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Old 9th January 2009, 03:08 AM   #11
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I'd love to give a set of JJ a try, but I'm not about to buy them if I'm not confident they are going to work well in my amp.

I put back in the 5AR4 and the Sovtek EL34 and took some quick measurements:

459 volts plate
465 volts screen (wired UL)
38.3 volts cathode (810 ohm resistor)
660mV across R18

Math works out to 47.3 mA total current for the tube, of which 6.8 mA is going through the screen. I believe that is 2.9 watts on the screen - isn't the EL34 supposed to be rated for 8 watts Pg2?

If I just ignore it and leave it with little glowing spots on the screen grid, will it eventually settle down and behave? Or will a grid wire melt, fall, and touch an adjacent grid causing a hideous short and a fiery explosion of sparks?
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Old 12th January 2009, 08:30 PM   #12
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For what it's worth, I took some similar measurements using a beam tube. While the plate voltage and overall tube current is nearly the same, the screen current is about 1/3 of the pentode. This isn't unexpected. I have not yet made any changes to the amp. I have tried searching for information regarding pentodes and methods of triode connecting them. I found a few posts which nearly sum up the way I'm feeling about the whole thing right now:

"...regarding red screens, in my case with EL34s. Not to discourage their use, but after some experience there I went to beam tubes."

"Though they were comparatively sensitive I went off EL34s because I had glowing spots on G2 when I did not want them."

I should still try running it in triode mode again, perhaps with more than 100 ohms between plate and screen. It would be very easy to use a 250 ohm resistor (instead of a jumper wire) between Tx-PRI O2 and O1. In another post, I found some recommended (?) values for the screen resistor:
EL34 Screen Resistors
From experience:
EL34 Triode Mode - use 150R minimum
EL34 Ultralinear - Use 1K minimum


I also found some references to the use of zener diodes to build a voltage dropping network for the screen connection. It seems to be credited to Bill Perkins, and is sometimes called the "Triode Trick". There is an excited thread discussion here, where some members involved feel it is nothing more than a "stinking pile of audiophoolery." I'm still intrigued by the concept, and tempted to try it for myself. I have to wonder why two series connected 470uF/16v caps were used, instead of just a single 35 volt rated part. I also haven't quite figured out the backwards (?) 1N914 diode at the top of the network.

Here's a link to an article describing how to build the zener diode, triode trick:
http://www.t-linespeakers.org/tubes/triode-trick.html
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Old 13th January 2009, 12:33 AM   #13
w5jag is offline w5jag  United States
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In my Simple SE with EL34, triode wired, I use 620 ohm cathode resistors. I don't recall what my screen resistor is, probably whatever was specified in the parts list.

I have found that of my stash:

Mullard (genuine, not Russian)
Tesla
Siemens
General Electric (both Matsushita and Mullard made)
JJ (E34L)
SED
Groove Tubes (Shuguang EL34B)

will make their rated dissipation without plate glow or screen spots at voltages in the mid to mid upper 400's.

Shuguang EL34A will glow at less than 25 watts and mine show hot spots on the screen. Despite that, I have not killed them yet, although I have not tried real hard.

I have not tried many of the beam variants. I lit off an Ei 6CA7 HARD up around 550 volts taking a power transformer with it and that kind of soured me on 6CA7's. The Ei seem to work fine and sound good in the mid 400's though. The Groove Tubes 6CA7GE is another tube of dubious merit. Some work fine. Some don't work at all. The ratio in my small sample has been about 50:50.

I may be off kilter, but minor plate glow never really excites me very much as long as the tube does not take off. Cathode current is what I get concerned about - there are only so many electrons in the cathode. All the plate does is collect them. I tend to go high voltage, lower current, and not worry much about the plate dissipation if the cathode current is reasonable. I keep an ammeter in both my Simple SE and Tubelab SE to constantly meter current.

For big beam tubes, I would just use 6550 and be done with it.

