proper voltage at PI input ?

[Alan0354]

FWIW what you show is oscillation.

Applying NFB is difficult when you have a transformer within the loop ..... what happens in tube amps , so you can only apply so much NFB before it becomes unstable.
Thatīs why you donīt have problems with little or no NFB , which appear as you increase it (by decreasing resistor value).

Itīs a tribute to Fenderīs *excellent* transformers that they can use so much NFB (8:1 from 4 ohms tap) and stay clean and stable.

PS: adding a small cap in parallel with the NFB resistor may make things worse, by *increasing* NFB at higher frequencies.

If anything, lower HF gain locally, not end to end.

To that end, you can add a 47pF or 100pF ceramic or mica (x 1000V) cap from plate to plate at the PI.

That's exactly what I tried to tell Daz. He need to decrease the 4.7K gain resistor to keep the closed loop gain the same. Simply reducing the feedback resistor will cause oscillation as can you can get into 2 poles 0dB cross over and phase shifted 180 deg. He is reluctant to change that, so that's that.

Adding a cap parallel with the feedback resistor is a double edge sword. One really has to draw the Bode plot to see and to find the value of the zero frequency. Problem is finding the pole and zero of the output transformer is going to be challenging. That's the reason I suggested dominant pole compensation by putting a small capacitor across the PI plates also.

The picture from Teemuk is definitely oscillation, no if and buts about it. But I am no convinced about Daz's drawing yet. That's the reason I want a picture. Without that, I have nothing to add. I think we are going in circle at this point.

[Alan0354]

Well an EL34 is about three times as sensitive to input voltage as a 6L6GC, which may be the cause of problems.

You mean the gm is 3 times of 6L6? That is equivalent to raising the open loop gain by 3 times. That might do it.

[daz]

I had a 100k FB resistor till a day or 3 ago after listening to posts here. before that a 180k ! It doesn't really matter because i never use it with the variable pot that low. I usually have probably around 100-200k as NFB resistance, if i use any at all. So no reason to change the 4.7k. As far as my drawing, It's exactly as it looked on the scope. If i find my camera or get a new on and i happen to have the issue show again i'll post a shot. But it's not my concern since it isn't happening now and wasn't even happening before unless i play the thing with the VFB pot low enough which i don't i only got that wave while testing the amp and turn in the NFB all the way down into a region i never even use it.

[daz]

Well an EL34 is about three times as sensitive to input voltage as a 6L6GC, which may be the cause of problems.

Interesting. Well, look at the new schematic. I decreased the level into the PI a lot. But i wonder If it might be a good idea to add resistors at the el34 grids to ground to make the 5.6k grid stoppers into voltage dividers? Any reason i can't? They're after the couplers, but then i'm not sure if there are reasons concerning impedance that this wouldn't work. But they put ganged masters there so why not? It would just be a set master.

[Alan0354]

Interesting. Well, look at the new schematic. I decreased the level into the PI a lot. But i wonder If it might be a good idea to add resistors at the el34 grids to ground to make the 5.6k grid stoppers into voltage dividers? Any reason i can't? They're after the couplers, but then i'm not sure if there are reasons concerning impedance that this wouldn't work. But they put ganged masters there so why not? It would just be a set master.

Don't do anything if 100pF across the PI plate works. Closed loop feedback is a delicate thing, you tame it, it's all good.

Decrease input voltage into the power amp has nothing to do with stability. Stability is in the loop. You play with the grid resistor, you change the loop. Lower the gain does not imply more stability. Unless you really get into Bode Plot, you never know how it react.

[guitician]

Interesting. Well, look at the new schematic. I decreased the level into the PI a lot. But i wonder If it might be a good idea to add resistors at the el34 grids to ground to make the 5.6k grid stoppers into voltage dividers? Any reason i can't? They're after the couplers, but then i'm not sure if there are reasons concerning impedance that this wouldn't work. But they put ganged masters there so why not? It would just be a set master.

As you put a resistor to ground you will need to increase the size of the coupling caps to maintain the same rolloff, but I don't see why that wouldn't work.

[Dave H]

But i wonder If it might be a good idea to add resistors at the el34 grids to ground to make the 5.6k grid stoppers into voltage dividers? Any reason i can't? They're after the couplers, but then i'm not sure if there are reasons concerning impedance that this wouldn't work. But they put ganged masters there so why not? It would just be a set master.

I thank that’s a bad idea. It will mess up the bias voltage and the PI can’t drive a low resistance like that. It would turn it into a 5W amp with only PI clipping. It would work with a normal high resistance fixed post PI MV but as it is fixed the power amp would not be able to output its full 50W.

[daz]

I thank that’s a bad idea. It will mess up the bias voltage and the PI can’t drive a low resistance like that. It would turn it into a 5W amp with only PI clipping. It would work with a normal high resistance fixed post PI MV but as it is fixed the power amp would not be able to output its full 50W.

But people put ganged masters there, so why wouldn't this work? It's just the same thing but not variable. Might have to replace the grid stoppers with a different value, but it shouldn't affect bias on the north side of the coupling cap should it? Even if it did i could just re-adjust bias but i don't see why it would anyways.

