Twin-Amp biasing

[ioginy]

I was playing my amp and suddenly the volume starting grumbling and cut in half, then it started going in and out totally.

I assume it's my tubes, and I also assume it's my pre-amp tubes because all four the the 6L6's were burning nice and bright. My tubes haven't been replaced in a long time so I figured it would be a good idea to do the whole lot of them.

There is biasing controls on the back of the amp, but I don't really know how to do it. I've researched all around and it seems like everyone has their own way. I could take it to a tech but i really want to learn to do it myself.

If anyone has some input it would be much appreciated.

[Enzo]

The little glowing heater in the tube is a heater, nothing more. The fact it is glowing tells you nothing about the tube's condition other than that the heater works. COmpletely failed tubes can still sit there with a heater glowing merrily away.

So while it still could be a preamp tube, glowing heaters in the powr tubes is not evidence of it. I would always suspect the power tubes first.

Chances are your preamp tubes are fine, and at worst one of them is bad. The preamp tubes can last for decades. Power tubes wear out just doing their job, that is normal.

The bias controls are on the rear, but if you don't know whose plan to follow, why not follow the factory advice? You stick your volt meter into the holes as directed and twiddle the control until the specified voltage is set. I believe that is 80mv (millivolts). WHich is 0.08 volt.

There are three holes. They want you to set the voltage between two of them to 80mv, then move one prove over and balance the reading to zero. That works. But really all they are doing is making the two readings different ways. The hole on the end is just ground, then each of the other two holes are tube cathodes. Set the first one to 80mv, then you are setting the second one to the same. But they have you measuring between teh two cathodes, so if they are BOTH at 80mv, there will be zero volts difference, follow?

DO it their way and be fine. That is 40ma per tube, which is hot enough for most of us. Some guys might want you to tweak it up to 45ma, but really, it sounds fine at factory spec.