Valiant Audio Issue

[AB2EZ]

John

Hi!

1.  "Why not remove one of the 100K resistors to decrease the resistance to allow the correct feedback level not so close to ground on the wiper of the pot?"

The feedback circuit is (to an approximation that is good enough for this discussion) in the form of a current source. The voltage across the output of the (correct) 6146 produces a current that will be pushed through the combination of the existing three 100k ohm resistors (300k ohms of series resistance); and, then, through the resistance that corresponds to the combination of the 100k ohm potentiometer and the resistance of the branch leading to the 6C4. The voltage that will be produced across the 6C4 (cathode-to-ground) will be: the cathode input impedance of the 6C4 (roughly 400 ohms) x the portion of the total feedback current that flows into the branch leading to the cathode of the 6C4. In the circuit defined by the current schematic, almost all of the feedback current will flow into the branch that leads to the cathode of the 6C4, unless the wiper of the potentiometer is close enough to the grounded end to produce a resistance between the wiper and ground that is just a few thousand ohms (or less).

To make the feedback adjustable, without having to set the wiper of the potentiometer so close to the grounded end, you need to put additional series resistance between the wiper and the cathode of the 6C4. Changing the existing 2.2k ohm series resistor to a 100k ohm series resistor will produce the desired adjustability.

You should not remove any of the existing 100k ohm series resistors. Doing so will increase the total feedback current... and, therefore, increase the feedback (which is already much too high). If anything, you may have to add a megohm resistor in series with the existing three 100k ohm resistors, in order to reduce the feedback... but you can cross that bridge if you come to it.

The first step is to change the 2.2kohm resistor, between the wiper of the pot and the cathode of the 6C4, to a 100k ohm resistor.

2. "What does 10db mean?  At what frequency is this to be measured and how would you measure it?  Is it linear across the audio range?  If so, when why do it? I'd guess not."

You can look up the definition of dB on line, or in any "ECE-101" textbook. You don't need to know what 10dB means to fix this problem.

The gain around the feedback loop should be constant (but not anywhere near as big as it is now, unless the wiper of the pot is set almost all the way to the grounded end) at audio frequencies of interest (e.g. 50Hz - 5000Hz). Beyond the top of this audio range, the gain around the feedback loop must drop to less than 1 (unity), as the frequency increases, before the phase shift around the feedback loop increases to a total of 360 degrees (i.e. 180 degrees of additional phase shift, v. 180 degrees of phase shift with negative feedback).

After changing the 2.2k ohm resistor (from the wiper of the pot to the cathode of the 6C4) to 100k ohms, you should add a capacitor between the wiper of the pot and ground. The capacitor's value should be chosen so that the magnitude of its impedance at 5000 Hz is approximately 100k ohms. I.e., the capacitor should have a value of approximately 317pF. You could use a standard 270pF capacitor, or a standard 300pF capacitor. This added capacitor will roll off the gain around the feedback loop, by a factor of 2 for every doubling of the frequency, starting at around 5000Hz.
  
Even with the added capacitor, the gain around the feedback loop is so high (unless the potentiometer's wiper is set almost at the grounded end), that the rolloff in the gain around the loop produced by this capacitor will be insufficient to bring the gain down below unity at higher audio frequencies (beyond 5000Hz) at which the phase shift around the loop increases beyond a total of 360 degrees.

Therefore, when you adjust the potentiometer (after increasing the value of the resistor between the wiper and the cathode of the 6C4 to 100k ohms)... starting with the wiper at the grounded end... there will be some setting that results in positive feedback/oscillation. You need to limit the setting to a value below that.

If, after making the above changes, the setting of the potentiometer's wiper (to avoid positive feedback/oscillation) is still very close to the grounded end... then you should add a 1 megohm resistor in series with the existing three 100k ohm resistors. After this change, if the maximum setting of the pot wiper is still very close to the grounded end, then replace the three 100k ohm resistors with two 1 megohm resistors (for a total of three 1 megohm of resistors in series, including the previously added 1 megohm resistor, replacing the three 100k ohm resistors)

3. "Why are the plate resistors of the driver different?  Is it to make the positive portion of the modulation envelope larger?  Is it because you're hanging the feedback loop off of one of the plates and need to be rebalanced?"

The phase splitter does not produce exactly equal (and opposite) currents in the two halves of the splitter circuit. The larger plate load resistor on the right side half compensates for the fact that the audio frequency current flowing in the right side half is somewhat less (in amplitude) than the audio frequency current flowing in the left side half.

