The AM Forum

Building this modulator.  I have the front end built and tested. Sounds great. Except: the 6V6 stage is drawing 62 mA of current (305 vdc).  Maximum for a 6V6 is 35mA at that voltage, and my driver transformer is getting warm after about 15 minutes. I checked the 470K resistor on the grid,  it is the correct value installed.
(https://i.postimg.cc/TYny9Twv/811a-modulator.jpg)

What is the purpose of that 750K/.001 combination coming off the plate of the 6V6?  That looks like a recipe for feedback disaster.

I'm betting you're running an oscillator and don't even know it.  Get on it with a scope, look at the plate of the 6V6.

  Bob,

    If you can rule out oscillation, then perhaps the DC operating point is too HOT. The 750K feedback resistor is negative feedback, but that .001 to ground on the 6SJ7 plate to GND should not be there. There will be appreciable phase shift as a result.
  
    The 6V6 at 305v plate and screen is within its voltage ratings, but with the screen voltage is higher than the App notes. If it is necessary to turn down the DC operating point, the cathode bias resistor needs to be increased to get the bias point down. Your schematic shows 250 ohms. You might try something between 470 and 1K to get the zero signal idle current down to 35ma or so. Looking at the curves, and with a little guessing, I'd bet 750 ohms will be about right.

Good Luck,
Jim
Wd5JKO

My thoughts exactly.

Other ideas:

1) Drop the screen voltage to about 250 VDC.

2) Try putting a 25 to 50 uF from Point 2 to ground.

3) If you want to do the feedback thing, shunt the 750K and 0.001, and connect them from the 6V6 plate to point 3.  Do not connect this network to 6V6 grid!

Good luck.

I removed the 750K feedback resistor, and it did not have any effect.  I am not getting and oscillation, in fact the thing sounds very "high fi" passing music!  I will pull that .001 out, and see what happens.  This print is straight out of the Radio Handbook (West Coast handbook) 12th edition page 185.  I did re touch the rectifier to make it solid state instead of a 5Y3.
Thanks.

Screen voltage probably too high, put a few hundred ohm resistor in series with the screen.  The cathode resistor should be increased

Looking at the data sheet for the 6V6 in class A operation, the static resting current is 34 ma.  The 34 ma is when the plate voltage is at 315V and the screen voltage is at 225V.  Your DC plate and screen voltage are approximately the same in your configuration.  I have done that in the past, but it required increasing the cathode resistor to get back to the 34 ma static plate current value. 

Secondly, I really do not care for the feedback scheme they are using.  With a plate resistor value of 220K, the .001 capacitor from plate to ground will definitely roll off the high frequency response.  With that scheme, you end up with an AC divider between the 220K plate resistor, the .001 to ground and the .005 coupling capacitor to the grid of the 6V6 based on each Xc values along with the 470K to ground grid resistor.   If you need or want negative feedback, I would do it from the secondary of the modulation transformer back to an earlier stage.   

However, if you want to stick with their scheme, at a minimum you will need to raise the value of the cathode resistor until you get back to the correct no signal plate current value of 34 ma. 

73,
Joe-W3GMS

             

Last night I lifted the 750K resistor again, and also lifted the .001.  Current starting climbing.  Turned off the hi voltage and put them back in, I am going to try the lowering of the screen voltage that has been recommended.  Thanks again.

Also, check the tube buy substitution!

Lowering the screen voltage will lower the resting current, but I don't believe that is your current runaway problem.  By the way, lifting those components should not change anything from a DC perspective unless it oscillating.

Joe

 

   I am attaching an image from the ART-13 audio driver. Looks like the West Coast handbook took it, and with a few modifications they ran with it. Notice the 6V6 cathode resistor is 250 ohms with only 190v B+.

   Using a screen dropping resistor is ok to lower the screen voltage, but it would need to be AC bypassed at the screen side with an electrolytic such as 20 uf. I'd just vary the cathode resistor from 250 ohms upward..as I stated earlier, somewhere between 470 ohms and 1K (2 watts) should do.

