Bias 2 matched pairs in 100 watt soldano?

[888guitars]

Is it ok to use two different matched pairs in a 100 watt amp? If so would you split the pairs up? One set on the outside, or next to each other? Or how do you recommend installing them to get a close bias between the pairs? If the 2 pairs are not that close together is it even possible to get it biased? This is on a soldano hot rod 100

[The Dude]

If the amp has a single bias control or no bias control, I would split them up. If the amp has 2 bias controls (one for each pair), I would put each matched pair on a side and match the bias current using the dual bias controls. Hope that makes sense. Whatever works best for getting each tube to bias the same or as close as possible.

Edit: I am assuming that they are the same tube type and brand- just 2 separate matched pairs.

[888guitars]

Just one bias pot. If I understand, i should use the pairs together from left to right. First matched pair in sockets 1&2, second set in sockets 3&4. Then try to match them with my bias probes between the center sockets (2&3) ???

[The Dude]

Ok- one bias pot. Assuming they are actually matched, which is questionable with QC these days, one matched pair in the inside sockets and one on the outside sockets. Compare bias current of each tube and average them to your target without exceeding maximum current on any one tube. You want the total current of each side (sum of two tubes) to be as close to the same as you can get it, so you might have to swap tubes around to achieve that.

All of this is unnecessary if the tubes are within a few milliamps here or there. (Exaggerating values for clarification) Let's say you have a 60ma, a 30ma, a 40ma, and a 50ma reading on each tube (again these are extremely unmatched and I used this only for ease of illustration). Put the 60 and 30 on one side and the 50 and 40 on one side so that each side has a total of 90ma. Then adjust the bias control so that the tube drawing the most current is not over it's maximum bias rating.

[888guitars]

That's what I did the first time. I could not get them close at all! 46 on one pair and 68 on the other. These pairs must be too far off from each other then I guess.

[The Dude]

Yes, 22ma difference is quite substantial. By chance are they Mesa tubes? You may want to just try another set and save these for a couple 50 watt amps. I have a variety of tubes in my shop and can usually match most anything. Often times someone can't afford a full set of tubes and maybe one broke or flaked out in some way. I can usually match their remaining tubes pretty closely with stuff I have laying around. It's not an optimum situation, but that's how it is.

[Enzo]

But also be aware that it doesn't matter NEARLY as much as people think. Folks have this idea that bias has to be set down to a fraction of a milliamp and must be oh so precise, and it just isn;t so. When they make amps, they don;t install matched tubes. Grab four tubes from the big box-o-tubes, and in they go.

[Chuck H]

That's what I did the first time. I could not get them close at all! 46 on one pair and 68 on the other. These pairs must be too far off from each other then I guess.

So, to be perfectly clear, the first time you tried these two individual pairs of tubes you placed one pair in sockets 1 and 4 and the other pair in sockets 2 and 3. And doing this you got 46mA on one side of the OT and 68mA on the other side of the OT?

I'm guessing this is not the case. Because if it is then you do not have matched pairs, you have either misunderstood the instruction, you are not measuring correctly or the amplifier is broken.

Do this:

Plug one matched pair in sockets 1 and 4
Plug the other matched pair in sockets 2 and 3
Measure the bias of each tube
Pick the tube showing the highest current and bias the amp so that tube idles at 75% of it's max dissipation.

Now take a reading of each tube again. The total current of tube sockets 1 and 2 should be very close to the total current of sockets 3 and 4.

[888guitars]

I had a bad tube in a so called matched set. Must have been dropped?

[Chuck H]

I had a bad tube in a so called matched set. Must have been dropped?

Not necessarily. I've recently had trouble with three new pairs of EL34's (out of three purchased, all different brands or models). It seems you just can't expect anything but garbage right out of the boxes anymore.?.