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I am working on my friends mig 50 that was popping fuses. I determined that one of the power tubes was shorted, so I popped in a used set of tubes and the amp seems fine. The one thing that I am worried about is that the plate voltage is about 545 volts. I know that this is torture to the typical 6l6. It came in with sovtek 5881wxt's in it. I just want to make sure that the replacement tubes aren't going to have an untimely death. Also, I noticed that the negative voltage on the grid on one of the power tubes goes up (more negative) by about 30 volts. The other tube stays constant. The OPT also squeals a bit when I put the DMM probe on the grid pin.
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in case you haven't already...check and monitor the voltages from the power and output trannies....I've worked on quite a few of the Migs, and as they age, their trannies start failing. -
one thing that I did notice is that the AC voltage was different by quite a bit from the 2 secondary leads to ground. From lead to lead it still puts out quite a bit of voltage though.Comment
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6L6 bias voltage
Can you inform us of the bias voltage reaqdings on each tube, pin #5?Comment
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+1 on what Gomez said - those Sovtek trannies are not the best and they do fail, I've seen more of those amps with bad OTs (and PTs) than with bad power tubes.Comment
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I set the bias voltage to -55. Bare in mind that this is with some crappy mesa branded chinese 6L6's that I tried to use in a peavey, but it didn't have enough negative voltage. Anyway's One of the tubes stays at a constant -55 when signal is applied. The other jumps up to almost -80. This does not happen when the power tubes are removed.Originally posted by Jazz P Bass View PostComment
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6L6 bias voltage
How is the bias when the tubes are at idle?
When applying a signal I would assume that you are also measuring Vac at the bias along with the dc value.
Does the quirky behaviour follow if you reverse the tube positions?
If not, check your screen resistors.Comment
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I think I have everything figured out. The amp didn't like the input signal I was giving it and was making the PI do funny things.
I am still concerned about the B+ being so high. The amp is meant to be run at 110vac. Wall voltage nowadays is about 120 or so. What would be the best 6L6 to use that can withstand that kind of voltage? Could the high voltage be to blame for the 5881WXT failing? Thanks for the help.Comment
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I think most would agree that the Sovtek 5881 is the 6L6 type most likely to withstand high voltages and other forms of abuse. Do a forum search here.Comment
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I would probably install a zener diode that'll drop the voltage down a smidge-like, say, 75v or so.Originally posted by antieatingactivist View PostComment
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Don't think the OP is aware of how to install a back bias diode although it's really not that hard I would think it's beyond his realm but I may be wrong. The 5881 will take over 600 volts easily without failing but it isn't the best sounding tube out there by a long shot. The Svetlana 6L6GC Winged C will take it and it's a 30 watt tube so even at 60 % should be pretty nice in that amp and they are current production. Of course NOS like an old RCA blackplate would be ideal.Originally posted by Prairie Dawg View PostKBComment
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