Tried a cathode bias resistor swap and got way more gain than I expected

I was working on a 5E3 build last weekend and figured I'd try a smaller cathode resistor on the power tubes to bump up the bias a bit. Threw in a 150 ohm instead of the stock 250 ohm. Well, the plates started glowing cherry red after about 10 minutes of playing at half volume. That was a new one for me. I shut it down fast but I think I cooked the tubes a little. Learned that smaller resistor means way more current, not just a slight bias shift. Has anyone else made this mistake and wrecked a set of tubes?

2 comments

2 Comments

the_sarah16d ago

@jordan653 nailed it with the multimeter tip. Dude, glowing plates are a rite of passage I guess lol. That 150 ohm was def a one way ticket to toob town.

jordan65316d ago

Wait, did you check your bias current with a meter before cranking it up? I did almost the same thing on my Princeton build a few years back. Ended up swapping in a 180 ohm resistor and kept my multimeter hooked up to watch the cathode current as I played. It settled around 38mA which was safe for my JJ 6V6s, but I had to tweak the screen resistors too to keep everything stable. That glowing plate thing is scary though, I had to replace one of my output tubes because I think I stressed it too much.