would prefer tube amp because transistor amps may rectify/become unstable near a transmitter I've found), or an old orphaned boat anchor of an AM transmitter their wife would be happy to see gone, I hope you keep me in mind.

I just ran a short QSO with VE3AWA today at 1pm EST on 7104. His noise level was S8, my carrier came in at S9 but voice quality was rough. I need to improve modulation somehow.

Thanks and 73s
geo
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k4kyv
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Don
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« Reply #15 on: November 22, 2009, 05:15:06 PM »

The nonlinearity in screen grid modulation occurs mostly near cutoff, so it should be OK on positive peaks until saturation resulting in flat-topping occurs.

Continental Electronics used a combination of control and screen grid modulation on some of their broadcast transmitters in the 50's and 60's.  This was supposed to be very linear, comparable to plate modulation. They always stayed away from high level plate modulation.

I recall Tom, K1JJ using a similar scheme  of control/screen grid modulation in the 1970's, with a 4-1000 IIRC.  Perhaps Tom could comment further on this.

I agree with Brian that most controlled carrier sounds like crap, although I have heard a few reasonably good sounding ones.  It works best with only 3 dB or so of carrier control, which still gives substantial saving in plate dissipation, safely allowing more output power on voice peaks, without generating the infamous pumping effect due to AGC action at the receiver.

Not all controlled carrier is screen grid modulation.  Before WW2, there were several widely used circuits that used plate modulation.  One scheme connected the DC plate circuit of the rf PA in series with that of the class B modulator stage.  As the modulator drew more current on voice syllables, its load impedance on the power supply decreased, correspondingly increasing modulator plate current and therefore rf plate current since both stages were connected in series across the power supply .  Another scheme used a saturable reactor that varied the a.c. voltage to the rf power supply plate transformer, controlled by the modulator plate current.
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Don, K4KYV                                       AMI#5
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« Reply #16 on: November 22, 2009, 05:51:48 PM »

Has anyone ever modified the sreen grid modulation on a Knight T 150 would the new WC3K DX 60 modulation work on the 150.

John W9BFO
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VE3GZB
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« Reply #17 on: November 22, 2009, 06:42:53 PM »

Geo, one last suggestion, have you considered cathode modulation of the RF amp? You basically connect the plate of the final modulator tube(s) to the RF final's cathode.

I tried Cathode modulation before, on the big 100+ watt breadboard rig, I ran into some problems with freq. drift and some internally generated noise/squealing caused by the modulator tube in some kind of reaction. I used a single 6AS7 for modulation in series with the 805's cathode, I used both sections of the 6AS7 in parallel.

I'm concerned that those problems might happen again if I try using Cathode modulation on this rig. Ideally I want to leave the Cathode/Control grid loop alone, isolated for RF purposes only - and apply modulation to some other part of the Xmitr.

I tried to see if I could make some kind of plate modulated thing happen this afternoon with a 35 watt solid state amp I have. But as soon as something RF starts near this amp, it becomes unstable and starts to break into HF oscillation on it's own, so that's a no-go there. I'm not even going to touch this small amp, it's just not worth the additional headache.

73s
geo
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w4bfs
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« Reply #18 on: November 22, 2009, 07:22:08 PM »

hi John ....just got your message ... just refreshed my memory with the WC3k mods to a dx 60

I think there may be a typo on the carrier control pot value ... 50 ohms seems way too low .... more like 5k or 50k

notice that the 6de7 stage a cathode is grounded .... this forms grid leak or contact bias which varies with audio level in and shifts plate voltage q point .... lift the cathode and place a 2k pot to ground and this will allow you to vary bias from grid leak to about 2 volts which should keep the plate q point steady and defeat the controlled carrier aspect .... some intermediate setting would allow some cont carrier op but not the usual 10 to 12 db these run which is annoying .... a single 6146 can gen about 15W carrier with plain screen grid mod ....hope this helps ...73 ... John


Geo and others, at el....

I am trying to understand how my DX-60 controlled carrier works.  Mine has the WC3K audio mods too.

See http://www.amwindow.org/tech/htm/wc3kmods.htm

Geo,  the resting carrier control in that mod really does help a lot, especially if you ever use it to drive an amplifier.  In general it helps to tailor the modulation quite a bit.  Looking at W2UD's dual control is very intriguing.  Maybe it could be implemented in the DX-60.  The WC3K mod includes "clamping circuit" that flashes an LED (now installed on my front panel) when you overmodulate.  I can tell you it is VERY accurate.  Bright red flashes occur with every flat lining of the envelope on the scope.  

I had a qso with WB4BFS one morning about converting the DX-60 to pure SGM.  John are you following this thread?  I'd love for you to refresh my memory on what it would take to do that.  Is the RC in parallel on the cathode responsible for the controlled carrier?  It couldn't be as simple as removing them could it?

John KX5JT
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VE3GZB
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« Reply #19 on: November 22, 2009, 07:36:17 PM »

I found that when I tried the controlled carrier modulation, what was "modulation" was sort of a very abrupt analog to digital reaction, where my analog signal (audio) was being interpreted into a stream of highs and lows (as I saw on the scope). Sort of like a Pulse Width Modulation.

But the effect on the sound quality was absolutely horrible! It looked great on the plate current monitor but it was impossible to call it modulation. Digitization seems more accurate a description.



What I ended up trying was placing a 15k resistor in series with the first 6DE7's (the tube I ended up using was not a 6DE7, it was a 6BZ7/6BQ7, which is what I had on hand to try) cathode-ground connection and reduce the 22 Meg resistor down to something less than 1 meg. This let the first tube behave in a more linear fashion. But it was still not good enough to give me more than perhaps 40% modulation.

