More electronics 101 here guys. Sorry about this!

How do I know which valve is bad in a guitar combo with 5 valves??

Any help greatly appreciated.

Simple answer: You don't, unless you have the needed experience and a schematic.

Jakob E.

I we're talking about noisy (microphonic) preamp valves one d.i.y. way of troubleshooting is to tap them lightly with the rubber-end of a pencil. The cilprit MIGHT make a horrible noise when you hit it. Otherwise the obvious thing is to replace them one at a time - when the noise stops you've found the nasty one.

OK, I have an honours degree in E&EE and a schematic of my valves here.

But thanks for wasting my time anyway. fuuck

Anybody else want to help out a fellow GS who has been away from the shop floor for too long??

The kit is a totally dead Mesa Boogie guitar combo. The push pull pair are EL 84s and the rest are all 12AX7-As.

Any (actual, real) help will be greatly appreciated.

Thanks in advance,

ouch !!!


what makes you think it's a valve ?


totally dead ? meaning no power ?

si

Hi Orange,

should have clarified. The unit does have power but absolutely no ouput whatsoever.

Did a quick check at the input and output stages for dry joints and found nothing.

I then just jumped to the (perhaps incorrect and certainly naive) conclusion that I had a bad valve. This is the only bit of valve gear in the studio so thats what I tend to think when it goes wrong.

Neil,
First off forget about solder joints. Mesa-Boogie builds them like tanks.

A little more info would be helpful. Is it out on both channels? Are you getting signal from the FX loop send jack? Can you put signal into the return jack and get audio? Are the tubes lighting up? And please forgive this question but is the speaker good?

Let's start with the obvious:
1) If one of the EL84's was bad you would still get audio. Pretty rare that both EL84's went dead at the same time.

2) Again for no audio at all you would have to have a totally dead PI tube, the last one in the chain of five or a dead V1 tube, the one closest to the input jack.

3) You need to establish you have first heater voltage. pretty obvious if they are all lit up, and then next check for the hi voltage to the plates of the tubes. (Pin 1 and 6 on the 12AX7's)

If all that is correct and it still has no audio, it's time to follow the signal flow with a scope and see where it dies.

Mesa's pots typically break when you hit them with your knee, and a broken treble pot will shut the entire amp down - signal wise.

Hope that helps.
Tony

...what tony said !......


si

Thanks preben and orange for the help but extra BIG thanks to Tony. You my man are what Geekslutz is all about.

Amp now fully working again dead PI tube it was.

I am never quite as desparate as when I come to Geekslutz and advice of this calibre is always very gratefully received.

Thanks again!
___
Neil.

P.S. Gyraf you can come out of the corner now, but in future try not to waste my time and your own!

Hi Neil,

I'm glad you survived the error-fixing of the amplifier.

From the info (and lack thereof) in your initial post, I mis-took you for a newbie - and stuff like tube power amplifiers are seriously dangerous beasts. I'd never recommend anyone without basic knowledge of electronics to try fixing or even error-tracing equipment with that high voltages.

I'm sure that you'd agree..

Jakob E.

OK, I have an honours degree in E&EE and a schematic of my valves here.

But thanks for wasting my time anyway.

why dont you take that ee degree out of your ass and show some respect. jackass.

you have a degree and you cant find a bad tube? 5 12ax7s and you dont have some old spares? you came off .... never mind.

Looking back I can definitely see how my first post came across as though I was a bit clueless (which to be fair I kind of was...).

No hard feelings.

This thread has been a bit odd...many words, little useful info.

Basic EE degrees typically provide very little in the way of useful info when dealing with the Real World.

But, beyond formal schooling, some of us "perfected" (NEVER!!) our trade via the University of SoHN (School of Hard Knocks...lol), and we attempt to answer carefully worded questions with potential solutions based upon our experience...

Bri

All is well. Pretty sure Gyraf was exaggerating to scare off a guy who was gonna turn himself into a charcoal briquette! LOL

I love happy endings!

if the amp has an effect loop, you could test the pre amp and power amp seperately. run the guitar into the effect return and plug an unconnected cable into the send (just to open the effect loop). You should here a clean tone, not much volume....if that works, your problem is with the pre-amp.....now plug the guitar into the input and plug the send into something line level with a VU meter, or a console...if there is sound or light, the problem is the power amp.....if you get neither, the problem may be the rectifier tube (usually blows the 3 or 4 amp fuse)..or if both work (and this can happen), it's the jacks in the effect loop not closing when disconnected. If the pre-amp is down, "borrow" the reverb driver tube (if applicable) and sub it for the other valves (the amp will run fine without the reverb tube for now and that gives an extra tube to play with).......if none of this helps, you may want someone with a scope.

Remember that there are potentially dangerous 500ish+ V DC hazards inside even once powered off so don't fool around.

These days, being an EE may be helpful if you want to tweak GuitarRig but tube's aren't anywhere in the curriculum ....knowing where to go to find out is though.

Andy

it wasn't your first post that was the problem, it was your second post. many of us have degrees in lots of things... we try to be humble when asking for help. oh, and not condescending as in your "PS:" in your 4th post.