Diagnosing a toasted tube

  • Thread starter Thread starter espskully
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espskully

espskully

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I have a Mesa Rectoverb head. No sound from the cab but a slight weird tube hiss that also comes from the head - I can't tell which tube. I pulled them all, inspected and there was no obvious blown tube.

Power tube? Pre amp tube?

I probably need to replace them all anyway.
 
If you are not getting sound then it's most likely one of the power tubes. Generally when preamp tubes go bad they get noisy or microphonic. You won't be able to tell a bad tube just by looking at it unless it's red plating or has a bad vacuum.
 
If there's no sound I have to think it's more than just a tube.
 
ocnor said:
I believe in a push pull circuit if one of the power tubes goes bad then you get no sound.
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nope .... you'll get sound .... it'll be really distorted because you're only getting half the sine wave. But you will get sound.
 
Lt. Bob said:
nope .... you'll get sound .... it'll be really distorted because you're only getting half the sine wave. But you will get sound.
Click to expand...

So then it will sound like one of those little 5 watt class A amps? :D
 
Rectifier tube?

"A bad rectifier will usually show itself in one of
two ways. (1) power light comes on, but NO sound at all comes from the amp"...

Scuze my ignance, but which one is the rectifier tube? There's the power tubes, pre amp tubes, and ?
 
It's rare (or unheard of?) to see a rectifier tube in a newer amp (newer meaning 1970s+) - if you tell us the model of each tube, we can prolly figure out which one, if any, would be a rectifier. Manufacturers started using solid state rectifiers long ago, mostly for cost/durability reasons. Most, but not all of my tube amps have solid state rectifiers. Some old ones (50's, 60's) have tubes. I just realized I don't know a thing about Mesas in this regard.

The rectifier tube (or solid state rectifier) is changing the current from AC to DC (I think), so yeah, if it blows, there's no sound or anything. Usually, the power light is running on the AC current prior to conversion, so it'll still light up.

You said the *head* was hissing? That'd freak me out.
 
my mark V has a choice between solid state rectification or tube depending on the power setting' It won't allow tubed for 90 watts but for 45 or 10 you cabn choose tube. I pretty much prefer the solid state rectifier' sound but the tube IS there.
So that Rectoverb could very well have a rectifier tube.
 
Isn't tube rectification part of the name game on Mesa Rectifier amps?

At any rate - call Mesa. They'll tell you what's likely going on.


lou
 
Sky Blue Lou said:
Isn't tube rectification part of the name game on Mesa Rectifier amps?

At any rate - call Mesa. They'll tell you what's likely going on.


lou
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yep ..... they're REALLY good about calling you back in a day or two.
 
Apparently the rectoverb doesn't have a rectifier tube.

I'll call them up. Thanks guys.
 
Either you blew your cabinet, or you have a bad 12AX7. I'd almost bet my allowance on it.
 
espskully said:
... a slight weird tube hiss that also comes from the head - I can't tell which tube...
Click to expand...

An old auto mechanic trick is to use a short length of hose as a stethoscope.

Find some hose, car heater hose, garden hose... a piece maybe 18" long, put one end to your ear and the other to the individual tubes. I've never tried it on tubes but have been able to tell if it's my alternator vs my water pump bearing on my car, stuff like that.

I read that you can use gel type crazy glue to seal up a tube in a pinch.
 
dintymoore said:
An old auto mechanic trick is to use a short length of hose as a stethoscope.

Find some hose, car heater hose, garden hose... a piece maybe 18" long, put one end to your ear and the other to the individual tubes. I've never tried it on tubes but have been able to tell if it's my alternator vs my water pump bearing on my car, stuff like that.

I read that you can use gel type crazy glue to seal up a tube in a pinch.
Click to expand...

Seal up what on a tube? If a tube has a leak to the evacuated envelope, it's already too late. Tube dead.
 
I know this is stating the obvious...but having at least one spare set of tubes is a worthwile investment...for testing and just for spares when one goes.
Saves a lot of "What do yo think it could it be????" time. :)
If the spares don't help...you know right away it's something deeper.

For lots of amps and tubes on hand, a tube tester is also not bad to have.
 
antichef said:
It's rare (or unheard of?) to see a rectifier tube in a newer amp (newer meaning 1970s+)....
Click to expand...

I don't think that's so much a case of older VS newer amps.
Lots of current manufacture amps are made with rectifier tubes.
Of my amps made in the last 10 years...4 have rectifiers. A couple are modeled after vintage circuits, but the others are modern designs.
It's mostly the feel that the amp designer is going for...rather than old designs VS new.
 
ggunn said:
Seal up what on a tube? If a tube has a leak to the evacuated envelope, it's already too late. Tube dead.
Click to expand...

I've never tried it and thought the same thing. I don't know if there's any times that has worked, just read that it did. I read that the Apollo missions never went to the Moon too so I don't believe everything I read.
 
Normally, if you can hear the tubes rattling when you play guitar and there is nothing coming out of the speakers, it is a:

1. bad speaker cable

2. bad speaker cabinet

3. bad output transformer

Make sure everything is plugged in. If the amp has an impedance selector switch, make sure it is not in some sort of middle position.

If the tubes are rattleing when you play the guitar, they are working. Something farther down the line is broken.
 
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