Win W5JAG
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Old 14th January 2009, 02:58 AM   #14
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Quote:
Originally posted by Ty_Bower
...Bill Perkins, and is sometimes called the "Triode Trick"... I'm still intrigued by the concept, and tempted to try it for myself.http://www.t-linespeakers.org/tubes/triode-trick.html
I went to Mouser to price out the suggested 1N821 diode. Uh, no. I don't think I'm going to spring for ten of these things at $4.94 each.
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Old 14th January 2009, 04:01 AM   #15
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The "triode trick" really doesn't make a pentode into a triode, or do any of the magic tricks that it is often associated with.

It can however be used to lower the screen grid dissipation when a pentode is triode wired or UL wired for that matter. The "magic voltage" or the right nimber of the "correct zener diodes" varies with the choice of output tube. Do you need to spend serious money on a collection of 6.2 volt zener diodes? I doubt it. It is true that the temperature coefficient of a zener diode is the lowest when the zener diode is 5.6 or 6.2 volts. Does it make a difference in this case? No. Do you need two OSCONS in series, well maybe if you believe that OSCONS are magical. Do you need the 1N914? Well that is another magic tale. I don't really subscribe to this one, but if you want to use a diode here, make it a 1N4001 type. The screen can draw enough current to blow a 1N914.

I have used this "trick" especially when abusing sweep tubes. I have used whatever zener diode or combination of zener diodes that I could find in my junk box. Noise isn't an issue since the diodes are bypassed by a capacitor. I use a decent quality (Panasonic) enectrolytic in parallel with a .1 uF polypropelyne. I have used a single 33 volt 5 watt zener diode on a Simple SE to "cool off" some glowing tubes when running B+ over 500 volts.

The larger the voltage of the zener (or multiple zeners) the lower the screen dissipation. Too large of a zener voltage will cause distortion on large signals. The screen voltage dips too low causing tube nonlinearity. I have even used gas regulator tubes like the 0A2 when running sweep tubes. Not really a good idea.

Quote:
I may be off kilter, but minor plate glow never really excites me very much as long as the tube does not take off.
A faint red glow is not usually a bad thing on an older tube. Some of the newer stuff isn't made of pure materials nor is the vacuum as hard. A glowing plate can cause outgassing of some of these imputities, which will cause an increase of grid gurrent, and you guessed it tube runaway.

Quote:
Math works out to 47.3 mA total current for the tube, of which 6.8 mA is going through the screen. I believe that is 2.9 watts on the screen - isn't the EL34 supposed to be rated for 8 watts Pg2?
Yes, it is. I have a pair of "Winged C" tubes that have screens that glow brightly when the voltage is anywhere above 320 volts. All of the modern tube manufacturers simply copy the old Mullard specs, but they don't copy the old Mullard quality control. Some tubes are far worse than others, but all of the electrodes need to be aligned correctly for proper dissipation.
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Old 17th January 2009, 09:27 PM   #16
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I haven't bought my zeners yet. I did wire it up for triode again (easier on the screens?) and added an extra 250 ohms to the 100 that's already on the board. I also plugged in the 5U4 instead of the 5AR4 to drop a few extra volts.

The darn screens still glow on the Sovtek EL34. I can tell things are headed in the right direction - spots which were yellow and orange and now orange and red. Still, there shouldn't be any spots on the screens at all, should there? I wonder how low I need to get the screen voltage before it behaves?

What's the failure mode if I just leave it running this way? Is it going to take out the cathode caps & resistors when it fries? Am I risking the output transformers? If the tubes poof, no big loss. I'd prefer not to damage the amp...
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Old 18th January 2009, 02:11 AM   #17
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Quote:
Still, there shouldn't be any spots on the screens at all, should there?
The screen grids should not glow. As you have found this is just about impossible on some EL34's.

Quote:
added an extra 250 ohms to the 100 that's already on the board
Some users have gone as high as 1K ohms. In fact some people claim that 1K in series with the screen is a "magic value" that reduces distortion in UL mode on the EL34. The Mullard data sheet does specify 1K ohms in UL mode and up to 1.5K in pentode mode. If you are using EL34's you can try a 1K resistor, even in triode mode. I specify a 100 ohm resistor in the parts list as a compromize since larger values tend to cause distortion with some KT88's.

