How to adjust the trimmer for the PI plate

[Tonegeek]

How do I adjust the 10k trimmer on the PI plates? Am I gong for equal voltage to the plates or is it a deal where I do it till it sounds right?
Thanks,
Whit

[Bob-I]

You really need a scope to adjust it correctly. Connect the scope to the speaker output, inject a sine wave at the input and adjust until you have equal waveforms for + and -

That said, I stopped using the trim because it doesn't seem to make a difference in the tone.

[jaysg]

Can't you take the power tubes out and balance it looking at the PI outputs after the pass caps?

[dobbhill]

Inject a sine wave signal (I use the power amp input) to the PI and measure the AC voltage at the power tube grids. I set it about 20vac and use the trim to set each grid the same. Don't forget the dummy load on the OT. The scope method is better because it balances the entire output section, including the output tubes. This method only balances the PI tube. Or you could use a THD Analyzer on the output, but that would be overkill for a guitar amp.

As always, YMMV and proceed at your own risk.
D

[Bob-I]

Jay and Dobb. In my experience the PI is not always balanced when the output is balanced. That's why I suggested checking at the speaker out. On my last build the PI is quite a bit off, where the output is right on the money. It's my belief that this is more important, then again I can be wrongg.

[glasman]

I have tried this a few ways.

Method One - Measure amp output of scope, inject signal at the loop return and adjust for symmetrical clipping. (I adjust the signal generator until I get the output JUST to the point of clipping. then adjust the trimmer for equal clipping.

Method Two - Inject signal at loop return. Display both PI outputs on the scope and use the scopes ADD function. I then adjust for MINIMUM signal.

I find that sonically, I prefer method two (at least this week). It seems to add more harmonic complexity with the overdrive enabled.

Of course I do reserve the right to change my mind tomorrow.

Gary