Common/Frequent Symptoms of Blown PreAmp Tube?

[Diminished Triad]

After a few minutes of playing my marshall my guitar sound begins to fade and is eventually replaced by a hum. EAch of the 4 channels goes down. The fuse does not blow. Each of the power tubes seem fine and they all glow equally and a sharp orange color. I can't clearly see the pre-amp tubes but my feeling is the power tubes are fine and it's my preamp tubes. Does this sound likely or are these symptoms of something else? Sound like a V1 problem?

Thanks for any help you can share!

 

[Diminished Triad]

Re: Common/Frequent Symptoms of Blown PreAmp Tube?

After a few minutes of playing my marshall my guitar sound begins to fade and is eventually replaced by a hum. EAch of the 4 channels goes down. The fuse does not blow. Each of the power tubes seem fine and they all glow equally and a sharp orange color. I can't clearly see the pre-amp tubes but my feeling is the power tubes are fine and it's my preamp tubes. Does this sound likely or are these symptoms of something else? Sound like a V1 problem?

Thanks for any help you can share!

Second round of checks tonight and we isolated problem to only when amp warms up. After it warms up medium loud hum begins and sometimes after turning amp off there is a delay and hum goes louder before going away.
Thoughts?

 

[glassman]

Re: Common/Frequent Symptoms of Blown PreAmp Tube?

The first thing to do is to isolate where the problem is at...remove all of your preamp tubes, leaving only the power tubes, and power up. If it still hums, your problem is in the power section, which could be a number of things. If not, replace the preamp tubes one at a time until you find the noise. once you recreate the noise, replace that tube with a known good one. If that fixes it, it's that tube, if not, it's most like in the amp.

 

[Diminished Triad]

Re: Common/Frequent Symptoms of Blown PreAmp Tube?

The first thing to do is to isolate where the problem is at...remove all of your preamp tubes, leaving only the power tubes, and power up. If it still hums, your problem is in the power section, which could be a number of things. If not, replace the preamp tubes one at a time until you find the noise. once you recreate the noise, replace that tube with a known good one. If that fixes it, it's that tube, if not, it's most like in the amp.

Thanks! Kind of surprised a Marshall went out this early but so did my Fender Super-Sonic a few weeks ago, sounds like very similar problem. Used at a couple of beach gigs and also after 2 weeks on a ship at sea on its way here maybe explains the coincidence. Look forward to having both amps back with us and jamming. Really appreciate the help my friend!

 

[Diminished Triad]

Re: Common/Frequent Symptoms of Blown PreAmp Tube?

Changed out all tubes.............amp is perfect. Not sure why, but came down to the very last tube. We thought it was for reverb but still don't know for sure. Will look it up one day. Amp runs great again. The Marshal JVM 410c is amazing.