I'm a bonehead. Please help.

[misterwhizzy]

So I had been doing a bit of tube rolling, and in the process, I screwed something up. I got no output from the amp. When I flipped the effects loop off, though, it worked. So I turned it off, not wanting to deal with it that night. Today I came home, took the chassis out and started measuring voltages. I was getting power, and I had signal continuity from input to output. Then I realized I had the amp on without the cabinet plugged in. Oops.

I quickly powered it down and plugged the cab back in. It turns out the signal loss was due to a loose cable, and I feel stupid. But now the output level is significantly lower, even after a power tube swap. How can I tell if I fried the OT?

 

[Little Pigbacon]

Re: I'm a bonehead. Please help.

A fried output transformer will be a nice golden brown around the edges.

 

[Red_Label]

Re: I'm a bonehead. Please help.

Wow... sorry to hear that. I've made the mistake of forgetting to plug speaker cabs in many times over the years (often in expensive boutique amps) and never have had anything bad happen. I appear to have been lucky.

 

[Drano Max]

Re: I'm a bonehead. Please help.

Tubes!!!

 

[misterwhizzy]

Re: I'm a bonehead. Please help.

Wow... sorry to hear that. I've made the mistake of forgetting to plug speaker cabs in many times over the years (often in expensive boutique amps) and never have had anything bad happen. I appear to have been lucky.

Me too, maybe. I'm still a bonehead, but that may be acceptable for a few more days.

I figured my best bet would be to check the plate voltage and the bias on the power tubes. Plate voltage was 366 Volts, no problem. First tube read 20.2 mA. A little hot, but not to the point of severe volume dropout. The second tube was about 11 mA. Apparently bonehead move number two was to put a bad tube in the backup power tube sleeve. Oops. Found a good pair, biased them correctly (well, to about 19 and 18, respectively), and all is right with the world once again. Or at least with the amplifier. Any further commentary might turn this into the first thread in Amplifier Central history to be deleted for being political.

 

[devastone]

Re: I'm a bonehead. Please help.

It usually won't hurt the amp as long as you don't put a signal into it. It's when it tries to push a speaker that it does the damage, kinda like putting a clamp on your nose and lips and blowing as hard as you can, at some point something has to give.