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Musicmaster bass amp loss of power

7/6/2005 12:26 AM
Musicmaster bass amp loss of power
Hi there, I recently rebuilt the preamp section of my EL90 Powered Musicmaster bass amp. It was sounding great untill just the other day when I got a drastic drop in vloume and a shrillness it the tone is up ata all. I took some voltages in the amp and some are a little high like ~314V on the plate of the 6AQ5A. I have 152 on V1A and 281 on V1B. The amp seem to be louder when ithe power is just turned off and it is discharging the caps. Any idea as to what the problem is?
7/6/2005 1:00 AM

also, all (3) of the tubes are glowing pretty dim.
7/15/2005 7:16 AM
Old amp troubleshooting rule: whenever amp worked fine for years up until a mod, and then worked only a little while after the mod, the problem has something to do with the mod, most likely. Since I don't know what exactly you did, but I can assume you rewired the preamp to more of a normal Fender style than stock, I would start the mod from the begining, re-checking each component for spec and redo each solder joint checking for cold joints, stray strands, melted tube lugs, over heated wires etc.  
There are some other peculiar MM bass issues that may be a problem also. They involve the interstage transformer, the rectifier diodes and the power supply cap grounds. The leads on the transformer are easily damaged. I always install two 5 way solder lugs between the power transformer and the tag board and move the filter caps off the board and make new grounds for them.These share grounds with other parts of the circuit and may be cold after the mods you did.  
If you have access to an audio signal generator, and a second amp plus a proper high voltage probe, you could trace the signal from stage to stage and see where it dies. You can also inject audio into the circuit at various places to see where it starts working well.  
These tests require a complete knowledge of high voltage safety precautions, schematics,layouts, tube pinouts etc. You must obtain a probe that completly isolates the high voltage dc from the desired audio ac.It is very dangerous but it works great.
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