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Revamping A Modulator Input Circuit




 
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #75 on: April 27, 2018, 03:12:17 PM »

I am sorry guys, I don't agree. Separate cathode resistors will give you less perfect balance IF YOU GROUND THEM.
A cathode resistor in each tube will linearize the tube characteristics, it is the best negative feedback system without any complications. BUT where these cathode resistors meet, DO NOT GROUND. That will give you un-balanced signals, second harmonics. The cathodes should be fed with a current source, an impedance very high compared to the cathode resistors plus 1/S, the cathode impedance. Than the sum of the two triode currents is ALWAYS exactly constant and you will have a perfect balanced system. less even harmonics.
You can reach that in two ways, feed the sum-point of the cathode resistors with a J-FET, that is an almost perfect current source, (take care that the FET has sufficient source - drain voltage to operate. A transistor biased at 15 mA will do as well) or connect with a LARGE resistor to a negative voltage, e.g. the bias supply. When you have e.g. a -100 V bias supply, that resistor should be 100/15 mA=approx 6.8 kOhm, 2 W
In HI-FI amps I use a J-FET with its source at approx -12VDC or more selected for the current I want with zero gate volts.
In your case, I should simply use a 6.8 kOhm to the neg. bias supply (if it is approx -100VDC)
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N1BCG
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« Reply #76 on: April 27, 2018, 04:20:24 PM »

I might not have described by intentions adequately, so here's another shot...

The feedback loop goes from the UTC S-21 modulation transformer in the plate circuit of the four 6146 tubes back to the input of the modulator, which now includes the new 6SN7 driver. My proposal is to use separate 560 Ohm cathode resistors so that the FB loop can be connected to the cathodes.

Don't make me draw this out. You wouldn't like by handwriting...
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PA0NVD
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« Reply #77 on: April 27, 2018, 06:23:47 PM »

Its like Phil said

kill any distortion up front before complicating matters with a feedback system

Why complicate with that feedback system, not required and high possibility for oscillations Feeding back over two stages with two transformers in the circuit is really asking for trouble.
Simply each cathode approx 470 Ohm (not critical, more resistance is more negative feedback = less gain) Tie these together and at this point 8K2, 3 Watt to your bias, -120VDC
Very good balance guaranteed, less second harmonics and with the two 470 Ohm resistors you can select the neg feedback, the gain.
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« Reply #78 on: April 28, 2018, 11:59:40 AM »


...The feedback loop goes from the UTC S-21 modulation transformer in the plate circuit of the four 6146 tubes back to the input of the modulator, which now includes the new 6SN7 driver. My proposal is to use separate 560 Ohm cathode resistors so that the FB loop can be connected to the cathodes...



Is this the concept?

JMO, but since this an ARS transmitter transmitting under conditions not found in a home listening room with a Dynaco Amplifer, it would seem that one would not need a 0.05% distortion figure for this audio system and modulator. Shocked


Phil

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N1BCG
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« Reply #79 on: April 28, 2018, 12:17:57 PM »

Thanks, Phil. That's pretty much the circuit I had envisioned although I wish I took NFB into consideration before my pristine soldering job on the 6SN7 octal socket ;-)

Since there's no FB winding on the mod iron, I can either:

1) Take a feed from two unused primary side terminals on the S21 mod transformer and cross them back to the two cathode resistors for balanced NFB.

2) Take modulated B+ from the secondary and loop that back to one of the two cathode resistors for unbalanced NFB.

There's going to be considerable resistive attenuation with D.C. blocking caps in either case.

So, 680 for cathode resistors? The B+ that's appearing at the top of the 18k plate resistors is around 260V which yields a plate potential (to ground) of 125V @ 7.5mA.
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« Reply #80 on: April 28, 2018, 04:11:41 PM »

Consider this:

Most of your even harmonic distortion will be very low due to Push-Pull

Most distortion products (if any) will appear on the secondary of the Mod. transformer

So I would go back to the single cathode resistor and send feedback to it as suggested below with a DC decoupling cap so as not to upset the 5V bias on the 6SN7.


Phil - AC0OB

* Speech Amplifier 6SN7 with Feedback Ver. 123.pdf (66.71 KB - downloaded 26 times.)
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KA2DZT
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« Reply #81 on: April 28, 2018, 05:36:18 PM »

I think this xmtr has four 6146s in the final.  With that much plate current the two secondary windings on the S-21 mod xfmr should be run in parallel.

He will need a high voltage cap to isolate the modulated B+ off the feedback circuit.  He could connect to the tap on the mod xfmr to lower the modulated B+ on the FB circuit.  Even doing that he will still need a cap with at least a few KV rating.

