The newer Triwatts (such as mine) came with an output transformer capable of supporting either 6v6+kt66 tubes, or EL34. I originally built my Triwatt for 6v6/kt66 operation. Yesterday I made the changes to use EL34 tubes. This is a very simple change: a couple resistors in the bias circuit, and swapping the transformer leads.
After the change, I powered it up and started to bias the tubes. They were running really hot, over 55mV, in the initial setting. I started turning the adjustment pot to bring that down, and while doing this, I heard a "pop" come through the attached speaker. I didn't see any smoke or smell anything that suggested a serious problem, so I finished with the bias procedure. Then I plugged a guitar in, and... no sound.
After a bit of troubleshooting, I determined that preamp tube V3 was bad. V3 looks like gain recovery from the tonestack, and I think the other triode is some kind of voltage regulator (that's what I've hard, not sure about that; but I can say, the plate voltage on V4 was way too high, by 100V or so). So anyway, I put in a new 12ax7, and everything is fine. I then played it for a good hour or so, without issue. (Could just be expectation bias, but I do think it sounds more authentic with the EL34s.
)My question: is it pure coincidence that that tube just so happened to pop and die right when I was biasing the new power tubes? I can only guess how much play time was on that tube - maybe 200-300 hours? Likely less. As I said, those EL34s were biased too high for a few minutes. I would think that wouldn't effect the preamp tubes, but I don't understand the circuit well enough to say that for sure. So I'm just hoping the folks here might have some ideas. If it's just a shoddy tube that was destined to die sooner or later anyway, that's fine. But I'm more concerned about other potential subtle problems.
Thanks!