SE EL34 design considerations

[panhead]

I've built a few SE EL34 amps, but one of them has come back to me a couple of times with power issues. It has arced a JJ EL34 plate to screen, and most recently wiped out the power transformer. Wiring is correct. The situation got me looking more in depth into some design parameters, such as the relationship between plate and screen voltages with different tubes (and making them live longer) - lowering the screen voltage in the power supply vs. at the screen resistor, etc. Also what effect grounding the supressor grid has on plate and screen current. I'd appreciate hearing others' views or experiences.
Thanks

[DummyLoad]

Richard has a similar issue with a 6L6GC pattern amp. He has smoked 4 OTs. The original, a generic replacement(forget the brand) and two edcor 15W OTs.  He's running over 400V B+. I'm betting that's the issue.

--DL

[tubeswell]

So, what plate and screen voltages are you running with the EL34?

[panhead]

Measured about 385v on the plate and 382v on the screen at idle. I have a choke between the screen node and the B+ node. I'm thinking of adding a 1k 5w resistor in series with the choke. I also have a 1k 5w screen resistor, which I'm going to try replacing with a 2200 ohm resistor, to hear what kind of effect this has (compression?) and see what it does for tube life.

[DummyLoad]

Measured about 385v on the plate and 382v on the screen at idle. I have a choke between the screen node and the B+ node. I'm thinking of adding a 1k 5w resistor in series with the choke. I also have a 1k 5w screen resistor, which I'm going to try replacing with a 2200 ohm resistor, to hear what kind of effect this has (compression?) and see what it does for tube life.

cathode current? cathode bias? fixed bias?

[tubeswell]

The typical published value for screen current in an EL34 is about 1/7th of the plate current. Assuming you are cathode-biased and running the tube in Class A into an optimal load resistance, which in your case would be (385V/(25W/385V) = 5k9, for SE operation, you want to set the stage up so that the screen current is idling no more than 1/7th of what the plate current is idling at. Therefore if your CLC filter and g2 resistor is not getting the screen current to idle at 1/7th, or less, of the plate current, you need to take steps to make sure that it will.

[jojokeo]

On one of mine I upgraded my PT almost exactly a year ago to a 325-0-325 PT from a 260v one. It is power scaled and I have had no issues running these voltages/values:
Plate/Node A = 397Vdc
Screen/Node B = 394Vdc
Preamp/Node C = 285Vdc

Using a 400r/10w cathode resistor measuring an actual 394r:
Plate-Cathode = 348Vdc
Cathode-Ground = 27.1
Pdiss = 23.94w
Screen resistor = 1k/5w

Don't know if this helps or not? I went through old notes where the original voltages may have been running a KT77, then the lower measured voltages using a Mullard EL34. Then I have additional voltages using a 6L6GC and a JJ EL34. Each tube's voltages measure a little differently.