Symmetry Pot

[TIMBO]

Hi guys, Just for curiosity sake, the FiSonic has this arrangement on the PI. Can any one educate me in why the builder would include this into a build rather than just putting in the normal resistor arrangement. What added mojo could this pot add.

[sluckey]

That pot is used to make sure that you hit each power tube grid with exactly the same drive level. Very desirable in the hifi world.

[TIMBO]

So when powered up, adjust so that each triode plate sees the same voltage

[sluckey]

I would inject a sine wave test signal into the amp. Then I'd look at the signal on each grid of the output tubes with a scope and adjust the pot for exactly equal amplitude.

[TIMBO]

Thanks for that Sluckey, But that is way above my pay grade. I have adjusted so each triode plate is sitting on 179v. Thanks

[HotBluePlates]

So when powered up, adjust so that each triode plate sees the same voltage

What Sluckey said. It's not a d.c. adjustment, it's an a.c. adjustment.

The long-tail needs unequal plate loads for exact balance when only one of the two triodes has its grid driven (as is typical in guitar amps). So the only way for you to know balance has been achieved is to inject a drive signal and monitor output tube grids for equal a.c. voltage.

I might put a meter's red lead on one grid and black lead on the other, and fiddle the pot for minimum measured a.c. voltage (i.e., minimum difference in drive voltage).

[Willabe]

Since it's a guitar amp and not Hi-fi can't you just set it by ear?

Just set it where it sounds the best.


           Brad     

[TIMBO]

Thanks Brad, I found a little info on the net and it was suggested that is one way to set it. 

[Glennjeff]

No need to use an oscilloscope, just feed it a low frequency signal, like 110 Hz - Lowest A harmonic, and measure with multimeter set to AC. Adjust for identical readings, grid to ground. Do that at a couple of low frequencies say 160 Hz (Low E 2nd harmonic) and A 220 and bob's your uncle.

Hot Blues Plates adjust for minimum signal with red multimeter test lead on one grid and black multimeter lead on other grid, set to measure AC is effectively the same and you don't need a signal gen for that just pluck the low E string or pipe something from a mp3 player.

[sluckey]

I might put a meter's red lead on one grid and black lead on the other, and fiddle the pot for minimum measured a.c. voltage (i.e., minimum difference in drive voltage).

That would work if the signals were in phase. But, since they are 180° out of phase...

For example, if you have 25VAC on one grid and you have opposite phase 25VAC on the other grid, your meter would read 50VAC phase to phase.

[HotBluePlates]

Dang you're right! Outsmarted myself... 

[TIMBO]

Thanks guys, I don't have a scope, so the ear and Glennjeff idea is the method i'll have to use for the moment. I set the pot at centre and turned it either way and was able to get a spot that I think sounded good, notes finger picked were clear and crispy and power chords were not too distorted. I then hooked up the MM as Glennjeff asked and struck the low E string and the voltage would fly up to about 1.8v. This was not that easy as it would as you expect as it differed to how hard you struck the string but a good average got me to close to my ear setting.Thanks

[Glennjeff]

Duh. No amount of trying avoids the dyslexic moment.