PI equal but opposite

[jeff]

 I've been reading up a little on distortion and want to try something out but not sure this will work.

 From what I gathered 2^ndorder distortion is caused by a tubes inablitiy to amplify the positive and negative portions of the signal equally. It is pronounced in SE amps because they only use one tube but is cancelled in push pull amps because the two tubes are equal and opposite. I was wondering if you could introduce some 2^nd order distortion by lowering the gain to one of the tubes in a push pull amp. I notice that in alot of LTPIs one sections plate resistor is 100K and the others is 82K and this is to balance the gain of the in and out of phase signal. I've also noticed that a marshall 18Watt uses two 100Ks.

 So this is what I was wondering: Since I don't want to have a pot to adjust the plate resistor(don't like the idea of a pot with high voltage on it, and my pots are 1/4 watt) could I use a pot between the phase interter and power tube's grid to lower the gain of one side of the PI adjusting the balance? I would use a 250K on top of a 750K resistor so I could turn it down but not all the way off, just to hear the difference. I'd put it after the PI's coupling cap then use another cap to connect the pot's output to the power tube's grid to keep the bias voltage off the pot.

Has anyone tried this? How'd it sound? Worth it?

Maybe I'm way off here but It sounds like an experiment I'd like to try.

[phsyconoodler]

On fixed bias amps with negative feedback you find the 82k/100k plate resistors to balance the PI but on an amp like the 18 watt it is cathode biased and has no negative feedback so it doesn't need balanced plate resistors.
  You can induce an imbalance with a pot if you just use a 1/2 watt trim pot and adjust it where you like it.
 You can also fiddle with the PI bias by simply removing the grid resistor and installing a 50k pot.It alters the bias of the PI and is also called 'dampening' by some amp builder's that have used it.I installed that control on a 2204 that i built a while ago and it allowed me to get some lower volume grind.It was OK but not earth shattering.
  I don't see any real value is fussing with the PI for the purpose you want.
I would rather move on to the preamp or use VVR,but that's just me.

[jeff]

You can also fiddle with the PI bias by simply removing the grid resistor and installing a 50k pot.It alters the bias of the PI and is also called 'dampening' by some amp builder's that have used it.I installed that control on a 2204 that i built a while ago and it allowed me to get some lower volume grind.It was OK but not earth shattering.

Does this change the balance of the positive and negative signal or is it more of a master volume?

 I got to try that VVR. I hear it mentioned all the time and should try it out for my next build. I assume this is more like a output volume control. I'm interested in knowing if driving one tube harded than the other is a good idea in a push pull circuit.

You can induce an imbalance with a pot if you just use a 1/2 watt trim pot and adjust it where you like it.

That's a good idea. I'm better at A /B-ing something than A, open it up and replace a part, B-ing

[jeff]

 I understand that tubes are not perfect and the circuit is not perfect and that most PIs aren't prefectally matched. I just wanted to hear how playing with the balance would affect the sound.

[sluckey]

The easiest way to do this is to replace the two grid leak resistors with a pot. For example, you have a cathode biased PP amp and the grid leaks are each 220K. Replace the two 220Ks with a 500K pot. Connect each outside lug to each grid and connect the wiper to ground. (Just a simple balance control). Now you can balance or unbalance the grid drive signals any way you like. Works the same for a fixed bias amp except you would connect the wiper to the bias voltage.

[jeff]

Cool idea, thanks.
        Jeff

[38Super]

From what I gathered 2ndorder distortion is caused by a tubes inablitiy to amplify the positive and negative portions of the signal equally. It is pronounced in SE amps because they only use one tube but is cancelled in push pull amps because the two tubes are equal and opposite. I was wondering if you could introduce some 2nd order distortion by lowering the gain to one of the tubes in a push pull amp

You can adjust the phase gain balance a number of ways.  Could experiment with plate load resistor mismatch in PI, could use slightly mis-matched (heresy !) PI triode stages or slightly mismatched output tubes.  Smaller plate load resistors (~75K) in your 12ax7 preamp stages will also add some 2nd harmonic distortion.  It works - I have done it.

If you are concerned about 120 Hz rejection in your mismatched output stage, remember, you only have effective rejection when both tubes are conducting.  On large signal swings in an AB output stage, one tube or the other will be off during part of the waveform.  During these events, the common mode ripple rejection is lost.

Everyone talks about matched pairs and ultra matched triodes in your PI, but you know Fender and Marshall didn't spend a lot of time hand picking tubes for their amps that we call "vintage" today.

Experiment and you may find that you like more 2nd harmonic distortion.  Even harmonics are sweet, but you still need some third and fifth in there if you want some crunch.   


cheers,

rob