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Making Sense Of Bias Numbers



Hello everyone nice to be here. I have a question for anyone that might be able to help. I have a '65 Fender Twin Reverb Re-Issue and I am trying to bias this for the first time. LEt me explain what I have done and the numbers I have for intepretation. First I took the plate voltage of the amp. Tube 1- 451.4 Tube 2- 451.4 Tube 3- 448.2 Tube 4- 448.6 I got this by taking my DMM on end to ground the other to pin #3 on each power tube. Next I took the DMM and set it to Dc Millivolts and placed one lead on the board on the B+ line (Red) and placed one lead on the tube that had each side of the output transformer on it. On the brown side I got 36.01 mv on the blue side I got 33.86 millivolts. Now with all of these numbers what is the next step to determine what each tube is pulling and what do I need to adjust the bias to (I am using 6L6 GC's) Sorry this is so long for my first post but I sure would appreciate some help. I also hope I gave enough information. :rolleyes:


Old Tele Man,
I just re-adjusted the bias now I am getting Power Tubes 1-4: 446.5, 446.1,447.3,448.1 and Blue Side (Tube 1&2) 73.4 milliamps, Brown Side (Tube 3&4) 72.4 milliamps but the bias is up as high as it can go. So if I take these numbers I believe that the new bias numbers are Blue Side: 32.75 (16.38 watts per tube) and Brown Side: 32.41 (16.21 watts per tube). Now having said all that don't the tubes need to be about 23 watts per tube, and if so what do I do to get it up to that range. Oh by the way I have Groove Tubes 6L6 B's, I plan on re-tubing but I want to know how to do this before I buy some good tubes no the cheapies that are in them. Any help form anyone is appreciated.:) :) :)

John Phillips

Originally posted by mickeyd415
I am getting Power Tubes 1-4: 446.5, 446.1,447.3,448.1
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.


that is my mistake I meant milliamps not millivolts. Sorry. BTW, I just finished with the bias and all I have to say is WOW!! I now have some life out out the amp. It made a large difference, now on to some good tubes, thanks to all who helped.:dude

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