Blown (?) 2A3

[Jameson]

I turned on my Stereomour today and heard a weird hissing out of one channel. Walked over to the amp and saw a purple arcing flickering around the components inside the Electroharmonix 2A3 tube. Pulled it, replaced with the original Sovteks the amp shipped with. All is good. A couple questions (I think I know the answer but it's been a while and want to double-check):

• Does that sort of failure seem like it is a symptom of a larger issue with the amp, which has been happily humming along for a couple years?
• The amp sounds ok. Anything I need to check voltage- and resistance-wise to make sure the failing tube didn't take anything else out with it?
• Is it necessary to find a balanced pair of 2A3 replacements, or is that what the trim pot is for?

 

[Paul Birkeland]

Is the silver flashing still silver in that tube, or has it turned white? 

If that kind of problem happened and didn't blow the fuse, I don't think I'd worry too much about the internals of the amp.  Sometimes the cathode bypass cap on the 2A3 can take some abuse from that kind of fault, but it will take some time for that to happen. 

There is no need to buy a matched pair of 2A3s, as each socket has separate biasing.

 

[Jameson]

Is the silver flashing still silver in that tube, or has it turned white? 

Still looks silver to me. Looks identical to the other tube (hence the tape).

If that kind of problem happened and didn't blow the fuse, I don't think I'd worry too much about the internals of the amp.  Sometimes the cathode bypass cap on the 2A3 can take some abuse from that kind of fault, but it will take some time for that to happen. 

Awesome, great news. No fuses blown.

There is no need to buy a matched pair of 2A3s, as each socket has separate biasing.

Ah right, I guess I need to re-bias now that I've swapped in new tubes. I knew there was something more to it.

Thanks as always, Paul.

 

[Doc B]

Just being perhaps overly picayune here. For me "blown" implies a blown (open) filament - the tube would no longer glow. This arcing I would more likely characterize as shorted, that is, a short between grid and filament or grid and plate.

 

[Jameson]

Just being perhaps overly picayune here. For me "blown" implies a blown (open) filament - the tube would no longer glow. This arcing I would more likely characterize as shorted, that is, a short between grid and filament or grid and plate.

It's a good distinction, thanks for making it. I'd put blown with a question mark after it because I really didn't know how else to describe it.

Is a shorted tube salvageable in any way or should I keep it to smash on the ground as a distraction in the event I'm confronted by a roving band of street toughs?

 

[hmbscott]

Internal arcing typically is due to one of two causes. Over-voltage applied to the tube by the amp circuity, which seems implausible in this case. Or loss of voltage hold-off capability internal to the tube, which implies the tube is no longer fit for duty. This later case seems the most likely.

BTW, the inability to hold off voltage can be due to several things including, some physical change to the electrodes, like breakage and movement closer together, a coating pealing and curling (moving closer to another electrode, something generating particles internally that charge up and then fly around due to electrostatic attraction, etc. or a partial loss of vacuum (increase in pressure). The increased gas pressure inside the tube ionizes, creates plasma, becomes conductive and then supports arcing. The increase in gas pressure can be due to a slow vacuum leak, or can be caused by heating a surface inside the tube that then releases gas. Both sources of gas are counteracted by the silver getter coating who's job it is to chemically absorb excess gas thus preserving vacuum. Eventually the getter's capacity to absorb gas will be exhausted and the gas pressure rises uncontrollably, ending the life of the tube in a manner similar to yours.

I work in the tube industry making microwave tubes, hence my familarity with this type of thing.

 

[Doc B]

Jameson says the getter looks intact so it's not gassy. More likely a short from filament sag due to heat cycle fatigue or something else coming loose or getting out of shape with age as you suggest.