   You could also pop in a 6L6, or EL34/6CA7 and then "hone the bias point" with whatever tube you choose. Keep the driver transformer primary current within ratings for the tranny, or use Parafeed with an AC coupling Cap, and high inductance audio choke to B+

Jim
Wd5JKO

You could probably solve a lot of your problems by lowering the plate voltage.  Redesign your power supply.  Remove the input capacitor and the resistor off the rectifiers.  Connect the choke directly to the rectifiers and then the filter cap, put the resistor between the two filter caps.  This will probably drop your plate voltage down below 250 volts.  Add a 100-200 ohm resistor to the screen of the 6V6.  The cathode resistor should be around 330-370 ohms.  Like others have said remove the feedback resistor and cap.

Lastly, check the cathode by-pass cap, it may be shorted or in backwards.  Seems you have little to no cathode bias.  The cathode voltage should be around 12-13 volts.

Fred

My question boils down to, why should I have to change anything.  If this circuit was published by a reputable firm YEARS (1956) ago, and has not been changed in subsequent volumes, why should I have to adjust it.  It should work as-is, ARRL plans always do!

"We have a winner".. replaced the cathode by pass resistor with a 330, brought it down, increased it up gradually...finally put in a 1K resistor, and resting plate current is 30 mA. thank you Jim, Joe and all the others. 

It is absolutely correct Bob, the 6V6 will draw approx 65 mA when both the anode and g2 are at 305 Volts and the cathode resistor is 250 Ohms (that results in approx -16 V G1). When you look in the characteristics, you have to look in the triode connected characteristics because the Anode and G2 have the same voltage.
I don't have the book where the diagram is published, but I think that the circuit voltage was desiged for a lower voltage like 250V instead of 305V
When you lower your G2 to 250V and increase Rk to 390 - 470 Ohm, the anode will draw approx 35 mA
You can also increase the Rk only to approx 690 Ohm to draw 38 mA and leave the Ug2 the same

Ok,

A 1kohm cathode resistor is a bit high...

First step is to put in a known good 6V6 (assuming the plate voltage is not substantially over
tube manual specs).
Consider changing to choke input filter and get the B+ lower, like the ART-13 schematic shows?

Second, you want either a series resistor with the screen (you will loose some output, but
save the tube from early demise), something between 100ohms and 1kohm - use ohms law
and the tube manual to find the max screen current then find a reasonable value resistor.
The purpose is to limit the max screen current.

Idea 2.5 use a divider between the B+ and ground to set a screen voltage lower than B+. Add a
bypass cap to ground.

Idea 2.75, the tube will likely prefer to be run with a lower cathode resistor... or consider changing
to fixed bias or a combination of fixed and cathode bias.

That 0.001 to ground is not the most elegant way to roll off unwanted highs.
There may be better ways - but here's where a scope and a signal generator are very useful to
sweep the circuit and find the LF rolloff and the HF rolloff - without the 811a running. And, before the
driver transformer.

I'd also look at the way a square wave looks on the output of the driver transformer - a 1kHz square wave
will tell a lot. Look for leading edge tilt and ringing. If you see a lot of either or both, see what (if any) effect
the 0.001 has on that.

Also, sometimes the driver transformer will behave better if there is a light resistive load on the secondary...
under drive conditions, the grid of the 811a looks like a high Z until suddenly on peaks current gets drawn.
So the tube goes from nil current to CURRENT, a light load may help to make that transition less severe. It
may also help to make that square wave look less squirrely (if it is). Easy enough to experiment with.

The circuit at the top of the thread is a photocopy of the actual diagram from the 1956 handbook.  Other than my changing the 5Y3 tube to a couple of diodes, it is 100% as originally drawn. No values changed, nothing.
I just listened to music thru the amp for half an hour, and all is well with the 1K by pass resistor. 32 mA plate current max after 5 minutes of heating.

Bear, I am going to try that this week.  For my own peace of mind, I would like to know what the response of the thing is, and yes that feedback is going away.  I was thinking that the .001 was to cut down on RF for some reason.

I have found many of the Amateur Radio cookbooks to have errors in them and many an amateur has had to tweak those circuits to get them to work properly.

So don't take them as absolute gospel.

As other's have stated, the 0.001 uF cap must have some high frequency roll-off to limit the upper end.

According to the GE tube data here is the circuit I would set up for the 6V6 stage:


Phil - AC0OB

I presume the control grid voltage of the the 6V6 is zero, no leaky cap.

