dying '77 peavey classic 50 212

[25orsixtwofour]

so, a while back, i blew the power tubes out of my beloved Peavey, i scabbed in a pair of tubes that i had lying around, one of which had the set pin broken. i was not as careful as i should've been in putting that one in, and fired her up with a tube offset one pin. needless to say, the fuse blew and i immediately turned her off. i got a new fuse, and a new set of 6L6's, but there is no real volume, it'll play a bit, but the pre's and the master have to be dimed.

anyone else bugger up an amp like this? i feel like it's fixable, but i want to know some possibilities of components i fried. i think because of the fuse blowing, that the damage is probably localized to components behind the fuse. wishful thinking?


HELP!

[Gtr_tech]

You may have inserted a tube so it put plate v on the heater circuit......or somewhere else it didn't belong. Pull the tubes and check for proper heater voltage. With the chassis out, look for any burned components.

BTW, its not a "classic 50". Its the older "classic" model with the SS front end and 6L6s in the output section.

When using octal tubes that have the keying pin broken off, use *extreme* care to make sure its lined up in the socket as it should be. There's also these which when installed correctly eliminate the possibility of inserting an octal tube wrong with broken keying pin. P-STKY-1 is the part # at antique electronic supply.

[25orsixtwofour]

hrmmm

how do i test the heater voltage? i know, "if you have to ask..." also, i cracked the chassis open, and nothing looks burned up. what could be fried if the heater voltages are off?


thanks for all your help!

[Billy R aka DynaFreak]

a good first step would be to pull the power tubes then carefully (safely) check voltages on each pin with the amp powered up. That will help inform the next step.

[nashvillebill]

Note that this is likely one of the Classics that controlled bias automatically by a servo system, so pin voltages could be quite goofy without the tubes in.

My advice, for what it's worth, would be to call Peavey and get the schematic (they are very good about giving out schematics), give them the model and serial number off the back because there are several versions of Classics in that time frame. With the schematic you can measure voltages at the appropriate points and go from there.

I'm not sure what you meant about "blew the power tubes out", describe exactly what happened to cause that?

[25orsixtwofour]

yeah, the power tubes just warmed up and started arcing like a light show, i figure they were just super old. they were some used silvertone tubes out of an old 1484. thanks for all your help with this, i should have her up and running in no time.

[Enzo]

Look on the front of your amp. Does it say "Classic."? Or does it say "Classic VT"? Or does it say Classic VTX"?

I don't know of any Peavey amp with fancy bias. Most of them are not even adjustable. The early Classics had a basic adjustment, the later ones did not. The VTX version had cathode signal drive, like the Music Man amps, but there was no bias servo action.

[nashvillebill]

I don't claim to be a servo expert, I based my response on this thread (though it specifically discusses the Heritage, I believe it's the same circuit) which calls it an AC servo...http://www.diyaudio.com/forums/tubes...-question.html

The Peavey patent which included their bias design was 4,439,742 which I found online, and has a complete thorough description. Again, I'm not claiming to be an expert so I'm not trying to be argumentative--all I know is that it's complicated...

[Enzo]

Yes, the cathode drive circuit is the same, or close enough. I disagree with the analysis in the other thread, but I claim no particular expertise there. There are the main cathode transistors which control the current through the tubes. The additional transistors clamping those bases are just limiters, like on most any solud state powr amp you find. I don;t see them as dynamic bias adjustments.

If power tubes were arcing inside, they are shot, and certainly the cathode transistors could have been damaged.

[25orsixtwofour]

She just says classic on the front, it has a bright, series, parallel and normal input, a solid state front end, and a 2x 6l6 power section. yeah, i had the amp up and running after the tubes went out, but i switched the new 6l6's that i had for some older ones, one of which had a broken set pin, and the rest is history.

i'll be checking the amp over the thanksgiving weekend with a borrowed multimeter, we'll see what we come up with.

thanks again for your help!