Fender Twin Pro 100 Biasing...

[mrr3000gt]

Hey gang,
Long time no see! I have just bought a new Fender Twin Pro 100 and noticed there is some strange method to bias it involving reading some voltages on the back of the amp that seem to have little to do with the actual plate dissapation from what I can tell. I worked on a Vox AC100 head that was biased in a similar manner (never buy the VOX AC100 - a look at the cheapness on the insides will make you cry).

So whats up with the way the Pro 100 Twin is to be biased? I am looking for the explaination for why it is different from 99% of the other Fenders I have worked on for some time.

[sluckey]

Look at page 5 of this manual...
http://support.fender.com/manuals/guitar_amplifiers/94_Twin_Amp_manual.pdf

[mrr3000gt]

I already know that: I was more or less looking for a technical explaination for why biasing is performed in that manner.

[sluckey]

You set the bias adjust for the specified amount of idle current thru the tubes. Then you adjust the bias balance for zero volts which insures the current in the top half of the OT is the same as the current thru the lower half of the OT.

The bias adjust operates the same as an AB763 that has been modified with 1Ω resistors under the cathodes. The bias balance takes it a step further (an improvement) by allowing you to balance the current thru tubes rather than just swap tubes to match current.

[mrr3000gt]

Thats what I was looking for!

I wondered how to comensate easily for different impedances in OT primaries - my other amplifier has a small - but measurable - difference on each leg and I compensate with putting different combinations of tubes with minute differences in current draw so the measurable difference is virtually nill.

I practically run a simplified design of experiment with one factor to determin the best placement of four tubes. A DOE with just two tubes is just plain silly.