Mesa Mark IV issue

[spirit7]

Hi All,

My Mark IV's lead channel has started being funny.

There is a constant, noisy hum in the background when playing, and if I turn the master volume up past 2 it descends into a high-pitched squeal!

I replaced the V5 preamp tube (which deals with the lead channel) and it doesn't seem to have helped the problem. All the other channels are fine.

Could it be the power tubes? They haven't been replaced since 1995

-C

 

[philb]

Could be a bad contact on the preamp tube socket. Try wiggling (sp?) each preamp tube and see if it comes and goes.

 

[mrkmas]

I wouldnt think its the power tubes personally unless the issue occured on all channels, however I am definately not an expert with Mesa's. Easy way to narrow it down is to run the effects loop out into the effects loop in on one of your other amps to check if the preamp or poweramp is the issue. If you can run the lead channel past 2 without the squeal, its safe to say the issue is the mesa poweramp.

http://www.mesaboogie.com/manuals/Mark%204.pdf
pg 17 shows the tubelayout. V5 according to this is the phase inverter which would affect all channels.
All of the lead preamp tubes are V1( which would affect all channels so I doubt that is the culprit). v2,3 and 4.

V2, V3, V4 are what I would put my money on as being a bad tube.