Does bias change from a quad to a pair?

[charvel750]

Guys, I'm running a 100 watt tube head and one of the tubes dies in the middle of playing. Not even sure when it happened, but I noticed it was not lit up as I powered it down at the end of a set. So, when a matched quad dies on a 100 watt Marshall (1978 JMP), and the other 3 tubes keep going, does the bias change?

Someone said to remove the 3rd tube to "balance" it out and run 50 watts. If you do that, does the bias get affected?

[Aaronharmon]

No, not the bias, but the impedence does. At lest that is my understanding.

[Aaronharmon]

"Matched" Tubes are silly anyway. If you bias your tubes and they are "matched" So, why buy "matched" tubes?

[charvel750]

"Matched" Tubes are silly anyway. If you bias your tubes and they are "matched" So, why buy "matched" tubes?

You set the amp to match what the tubes are rated at. So if you used "unmatched" tubes you can never get a true bias adjustment.

[Cygnus X1]

Tube bias is quite simply, the operating point of the tube. Whether it is a single tube, a pair, a quad, or whatever pairing you would choose from there.

The multiples derive from the push-pull design. If there are four tubes, and one fails, then pull the mate to make it a dual. DO NOT run it with three tubes.
The matching is there to keep the operating bias of the pair within an acceptable range.

Quads should be biased as two pairs....so pulling the tubes of the offending pair should not affect the bias of the working pair. Normally...but some choose to bias the four together. Either method, and the impedance going to the primary side of the output transformer will change a bit. This does not mean you should run your speakers on the secondary on a different setting. You can run the amp in "limp mode" to get through the gig, but no more. If anything, the biggest risk is going to change the load on the phase inverter, and you will start popping preamp tubes, as well as risking the life of the output transformer.

Quads set up as dual pairs on the bias can be run safely, provided the bias is set just a bit lower. Biased as as quads...things get risky.

[Cygnus X1]

"Matched" Tubes are silly anyway. If you bias your tubes and they are "matched" So, why buy "matched" tubes?

Because they are biased in the amp as a pair, not individually.
In push-pull, one tube may "work" harder than the other, and it will overheat and fail. There are balance pots in some to make up for the difference, but it is still best to start off with a matched set.

[DonP]

Ideally each tube would have it's own individual bias. My DSL100 has bias for the left and right banks. But adding an individual bias would increase cost, cut into profits, etc.