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Post subject: Super 60 Problem
Posted: Sat Oct 23, 2010 11:58 am
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Hey all - I have a super 60 that I bought a few years back. It had a nasty hum problem when I got it, and being an electrical engineer I figured I could handle figuring it out. So i opened the thing up and found that around the output tubes there were some resistors that looked pretty fried. I checked out the schematic and here's the parts that were fried.

R159/160 (which appear to be 47ohm pull downs for the filament voltages. R146/R147, which appear to be some sort of biasing resistors (470ohm) to the output tubes.

R168 was soldered directly to the board (with no room for convection on the bottom) and it was labeled ad 22kOhm - there was some pretty significant burning of the PCB. The schematic has this part listed as 30kohm, and it is part of a voltage divider circuit to provide a dc voltage to the output tubes.

Based on this (and some other visual cues) that someone before me was hacking on the amp and maybe didn't know what they were doing.

So - I replaced all the parts per the schematic (with exception of R168, I bought those resistors before I looked at the schematic, so it stayed at 22k) and buttoned it back up and turned it on. The good news was that the hum was gone. the bad news was that ONE of the tubes was boiling lava hot. so much so that it discolored the MESA silkscreen from white to a nice brown color.

Any ideas? From what I can tell, the 22k resistor would cause the voltage to the tubes to be lower than expected. Would this cause just one of the tubes to be biased "too hot"?


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Post subject: Super 60 problems
Posted: Sat Oct 23, 2010 1:17 pm
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Joined: Tue Jun 29, 2010 1:20 pm
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Hi kpattison,

If I have the correct schematic, R159 and R160 are indeed pull down resistors fro the filament supply. R146 and R147 are screen grid resistors, and often are smoked when an output tube goes bad, nothing to do with bias. R168 looks like the plate voltage supply for the preamp tubes (C). Changing the value of R168 would change the plate voltage to the tubes. A lower value would give less voltage. I would put a 30K in as the schematic shows, for the right voltage, and get two new output tubes, those tubes are probably shot. Maybe the amp doesn't have the optional bias adjust, and someone tried to raise the idle current by changing R168...??? :idea:

You can pull the tubes and run your voltage checks without them in the circuit. Make sure that all of the previous components are still OK before putting in the tubes to try. :)

Edit:
It looks like R158 would be the resistor to change the bias voltage with, I don't know why the R 168 was changed. :? The schematic shows that when adding the optional bias adjust, you should remove R158, and replace with R170 and R171.

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