replace which tube and why?

[architele]

I'm looking for some schoolin' here....

I have a spare mesa studio 22 in my practice room. It was accidently left on for about a week and a half with no guitar plugged in, and standby NOT on.

It now is not putting out a signal. I think one of the tubes is shot.

THere is speaker hiss and if I shake it there is reverb "crash" as normal. If I plug my tele in and crank everything to 10, I can here a barely audible scratchy signal.

which tube might cause this and why?

I know I can figure it out by replacing them one by one, but I'm curious what might have happened and how it might have happened

thanks

[Joe-Bob]

The one that doesn't glow?

[tazzboy]

Replace them all.

[PRNDL]

It could be any of the tubes, or the power supply circuit.

The easiest test is to tap on each tube and listen. Bad ones will usually make some sound.

Usually an amp tech begins by measuring all of the voltages, including power section and tubes.

[pottedmeat42]

Originally Posted by architele

I'm looking for some schoolin' here....

Not trying to be a smarta**, but are you certain that the amp worked correctly up until the time that it was left on? That may help the analysis. I don't know why leaving it on should kill anything. The filaments are functionally the same as a lightbulb, right? Why should that wreck the tubes? I'm pretty sure that back in the day, there were tube-powered industrial PA systems (subways, freight yard, etc.) that stayed on 24x7 without incident. I don't fully accept the premise that leaving it on would kill it. Just thinking out loud. I do hope that it turns out to be something minor, like a tube. =jason

[skeksis]

first, eliminate silly stuff, so try a different cable and a different guitar to make sure that the amp is really the problem.

next in order of suspicion would be the preamp tubes.

check page 5 of this document:
http://www.mesaboogie.com/manuals/St...022%20Plus.pdf

does either the lead or rhythm channels work, or neither? those are the sorts of diagnostics that should help you pinpoint which tube to change. also, you could just as easily get a spare 12ax7 and swap till the problem is fixed.

[franchelB]

Originally Posted by tazzboy

Replace them all.

You're better off replacing them all...
better safe than sorry.

[architele]

simple as that. it was the preamp for the "rhythm" mode - I never even thought to hit the footswitch for the gain channel...

I am quite new to tube amps, so I appreciate your feedback.

[BottyGuy]

Mesa's have a lot of tubes (I have a Lonestar Special), I wouldn't replace them all unless you have to. You need to follow skeksis's advice and read the manual, it is pretty helpful. You can also get the schematic at:

http://www.schematicheaven.com/boogi...ogie_22cal.pdf

First check out Joe-Bob's suggestion, you've left the heaters on all week so maybe one of the wires burned out (the heaters are basically cathode toasters). If one did the orange glow will be missing.

Since your reverb is working the power tubes and V4 (reverb driver and recovery) are working. Also the phase inverter V5 should be fine.

Your lead channel is the only one that uses V1, if the rhythm channel works it it probably V1.

Both channels use V2, but the rhythm uses both halves, one for its input preamp. The both channels used the other half as a preamp stage. So if only the rhythm is dead its probably V2.

If both channels are dead its either V2 or V3, and if they both glow its a crap shot, try one at a time, then both.


Dang I should type faster! you replied while I was writing!

[architele]

thanks anyway, I appreciate your description, as that was kinda what i was interested in the first place.

thanks for taking the time.