Bad Power Tube?

[fearhk213]

My amp just started doing something odd & I'm thinking it's a bad power tube, so I'm wondering what you all think. It sounds fine at low tv volume. Once I get it to a volume that would be just under gigging volume it sounds fine for a little while, but within 30 seconds or so of being louder it cut out almost like something shorted and there was a very loud buzzing sound through the speakers. I couldn't get to the standby fast enough.

I had recently rewired a cab, so I thought maybe it was a bad connection that was rattling loose with high volume, but I checked all the connections and tried other amps at stupid loud volume and had no issues. Tried the culprit again, and it did the same thing - fine at low volume, but ended up with that same loud buzz after getting loud again.

I checked the bias. One pair had drifted hot, which makes me think more that it's power tubes, so it's biased correctly now. I also swapped the phase inverter. Neither of these did the trick.

Anyone experience something similar? Sound like power tubes?

 

[Racerxrated]

Could be a preamp tube as well. Try to swap all the pre tubes first with known good ones. Then power tubes.

 

[Rick Lee]

When I've had a bad power tube, it either blew a fuse or it stopped glowing and the top became frosty looking because it had lost vacuum. When I've had bad noises, it's been a preamp tube.

 

[311boogieman]

What amp is this?

I'm assuming you don't have any spare power tubes or you would have already tried that.

I'd get some nicely matched power tubes. Swapping a known good preamp tube through the deck is a good idea too.

 

[fearhk213]

Started with the phase inverter, but that wasn't it. I just ended up swapping the cathode follower and the pair of tubes that drifted and that did it. Not sure if it was the preamp tube or pair, but as loud as it was I was getting nervous I was going to damage a speaker or worse, so I decided against going one by one.