Mesa Single Rec Issue

[Steve Conner]

It's a shame to hear you're having such a nightmare with this thing, jak0lantern I don't know what's wrong with it, but there surely is something wrong with it. Tube amps shouldn't be so unreliable, they should generally just work, and the power tubes should last for a good few hundred hours of playing even if you dime it.

Either bad biasing (causing the tubes to run way too hot) or parasitic oscillation seem like reasonable explanations. Do you see the plates (the large grey metal piece inside the power tubes) glow red hot? Look in a darkened room to see if you can see any redness on the grey metal. This is a sure sign that the tube is being overrun with too much current. Don't confuse it with the heater in the middle of the tube, which is meant to glow.

[jak0lantern01]

I assume the plate is the bottom most part of the tube. I have not noticed any glowing, but there was substantial heat and the sound went to @!#$. Assuming there is too much current, what causes this? I'm beginning to thing that this amp had problems from day one, I've been through way too many tubes over the last two and a half years. I'm getting it into the shop tomorrow (damn, it's starting to sound and feel like my car).

[Enzo]

Several thoughts.

The plate is the metal structure in the center of the tube. More specifically the outside of that structure. if the gray metal looks like it is getting red hot in some spot, that is the overheating we are concerned with. The red spot will usually be right half way up the side of the thing. Think of it as aiming at the rear of the label from inside the tube.

WHy can you say it can't be the tubes? It most certainly can. All it takes is one bad tube, and that can kill the bias to the one next to it. Your amp has worked for a couple months you say. Tubes can come out of the box new and be bad. Or they can fail in two months. Or they can work for 30 years.

I forget, did you mention the brand tube?

And when they fail, they either stop working, they blow fuses, or they do what you describe your amp to be doing. Try a set of tubes you KNOW to be good - even worn ones.

Tubes burn up because they have failed internally, or because the amp is making them conduct too much current. A common reason for this would be a loss of bias. AN intermittant connection from the bias supply to one or more of the tubes would allow them to seem right at one time and not be right another time. Your tech should check current flow through each and every power tube in the amp. He should check the bias voltage to make sure it is there, clean, and up to level. He should also check all the screens for voltage. Screen resistors are often damaged when tubes fail, and a new tube stuffed into that socket won't be working right after that. SO new tubes alone are not always all that is needed.

And the bias voltage itself might not be interrupted, but if the filter cap for the bias is loose, the loss of filtration has the net effect of lowering the bias voltage.

I don't know whether I am correct or not, but I reserve the term parasitic for little temporary oscillations that ride - like parasites - the peaks of waveforms. Little (short) chirps on peaks mostly. I don't call steady oscillation parasitic. In those cases I jusy say the amp is oscillating, or that is has become unstable. In other words, All unwanted oscillations are not termed parasitic in my shop, just the ones that ride the signal waveform.

So, I don't think parasitics would do this to your amp, but steady oscillation could.

[Andy]

...it had a good hum even on standby...

this could be a sign of a bad output transformer. if one half of the primary winding is shorted to ground, and the standby switch is between the plate and screen nodes (no idea if it is or not) you could have some serious hum on standby, and it would probably sound like crap most of the time. not really sure if it would make it go through tubes faster though.

[jak0lantern01]

Perhaps you didn't reveal the entire thread. This has been an ongoing issue. Oh, did you catch the part about the odor coming from the amp and being almost too hot to hold your hand against? I've been going through way too many tubes with this amp, and I baby it. What are the chances that I've purchased so many bad tubes?

[Steve Conner]

Well, some tube amps do smell and get pretty hot. "Tubes down" combos are notorious for running hot, because the hot air from the tubes rises up into the chassis. I don't own an AC30, but I've heard from reliable sources that you can fry eggs on top of one As for odours, I have an amp with an unfinished wood cabinet that smells strongly of pine after it's been on for a while. I suppose I should fix it, but I like the smell

Fan cooling helps all of these things a great deal, but Mesa are a reputable company that should know how to manage heat in their amps. So it should work fine the way it came from the factory. If I were you (and assuming you bought it new) I would try calling Mesa and complaining like hell until they take it back to the factory and examine it themselves. These amps aren't exactly cheap, so surely you deserve to be treated nicely for buying one...

What is your line voltage BTW? If it was seriously high, like 130+, I can see how the amp might run hot. But again a good amp designer should check that his product works properly over the full range of line voltages.

[jak0lantern01]

I'm not sure about the line voltage, but 2 months ago the thing had a catastrophic breakdown. Also, I wish my amp smelled like pine. It smells more like when something electrical is cooking. Damn Mesa Boogie, they are closed for the holidays. I'm dropping it off today at a different shop to look at it.

[jak0lantern01]

OK, I just dropped off the amp with a new shop. Guy was friendly and seemed knowledgeable, so it was a good start. He basically ran down a list of possibilties with me, and I should have it back within a week. I'll update when all is complete.

[Mars Amp Repair]

OK,
Maybe parasitic is not the technically correct term (I don't really know)... the point not being lost, an undesired-destructive oscillation could be a cause for eating up tubes....glen