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Old 20th November 2004, 08:21 PM   #1
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Default EL34 plate glowing?

I just rewired my ST70 and installed a Curcio premium driver board. After using it for the first time I noticed that the plate on one of EL34s is now glowing. Never saw this before. None of the other tubes are doing it.

I replaced the tubes on that side with the tubes that I got with the amp and they seem fine. The glowing tube is a new Electro Harmonix that has probably less than 15 hours of use.

Am I losing a tube or might there be something wrong with the amp? I haven't moved the tube around to see if the glow follows yet. I'll have to wait until it's dark tonight. I think the amp sounds great.

The picture shows an intense purple glow but to the naked eye the color is orange and not as bright. Might be my camera picking up UV or IR.

Click the image to open in full size.
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Old 20th November 2004, 08:51 PM   #2
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Well, you certainly blew bias SOMEWHERE... that looks like NONE!

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Old 20th November 2004, 08:57 PM   #3
SHiFTY is offline SHiFTY  New Zealand
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I think that tube is either toast or has no bias voltage on it, have you checked your bias wiring? Also are all the coupling capacitors new?

Otherwise the tube may have an internal short in it. You might want to see if you can get it replaced...
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Old 20th November 2004, 10:42 PM   #4
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If I "blew the bias somewhere," would my bias voltage indicate that? It's at the correct bias voltage even after I replaced the tubes with new ones. Only one tube has the problem.

I looked on audio asylum and a similarly described phenomenon was speculated as being mismatched tubes. The guy having the trouble moved the tubes around and the problem went away.

These were matched pairs and I tried to be careful not mixing them up when I pulled them to do the rebuild. I'll double check my wiring and make sure I didn't screw something up while troubleshooting my last problem.

The amp sounds great. No discernable sound issues. With the new driver board, it sounds a lot better than it did before.
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Old 20th November 2004, 10:55 PM   #5
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This guy is sitting, quietly listening while his EL34 is burning at 80W plate dissipation

Definition of bias: generally the voltage between grid and cathode, or cathode current. Difference is contextual. Can apply to other elements if specified ("screen bias"). So stick a DMM or VTVM in there and see what happened to it.

If bias blew period, all of them would appear so (and hopefully pop the fuse, or failing that, the rectifier or PT). If a coupling cap or grid leak failed, it would do this.

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Old 21st November 2004, 12:05 AM   #6
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If you've got a glowy tube and the bias voltage at the socket looks OK, then you either have a bad tube or a bad socket.
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Old 21st November 2004, 12:22 AM   #7
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Do you have the capability for bias adjustment for each tube or for each pair? If its each pair there is a good chance of tube mismatch in my opinion.

My VTA St70 board has bias adjustment for each tube. I had the same problem you have when I had the stock board with a set of EI tubes. When I switched the driver board I was able to adjust each tubes bias and problem is gone. If your board doesn't have that capability you can modify it.
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Old 21st November 2004, 12:47 AM   #8
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Unfortunately, I can only set bias per side.

Sockets are new ceramic types.

V2 pins 1/8 and V3 1/8 are all tied together. The bias resistor comes off V3 to ground. Bias voltage measurement is off of V2. So I measure through both tubes to get to ground. V2 is the hot tube.

Any hints how I could I measure each tubes' current draw separately and see how balanced it is?

I replaced the EH tubes with a pair of Natl. Elec. tubes that came with the amp. I just grabbed two at random. Neither of these tubes had a glowing plate after significant runtime.

How do you mark your tubes to keep pairs together and what not? Given how hot they get, what's the best method?
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Old 21st November 2004, 02:05 AM   #9
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The Curcio boards have an option for balancing idle currents. It should be in the documentation you got from him. I assume you do have the two output ubes' cathodes tied together?

Any marking you do is best done on the tube base. I use a bit of white nail laquer.
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Old 21st November 2004, 03:32 AM   #10
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I use black magic marker on the top of the tube. It can be taken off when the tube is cool with some WD40 or laquer thinner.
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