Making Sense Of Bias Numbers

Firstly, this isn't possible, so you are getting some metering error.

The reason it's not possible is because tubes 1&2, and 3&4, have the plate connections paralleled with a fairly thick piece of wire. So the plate voltage on tube 1 must be exactly the same as that on tube 2, to a very high degree of accuracy. It doesn't matter what each tube is doing individually; the result is applied to both tubes equally.

You shouldn't try to deduce anything from variances which are within the meter accuracy - at best about +/- 0.5% with most normal multimeters. That's over 2V on a measurement of around 450V.

Reading to four significant figures will only confuse you. Even three sig figs (ie 1 part in 1000) is overkill; two (ie 1%) is all you should ever need for tube-amp work. In other words, you only need to know whether the voltage is 445 or 450, not whether it's 447 or 448. Likewise with current - there's really no important difference in .1mA, and not much in 1mA either - tubes are usually not matched to better than 5% anyway.

Like Old Tele man says, there's actually a fair range of 'correct' values for most amps, and as long as it's within that you'll be fine - after that it's only preference.


(But - Class B is not achieved at 50% of max dissipation. Class B is when each tube conducts for 50% of the cycle - which actually means that at idle it's cut off, ie no conduction. The problem is that this causes severe crossover distortion, so it isn't used for audio amps. That's the reason Class AB came about - by running some current at idle, the COD can be eliminated while still giving most of the power advantage of Class B. In some amps, you can run well under 50% of max. dissipation and not get COD. I agree that generally they don't sound quite right until they're up in the 50% region though.)

 

Hey, the first post says 36 mv and the second swaps that to 36 mA .. unless the transformer resistance is 1 ohm, that's incorrect.