Somehow I'll have to do something to improve my situation. Just not sure what now.

73s
geo
hi John ....just got your message ... just refreshed my memory with the WC3k mods to a dx 60

I think there may be a typo on the carrier control pot value ... 50 ohms seems way too low .... more like 5k or 50k

notice that the 6de7 stage a cathode is grounded .... this forms grid leak or contact bias which varies with audio level in and shifts plate voltage q point .... lift the cathode and place a 2k pot to ground and this will allow you to vary bias from grid leak to about 2 volts which should keep the plate q point steady and defeat the controlled carrier aspect...
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John-O-Phonic


« Reply #20 on: November 22, 2009, 08:16:26 PM »

hi John ....just got your message ... just refreshed my memory with the WC3k mods to a dx 60
......
notice that the 6de7 stage a cathode is grounded .... this forms grid leak or contact bias which varies with audio level in and shifts plate voltage q point .... lift the cathode and place a 2k pot to ground and this will allow you to vary bias from grid leak to about 2 volts which should keep the plate q point steady and defeat the controlled carrier aspect .... some intermediate setting would allow some cont carrier op but not the usual 10 to 12 db these run which is annoying .... a single 6146 can gen about 15W carrier with plain screen grid mod ....hope this helps ...73 ... John

Ding Ding Ding!! The light bulb goes off! Thank you John!  This certainly helps clear out some confusion in my mind!  Such a simple and elegant fix!

Geo, it is not my intention to hijack the thread you started, but I just felt that my questions were certainly on topic and relavant and the answers would help both of us!

73

John KX5JT
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John-O-Phonic


« Reply #21 on: November 23, 2009, 02:45:01 AM »

notice that the 6de7 stage a cathode is grounded .... this forms grid leak or contact bias which varies with audio level in and shifts plate voltage q point .... lift the cathode and place a 2k pot to ground and this will allow you to vary bias from grid leak to about 2 volts which should keep the plate q point steady and defeat the controlled carrier aspect .... some intermediate setting would allow some cont carrier op but not the usual 10 to 12 db these run which is annoying .... a single 6146 can gen about 15W carrier with plain screen grid mod ....hope this helps ...73 ... John

BFS John, for the SGM on the DX-60, are you speaking about the first 6DE7 (a) triode stage?  BTW, the pot on the cathode follower load in the WC3K mod should be a 50K ohm pot not a 50 ohm (replacing a fixed 33K ohm resistor).  A typo indeed.

TJL Terry.. I am intrigued by the feedback idea!  If you still own that modified DX-60 I'd love to make a schedule with you sometime to hear it on the air!

KYV Don... I recall you mentioning on a few occasions that you have increased the input impedance of your mic input stage to something on the order of 20 megs with a resistor to better match the Hi-Z D104 Crystal mic and get better low frequency response. Certainly this could apply here, but I wonder where on the input of the 12ax7 I would apply this? I'm assuming I would replace the 2.2 megohm from the grid to ground.

73 from JT John  




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AMI#1684
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« Reply #22 on: November 23, 2009, 09:38:06 AM »

You cannot completely screen modulate 100% without distortion.  The modulation linearity of the class-C stage goes south somewhere about 85-90% in the negative direction.  It is possible to maintain good linearity on positive peaks that extend above 100%.

Don,
      you can easily get 100% in both directions if you bias the final well into class C, eliminate the grid leak resistor, and also modulate the control grid biass in phase with the screen. About 25% of the modulating voltage applied to the control grid works just fine.

I've been running my "scream modulated" 4-1000A like that for quite a few years now. It will go 100% negative without that funky crossover distortion normally found in screen mod rigs. It also has no problem going well over 100% positive as well..

there is a segment in the handbooks that says you should apply "a small amount" of the modulating voltage to the control grid as well.

I run 3200v on the plate, -150 or so on the control grid and around 290-300v on the screen. Drive it with a 6146 to 40 mills grid current, and load the hell out of it. It loafs along at a half gallon out and 100+% swingin the munky.

You and many others here have heard it, no one ever complains about my audio.

                                                         the Slab Bacon
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« Reply #23 on: November 23, 2009, 11:16:06 AM »

Slab,

Now that you've settled on a good 4X1 screen modulated design - no grid leak, heavy class C, heavy loading, etc., what do you calculate the input/output final stage power % efficiency to be?  (Both dead carrier and peak power audio tone calculations)

T
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« Reply #24 on: November 23, 2009, 12:15:49 PM »

Slab,

Now that you've settled on a good 4X1 screen modulated design - no grid leak, heavy class C, heavy loading, etc., what do you calculate the input/output final stage power % efficiency to be?  (Both dead carrier and peak power audio tone calculations)

T


Tom,
        I have never calculated the efficiency at peaks, but at resting carrier it is around
40-41% which is better than what the handbooks tell you it should be. IIRC the handbooks tell you you should be getting 30-33% efficiency. It is still loaded heavily enough to get well over 100% on peaks (on the scope). I think running the higher voltage has something to do with it.

Becuz the screen's impedance presents a varying load to the modder, I did wrap some feedback around it. I also installed 2 meter probe jacks on the output of the mod tranny to allow me to load down the modder as the plate voltage changes. (Higher Ep = lower Ig2) I just stab in a 10w dogbone to compensate for any necessary changes.

I have never ran any efficiency tests at peak audio. It of one of those things I want to do, bud never found that elusive little thing called a round tuit. It would be very interesting to know since this type of modulation is sometimes called "efficiency modulation."

However it has worked so well I have taken to "if it aint broke, dont fix it".

                                                            The slab bacon
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