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Tube overheat problems... [message #27252]

Thu, 16 March 2006 17:43

Perplexied
Messages: 1
Registered: May 2009

Esquire

Hey I just got a Sovtek Mig-50 for really cheap cuz it wasn't working. I guessed it was the tubes so I changed them and it came back alive. Then one of them started to overheat and glow bright red. I took the amp apart and noticed a resistor had come off, so I soldered it back. The Thing worked fine, but I fire it up now and and the same problem w/ the tube overheating keeps happening. I've tried changing tubes again, swapping sockets, and it STILL keeps happening. It's worse when the amp is cranked, but that's normal, i guess. Any Ideas?

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Re: Tube overheat problems... [message #27253 is a reply to message #27252]

Fri, 17 March 2006 08:30

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

You probably have "bias voltage" problem on (at least) one output tube.
I think it`s a "Sovtek" 5881 based amp, with high anode voltage, about 500V - correct me if I`m wrong.
Pull out both output tubes and measure voltages on both 5881 sockets:

-between pins 2&7 (heater) - 6,3V AC
-pin 3 (anode) - about 500V DC (carefull!)
-pin 4 (g2) - about 500V DC (carefull!)
-pin 5 (g1) - about -50V DC (negative to ground)
-pin 8 is the ground (check it)

If something isn`t like above, probably -50V bias voltage on the pin 5, then you must check where you "lose" it.
Also, check the screen grid resistor, connected to pin 4. Mostly, it is wired between the pins 4 (g2) and pin 6 (empty, used as a soldering point). Be carefull - high voltages!

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