Er, your schematic isn't in an ARRL handbook...the Jones to Orr publications had nothing to do with them.  You notice there is no 'league' diatribe in them. ;)

73DG

Bob, if you need any parts for your project, please PM me.

d

   :) Hey Dennis, "The circuit at the top of the thread is a photocopy of the actual diagram from the 1956 handbook.  Other than my changing the 5Y3 tube to a couple of diodes, it is 100% as originally drawn. No values changed, nothing". (Actually I did change the coupling caps from a .005 to a .05)

The Handbook I referred to is the Radio Handbook 1956 edition. I had written that in one of the posts, and in editing, I must have taken it out.  And in my tirade, I was pointing out that in the ARRL Handbooks, everything (I have built)  has worked out of the box, using their plans.
I would think the Editors and Engineers folks who published the left coast book, would have checked everything over time.
 
No harm, just lessons learned and I appreciate the folks like you who help me get this stuff done.

Somewhere it is written, " as a whole, the group collective knowledge is at genius level".  I firmly believe that.

Bob,

I think your transformer, T3, is too high in voltage. If you're getting 305VDC, the transformer is probably about 430VAC ct. If you were to move the 8uF cap at the input of the filter choke to the output of the filter choke, you should get about 190VDC.

However, if you're using a transformer with the same voltage that is called for in the Handbook, then the placement of the 8uF capacitor at the choke input may be the "error".

Either way, 305V is too much on the screen of a 6V6, which is rated at 285V maximum, but probably should be run no more than 250V in that circuit.

If the circuit is designed for 190V and you have 305V on it, then you'll have higher voltages on ALL the stages, not just the 6V6. You can measure the voltage drop across the cathode resistors of those earlier stages and calculate the current, then check it against the data sheets for those tubes.

I hope this is helpful to you.

Don

Hey Don,
Good to hear from you.  The list of parts asked for a 700V center tap 90 mA transformer.  I looked at all my stuff and found a 720 (no load) unit bit it is probably more like 120 mA capable.  As best I can tell) I wired it just like the print from the Radio Handbook. I can certainly move that cap, if you think everything will play nicely. I just need about 5 watts out to drive the grids of the 2 811A's.

Bob,

It is unlikely the xfmr you're using is 720vct.  You would be seeing B+ voltages over 400 volts using the cap input filter.

If there are any numbers on the xfmr I might be able to find the specs for it.

Fred

    There has been no mention of what it takes to drive class B grids. The good news is that the 811's are running zero bias, and that means in push pull, the load on the audio driver will be there throughout the audio cycle. The load will not be constant however since the 811 grid current will rise with the rising grid voltage. In order to keep the voltage from sagging, the driving stage needs to have a low source impedance. Without negative feedback, that means the driver would have to be a low mu triode such as a 2A3/6B4, 12B4, or a triode connected Beam power tube such as a 6V6, 6W6, 6L6, etc. Of the three Beam Power tubes, the 6W6 would have the lowest plate resistance.

This link by John WA5BXO illustrates my point:
https://www.qsl.net/wa5bxo/driver1.html

   Go back to the original circuit at the top of this post. Then look at the ART-13 audio driver schematic that I posted. Both have a 750K feedback resistor in there to make a form of negative feedback. I agree that that .001uf Cap needs to go, but that feedback resistor could/should come back. Sure, other forms of feedback could be used, but hey, they used it on the ART-13, in a radio that helped save the world during WW2!

   The Gonset G-76 uses a similar form of feedback in the audio driver. The audio driver there is a 6CM6 which is very similar to a 6V6 and 6AQ5. My recent posts about that show two NFB loops that I tweaked into that rig.

  
Jim
Wd5JKO

Bob,

A 700 vct transformer, wow. I'd think it should be about half of that. 350 vct maybe. I'm at a loss. I can't figure out how you end up with 305V without using a choke input filter if the transformer is 700 vct. A bad filter cap maybe?

Try making more measurements. I like to check the values of components before I use them. Not just my junk-box stuff, but new parts as well. I've had NOS power resistors, right out of the original box, be open circuit. Check voltages, voltage drops, run the calculations. As a newly built piece, don't overlook the posability of a miswire. We've all done it!

