Yamaha T100 Soldano Biasing

[arbik]

Greetings,

Sorry if this is too rudamentary of a question. I'm new to the tube amp scene, and did a search on the forum to see if I could find the information I was looking for, but the search feature didnt come up with any.

I have a Yamaha T100 Soldano Amp. One of the tubes gets VERY hot, but the others are ok. I changed the tubes, but the same thing is happening. I have read some literature about this, and I'm thinking it might need to be rebiased. I'm pretty handy with electronics repair work, and have biased some audio amps previously, with direction from experts or the manual, so I'm not completely new to this. However, I dont know anything about the T100 circuitry.
Does anyone have instructions on how to bias this amp? I downloaded a schematic of this, but need some input as to what type of bias it is, and if it has a trimpot, or I need to bias is with a fixed resister, etc.

Any help would be greately appreciated.


Thanks.,,

-arbik

[steve]

I had the head version of T50 Amp and it was the amp I learned to bias on.

I don't have it anymore, but the schematic shows a 25k bias trim pot.

When I biased mine I used the transformer shunt method, but if I had to do it now I'd get some 10 ohm 1% resistors and solder them in series from pin 8 of all four power tubes to ground. Then just measure across these and use Ohm's Law to figure out how much current each tube is drawing.

Be careful when biasing that amp. With my wall voltage, there was about 500v on the power tubes' anodes.

BTW, I experimented with different tubes in that amp and found that the only tubes that made a difference were real Tung-Sol 5881s. I didn't leave them in there too long as I didn't want to fry them. All the other tubes I tried: Chinese Mesa Boogie, Sovtek "5881", GE 6L6GC, and the original Yamaha Chinese 6L6GC sounded all pretty much the same.

HTH,

steve

Originally Posted by arbik

Greetings,

Sorry if this is too rudamentary of a question. I'm new to the tube amp scene, and did a search on the forum to see if I could find the information I was looking for, but the search feature didnt come up with any.

I have a Yamaha T100 Soldano Amp. One of the tubes gets VERY hot, but the others are ok. I changed the tubes, but the same thing is happening. I have read some literature about this, and I'm thinking it might need to be rebiased. I'm pretty handy with electronics repair work, and have biased some audio amps previously, with direction from experts or the manual, so I'm not completely new to this. However, I dont know anything about the T100 circuitry.
Does anyone have instructions on how to bias this amp? I downloaded a schematic of this, but need some input as to what type of bias it is, and if it has a trimpot, or I need to bias is with a fixed resister, etc.

Any help would be greately appreciated.


Thanks.,,

-arbik

[Thorny]

If you are putting in a good set of matched tubes and one gets hot, your amp has a problem and needs to go to a tech. They need to check the resistors on/near the tube socket - one is probably damaged or burned from a previous tube that has gone bad. That is one thing that will cause this.

[Enzo]

I agree. if the bias is way off, ALL the tubes will get red hot, not just one. And if you changed tubes and again just one gets hot, it ain't the bias. Especially if it is the same socket each time.

I suspect that - probably due to a bad tube in the past some part has failed, like a resistor went open, or possible a socket pin has loosened, and it is denying bias voltage to that particular tube.