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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
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.
Tim
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.
Tim
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.
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.
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?
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?
I don't know what's going on. Threw the amp back on the bench (still with the NE tubes) and checked the wiring. All connections good, nothing loose. Turned it on and check bias and it measured 1.5V on the V2/V3 side! Quickly adjusted it back to 1V. Did I have the voltmeter connected to the wrong channel last time I measured it? Who knows.
Now with the EH tubes back in, former plate glower in the back socket, and bias set at 1V, all looks good after an hour of operation.
Also moved the 6922s around on the driver board until the plate resistors measured closer to each other.
The driver board has no adjustments. I know that the cheaper upgrade driver board had all sorts of optional adjustments. Maybe because this board is regulated?
Now with the EH tubes back in, former plate glower in the back socket, and bias set at 1V, all looks good after an hour of operation.
Also moved the 6922s around on the driver board until the plate resistors measured closer to each other.
The driver board has no adjustments. I know that the cheaper upgrade driver board had all sorts of optional adjustments. Maybe because this board is regulated?
Black Sharpie works great on the glass and wipes off with naptha (lighter fluid).
I once had an RFT EL34 do that. Fortunately the imbalance it caused created some hum, so I realised that something was wrong. Initial checks showed no circuit problems so I swapped valves in that channel, hoping to see whether the problem followed the valve or not. It hasn't done it again, so I am none the wiser! I suspect a socket fault.
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