The balance trimmer this stranger

[jazzbass]

Dear Ampgarage enthusiasts,

I tried to understand the function of the "balance trimmer" by searching the discussions published on Ampgarage but I had no luck.
Can you tell me the keyword for research on what is needed and how it must be practically set up to be effective and why?

Thank you, a big hug.

Franco

[martin manning]

Pinned in D discussion section:
https://ampgarage.com/forum/viewtopic.p ... 89#p416389

[jazzbass]

Thanks Martin,

I wasn't able to set up the search correctly. I will read the discussion thoroughly even if I think the Ampeg method is a bit complicated for me.

A strong hug.

Franco

[Raoul Duke]

Franco,
I’ve read a lot of your posts and I’m confident you can figure this method out for certain

This adjustment is subtle, but IMO very important to the overall sound. This method gets me there every time and I usually don’t have a clue what I’m doing, lol…

[jazzbass]

:lol:Thanks Raul Duke,

I think I will have to give the topic due attention, it seems to be really important.

Thanks and a hug.

Franco

[sergio]

Hi Franco, It depends on how you want to proceed, IMHO to set the trimmer of the phase inverter, you should have the final outputs perfectly matched in terms of transconductance, and an excellently made output transformer! otherwise the adjustment makes no sense.

many guys here adjust it , by ear!

[jazzbass]

Grazie Sergio,

Thanks for the reply.
Pretend I know nothing about transconductance, how do you get the power tubes perfectly matched? probably buy them paired?
For us poor mortals it is necessary to believe that the output transformers are of excellent workmanship but I believe that we should be satisfied with what is offered by the market.
Certainly adjusting the trimer by ear is a solution but how should a correct adjustment be made?

Thanks for all your contributions.

a hug

Franco

[GAStan]

Hello Franco,
Can you describe the amp you wish to set the trimmer on? What kind of power tubes does it have? How many? Do they have 1 ohm resistors installed on the cathode?

What test equipment do you have available?

[martin manning]

Franco, DC balance and AC balance are two different things.

First, get the DC balance (idle bias current) on each side of the power stage to match as closely as possible.

Use matched tubes and/or swap pairs from side to side for the best balance. With four power tubes, this means placing the tube with the highest current on the same side as the tube that has the lowest current.

Second, adjust the PI trimmer for AC balance.

Transconductance is just the change in current flow divided by the input signal voltage that caused it, and the PI trimmer adjusts the signal voltage delivered to one side of the power stage vs. the other.
Assuming you have a 1 ohm resistor and a test jack on each cathode, the current is measured at the power tube cathodes. If there are four power tubes, short the test jacks on each side together so that the voltage reflects the total current flowing on each side.
Connect a DMM set to measure DC mV from the cathode(s) on one side to the other, across the test jacks.
Connect a load and feed a 1.5-2V RMS sine wave signal into the power amp input.
The DMM will show the DC part of the signal appearing across the cathodes.
The trimmer is adjusted so that the DC voltage is zero, indicating that the sine wave is centered on 0V DC, with equal current pulses on each side.

Facile come bere un bicchiere d'acqua ;^)