He could also run the feedback to the grid resistor.  Dividing the 470K grid into two parts.  This will have a much higher impedance on the feedback circuit allowing a much lower value FB isolation cap.  Still the cap would have be a few KV rating,  but much easier to find a low capacitance cap with the necessary high voltage rating.

Although phase may be an issue.
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« Reply #82 on: April 28, 2018, 07:05:44 PM »

DZT: I think this xmtr has four 6146s in the final.  With that much plate current the two secondary windings on the S-21 mod xfmr should be run in parallel.

I would be more concerned about the primary's current carrying capability. The S-21 is rated for 115 Watts max, here we have the capability of producing up to 192 Watts:

http://www.bunkerofdoom.com/xfm/UTC_1949/UTC49.pdf    Page 15.

DZT: He will need a high voltage cap to isolate the modulated B+ off the feedback circuit.  He could connect to the tap on the mod xfmr to lower the modulated B+ on the FB circuit.  Even doing that he will still need a cap with at least a few KV rating.may be an issue.

HV cap not needed if he uses a voltage divider attenuator.


DZT: Although phase may be an issue.

For sure the proper side of the secondary will need to be selected.


Phil
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KA2DZT
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« Reply #83 on: April 28, 2018, 08:05:21 PM »

Phil.

Right, I overlooked using a voltage divider to ground.

I have a S-21 here,  not quite a big enough mod xfmr both on the primary or secondary side for 4x4 6146s.  The UTC S-22 would be a better mod xfmr for this xmtr, but he probably doesn't have one.

I'm not a big fan of UTC S series xfmrs or chokes.  I have a number of them here and never consider using them.  The S series chokes are about the worse of all the common chokes from other makers.  The earlier UTC PA series xfmrs and chokes are very good.  I have more than a 1000 xfmrs and chokes here and did a lot of testing on them.

Fred
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« Reply #84 on: April 30, 2018, 10:51:46 AM »

Here is one potential Feedback arrangement:



Phil - AC0OB

* Speech Amplifier 6SN7 with Feedback Ver. 123.pdf (70.27 KB - downloaded 30 times.)
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N1BCG
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« Reply #85 on: April 30, 2018, 11:27:21 AM »

This is all good info, but, the specs on this transmitter are unique:

RF Supply: 600V @ 300mA (120 Watts carrier)

AF Supply: 650V @ 120mA quiescent

I've attached a simplified schematic of the feedback strategy. The resistors from the mod transformer to the 6SN7 cathodes are comprised of high voltage resistors in series. The total value is chosen to provide 3dB of negative feedback and a very slight increase in bias voltage.

I don't feel that this would present a challenge for four 6146 tubes tasked with providing 60 Watts of modulation, plus headroom. With no feedback, the modulator with the UTC S21 produces this response:

Hz:  20   30  50  100  250  500  1000  2500  5000  10000   15000   20000

dB:  -8   -4  -2    0     0      0    0 (ref)  0       0       0         0         -1     



* IMG_7843.JPG (1939.15 KB, 4032x3024 - viewed 44 times.)
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« Reply #86 on: April 30, 2018, 01:35:01 PM »

RF Power Input for a parallel 4 tube 6146B Quiescent = 600V X 300 mA = 180 Watts; Practical RF power output@Quiescent = 180 Watts X 0.74 = 133.2 Watts in Class C.

RF Power Input for a parallel 4 tube 6146B = 600V X 560 mA = 336 Watts;
Practical RF power output@100% modulation = 336 Watts X 0.74 = 248.64 Watts in Class C.

Modulation Power Needed for 100% Modulation >= ~ 124.32 Watts;


For a 4 tube Modulation system in Push-Pull parallel Class AB1, Quiescent input power is 650V X (2 X 48 mA) X = 62.4 Watts;

For a 4 tube Modulation system in Push-Pull parallel Class AB1, input power@100% Modulation is 650V X 500 mA Max. = 325 Watts (assuming Mod. B+ stays at 650 Volts);

Practical Modulation Power Output@100% = 325 Watts X 0.64 = 208 Watts.

Mod. Power levels@100% Modulation are still above S-21 capabilities.


Phil - AC0OB
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KA2DZT
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« Reply #87 on: April 30, 2018, 05:41:39 PM »

Probably better to remove at least one 6146 from the PA and two 6146s from the modulator.  This will bring the current load on the TV xfmr down.  It is rated at 400ma with a FW ct power supply.  Using it with a FWB power supply and loading it to 400ma at 600v doubles the VA load on the xfmr.  But these RCA TV xfmrs can handle the overload easily, especially when not using the filament windings.  Although I'm not sure it the filament windings are being used.

