Hi, gang - I had something happen to me today that I can't readily explain. Does anyone else have any insight they want to share?
I put a brand new matched quad of EH EL34s in my 100W Marshall today. I usually run it as a two-holer, but I wanted to make sure all 4 tubes were OK. After a good robust warm up, I hit the standby switch and slowly brought the bias up while watching plate current (voltmeter shunt method). When I got to 15W/tube (30-odd mA @ 475V), I stopped and let them sit for a while. When I got back to the amp 15 min later two of them, both on one "side" were bright red! I shut down, swapped 1 & 2 with 3 & 4, and the problem followed the tubes. Damn. 2 dead tubes right outta the box. Oh well, it could have been my amp! I shut back down.
I got busy doing something else for the rest of the day. Tonight, I turned the amp back on because I couldn't remember which side I left the bad tubes on. None of them glowed. I checked the plate current again - still 60ish per side @ 475V. I played it for a bit - it sounds OK. Left it running for about an hour - still OK. I can't measure each tubes current individually (gotta install those 1 ohm resistors, someday), but as I type this, all still appears well. The 4 screen voltages are within 2 volts of each other.
What the hell just happened here???
W
EL 34s red plating
Moderators: pompeiisneaks, Colossal
Re: EL 34s red plating
That is a strange one and will probably drive you nuts with paranoia until you figure it out.
If it does it again, try swapping one of the tubes over to the other side and see if the problem always follows the tubes or if it is always on one side or the other.
Because it appears to be something with the tubes I would look at the pins of the tubes. Are they clean? Soldered well? (the solder inside the pin).
The EH are made by sovtek, right? I haven't used any sovteks in years but I do remember trying them when they first became available and noticed that their pins were larger in diameter than most other tubes. As a result, they would expand the contacts in the socket so that other tubes would not always make good contact. Some of them also had a blob or overhang of solder on the pins that would exacerbate the problem. Just rambling here, but maybe check the pins and the sockets closely and see if you can find anything that might be suspect.
If it does it again, try swapping one of the tubes over to the other side and see if the problem always follows the tubes or if it is always on one side or the other.
Because it appears to be something with the tubes I would look at the pins of the tubes. Are they clean? Soldered well? (the solder inside the pin).
The EH are made by sovtek, right? I haven't used any sovteks in years but I do remember trying them when they first became available and noticed that their pins were larger in diameter than most other tubes. As a result, they would expand the contacts in the socket so that other tubes would not always make good contact. Some of them also had a blob or overhang of solder on the pins that would exacerbate the problem. Just rambling here, but maybe check the pins and the sockets closely and see if you can find anything that might be suspect.
Re: EL 34s red plating
I'll take a look at how they fit the sockets in the morning. I didn't notice anything funky with the pins, but I have to admit I didn't look at them too closely before I plugged them in.
For good measure, I'll re-tension the sockets as well. It's probably just about time. For the amount that amp gets beat around it doesn't get much maintenance.
I bought the EHs in a moment of weakness, because they were available here in town. If I still have misgivings about running tubes that have bright red plates in their past, I'll take 'em back and do what I should have done in the first place - mail-ordered some Winged C's.
W
For good measure, I'll re-tension the sockets as well. It's probably just about time. For the amount that amp gets beat around it doesn't get much maintenance.
I bought the EHs in a moment of weakness, because they were available here in town. If I still have misgivings about running tubes that have bright red plates in their past, I'll take 'em back and do what I should have done in the first place - mail-ordered some Winged C's.
W
Re: EL 34s red plating
Well, I'm not sure if I've figured out anything by now, but I am done!
I examined the tube pins - a couple of them had a bit of solder on the outside. I cleaned them up, but I doubt that was the problem. The actual joints (inside the pins) all looked good - didn't touch them. I re-tensioned the sockets. Nothing out of sorts there either, but I did notice that 2 of the sockets had a little "wiggle room" under the retaining rings. Tightened them. Examined all the solder joints to the sockets. Touched a couple of them that looked less than great, but even the worst one looked OK. Re-biased for 2 tubes @ 70% (37.5 mA @ 475V). Ran a tone into a dummy load at 50%, 75%, and 100% power for abt 20 min each, in the dark, and noticed nothing out of the ordinary.
Maybe its possessed???
W
I examined the tube pins - a couple of them had a bit of solder on the outside. I cleaned them up, but I doubt that was the problem. The actual joints (inside the pins) all looked good - didn't touch them. I re-tensioned the sockets. Nothing out of sorts there either, but I did notice that 2 of the sockets had a little "wiggle room" under the retaining rings. Tightened them. Examined all the solder joints to the sockets. Touched a couple of them that looked less than great, but even the worst one looked OK. Re-biased for 2 tubes @ 70% (37.5 mA @ 475V). Ran a tone into a dummy load at 50%, 75%, and 100% power for abt 20 min each, in the dark, and noticed nothing out of the ordinary.
Maybe its possessed???
W