phase inverter balance potentiometer

[pula58]

O/.K. So , I have a Fender AB763 style build. For curiositys sake I installed a 10K multi-turn trim pot so I could adjust the output balance of the phase inverter circuit. So, I adjusted the pot until the AC SUM of the PI outputs was as close to zero as I could get while cranking out about 40W or so (in a 50W amp).

Seems to me that this makes the 2nd harmonic smaller, But isn't the 2nd harmonic the thing that makes tube amps sound so warm and good?

Anyhow, I did the trim and have been playing the amp and I have not discerned anything dramatic (yet) in its reponse. What should I be looking for in terms of before/after comparison?

Thanks!

[katopan]

But isn't the 2nd harmonic the thing that makes tube amps sound so warm and good?

And that's why some (lots?) of PI imbalance is a good thing in a guitar amp. I'd be using the pot to tune out any balance so it sounds good by ear.

[Structo]

I think most of us adjust that by ear.

But first of all, the guys here that really know their stuff say that the tube should have pretty closely matched triodes to begin with.

I wouldn't use a 10 turn trimmer for this.

Most use a 10K or even a 5K trimmer if the tube is balanced.

Adjust the trimmer while the guitar is pretty loud and you are listening for good sustain and bloom of the note.
Make small adjustments and test it for a while before adjusting more.
Due to the caps in the circuit it takes a second or two before the adjustment takes hold.

[LOUDthud]

Tubes are not perfectly linear devices so they tend to distort any signal sent to them. A single ended tube amplifying stage tends to produce even harmonics because the distortion is asymmetrical. A push pull stage however, tends to distort the signal symmetrically and that produces odd harmonics. So by tweeking the balance of the phase inverter, you are decreasing the asymmetry and thus the even harmonics.

[jjman]

Balancing the outputs of the PI won't result in balance in the OP tranny if the OP tubes are not balanced in their ac performance. "Matched" OP tubes are often "matched" only for dc idle and not ac performance. If you want to calibrate for a "balanced" output you would need to scope the outputs instead (or also.)

[VacuumVoodoo]

Phase inverter being inside a NFB is affected by it so that any asymmetry is reduced by the feedback factor. i.e if gain difference between PI sides with no NFB applied is 10% and feedback factor is 20 it will reduce that difference to less than 1%.
Adjustment of PI symmetry has strongest effect in power amps with no NFB.

[teemuk]

But isn't the 2nd harmonic the thing that makes tube amps sound so warm and good?

No.

I know it's a common myth but don't get too caught on the idea and on the things that common myths say about harmonics.

You can use any decent wave editor software and distort just one of the half waves of signal and listen to the outcome (that's LOTS of 2nd order harmonic distortion). You can even do that with a plain asymmetric solid-state diode clipping circuit (still LOTS of 2nd order harmonic). Rest assured, it doesn't sound all that wonderful.

As said, don't get too caught on what common myths teach about harmonics and distortion. For example, in reality the revered push-pull tube power amp clipping is mostly introducing odd order harmonics and plenty of high ordered ones too; more the more you overdrive things.

[rdjones]

Fiddling with multiturn pots gets old quick.

Since it's a Fender style a chassis mount bias pot should fit in.
Just don't confuse it for the actual bias adjustment.
Or maybe use a 100 Ohm hum balance on the cathodes.

rd