Don

I did some more FIG-ER-IN' and here is what the power supply should look like to develop 315V at 55 mA for the Speech Amplifier and Driver:

Current for Speech Amplifier/Driver = 48.7 mA
Current for 50k Bleeder = 6.3 mA
Transformer Secondary = 470VAC@100 mA with CT to ground (235 VAC each to ground)

R1 = 200 ohms 2W
R2 = 50k 5W

C1, C2 should be modern 10 uF @ 450V.

The values in brackets are for a 500VAC CT transformer  (250 VAC each to ground).


Phil

Here for all the see, is the actual complete print from the Radio Handbook.  Notice that nowhere do they mention the the 6V6 cathode bypass should be a 2 watt resistor.  After looking at several variations on this design (a basic 6 watt amplifier) I noticed that the RCA tube manual from 1959 points out that it is a 2 watt job instead of the 1/2 watt used throughout the rest of the amp.  Also, you can see that the components I put in there are what was asked for. I am going to try to read some voltage tonight, and see what to do.

One more thing, I noticed also that WRL on the Globe King modulators, used a single 6L6 with 400 on the plate and 350 on the screen with a 390 ohm, 2 watt resistor and 25 mfd at 25 cathode bypass, to drive the two 811A's!

(https://i.postimg.cc/GmHBBkhb/img323.jpg)

Could be a circuit that they printed in the book but they never built it and try it.  I tried a driver circuit from their hand book and it didn't work right.  I designed my own driver circuit for my 811a modulator and it worked perfect.

Seems to me that they copied the circuit from the ART-13 xmtr but they included a power supply that they designed.  The mistake they made was using a power xfmr with too high of a plate voltage.  That's why you had problems and had to increase the cathode resistor to 1000 ohms.

That circuit in the ART-13 worked on 190VDC,  you're trying to run it with over 300 volts B+.  That's why you're having so many problems.

Myself and many others have told you to lower the B+ voltage but you keep insisting that because you see it in the book you shouldn't have to change anything.  That's the mistake you're making.

Fred

I designed and built my first 6V6 audio amp back in 1964.  I still have the amp and it still works.

Looking at my RCA tube manuals, Class A1 amp. the 250 Ohms cathode resistor appears to be a proper value for triode connection when you have 250 Volts on the plate and screen.  With this condition the zero-signal plate current is stated as 45 ma., and the zero-signal screen current as 4.5 ma.  This a total of 49.5 ma cathode current.  IR drop = 250 Ohms  X 0.0495 A = 12.375 V.  The grid voltage with this condition is listed as -12.5 Volts, so all seems correct for 250 Volts DC, however no B+ voltage listed on that schematic.

It’s o.k. to run a higher plate voltage on the tube with the following condition.  The max rating is 350 V., but the highest condition shown in the manual is 315 Vdc.  Above 250 V. operation the screen voltage is divorced from the plate voltage and the manual lists 225 V screen at 315 V. plate voltage.  Here the zero-signal plate current is 34 ma. and the zero-signal screen is 2.2 ma. for a total cathode current of 36.2 mA. The grid bias is listed as -13.0 V.  Cathode R = 13/0.0362 = 359 Ohms.

A tube at this power level, even RCA HB-3 does not give a screen dissipation rating.  I wouldn’t run the screen at 300 V along with the plate and just increase the cathode bias as you may be over-dissipating the screen grid. 

So you have 2 choices, lower the plate voltage to the whole circuit to 250 or cut the screen voltage down to specs and re-adjust the cathode resistor value.  I would consider running a full divider for the screen and not just a series resistor since we are dealing with low power, healthy bypass capacitor value - a few uF.

As most recently Fred and Tom have pointed out, lowering the operating voltage is the best way to go, and I shall.  I found a more appropriate transformer in my stash, and will begin "re construction" tonight.  I wasn't happy with some of my sloppy wiring anyway.  I like for my home brews to look "factory".  Also, I did find where the Radio Handbook pointed out that all the components were taken from a Collins 26 S-1 speech input module used with the ART-13 set.

Again, and again, thank you all for reading this topic, and making your contributions.
And I hope you all enjoy the Holidays coming.

