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Circuit problem on a modulator




 
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W4RFM
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« Reply #25 on: December 06, 2018, 11:18:52 AM »

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!

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« Reply #26 on: December 06, 2018, 01:30:32 PM »

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.
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Tom WA3KLR
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« Reply #27 on: December 06, 2018, 02:48:21 PM »

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.
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« Reply #28 on: December 06, 2018, 05:09:00 PM »

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.
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« Reply #29 on: December 06, 2018, 09:34:57 PM »

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.


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. Roll Eyes


Phil

 



* 6V6GT AUdio Driver Stage.pdf (141.79 KB - downloaded 9 times.)
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« Reply #30 on: December 07, 2018, 10:01:06 AM »

 Wink Agreed Phil, thanks.
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« Reply #31 on: December 07, 2018, 02:56:42 PM »


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
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* 811A.pdf (411.72 KB - downloaded 12 times.)
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« Reply #32 on: December 07, 2018, 04:03:34 PM »

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.
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« Reply #33 on: December 09, 2018, 01:19:17 PM »

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.


* GeorgeE.JPG (784.31 KB, 2048x1536 - viewed 22 times.)

* GeorgeF.JPG (778.81 KB, 2048x1536 - viewed 26 times.)
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« Reply #34 on: December 09, 2018, 02:00:27 PM »

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...

...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...

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
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« Reply #35 on: December 09, 2018, 02:23:17 PM »

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.
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« Reply #36 on: December 09, 2018, 03:00:36 PM »

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."  Grin


Phil





* 6V6GT AUdio Driver Stage.pdf (172.13 KB - downloaded 15 times.)
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« Reply #37 on: December 09, 2018, 04:31:41 PM »

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.
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« Reply #38 on: December 09, 2018, 09:45:41 PM »

Jim,

I used a 25w mod transformer for the (3.8W 811) driver xfmr.

Now that's what I'm talkin about!
Proper margin!
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« Reply #39 on: December 10, 2018, 01:57:33 PM »

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.

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

* 6V6GT AUdio Driver Stage.pdf (207.04 KB - downloaded 10 times.)
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« Reply #40 on: Yesterday at 08:34:18 PM »



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
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« Reply #41 on: Yesterday at 09:00:42 PM »

Bob,

The finished 811 modulator looks great,  very nice job.

The rest of the project should go well

Post some pictures

Fred
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« Reply #42 on: Yesterday at 09:51:05 PM »

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
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« Reply #43 on: Today at 10:13:39 AM »

That is what I am hoping for, a 4-125 running at about 212 watts input, (Supposed) to be a match for this modulator.
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« Reply #44 on: Today at 12:15:35 PM »

so which circuit won the prize ?
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« Reply #45 on: Today at 03:41:58 PM »

so which circuit won the prize ?

Yep, lots of "switcherooing" here.  Grin


Phil
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