Jim has pointed out that an even better design (but probably not needed to be done now) would have been to make the left side plate load resistor adjustable... using 50k ohm potentiometer. That way, you could fine tune the value of the adjustable resistor to make the amplitudes of the output voltages, on each side of the splitter, the same. As it stands now, one has to trust that whoever picked the left and right plate load resistor values did so by measuring the amplitudes of the left and right output voltages... and selecting the resistor values to cause them to be close to being equal... using an oscilloscope to make the measurements.

Stu

What I posted about the phase of the feedback in relationship to the cathode to grid voltage of the 6C4 kept bothering me.  I worked on that in my head laying in bed last night and it became apparent that I must have been thinking wrong.  When I read Stu's comments, the light came on.  Yep, I was wrong.  I think we'll take another stab at it with a renewed understanding...except I have questions.

1.  Why not remove one of the 100K resistors to decrease the resistance to allow the correct feedback level not so close to ground on the wiper of the pot?

2.  What does 10db mean?  At what frequency is this to be measured and how would you measure it?  Is it linear across the audio range?  If so, when why do it? I'd guess not.

3.  Why are the plate resistors of the driver different?  Is it to make the positive portion of the modulation envelope larger?  Is it because you're hanging the feedback loop off of one of the plates and need to be rebalanced?

Questions, questions, so many questions and so little time.

John in Tucson
  

I can see how this might have confused you (or anyone else, including me).

The cathode of the 6C4 is a negative feedback point.

The grid-to-cathode voltage on the 6C4 is causing current to flow. This current flows through both the cathode resistor of the 6C4 and the plate resistor of the 6C4. The resulting cathode-to-ground voltage is in the direction that causes the grid-to-cathode voltage to decrease. I.e. the grid to cathode voltage being applied to the 6C4 is: [the grid-to-ground input voltage - the cathode-to-ground (feedback) voltage]

When the larger loop's feedback signal (from the output of one of the 6146s) is applied to the cathode of the 6C4, it must have the same polarity as the open loop 6C4 cathode-to-ground voltage... in order to produce negative feedback around the larger feedback loop.

Stu

Turns out that when we compared the phase of the feedback with the signal on the cathode of the 6AU6, it appears that the feedback was in-phase.  So, we swapped it out.  Then everything looked good until we hooked it back in place and then it was really hosed.  Now, maybe I'm wrong, but to me, "negative" feedback means that the waveform coming from the plate of the 6146 will be out of phase from the mike audio on the cathode of the tube.  Didn't look like it was right.  We switched it and it was even worse.  So, for whatever reason, the feedback was disconnected altogether and it behaves normally.  That may explain why it was disconnected when Dave got the unit from the seller in the first place.  Now...here's the issue:  The audio is a bit raspy on peaks with Dave's voice without the feedback.  Why did it screw up?  Don't know.  But the good news is that the mod transformer and the audio chain is good.  I even figured out why one plate resistor of the phase splitter is a 51K and the other is 39K.  What do you think?

[WD7F]

When I asked what the 10 dB was, I meant in respect to what? Assuming you meant the audio level, we ended up with about 6dB at 1KHz, maybe a bit less at 3.5K.   I just scoped it at the grid of the 12BH7 as a reference, assuming the signal at 50% amplitude is 6 dB.  Anyway, we thoroughly checked it out and it was apparent that we had way too much feedback with the 2.2K and tried the 100K as you suggested.  That decreased the feedback enough that we were just past mid of the pot when it started to oscillate.  Patching in one with leads hanging led us to believe it should be about 150K, but when we soldered it in, it wasn't enough.  Had a 180K on hand and it helped but not quite.  We couldn't find a 200K but tried a 220K and now I'd guess we are getting about 6dB of decrease at full swing of the pot and it's behaving.  The cathode of the 6C4 is bypassed with a 300 pF as shown.

Looking at the W1HZK and WA1HLR Timtron mods, I don't see the purpose of the negative feedback other than to improve the performance.  What's your opinion of what it's suppose to do?
John in Tucson
 

Separately, as I indicated in an earlier post... I think the feedback loop has much too much gain, unless the wiper of 100k ohm "the old clipper" potentiometer is almost all the way toward the grounded end. With too much gain around the loop (40db, in this case, unless the wiper is almost all the way toward the grounded end), there will me oscillation caused by phase shifts around the loop at higher audio frequencies. I.e. the loop gain will not decrease to below unity fast enough to prevent positive feedback (due to a 180 degree phase shift) at higher audio frequencies.

There are a couple of ways to fix this.

One way would be to replace this potentiometer with a 5k ohm potentiometer.

