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Ty_Bower 4th October 2008 02:50 PM

Simple SE and EL34
Here's a question for those who have a Simple SE with an Allied 6K7VG, Hammond 274BX, 374BX, or other equivalent 750VCT power supply. What size cathode resistors would you use to run EL34 output tubes? I thought I spotted a couple of people running with 560 ohm parts, but George's table suggests that might run them at too high dissipation. It seems I ought to be using no less than 750 ohm.

Any comments?

Tubelab_com 4th October 2008 06:45 PM

I have 560 ohm resistors in my EL34 amp. The tubes are older JJ's and there is no glow in a dark room. It is about 2 years old now, but it does not get daily use. I have tried other EL34's and some show a very faint plate glow in a dark room wnen the line voltage is high (123 volts). You may use a 680 ohm for a more conservative bias current.

Ty_Bower 4th October 2008 07:19 PM

That's seems consistent what what I've read in other posts.

I recently purchased a set of Electro Harmonix EL34. I immediately noticed glowing spots on the screens within minutes of powering up the amplifier. I've upped the cathode resistors to 810 ohms and briefly tried them again. Idle current is 52 mA. Screens are still glowing. This isn't normal, is it? I never saw any glowing screen grids with my 6CA7EH, 6p3s-e, or KT88.

whitelabrat 5th October 2008 02:34 AM

I've used SED EL34's with no fuss. The only glow I've ever gotten were from a random pair of Shuguang 6L6GC's. This all using a 560 ohm cathode resistor.

Ty_Bower 8th January 2009 01:45 AM

Simple SE and cooking my screens
Resurrecting an old thread... I'll try to keep it brief.

A while ago I tried a pair of Electroharmonix EL34 in my Simple SE. As noted above, I observed glowing grid wires, visible through the plate holes. I wrote off the experiment as a failure. Determined to find some EL34 that work, I bought a set of Sovtek EL34G. After doing a little research I discovered these were not the honeys of the tube world, but they were cheap.

I suppose I shouldn't be surprised to discover they also exhibit glowing grids in my Simple SE. Disappointed perhaps, but not entirely surprised. I quickly shut the amp down, pulled out the tubes, and stuck in a pair of 6CA7EH. I was startled to observe they also had tiny glowing spots on their screens! I had to work a little harder to find them, but they were definitely there. I pulled out the 5AR4 rectifier and stuck in the 5U4GB. Let the supply caps get kicked in balls during startup, who cares! Stuck the Sovtek EL34G back in. Anyhoo, they glow slightly less brightly but the problem is still there.

So, what's going on with my amp? Why are my screens cooking? How do Dynaco Stereo 70 owners survive? I'm considering increasing the screen grid resistors R18 & R28. What's a good value? 1k? Clearly, the lower B+ value resulting from the higher drop of the 5U4 helps a little, but I worry about long term stress to the caps from the startup surge. The blue light show during startup is a little exciting too. First the 5U4 gets a pale blue glow that dances around the inside of the glass for about 1.5 seconds until it's filament gets hot, then the output tubes get a WICKED BRIGHT BLUE GLOW all over the mica and out the plate holes when they're hit with 560 volts. Finally they warm up too, and all the voltages fall back down where they belong and everybody settles down.

Does anyone (who owns a Simple SE, and believes they are safely operating EL34 tubes) want to take responsibility for my Sovteks on loan, and test them out in their amp? Would that person be generous (foolish?) enough to let me test their tubes in my Simple SE? I really need to figure out if I've just had a run of tubes that can't hack it or if there is something else going on in my amp.

Details... outputs are Transcendar 10 watt 5K:8, currently wired for UL operation but I've also tried triode. Power is Hammond 374BX. I used a Hammond choke (159Q) in the power supply instead of the 150 ohm resistor. Line voltage usually tends on the high side here, probably over 125 most days. Simple SE was built out more or less stock. Read the whole story here.

So much for keeping it short...

chrish 8th January 2009 03:42 AM

I have noticed glowing plates on my amp when using Russian 6P3S tubes. I ended up using a switch to select cathode resistors. 820R is permanently in the cct, then there is a switch to place either 1.0K or 2.2K in parallel. This then gives a choice between 820R (6L6), 597R (EL34) or 450R (KT88). For info, I am using the JJ E34L, a higher voltage version of the EL34, so this may make a difference. I think that the photo of the tubes tried by Tubelab on his site show that he was using this same JJ E34L. For what it is worth, I think that the E34L sounds the best in my system, better than the 6P3S, 6P3S-EV, and JJ KT88. See this post switcheable cathode bias Simple SE

Ty_Bower 8th January 2009 10:51 AM

I've heard you mention you prefer the sound of EL34, so I'm determined to give them a try.

I'm not having issues exceeding the dissipation limit of the plate. My problem is definitely with the screen grid. At first I thought it only happened to the couple of pairs of EL34 that I tried, but then I saw it happening in my 6CA7EH. The next thing I suppose I need to do is measure the voltage drop across R18/R28 and calculate the screen current. Perhaps that will give me an idea of what is going on.

Funny thing is, the tubes in the photos below perform flawlessly in my Simple SE no matter what, despite their being rated for no more than 250V. The plates will shows signs of over dissipation if you don't keep the cathode resistor sized right, but as you pointed out that's easy to fix - just use the right cathode resistor. Plus, they've got great big inspection holes in the plates. If anything was glowing in there that shouldn't be, I would see it.

chrish 8th January 2009 11:36 AM

Just had a look at the JJ E34Ls in there now. No glowing screens. I did notice in the past however that the one winged C that had a glowing plate also had glowing screen wires. I figured it was a bad tube and discarded it as the other one was fine.

Zibi 8th January 2009 04:41 PM

dubdub 8th January 2009 10:18 PM


I am using JJ el34 in my simple SE but I am having strange glow problem:

(BTW thanks for your reply on this)

the amp is making beautiful music now but if swap the tubes over, one will begin to glow red on the plates while the other will be fine-so I have to leave the tubes in the same channels and not swap them over.

Would you give give JJ el34 a try to see if you still see glowing screen grids?

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