Sovtek Mig-60 No Output

[jaysengsr]

Hey fellas, i recently picked up this Sovtek Mig-60 with a matching 2x12 cabinet for free from a friend's rental house that the previous tenants left behind. I got the stuff home, and immediately yanked the chassis out to check things out and noticed a burnt up screen grid resistor. I'm pretty new with tube amps, but am VERY comfortable with electronics and schematics.

After grabbing the scematics for the Mig-60, i went over everything and replaced resistors that were out of spec (only 2). I bought new tubes for it, as one of the stock EL34s (the one that had the burnt screen grid resistor) had a strange purplish-glow at the bottom, i figured they were 13 years old anyway.

Long story short, i threw the tubes in today, and i get nothing, nada, zip, zero, zilch out of the caibinet. no hum, buzz, nothing. Is there any preemptive tests i can do to check out if it's the OT or PT before i shell out a hundred bucks or so? Or, is there anything else i should check out before i just make a quick dollar selling it as is?

thanks for any help,
jay

[Enzo]

Connect a different speaker to it with a different speaker cord for starts, and verify the existing speaker works too.

If the speaks are OK< then this amp is no different from any other. Got B+ everywhere? Since the screen resistor was burnt, then at least that tube was shorted. You have new tubes, but make sure B+ gets to both screen pins on the sockets. Check B+ throughout the amp.

[jaysengsr]

the cabinet is used everyday with no problems, it's connected to practice amp (Vox AD30VT). i get a solid 350V on each screen. and B+ everywhere. not sure what's up with this one.

[Enzo]

If there is B+ on the plates of the power tubes, the output transformer primary is not open. it is unlikely the output tranformer secondary us open, but you could check. And make sure the speaker jack is not shorted. The winding itself will measure shorted due to the low resustance of the wire.

OK, B+ on pins 3 and 4. Is there bias voltage on the pins 5? And is there continuity to ground at the cathodes? If so, there should be some little noise coming out, and if you toched a screwdriver or something to either grid - pin 5 grid - should be a little hum produced. If no hum, them there is an issue in the OT, Power tubes, or output jack/wiring.

If that part works, we move back through the amp, checking for B+ at every plate and also cathode voltages at each. ANy tube with zero cathode volts and a high plate voltage is not conducting.

If all those DC voltages are reasonable, apply a signal to the input jack and trace it with a scope or AC meter through the amp.

[jaysengsr]

yeah, i'm convinced it's the OT. i get B+ everywhere i should and nowhere i shouldn't. the scredriver trick on the Control Grid pin is not working at all. i had noticed that i had a short in the 8ohm output jack wiring, so i fixed that and still nothing. guess i'll be grabbing a new OT. any recommendations for this amp?

[Enzo]

I don't swap OTs lightly. Pull the secondary wires off all the jacks and use clip wires to connect a speaker.

If you still think it is the OT, disconnect this one, and clip in another OT. Just about ANY will be OK for a test. All we need is to see if sound results. Impedance doesn't matter, tone doesn't matter. We need to fire it up for 10 seconds and hear sound - or not. If we hear sound, but not with the other OT, then indeed the first OT was bad.

Go over to RG Keens Geofex site, I believe he has some transformer test procedures.

[jaysengsr]

i think i've found the culprit, i ran all the little tests on the Geofex site and nothing came up wrong, however, while tracing everything on the secondaries from the OT, i found a thrashed capacitor, C11, which is incorrectly labeled as a second C10 in the schematic, mind you. it's a .047 630V film cap which i guess i'll replace with an orange drop

looks like i need to buy some decent screen grid resistors AND a .047 630V tone cap, which is A LOT better than a new OT.

also, can you make any sense of this tone stack? it's obviously been previously modded and looks nice, except for the 180pf cap that just looks out of place to me. the schem. calls for a 470pf. just curious.