[dngl]

OK, carlos made a post that got me interested in re-biasing my melos:

"You can also change how a Melos sounds by adjusting tube bias. If you look at the board from the side closest to the tubes you'll see two trim pots to the top right. They'll either be blue, yellow, or white in color, they are the bias pots. The Plate/Anode are pins 1 and 6 of the tube socket (clockwise from the notch), adjust those pots to find what works best for you, measure the voltage, and match both channels. Obviously the higher you go the harder you drive the tube, usually around 60v for 6dj8s and 75-80v for 6922/7308 have been my preference with the amp. If one of your channels sounds slightly distorted, the channels imbalanced, and/or the tubes different in brightness (literally, not figuratively) adjusting bias usually solves the problem."

I have a multimeter... and have no idea what to do. I removed the tubes and found the trim pots. Any help would be greatly appreciated.

 

[carlo]

Measure from the pin to ground. The ground trace is the heaviest trace closest to the edge of the board thats closest to the tubes (hope that makes sense).

I had a brain fart: counterclockwise from the notch when looking down at the socket. Still pins 1 and 6.

 

[dngl]

thanks carlo, especially for the help on IRC.

I have been turning the pots little by little, eventually to significant amounts, and I can't seem to coax a voltage change. Any idea what may be wrong?

 

[carlo]

Jon,

Last night you were getting about a 5v swing right? Is it not changing at all anymore? Measure at both pins (1 and 6) of each tube.

 

[dngl]

I've measured at both pins and can't notice a voltage change. I had the same problem last night, the 5v thing must have been a miscommunication.

 

[carlo]

Jon,

Hmmm. Might be a screwy bias IC since I somewhat doubt that both trim pots decided to crap out. You could replace it but the voltages you were reading before are fine and since both channels were matched you're okay. On Zanth's SHA-X there's a bias pot (4 total) for each tube and each output FET, since Melos' are funny like that your particular amp might be adjusting the FETs only. Or maybe it isn't, too hard to call unless you wanted to remove the board, and since the amp's been your reference based on its sonics its probably not worth the trouble. A good experiment would be adjusting the trim pots while a signal plays and listen for any difference, if so explore further. Let me know how it goes, I'm curious.