Mark IV volume swelling/gain swelling

[guitarzan1143]

Playing my mark IV, i thought i was just hearing things ( turning my head, sitting in a different spot of the room, etc), i thought the amp was fluctuating in volume as i was playing it. Well i captured it on recording with an SM57 and now am certain of it. it just has a slight loss in volume/gain for a few seconds, then i jumps back up. i think it does it on all channels. I just put new mesa power tubes in, preamp tubes are kind of old. could preamp tubes be causing this, or is it time to get the caps and things changed? its a mark IVa... so 1991ish.

 

[aaronlyon]

Mine, too. I posted earlier about this. Now I have confirmed it happens on all three channels. Always when I first power up (I always let it Standby for a minute before playing). Usually goes away after a few minutes. Sometimes it doesn't happen at all, a couple times it was really bad--the definition of an intermittent bug! It's definitely not outside the amp--if I unplug the guitar from the input jack and turn the master up, I can hear the normal hiss of the amp rising and falling when this problem is happening.

 

[guitarzan1143]

I switched the power tubes out and it works fine now. guess there was a bias issue?

 

[geddyentwistle]

good observation and cure


i just got a used mark iv, love it, but was thinking the same about volume swells....

and it still has original tubes so probably needs a tune up


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