Mesa Mk4 lack of gain, poor sound

[jestaudio]

Picked this one up for a reasonable price knowing it needed a full service amongst other things, alarm bell started ringing when the footswitch exhibited some funny behaviour , Leds, not fully illuminating , functions failing to switch, the amp sounded gutless, sterile, real lack of gain on drive channels,
Swapped the preamp tubes out for known good ones from my stash, then noticed mismatched 6L6 , 3 mesa one Ruby, swapped in a known good tube to no avail, still sounded crap,
Ended up popping for new tubes in and viola a fully functioning footboard and buckets of gain, why am I posting all this blather about a solved problem, mainly because I don't really get how a duff set of power tubes could cause such havoc

[silvertonebetty]

Tubes are going

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[xtian]

Wild guess: Mesa has no bias adjustment in their amps. So if you had badly mismatched power tubes, maybe one or more was drawing too much current, dropping voltages system-wide, and making the LEDs and switching system misbehave.

[Stevem]

I don't understand your use of terms as how it relates to amp functions.
Gain does not come from output tubes as they have no where near the gain of preamp tubes and will have way more of a effect on the amps volume output and punch.

[silvertonebetty]

Picked this one up for a reasonable price knowing it needed a full service amongst other things, alarm bell started ringing when the footswitch exhibited some funny behaviour , Leds, not fully illuminating , functions failing to switch, the amp sounded gutless, sterile, real lack of gain on drive channels,
Swapped the preamp tubes out for known good ones from my stash, then noticed mismatched 6L6 , 3 mesa one Ruby, swapped in a known good tube to no avail, still sounded crap,
Ended up popping for new tubes in and viola a fully functioning footboard and buckets of gain, why am I posting all this blather about a solved problem, mainly because I don't really get how a duff set of power tubes could cause such havoc

After reading everything i say you bought a terd . Jk take it to a tech. If its been giged it could need a recap job. Its the tubes that cost$$. My mesa is getting recspped and tubed

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[jestaudio]

Wild guess: Mesa has no bias adjustment in their amps. So if you had badly mismatched power tubes, maybe one or more was drawing too much current, dropping voltages system-wide, and making the LEDs and switching system misbehave.

Yeah that makes sense, certainly banging in a matched quad solved all the issues,

[jestaudio]

Picked this one up for a reasonable price knowing it needed a full service amongst other things, alarm bell started ringing when the footswitch exhibited some funny behaviour , Leds, not fully illuminating , functions failing to switch, the amp sounded gutless, sterile, real lack of gain on drive channels,
Swapped the preamp tubes out for known good ones from my stash, then noticed mismatched 6L6 , 3 mesa one Ruby, swapped in a known good tube to no avail, still sounded crap,
Ended up popping for new tubes in and viola a fully functioning footboard and buckets of gain, why am I posting all this blather about a solved problem, mainly because I don't really get how a duff set of power tubes could cause such havoc

After reading everything i say you bought a terd . Jk take it to a tech. If its been giged it could need a recap job. Its the tubes that cost$$. My mesa is getting recspped and tubed

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New power tubes solved the issue, the question I asked was the root cause, as another poster has already alluded it stands to reason duff tubes were drawing the power down internally, certainly with new power tubes its night and day now and very much not a turd

[jestaudio]

I don't understand your use of terms as how it relates to amp functions.
Gain does not come from output tubes as they have no where near the gain of preamp tubes and will have way more of a effect on the amps volume output and punch.

I,m fully aware that overall gain has little to do with the power section unless driven hard, however the fact remains new power tubes solved the issue , I suspect Xtian has hit the nail on the head with his not so wild guess, I may even get the old quad tested and see exactly how far apart they were

[Jered]

I had a similar problem with a Bogner/Line 6 Alchemist. As soon as the tubes would start drifting it would mess with the voltage/current to the relays on the board. The channel switching, clean, high gain. and boost channels would stop working with the assigned footswitches and just randomly switch on thier own.
The power tubes had to be biased at 350mVdc or this would occur. Sooooo frustrating! And did not make it easy to shop for power tubes. Unless of course you were buying them from Bogner/Line 6.
Decent sounding amp when it worked, I have the 2 X 12 combo. Currently in the garage collecting dust.

[jestaudio]

Got a friend to pop the old tubes on a test rig, good call from Xtian, one tube absolutely screwed and pulling far more than it should, got it booked in for a quick once over to make sure there's no permanent damage