Fender Deluxe Reverb - Bad tube destroys screen resistor - Troubleshoot, repair, match and bias
8,286
Published 2018-07-30
The shop amp is back on the bench for repairs after an event during use which was a loud pop followed by red plating of a finals tube. The amp was shut down and the tube replaced but to no avail. There was nothing more to be heard from it.
I focus my troubleshooting with a visual inspection in the vicinity of the finals and immediately locate a melted screen resistor. This is not unknown. Sadly, the original tube that did this was not delivered with the amp and I wish it was, because it would have easily identified the mechanical failure in the tube that caused the current through that resistor to flow to ground.
The owner gave me all new (NOS) tubes to replace the finals with and match by request so after repairs are made I move the project to a matching and biasing event.
This is the first project to use the Hickok 6000!!! A full tube testing event is accomplished with the existing tubes first. We further see that many of the re-label tubes are not what they seem.
Just to be safe, voltages are taken to ensure good operation on the B+ before the finals are added.
This brings the project to the bias and matching event, noting the B+ improved with the 6V6's added. I dial it back the bias to 25ma for matching per tube. In the end the closest match came to .3ma match
A few hours of testing shows all functions working and everything stable.
All Comments (10)
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12AT7 I believe had a little more current handling capabilities. This is why you would see them as drivers in some circuits. Also prefixes and suffixes on vacuum tubes used to mean certain things but in modern times may or may not mean anything, and many times are just marketing gimmicks. One really has to wade thru the b.s. with modern tube manufacturers.
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Real nice! Always pick up something new from these sort of videos. Best regards from the west coast of Canada! ss
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Great video a Big👍 up.
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According to RMorg, the 6201 is equivalent to the 12AT7W which is a ruggedized version of 12AT7 for computer applications. Thanx for posting this - always keen to learn. What's the wattage of the screen resistors please?
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Maybe a little Windex on the meter display?
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More guitar amp repair videos please.
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OOOPS, The bias setting should have been set up in this video as 25mA not 35mA. My Mistake!!!
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I have never had any problems with Sovtek tubes.
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Well I don't trust a 20 year old+ screen resistor or droppers man...just change them....something close man....that 470ohm aintttt doing ya any favors either....1k-2k2+ man. I've seen smoked arced burnntttt trust me oh wonder why lol