From the GE data on the 6V6 the actual simple calculations show otherwise:

https://frank.pocnet.net/sheets/093/6/6V6GT.pdf  (see page 2, Class A1 amplifier, characteristics and typical operation).

Specifications: Vp = 315V@Ip = 34 mA, Grid Bias = -13V, so Vk = 13V,  Average Screen current Isg = 4 mA@Vsg= 225V.

There is nothing wrong with running the tube at a Vp of 315V as long as the screen current/voltage is limited.


Total current through the cathode = 38 mA.

Rk = 13V/0.038 = 342 ohms so I chose 330 ohms as nearest common resistor.

PRk = I^2XR = 0.001444 A^2.Ohms = 0.477Watts = 477 mWatts, choose 2W for headroom

Rsg = 315V-225V/0.004 = 90V/0.004 = 22.5k, choose 22k ohms as nearest common resistor

PRsg =1.6e-5A^2X22k ohms = 0.352Watts = 352 mWatts = choose 2W for headroom

So I could care less what that ridiculous original schematic shows, the actual simple calculations show otherwise.

Someone in that publication did NOT do their homework. ::)


Phil

 

 ;) Agreed Phil, thanks.

Don,

    I attach the 811A RCA data sheet. It shows that for Class B with 1250 on the plate, the peak grid to grid voltage needed is 145 volts. That could be read as 72.5V peak to each grid with respect to ground. The driving power stated is 3.8 watts.

    I am wondering on what you are using for T1 interstage transformer. The turns ratio becomes important here, arguably more important than the primary load impedance placed upon the 6V6. Since the 6V6 operated as a Beam Power tube has a high plate resistance, a high stepdown ratio at T1 would be helpful to stiff up the drive at the 811 grids. That means a higher plate voltage on the 6V6 would be beneficial. I agree with Phil and others that the screen voltage should be dropped.

   My earlier post (22) covered the need to have a stiff drive signal at the 811 grids.

Jim
Wd5JKO

Jim,

My 811a driver uses a 6L6 running on 300 volts plate voltage, screen also on 300 volts through a 100 ohm resistor.  I used a 25w mod transformer for the driver xfmr.  The xfmr had a step impedance secondary that I used for the 6L6 primary.  The PP primary is used to drive the 811a grids.  I used a large amount of negative feedback from the 6L6 plate back to an earlier stage.  While designing the driver I loaded the driver xfmr with two 130ohm resistors to substitute for the 811a grids.  Using a scope I could tell how much peak voltage I was getting on the 130ohm load resistors.  It worked perfect with plenty drive voltage to the 811 grids.
I forgot what step-down ratio I used but it was about 2-3/1.  Primary to 1/2 secondary.

Fred

I checked my notes and the ratio was 1.8/1 primary to 1/2 secondary.

WELL! I've been following this thread because just this morning I started drilling holes where a power transformer will live for this EXACT SAME MODULATOR. First. I have the 1949 AND 1963 Radio Handbooks in question and can't find the provided diagram in the '63, BUT said diagram from the '49 IS IDENTICAL with the exception of the replacement of the 5Y3 and changing the .005 caps between stages with .05s. I'm planning on doing the same with the caps. As for the power transformer, I have the exact part called for...a Stancor P-6012 NOS rated at 700 VCT and 90ma. A few years ago (before my heart started acting up) I ran this same diagram past the group and the main advice I got was to reduce the feedback capacitor from .001 to 500 pf. I'm making a couple more changes because of parts-on-hand, namely using 25 uf on all the cathodes rather than 20 uf. I'm not going to run the 811A's at 1250 volts yet. In fact I have a couple 600 volt supplies...one for a parallel 807 amp and the other for the modulator.  I may shift to 809's and plug in the 811A's when and if I build a larger amp. For now I'm just rehoning my construction skills after a long illness. Second, I have connected with you in the past, Bob. I bought the incomplete 4-125A amp from you. I gutted it, reversed the chassis and built the parallel 807 amp on it (actually using 5933's for a 120 watt AM job). Looks familiar??!? I may yet build that 4-125A job OR an 813 amp to use with THIS MODULATOR after replacing the ART-13 iron. SO. I haven't started wiring yet and have not cut out the hole for the Stancor. I was going to do that this aft. Maybe I should hold off and look for something with a lower voltage OR have you corrected your problems and if so, what did you finally come up with? Finally, my driver transformer is a 15 watt Stancor "Poly-Pedance" universal driver with taps for 3:1 for 1/2 secondary as recommended in the diagram.

