OT bad ?

[Valvehead]

i picked up a broken Crate blue voodoo to flip last week. BV-6212. I put some EL34's in it and it fired up and sounded ok. It needed a few of the pots fixed. Got those repaired.

So im playing through it, LOUD, the Master was on about 8

and the volume faded slowly down to zero in about 3 or 4 seconds.

So, i found 3 bad diodes, one was a 1N4148 , the other 2 (next to the power tubes) were 1N4007 . And a 220k was shorted. These are all right by the power tube sockets.

I replaced them all, and i have NO volume from the amp . If you crank the master to 10 and stick your ear against the speaker, you can barely hear signal (both channels).

Voltages and bias looks ok . Parts on the power tube board test good.

i tested the OT =
between the CT and each plate lead.
one measured 74 ohms , the other measured 175 ohms.
there are no shorts from pri to sec, and no shorts to ground.

Think i toasted the OT ?

[Enzo]

I doubt it.

The likelihood of a 220k resistor being actually shorted is very remote. What is in parallel with the resistor, a filter cap maybe? There are 220k bleeders across all the HV filters.

Aer you sure they are 220k?

An amp fading out over a few seconds sounds like the heater power failed, so the tubes faded out as their heaters went dark. There are two 220 OHM resustirs in the DC heater supply, not to mention some 1N4007s. ANy chance it is them?

meanwhile ther are pairs of 1N4007 acting as flyback diodes at each power tube plate. If they failed, I would make sure the ciring to the speaker jacks is 100%, because what usually kills those diodes is a lack of a speaker load.

[Valvehead]

sorry--dont know why i said 220k.

the resistor was discolored, looked like 220k.

talking about R91, which the scem says 12 ohms



D10 was the dead diode 1N914 on the scem-but the amp had a 1N4148

also found the 2 , 1/4 amp fuses blown + the 6 amp fuse blown

Im off to the parts store....

[Valvehead]

I ended up replacing the 2 , 12 ohms and 3 diodes . The amp is back to life and everything seems normal !

Glad it wasnt the OT--they are very hard to get, crate doesnt even have any!

[Enzo]

1N914 and 1N4148 are interchangable in the stuff we work on.

Those 12 ohm resistors adn the 1/4A fuses are in the cathode leads of the power tubes. ONly thing can burn them up is power tubes conducting like mad. Combine that with a blown heater fuse, and I'd have to think you had a shorted power tube.

[Valvehead]

hmmm, the tubes that are in it now are the same pair that were in it when it blew. They seem to be working great.

[Enzo]

Well, if someone can come up with an alternate scenario, I'll listen.

[Euthymia]

Well, if someone can come up with an alternate scenario, I'll listen.

Temporary loss of bias? Turn it on, play, lose bias, sound fades, protection resistors smoke....

Now that it's functioning I'd fire it up and make sure all the tube socket connections are solid, especially pin 5.

And I'd surely put those EL34's on my tube tester to make sure the experience hadn't irreparably traumatized them.

[Valvehead]

tube tester??

thats 40's talk there...........





Haha-----just kidding. wish i had one actually . i just keep "known working" tubes around if i can........



dont know Enzo--i fired her up tonight to double check--amp works great.................