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Here's an intermittent problem I have seen with my late 70s silverface deluxe reverb - one of the 6V6 tubes will, after a few minutes of operation, glow bright red and the sound will fuzz out and diminish. I can switch to standby or switch amp completely off and then back on and problem usually goes away, but not always on the first try. What's going on here?
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What you are describing is called "red plating", when the large metal plate in the center of the tube glows red from overheating. It can be caused by a bad tube, or can also be caused by a loss of grid bias to pin 5.
Try switching the position of the 2 6V6's and see if the same tube red plates. If the same tube in a different socket still does it, then I would suspect a bad tube. If the problem stays in the same tube socket, then there is a problem with the amp. -
How OLD are those 6V6s in there anyway?Education is what you're left with after you have forgotten what you have learned.Comment
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The tubes are only a couple of years old and I have swapped them out with another set with the same results. I think the problem is internal. What would cause, how do I fix a loss of grid bias to pin 5?Comment
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DRs run 6V6s pretty hard, a couple of years of regular, hard use could be enough to finish them.
What currents are the 6V6s drawing?
Check that you have negative voltage at pin 5 of each power tube, you only need to "fix" a loss of negative dc grid voltage if no voltage is present. If the voltages are dissimilar you may just need to adjust the bias balance pot, but you should really set bias current by measuring plate current with bias probes or 1ohm cathode resistors on the tube sockets.
Bias probes are the best bet, save you having to open up the amp, then you can ensure that both tubes are drawing equal current (+/- 5mA) just by adjusting the bias balance pot. If both/either draw much over 30mA then a tech will need to rebias the amp.
As you don't sound too familiar with the workings of tube amps read up on safety procedures best practices before opening it up - tube amps operate on lethal voltages.Comment
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Because according to your earlier post, only one tube shows signs of red plating, there is a problem with that tube's socket or associated wiring.
It could be as simple as a dirty tube socket, an open or off value resistor, a broken wire, or it could be a leaky coupling cap.
Without opening up the amp and metering a few things, there is no easy way to diagnose the problem. If you are qualified to work on it yourself, then let us know and we'll help guide you in the right direction. If not. its time to take the amp in to the shop.Comment
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Thanks for this forum
Guys, this forum is a great resource. Thanks to your advice, I purchased, built and fell in love with a Mission 5E3 kit. That experience and a thorough scouring of this board and other research gave me the tools to diagnose and repair my Deluxe Reverb problems, which included a crapped out resistor on the tube socket. While in there, I converted the amp's phase inverter to blackface specs in addition a few other minor changes and replaced the vibrato roach and got that circuit working for the first time in the 20 years that I've owned the amp. I also learned how to bias it and with a fresh set of tubes, I'm very pleased with the amp's sound now.
Now, the DR is ready for a cosmetic restoration and I am also ready to tackle another amp building project. I wish this forum and the Internet were around when I first started playing guitar, but I guess Al Gore hadn't invented it yet.
Big thumbs up to y'all.Comment
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