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50AE 25th December 2011 09:05 AM

Blown power tube - a short story
First of All, Merry Christmas!

As a beginner with tubes, I will share my first bad experience here.

It was the 23th December. A very kind Hi-fist promised me to give one of his valve amps to listen to for some time. While I didn't feel comfortable to take it for nothing, I offered him a set of fireworks for the NYE and we were even.

I took it. It was very nice looking, though the guy warned me it hasn't been powered on for a year. Never mind, I let it warm up from the cold for about a half an hour and then I turned the heaters on.

Everything seemed ok, but after a minute, what a surprise - the heaters of the left channel started to dim. They completely turned off. WTF, I called him and I told him what happened. "Sorry about it, try to open it and see what you can do", he said.

I did take the lid off and started to analyze the circuit and then - "Bingo!", I thought. I found a ripped soldered wire connecting the rectifier for the heaters. Then I soldered it and i thought - "This is it, it has to work now"!

I assembled it, put the valves back on, turned it on and for my surprise - the heaters weren't glowing again. I stood for about ten seconds looking at them and hoping they will turn and on and then..
Oh God, I was scared as hell, it came from the 6S4S power tube. "I screwed the amp now", I thought. "Now I have to buy him new tubes, Merry Christmas indeed"

I turned it off and I calmed down. I opened it up again to see that the same solder was ripped off again. Then I started thinking - "Something must be heating to dissolder the joint, bust what and why? Yeah, the diodes must be heating! If they do, then there must be a short circuit somewhere!"

But it wasn't from the tubes! Their heaters were ok. I followed the heater circuit completely and I found an electrolytic cap that was grounding the heaters with resistors in series.
I did measure the cap and guess what - it was shorting.

Now I don't really know what happened to that tube, but the parts inside look like moved and asymmetrical. There is something ringing inside when shaking the valve itself. I think it's screwed. I called the guy and he said he was sorry, I promised him I will buy a new pair.

Lessons learned
-Power tubes (and maybe preamp) can be screwed from a not glowing heater
-Always check old but important electrolytics.

KatieandDad 25th December 2011 10:17 AM

A valve with no heater supply simply wont emit electrons unless very high voltages are applied.

There are of course cold cathode valves but they don't appear very often in audio circles to my knowledge.

It is possible that something in the valve has physically failed causing a short between the heater and some other part at higher voltage.

A common practice was to bias the heaters at some voltage above 0V. If this circuitry has failed then the heater may become an active part of the valve.

male42 25th December 2011 11:34 AM

removed after a short circuit, rectifier diodes, check for short circuit or a job. then find pinout for the lamp, and verify that all stresses: heat, grid and anode. if everything is normal. lamp during measurement remove. if all the voltage of approx. you can put the lamp in place. and listen to music.

M Gregg 25th December 2011 05:35 PM

Just for interest,

If the heaters fail on a tube amp:

The HT (B+) will stay up on the PSU caps if bleeders are not fitted.

The B+ will be higher with no load and the PSU caps could blow with over voltage..

The power tubes " could" flash over if the B+ is left at a high level.

So number one isolate and test all PSU supplies before you touch anything...

Disconnect the B+ AC supply to the rectifier and insulate it.

Then get the heaters going<<<remember that heater supplies can be up at 70V above ground so take care!

Only when the heaters are working reconnect the Dc B+ and run the amp..

Never run the amp with no load you may stuff the output Tx..

All just for fun :)

M. Gregg

Conrad Hoffman 25th December 2011 07:00 PM

Shorted caps are pretty common on equipment that hasn't been powered up in a while. The oxide film thins and the cap can't support the operating voltage without either heating up and failing, or just shorting out. Thus the advice to power up slowly using a Variac if the equipment has been sitting around. The oxide film rebuilds as the voltage is increased, but by going very slowly you don't get the excess current and internal heating in the cap, and the applied voltage doesn't get ahead of the caps ability to withstand it. This is a much larger issue with HV caps than LV.

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