pot in the P.I. plates.

[odourboy]

Correct, if using a scope, the only way is to measure the output after the tubes and tranny. Merely setting DC balance on the PI is meaningless.

Andy Fuchs originally showed me how this is done with a dummy load.

Could you describe the scope technique you use? (Or do you perfer the audible 'sweet spot' approach?)

[Funkalicousgroove]

Plug the amp into an appropriate dummy load, Put a 1Khz signal into the front end of the amp, clean setting, all tone controls dimed, switches set to the most on position, ie mid on, rock mode, deep off, brite on etc. take your scope reading off the output. turn the trimmer until the positive and negative sides of your sine wave are equal.

At that point you can turn the volume up until the sine wave bareley flattens out at the top and take a measurement of the AC output, take that number and square it, then divide by your output impedance(dummy load should be pureley resistive) and you'll have your RMS output wattage.

Then adjust the trimmer by ear till it sounds good

[odourboy]

Then adjust the trimmer by ear till it sounds good

LOL! Thanks Funk.

BTW, my 'Funk Build' is up and limping as of today!

[heisthl]

Plug the amp into an appropriate dummy load, Put a 1Khz signal into the front end of the amp, clean setting, all tone controls dimed, switches set to the most on position, ie mid on, rock mode, deep off, brite on etc. take your scope reading off the output. turn the trimmer until the positive and negative sides of your sine wave are equal.


Then adjust the trimmer by ear till it sounds good

Yeah, you can adjust for maximum clean power but even though you're joking about using your ears, you're right, that maximum clean power setting is not the spot that sounds the best.

[Funkalicousgroove]

BTW, my 'Funk Build' is up and limping as of today!

[/quote]


Congrats!!!

[FUCHSAUDIO]

DC balance is not very significant IMO. You have the output tubes and tranny upstream. They have a measureable effect to. I'd measure AC balance with a scope.... What volts are on the tube is not of much consequence.

Ac balance (signal level) and DC balance (tube plate voltage) are two different measurements. The more important of the two is AC balance. I use a Leader Stereo AC volt meter, and measure for the same voltage from both halves of the PI. Another way is to use a well calibrated dual channel scope, set one channel out of phase with the other, and adjust for '0' volts. The output transformer (it seems) is less important. I had set this trimmer at clipping, but no longer do. I set the amp for 5-Watts or so output, and measure right at the PI plates. You can swing the AC voltage about 3db back and forth with 100-K plate loads and a 20-K pot, which is significant IMHO. Some amp builders have posted that "the trimmer is useless and not required", I strongly disagree. Clearly affects OD and clean tones quite a bit imho.

[heisthl]

Ac balance (signal level) and DC balance (tube plate voltage) are two different measurements. The more important of the two is AC balance. I use a Leader Stereo AC volt meter, and measure for the same voltage from both halves of the PI. Another way is to use a well calibrated dual channel scope, set one channel out of phase with the other, and adjust for '0' volts. The output transformer (it seems) is less important. I had set this trimmer at clipping, but no longer do. I set the amp for 5-Watts or so output, and measure right at the PI plates. You can swing the AC voltage about 3db back and forth with 100-K plate loads and a 20-K pot, which is significant IMHO. Some amp builders have posted that "the trimmer is useless and not required", I strongly disagree. Clearly affects OD and clean tones quite a bit imho.

Andy - your post leads me to believe the best tonality comes at exact AC balance. I was of the opinion that this was not quite true and a little deviation actually allowed for more harmonic complexity. I will pursue this further on my next build and see if the "ear point" and the " AC balanced point" are the same.

[FUCHSAUDIO]

Andy - your post leads me to believe the best tonality comes at exact AC balance. I was of the opinion that this was not quite true and a little deviation actually allowed for more harmonic complexity. I will pursue this further on my next build and see if the "ear point" and the " AC balanced point" are the same.

That's why I always put "imho". It's my feeling, but perhaps not always others.

[odourboy]

While you guys were stuffing youselves on turkey and cheap wine , I tried a little experiment which seems like it might have yielded some results.

I set my PI trimmer balance using Andy's scope method for a symetrical wave (using a 500Hz sine wave test tone). I then hooked up a real-time spectrum analyzer (a cheapy free-trial software thing) across the dummy load to see what i could see. The analyzer showed a nice abundance of 2nd harmonics. By turning the trimmer, I could get an easily observable drop in 2nd harmonics. I also found that scope setting was near the optimal point, and with a small deviation from the 'scope balanced' point, I was able to get just a hint more harmonics (which leads me to believe that the my 'scope baclanced. position, my not have been the actual point of perfect balance since it's tough to do perfectly with a scope).

Anyway, I just thought you all might find this interesting. Carry on!

[FUCHSAUDIO]

Note to self: eat less food on Thanksgiving...lol.

Anyway: I wanted to suggest you use the loop input (or if you have a passive loop, or the ability to enter the PI with your signal generator directly). It's really easy to overload and otherwise mess up the ac waveform going through the front panel input. If you are careful, bypass the tone controls (use a gain boost), watch the incoming levels etc, you can do it. BUT, entering the PI directly is easier, and may yield better results, again IMHO.

[odourboy]

Note to self: eat less food on Thanksgiving...lol.

Anyway: I wanted to suggest you use the loop input (or if you have a passive loop, or the ability to enter the PI with your signal generator directly). It's really easy to overload and otherwise mess up the ac waveform going through the front panel input. If you are careful, bypass the tone controls (use a gain boost), watch the incoming levels etc, you can do it. BUT, entering the PI directly is easier, and may yield better results, again IMHO.

Thanks for the tip Andy. I'll try that. (Easy because I do have direct access to the PI via the rear jack.)