Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Dreams on February 25, 2015, 04:15:26 am
-
So before I launch into wild theories and hypotheticals, is there anything that would cause all four grid resistors on the power tubes of an amp to burn up at once?
Amp is a Peavey 5150. 100 ohm screen resistors, 2.2k grid resistors, 100pf cap on each side before the grid resistors to ground. Everything tests fine except the grid resistors.
I was getting ~485v on the grids. Pulled all tubes and I get -55v. Tested the tubes for shorts with my meter. Two of the tubes are shorted from grid to screen. I put the other two tubes back in and sometimes I get -55, sometimes 485v, but they don't test short with my meter at least, even though I know that can be misleading. They're probably bad too.
So my working (working late) theory is that one of the tubes shorted grid to screen, taking out the grid resistors on that side, and then someone started swapping the tubes around for whatever reason, and that took care of the rest?
It's late where I am so I'm probably leaving something out or just not thinking full-on, but anyone got any thoughts?
Thanks, and good night y'all.
-
Sounds logical to me. Time to put in a fresh set of tubes.
-
What would make you suspect the OT? I mean, when something fails in the output section I check it anyway, I'm just wondering if there's something specific you're thinking of.
-
Also, thanks Sluckey. That's where I'm at, too. After I fix up all the burned stuff, that is.
I'm gonna replace the cap that the filament sits on as well. 6-whatever volts AC on 24v DC, and the cap is rated 25v. It seems fine, but that's marginal, and it's a pain to remove this board (the power tube sockets are pcb mounted and riveted in, so it's more like the board is socket mounted) so I'm just gonna do that while it's open.