[daz]

Don't do anything if 100pF across the PI plate works. Closed loop feedback is a delicate thing, you tame it, it's all good.

Decrease input voltage into the power amp has nothing to do with stability. Stability is in the loop. You play with the grid resistor, you change the loop. Lower the gain does not imply more stability. Unless you really get into Bode Plot, you never know how it react.

I'm not trying to stabilize anything. I just want to find the best way to configure the PI>>>on back so that the tone and volume are as linear as possible. I know this amp's tone changes more than most i've had and i want to find out why and fix that. I thought the large signal lever was part of that and it appears to help, but I'm still looking for ways to get the higher volume tone to retain more of the complexity and squish i get at lower volumes. Not ALL, i understand the way the ear works and all that. Just more in line with other amps i've had that were better in this regard.

[Alan0354]

I'm not trying to stabilize anything. I just want to find the best way to configure the PI>>>on back so that the tone and volume are as linear as possible. I know this amp's tone changes more than most i've had and i want to find out why and fix that. I thought the large signal lever was part of that and it appears to help, but I'm still looking for ways to get the higher volume tone to retain more of the complexity and squish i get at lower volumes. Not ALL, i understand the way the ear works and all that. Just more in line with other amps i've had that were better in this regard.

I put the way I test mine in post #33. Once you set the gain of the power amp, then measure the input dynamic range as described in post #33. Then use resistor in series with the master volume to match the max output to the input dynamic range of the power amp.

I woulds drive the power amp a little beyond so you do get a little of the power amp distortion towards the end of the MV range. I did put a resistor in series with my MV to reduce the output by half, I like it better, but it is still over drive the PA, just not by 5 times as before.

[daz]

Same here. Look at my latest schematic above. 120K in series with the MV input, then a 220k from the same input lug to ground to form a VD at the maters input. The wave looked the same till about 1/2 way up or so then began clipping. I might still wanna lower it more in another way rather than go too low on those added resistors. thats why i asked about the el34 grids and doing a voltage divider there to simulate a set dual ganged master.

[Dave H]

But people put ganged masters there, so why wouldn't this work? It's just the same thing but not variable. Might have to replace the grid stoppers with a different value, but it shouldn't affect bias on the north side of the coupling cap should it? Even if it did i could just re-adjust bias but i don't see why it would anyways.

It wouldn't work because you said to add a resistor from grid to ground. Unless I'm mistaken that makes a potential divider with the 220k resistor from the bias supply and divides down the bias voltage. On a fixed bias amp the added resistor should be between the grid and bias supply not ground then you don't need the 220k. Make the sum of the two divider resistors 220k and you won't have to change the coupling cap value but I still don't like it because all you will have then is PI clipping distortion and the power amp won't reach full power. A normal non fixed PPIMV can be turned up when full power is required. Didn't you say you didn't like PPIMVs?

[daz]

It wouldn't work because you said to add a resistor from grid to ground. Unless I'm mistaken that makes a potential divider with the 220k resistor from the bias supply and divides down the bias voltage. On a fixed bias amp the added resistor should be between the grid and bias supply not ground then you don't need the 220k. Make the sum of the two divider resistors 220k and you won't have to change the coupling cap value but I still don't like it because all you will have then is PI clipping distortion and the power amp won't reach full power. A normal non fixed PPIMV can be turned up when full power is required. Didn't you say you didn't like PPIMVs?

There are a ton of schematics showing this kind of MV. Google PPIMV then click images. And many are separated from the bias by the cap so they shouldn't change the bias. And yes, i hate PPIMV'S, but thats if they are the only MV in the amp because then the PI is slammed all the time. I have a pre PIMV, so the one i propose is nothing more that a divider to take the signal down To the point where with the MV on 10 the amp is just past clipping. It would be to set the max gain of the power stage wherever i want it then the pre PIMV controls it.

[J M Fahey]

Do you mean my preamp OD is high? Because some have said that, but i keep saying that's not so. I don't think you can be so sure just looking at my schematic because thats just not the case. That wav i drew and the one that was a actual pic i posted were done with the gain somewhere around 12:00 to 2:00. At that point theres about as much gain as a tube screamer. The OD it produces maxed is literally the same or less than a peavey classic 30's disto channel or maybe 20% more than a blues Jr's preamp maxed with the fat switch on. If you ever played one of those you know thats not a lot. Also, it will do that even with lower preamp drive, quite low. Also note it will NOT display that oscillation at the master's input, so how can the preamp be generating it?

You are misquoting me, big way.

In the response to your waveform drawing (my post #55) I didnīt say a single word about your preamp, or its schematic, nor need to.

All I need to see is the waveform to know whatīs happening.

How do I know itīs unstable, oscillating a certain way? Because I see that.

How do I know itīs WAY overdriven? Because I see that.

I *did* refer to your POWER amp, the PI contined within it and its NFB.

[daz]

I didn't misquote you, i simply wasn't sure exactly what you meant and to address you i had to assume what it was. First sentence is a dead giveaway. After that all you needed to say is "no, i mean your PA is overdriven".

Do you mean my preamp OD is high?