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Bass 400+ all tubes red plating

Posted: Fri Oct 19, 2018 8:26 am
by tizianod
Hi, I'm trying to repair a bass 400+ but i'm having trouble finding the issue. A friend of mine asked me to inspect his amp because last time he tried to play on it, he noticed only one row of output tubes was working. I gave the amp a good cleaning and inspection and fired it up on the variac. The fan and the pilot light starts to work. So far so good... The heater wires for the preamp tubes start to glow. At this point I'm thinking there's nothing wrong with the amp... but then... Nothing from the power tubes. I have 12 dead power tubes and i'm starting to get a dull popping noise from the speaker. The noises start to get louder and louder after a couple of seconds.

After a thorough inspection against the schematic I noticed that resistors R311 & R312 are missing. So in the past these got bad and somebody decided to just leave them out of the circuit... I soldered new resistors into place (100ohm/2watt) and when I start up the amp, still in standby mode, everything seems to work just fine. The heater wires of the power tubes start to glow. But when I switch the amp off standby mode, I hear a loud pop and the amp starts to buzz. Immediately followed by all power tubes red plating. I tried to measure the voltages on pin 3 and 5 but I get zero results.

I noticed someone has been inside the amp before. They already changed a filter cap and screen grid resistor in the past. So now I'm thinking what could be causing this.. It's a bias issue so what should I look for next... Screen grid resistors? Grid leak resistors? Grid stopper resistors? Diode? Bad filter caps? The resistors seem to look just fine. I'll give everything another inspection try to see if I missed something... But I would love to hear the opinions from you guys, maybe I'm missing something?

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Re: Bass 400+ all tubes red plating

Posted: Thu Oct 25, 2018 12:15 pm
by jediguitarist
I know nothing about Boogie repair, but I watched this guys vids.

https://www.youtube.com/watch?v=yDoI8rgd3K8

He mentioned red plating on a Mesa Boogie DC3.
Maybe it can help you solve the issue with the 400+