I tried resistors as high as 2.2K with the glowing pair of Winged C EL34's that I have, and they still glow. 2.2K caused some ugly distortion on heavy bass. The resistor started smoking too. I have just given up on that set of tubes.

Quote:
What's the failure mode if I just leave it running this way?
I suppose anything is possible. The glowing wires will cause contamination of the vacuum leading to a short lifetime of the tubes. Likely the distortion will increase over time. The tube current could increase due to grid current, and eventual tube runaway is possible. If the amp has a fuse in the line cord (2 amp on 120 volts) the fuse will usually blow before anything else is damaged. It is possible that a tube arc could occur. There was a thread on this forum several months back where a users amp was damaged by a defective KT88 causing the cathode resistor and cap to fail. I don't remember if that amp was properly fused or not. Damage to the OPT is highly unlikely.

I have managed to blow the screen grid resistor twice (EL34's both times) in early the early development stages of the Simple SE. That was when I was using 1/2 watt resistors. There have been no failures with 1 or 2 watt resistors. I also managed to blow a cathode resistor and capacitor rather violently when I intentionally put some known gassy 6V6's into my Simple SE that was running at plate voltages far too high for 6V6's.

Photos - the death of a Simple SE and 6V6's
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Old 24th January 2009, 02:29 AM   #18
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In case anyone was wondering, I have found a set of EL34 that appears to work just fine in my Simple SE. This seems to prove (to me, at least) that the glowing screen problem was with the junky tubes I tried, and not with my Simple SE amp. I received a loaner pair of JJ E34L tubes and tried them out tonight.

The holes in the plates are rather small, and it makes for difficult inspection of the screens. The red lettering that JJ uses for their logo further complicates matters - I keep catching glimpses of that logo out of the corner of my eye, and I need to look twice to see if the red color is coming from something inside the tube. In any event, I haven't been able to see any indications of the glowing screens that were prevalent in EH and Sovtek tubes I tried.

I will make a brief comment with regards to all the "quality control" comments I've read about JJ. The pins on these tubes do not have the nice rounded ends I see on nearly all my other tubes. One tube of the pair has rather square ends on the pins, and the other has conical sections at the ends. I doubt this causes much trouble, especially if one is cautious when inserting the tube. They do also feel slightly looser in the socket than my EH tubes. On the other hand, I've always felt my EH tubes were too tight in the sockets, and almost required unnecessary force to seat them fully.

The only other comment I can make at this time with regards to the construction quality of these JJ tubes is that the coloring of the tube is kinda odd. These are the "blue" JJ tubes, but it sure doesn't seem like blue glass to me. It appears more like one of those Swirl-an-Egg things you did as a child, with a sorty stripey blue smear down the side of the envelope. I'm guessing it's clear glass, and the blue is somehow applied after the fact. Purely a cosmetic thing, but still not what I would have expected.

It's too early for me to comment on their sonic qualities. Absolutely nothing offensive so far; indeed they sound very good. They seem to have a very natural (and neutral) quality to the sound. Thinking about it while listening to them makes me suspect that EH tubes I've been using previously might have had a touch of "bass in your face" kind of weight to them. Of course, the differences are much more subtle than I make them sound. I haven't heard a truly bad tube sound come out of the Simple SE, regardless of which tubes I've used.
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Old 25th July 2009, 03:47 AM   #19
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I found another set of EL34 that work great in my Simple SE. I lucked into a pair of Blackburn Xf3 tubes. No signs of screen distress so far, but I'm going to be watching them like a hawk. Initial impressions are that they sound very nice.

Click the image to open in full size.
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Old 25th July 2009, 04:30 AM   #20
rknize is offline rknize  United States
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Strange, I haven't had any issues with glowing screens on any EL34s I have tried in mine. Maybe I haven't looked closely enough. I usually run them as triodes. My B+ is about 460, I think. I've tried SED EL34s, Valve Art (Shuguang) EL34s, and 6CA7EH.
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