Another way to fix this would be to keep the existing "old clipper" potentiometer,  replace the 2.2k ohm resistor, between the wiper and the cathode of the 6C4, with a resistor whose value is roughly equal to the full resistance of the old clipper potentiometer. I.e. if the existing potentiometer has an end-to-end value of 100k ohms, then replace the 2.2k ohm resistor, between the wiper and the cathode of the 6C4, with a 100k ohm fixed resistor.

Turns out that when we compared the phase of the feedback with the signal on the cathode of the 6AU6, it appears that the feedback was in-phase.  So, we swapped it out.  Then everything looked good until we hooked it back in place and then it was really hosed.  Now, maybe I'm wrong, but to me, "negative" feedback means that the waveform coming from the plate of the 6146 will be out of phase from the mike audio on the cathode of the tube.  Didn't look like it was right.  We switched it and it was even worse.  So, for whatever reason, the feedback was disconnected altogether and it behaves normally.  That may explain why it was disconnected when Dave got the unit from the seller in the first place.  Now...here's the issue:  The audio is a bit raspy on peaks with Dave's voice without the feedback.  Why did it screw up?  Don't know.  But the good news is that the mod transformer and the audio chain is good.  I even figured out why one plate resistor of the phase splitter is a 51K and the other is 39K.  What do you think?

[k7iou]

Also I would like to add the old clipper pot is 350k not 100k so maybe a previous owner changed it?

[AB2EZ]

John

Hi!

I looked through this thread... and I didn't see any mention of "10dB" in my posts, or any other posts (except yours).

If I had to make a wild guess... I suspect that you were reading an article or comment somewhere that suggested the following:

As a rough rule of thumb, the level of the negative feedback signal should be adjusted (in your case, via the 100k ohm pot) so that: the audio input to the audio amplifier... to which feedback is being applied... has to be about 10dB higher when the feedback is connected v. when the feedback is not connected.... in order to achieve the same level of modulation. You would measure the audio input to the audio amplifier anywhere before the stage where the feedback is going to be applied. E.g. from the 6C4 grid to ground.

Another way to say the same thing is that: with feedback connected, the modulation index should be about 32% (i.e. 10dB less) of what it is when feedback is not connected (without making any changes to any of the audio gain controls).

In your case, you would start with the feedback connected, and the wiper of the 100k ohm pot at the grounded end (i.e. feedback level is 0). You would inject a sine wave (1kHz is fine) into the audio input of the Valiant, and you would adjust the audio level control(s) to achieve about 90% modulation. Then (without changing anything else) you would adjust the 100k ohm pot until the modulation was reduced to about 28% (i.e. 10dB less modulation). That would be a good setting of the feedback level.

The capacitor that I suggest you should add is not the same as the existing 300pF capacitor between the cathode of the 6C4 and ground. That capacitor doesn't do anything important that I can think of. At most, it bypasses RF from cathode to ground. The capacitor that I suggest you add should be placed between the wiper of the pot and ground. Although it is (coincidently) about the same value as the existing cathode-to-ground capacitor... it serves an entirely different purpose. It's purpose is to roll off the feedback level by 6dB per octave of frequency, starting at around 5000Hz. If you used a 220k ohm resistor between the wiper and the cathode of the 6C4, then this added capacitor, from wiper-to-ground, should have an impedance of around -j220k ohms at 5000Hz. The required value is, therefore, 144pF. You could use a standard 150pF capacitor. This value is less than what I suggested in my previous post, because you are using a 220kohm resistor from the wiper of the pot to the cathode of the 6C4. [If you decide to go back to using the 100k ohm value that I suggested... then, the proper value of the wiper-to-ground capacitor would be 270pF or 300pF].

The purpose of using negative feedback is to make the modulation of the carrier more linearly proportional to the audio input signal. If using negative feedback makes your modulated output signal sound better (to others who are receiving your signal), then that is good. If not, there isn't any real value to using negative feedback.

Stu

P.S. If the end-to-end value of the old clipper pot is 350k ohms... then using a 220k ohm wiper-to-6C4 cathode resistor (as you did) is better than using a 100k ohm wiper-to-6C4 cathode resistor.

[WD5JKO]

Something to add, and I hope doesn't contradict anything said is the output transformer primary will have a self resonance somewhere in the ultrasonic range. Special Hi-Fi output transformers will be up in the 100Khz range if not higher, and the  low end transformers that are built like a power transformer (like this one) might have a resonance in the upper audio range, say 20 khz. This is parallel resonance, so feedback taken off one 6146 plate will have to take this into account, by Tailoring the NFB circuit, and/or making a resistive load to put across the transformer primary.

