Chuck H's Zener bias mod

[eschertron]

Beating the tar out of the tubes, such as guitar amps do, the tubes don't often stick around long enough to become gassy

I've kinda wandered into that conclusion. Granted, my understanding is only a few hours old, but the sources agree that thermal runaway only happens with positive ions bouncing about in the tube. I'm not expecting to get 20..40 years of life out of a new production tube.

This begs another question: Thermal runaway, the tube overdissipates and electrodes melt. Bye bye, tube. Collateral damage? Can the OT suffer from this failure?

[Chuck H]

Thermal runaway, the tube overdissipates and electrodes melt. Bye bye, tube. Collateral damage? Can the OT suffer from this failure?

Usually.?. I think "mine" can.?. My personal amp is using Hammond iron. Known to be durable and has proven to be on a couple of occasions when I did something stupid. Typically, and we've read about it here many times, an over current condition in the tubes results in either a fuse blowing or some other board component failure. Rarely an OT.

[eschertron]

an over current condition in the tubes results in either a fuse blowing or some other board component failure

We're working towards convincing me that a HT fuse is good cheap prevention, esp. if we're flouting RDH4 on the grid resistors.

[g1]

Well, I don't think a mains fuse will blow in time to save an OT from a shorted tube.

[Chuck H]

Well, I don't think a mains fuse will blow in time to save an OT from a shorted tube.

But it usually does. That's actually what I was saying. Blown OT's are remarkably rare actually. Not little, cheap jobbies that are in amps made of pressed paper, but most REAL amp OT's are burly enough that they are the last thing to suspect very often. Ergo the common joke when someone asks "Did I kill my amp?". Someone here will always come in with "Probably the OT" but mean it sarcastically since it's the first thing most greenies suspect. I'm all for the HT fuse though as better protection.

But I'll admit that I do test any OT that's known to have been part of a power tube short as a matter of course. It's happened right in front of me a couple of times too. Great fun.