If you have the exact same PS choke called for, I would measure its DC resistance.

The only way a 200 ohm surge resistor will work with a 700VAC CT transformer and give about 315V is if the PS choke has a high resistance.

Most of these chokes had resistance from about 60 ohms to 150 ohms depending on how much copper and what gauge was used.

It is interesting that no voltages or voltage charts were indicated in those Handbooks or on the schematics, which I why I said you have to take those schematics with a grain of salt and run your numbers before finalizing your circuit.


Phil

Choke is a Hammond, 407 ohms, 14 Hy and 75 ma. I did a search and found that using a choke input filter with a 700 VCT tranny and a single 20 ufs cap, voltage under-load would be nearer 240 volts. Of course, the bleeder on that unit was 30K and not 50. Seems to me changing the PS to choke input makes the most sense.

The choke resistance must have been a major factor here.

Rerunning the numbers and averaging out choke resistances from both EDCORUSA and Hammond, it appears the choke resistances averaged about 350 ohms.

So at 350 ohms, a 55 mA current would drop about 20 volts across it.

So 315V+20V has to = 335V at the choke input.

The rectifiers would have produced a pulsating voltage with a peak ~ = 495Vdc, so (495V-335V)/.055 = a resistor value of 2.9k.

Now 2.9k - 200 ohms = 2.7k, so two resistors of different values in series could produce 335V at the choke input.

Did the authors/editors leave out the 2.7k by mistake? As Cousin Eddy said in Christmas Vacation - "I dunno."  ;D


Phil

I think I'll go ahead and use the power XFMR I have and then play with the filter a bit. There is more than enough room under the chassis to add a dropping resistor.  I have plenty of adjustable types. Even with 250 volts on both plate and screen, the tube will do 4.5 watts output. I'll just mess with it a bit and see how close I can get to the 315 pv / 225 sv.

Now that's what I'm talkin about!
Proper margin!

Sounds like a honkin' transformer to use. Please let us know the component values you used and the voltages you found.

The approximate 315 Vp /225 Vsg figure should give one about 5 Watts of output which should drive even an inefficient transformer with losses.

I suspect the 750k ohm feedback resistor was used to lower the gain about 29% and distortion somewhat for the 6SJ7/6V6 stage when using efficient inter-stage transformers.

If you find your choke resistance is higher than 350 ohms one can dispense with the 200 resistor and just use the 2.7k resistor.


Phil

(https://i.postimg.cc/MpxfRcGn/mod-3.jpg)

This one works! I got another chassis, this one worked better from the start. Replaced the 6V6 with a 6L6GC,                                
it rests at 54mA /300 vdc, the screen draws a resting 2 mA at 250 vdc, the thing sounds great.  
Sampling the audio at the 811A's grid point. Played music thru it for two hours, nothing gets hot to the touch (the tubes get warm - duh!)
Solved the interstage transformer problem by using a UTC mod transformer.  Got some valuable hook up information from Fred KA2DZT , who also does this trick, and it takes the worry out of overheating or saturation.

Now to finish gathering parts for the 4-125 transmitter.
Thanks to everyone who contributed and overlooked my impatience.  Getting old will do that to you.

Happy Holidays
W4RFM

Bob,

The finished 811 modulator looks great,  very nice job.

The rest of the project should go well

Post some pictures

Fred

Looks good, Bob!

Do I see an ART-13 mod transformer in there? Here's some info on that:
http://www.amwindow.org/tech/htm/modtran/misc/art13.htm
The turns ratio works out to 1.43:1 which should work fine with a common power supply.

Don

That is what I am hoping for, a 4-125 running at about 212 watts input, (Supposed) to be a match for this modulator.

so which circuit won the prize ?

Yep, lots of "switcherooing" here.  ;D


Phil

Very nice job Bob and glad things are moving along!

73,
Joe-W3GMS

It is beautifully made! Very respectable indeed. Love the glass pentode? up front there.