One method of dealing with this is to put a series R-C across the primary of the mod transformer. Make the 'R' roughly equal to 6146 plate to plate load impedance (maybe 5-10K), and about 1/10th the power rating of the modulator (maybe a 10W resistor). Then make the series 'C' such that the Xc is roughly the same as the 'R' value chosen at frequency 'F'. That frequency will be less then the resonant frequency. A guess might be 10-15 Khz. Realize that a sine wave sweep up above 5 kHz or so will dump more and more power into that resistor 'R' as the test frequency rises.

Jim
Wd5JKO

[WD7F]

I dunno' where I got the 10dB from then.  I have read it somewhere and I thought it was in your post.  I think we've got a handle on it.  Dave will probably want to find a 200K to try before he buttons it up.  I think the previous owner had disconnected it when it was behaving badly.  He was using a studio quality mike and audio processing as an input.  We asked him about it and he couldn't remember.  So, we've learned some good stuff.  Thanks for the input.
John in Tucson
 

John

Hi!

I looked through this thread... and I didn't see any mention of "10dB" in my posts, or any other posts (except yours).

If I had to make a wild guess... I suspect that you were reading an article or comment somewhere that suggested the following:

As a rough rule of thumb, the level of the negative feedback signal should be adjusted (in your case, via the 100k ohm pot) so that: the audio input to the audio amplifier... to which feedback is being applied... has to be about 10dB higher when the feedback is connected v. when the feedback is not connected.... in order to achieve the same level of modulation. You would measure the audio input to the audio amplifier anywhere before the stage where the feedback is going to be applied. E.g. from the 6C4 grid to ground.

Another way to say the same thing is that: with feedback connected, the modulation index should be about 32% (i.e. 10dB less) of what it is when feedback is not connected (without making any changes to any of the audio gain controls).

In your case, you would start with the feedback connected, and the wiper of the 100k ohm pot at the grounded end (i.e. feedback level is 0). You would inject a sine wave (1kHz is fine) into the audio input of the Valiant, and you would adjust the audio level control(s) to achieve about 90% modulation. Then (without changing anything else) you would adjust the 100k ohm pot until the modulation was reduced to about 28% (i.e. 10dB less modulation). That would be a good setting of the feedback level.

The capacitor that I suggest you should add is not the same as the existing 300pF capacitor between the cathode of the 6C4 and ground. That capacitor doesn't do anything important that I can think of. At most, it bypasses RF from cathode to ground. The capacitor that I suggest you add should be placed between the wiper of the pot and ground. Although it is (coincidently) about the same value as the existing cathode-to-ground capacitor... it serves an entirely different purpose. It's purpose is to roll off the feedback level by 6dB per octave of frequency, starting at around 5000Hz. If you used a 220k ohm resistor between the wiper and the cathode of the 6C4, then this added capacitor, from wiper-to-ground, should have an impedance of around -j220k ohms at 5000Hz. The required value is, therefore, 144pF. You could use a standard 150pF capacitor. This value is less than what I suggested in my previous post, because you are using a 220kohm resistor from the wiper of the pot to the cathode of the 6C4. [If you decide to go back to using the 100k ohm value that I suggested... then, the proper value of the wiper-to-ground capacitor would be 270pF or 300pF].

The purpose of using negative feedback is to make the modulation of the carrier more linearly proportional to the audio input signal. If using negative feedback makes your modulated output signal sound better (to others who are receiving your signal), then that is good. If not, there isn't any real value to using negative feedback.

Stu

P.S. If the end-to-end value of the old clipper pot is 350k ohms... then using a 220k ohm wiper-to-6C4 cathode resistor (as you did) is better than using a 100k ohm wiper-to-6C4 cathode resistor.

[k7iou]

John and I wrapped up the Valiant today, ran it through the wringer and found the culprit that started the chain of events. We tested on the bench reinstalled the cabinet and put back in the operators position. When I connected the D22, preamp and SR107 it went to heck. I checked the preamp and SR107 on the oscilloscope and it looked good. When I tested the D22 it was full of noise. The microphone cable on my American Microphone D22 was bad. The shield was broken at the solder joint and the audio lead was close behind. I repaired the cable and all is well.
The negative feedback circuit was repaired with the 220k resistor and the 150pf capacitor on the 350k old clipper pot.
The carbon track was removed from the terminal strip and voltages checked on the modulation transformer.
The audio is fantastic with my D-104 which has an equivalent aftermarket MC-320 element.
The audio did change with my D22 which uses the low z or high Q microphone connection, preamp and Shure SR107 so adjustments are in order on the equalizer.
I would like to thank John wd7f for his elmer'ng and patience.
Thanks to Stu and Jim and all others for your guidance and suggestions.
I learned allot.
73
Dave k7iou