This will bring the audio power down,  more in line with the S-21 mod xfmr.

Fred
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N1BCG
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« Reply #88 on: April 30, 2018, 05:42:59 PM »

RF Power Input for a parallel 4 tube 6146B = 600V X 300 mA = 180 Watts; Practical RF power output = 180 Watts X 0.74 = 133.2 Watts in Class C.

133.2 Watts / 2 = 66.6 Watts of audio needed for 100% mod, but lets say 80 Watts to be wild.

That should be a non-issue for four 6146s, and since the S21 is rated for 115 Watts of audio, all should be well.

But these RCA TV xfmrs can handle the overload easily, especially when not using the filament windings.  Although I'm not sure it the filament windings are being used.

The transformer is probably well within its CCS ratings for playing Honeymooners episodes for hours at a time. The filament windings aren't used, and the modulator screens are regulated at 150V.
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KA2DZT
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« Reply #89 on: April 30, 2018, 05:59:06 PM »

Typically the TV xfmrs ran with a considerable temperature rise.
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« Reply #90 on: April 30, 2018, 06:39:49 PM »

RF Power Input for a parallel 4 tube 6146B = 600V X 300 mA = 180 Watts; Practical RF power output = 180 Watts X 0.74 = 133.2 Watts in Class C.

133.2 Watts / 2 = 66.6 Watts of audio needed for 100% mod, but lets say 80 Watts to be wild.



But that was for Quiescent Power, at idle, no modulation.

The itch to finally apply audio may be shortly forthcoming. Grin

BTW, from my experience, the screen voltage for those RF finals is about 25 volts too low, unless you intend to operate this transmitter way below it's capabilities.


Phil


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« Reply #91 on: April 30, 2018, 07:35:14 PM »

well why not do like the big boys of broadcast did.

Also, having four 6146 in pp-par does not mean having to drive them to full output.

* BCTXmodFeedbacks.pdf (637.62 KB - downloaded 33 times.)
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« Reply #92 on: May 01, 2018, 08:13:03 AM »

well why not do like the big boys of broadcast did.

Also, having four 6146 in pp-par does not mean having to drive them to full output.

True, but were not talking Broadcast CCS, were talking Intermittent Service.

I once had a friend back in Mo. (now SK) that just loved 6146's and I drew up the schematics for him for a 4X6.

We had the Final tubes arranged in a 6" diameter circle and the Modulator tubes arranged in 4"X4" rectangle.

A 65 cfm muffin fan sucked hot air from the modulator side while a 110 cfm sucked hot air from the final.

However, we ran the Modulator in Class AB1 at 750 Volts.

Did I mention He loved those little bottles?   Shocked


Phil - AC0OB

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« Reply #93 on: May 01, 2018, 10:01:27 AM »

True, but were not talking Broadcast CCS, were talking Intermittent Service.

For the purpose of this discussion, CCS, please. I built a CW only transmitter to ICAS specs, but other than that, I only do CCS.
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« Reply #94 on: May 02, 2018, 09:18:14 PM »

The schematics were to show proven methods of taking negative feedback voltage from the modulator plates and applying it to the cold ends of the first audio input transformer windings. Other factors such as power level are not considered in my providing those old BC modulator schematics as one known way to improve fidelity.
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« Reply #95 on: May 04, 2018, 09:29:36 PM »

The schematics were to show proven methods of taking negative feedback voltage from the modulator plates and applying it to the cold ends of the first audio input transformer windings. Other factors such as power level are not considered in my providing those old BC modulator schematics as one known way to improve fidelity.

Agree totally Pat, as the "ladder" method of feedback was used in most Tube Broadcast Transmitters.

But some people seem to be rather cryptic.


Phil - AC0OB
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« Reply #96 on: May 05, 2018, 11:31:01 AM »

Agree totally Pat, as the "ladder" method of feedback was used in most Tube Broadcast Transmitters.

But some people seem to be rather cryptic.

The feedback ladder is exactly what I had in mind, thus the changeover to the separate cathode resistors given design limitations due to the single secondary of the input transformer. There was no intention of being cryptic, particularly given the generous and helpful suggestions that have been made here.

At this point, I'm experimenting with resistor values and the decision to use D.C. blocking caps or not, depending on how much the resistors increase the bias voltage on the 6SN7 @ ~ 3dB of NFB.
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