Looks great!! So Bob, I see a UTC transformer there in the speech amp. Did you eliminate the 6J5 mic preamp and feed the input transformer to the 6SJ7 second audio? I have a Triad transformer I would like to use. It's 250 / 600 to 85 K. Did you run the secondary to the volume control directly? Did you do any modifications in components in order to use the 6L6? I haven't punched any of the socket holes yet, so I could easily duplicate your circuit. Yeah, as suggested, can we see the changes to the schematic especially for the power supply? I can make the same changes in my version.

Mike
W9ZSL

Hello Mike,
No, I built the same exact circuit, (eliminating the 6J5 mike pre-amp stage) using .05 coupling instead of .005, and I removed the .001 feedback cap on the 6L6/6V6 stage. 
I couple the secondary of the UTC A-11 to the volume control on the front.  I went ahead and mounted a second pot in case -down the road I want an alternate audio input.
I didn't change anything on the 6L6 stage, but after lowering the voltage coming out of the power supply, it runs within tolerance, and no part (other than the glass tubes) gets warm to the touch.  I ran music thru it for hours to test all the parts, stopping occasionally to read voltages, and various currents on the tubes.

I debated using the Triad mic transformer with the 6J5 extra stage and setting things up for both crystal and dynamic mikes...or balanced / unbalanced input. Was curious to find out if you had enough gain using the UTC. Did you go with a capacitor or a choke input filter on the power supply? I brought up this exact topic a few years ago and the group suggested changing the .001 ufd cap in the feedback to 500 pf. I'll probably just can the feedback circuit completely. Keep us posted on the 4-125A amp. Like I said earlier, I have all the parts I need to build that unit you have in mind OR one using a single 813.

It appears I have plenty of gain using the A-11 step up audio transformer to the 6SJ7. I am keeping the 750K feedback resistor, and losing the .001 mfd.  The audio is hi fi, no kidding.  :)

Question :

On Bob's schematic, I see the 6V6 has the screen at plate voltage potential.
( no dropping resistor ).

If that is a No-No at RF, how does it work at audio ?

Is it just a characteristic of the 6V6 ?


Don W4DNR

Bob, I'm going to keep the 6J5 and use it as an extra stage with a switch to take it out of the circuit and route the Triad to the 6SJ7. I have an NOS Electro Voice 911 along with dynamics so that will give me the capability to use either one.

Don, when I rebuilt the design, I incorporated a bigger tube, and adropping resistor so that now the plate of a 6L6GC (which could handle 450 vdc if it had to) gets 300 vdc , the screen gets 250, and the 6SJ7 stage get 105.

Everything runs cool to the touch after half a day of continuous duty, (well the tubes get warm, but never any red plates).

Bob, I was just about to ask what your final schematic looks like, but I take it from above you just changed the driver stage from the 6V6 to a 6L6 and gave the screen a dropping resistor?

Nice chassis work.  :D


Phil

Thanks Phil.  I like the stuff I build to look as good as possible, which sometimes leads me to "find a reason" to start over.  I love building, but I dont like paying for another Hammond chassis :).

I have several friends whose work makes mine look like a beginner, but I keep on trying.

As for the schematic, you are correct, Lost the .001 in the feedback loop. lost the 6J5 mike pre-amp, changed to a 6L6GC tube, and played with the voltages hitting the various places.

Busy weekend here. Finished the exciter shown on the left. Both units still need panel marking. Modulator has been on the back burner. Front panel for that is at a shop in town that specializes in metal work. They will cut the meter hole and powder-coat. The exciter front panel was powder-coated by them and it matches the front panel of the 807 amp on the right.

It's done quite a bit for class A stuff and seems to be supported by the data sheet at least for 250V, but I'd also prefer a small resistor on the screen if the screen current is high. I guess measuring it would be the thing to do. Screen's rated 2W.

You might have missed my update on the second page of this thread, when I started over:
Replaced the 6V6 with a 6L6GC,                                 
it rests at 54mA /300 vdc, the screen draws a resting 2 mA at 250 vdc, the thing sounds great.

Perfect!

The units you show in the photos look great!  I'm building on my first transmitters, and I'd love to see your circuits. Would you mind sharing your schematics for the exciter and amp?

Ed, KB8TWH