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Old 28th December 2004, 01:04 PM   #1
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Unhappy JJ E34L failure(s)

Hello everyone

As of the summer of 2003 I've had three (two blue glass, one clear) JJ E34L's fail exactly in the same way; The internal connection from the cathode sleeve to the cathode pin became "unwelded" at the cathode sleeve.
There seems to be a very "serious" QC problem! I run my E34L's with a standing current of 35mA @ 440V anode, ULT PP. No cherry plates ever, those babies should never exhibit that type of failure! I understand thermal stress of cycling, turning the amp on/off, but I know I haven't exceeded what's considered "normal" cycling. Has anyone else had this happen?
Looks like it's time to get out the plastic and get me some EH-EL34's, or some "winged C" Svetlana's. Any suggestions?
Shame though, as the I really liked the sound/look of those critters! I choose the JJ's because they were reportedly/advertised to be able to withstand 600V on their plates! I was planning to mod my amp to apply close to 500V on the plates and get a little more "balls" out of it!
I also had another blue glass E34L fail. After setting the bias to 35mA, the bias current after 15-20 minutes would slowly start to climb waaayy past normal. 35 -> 50mA before settling down. Probably a gassy tube, or misaligned grid wires (sagging?).
I think maybe if I were to purchase the JJ's from a better dealer who tests and weeds out the marginal performing stock, I could do well.

Any comments would be welcome!

Happy New Year!

Wayne
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Old 28th December 2004, 01:19 PM   #2
SY is offline SY  United States
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That's just wrong. This is the sort of stuff that any self-respecting manufacturer would not let out the door. I've been using winged C's, not happy with the consistency unit-to-unit, but none of them have died on me.
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Old 28th December 2004, 01:21 PM   #3
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I might also add, out of 2 sets of matched quads I have two left! So now I'm using a matched JJ on the left (closest to my sitting position in front of the computer) and an unknown brand, Chinese or Russian (look like sovteks from the mica spacers) matched pair on the right. Just burns me up! And UPS is running behind schedeual due to the weather, airline companies computer crash (?). Dratt, I don't want to dig out my SS monster fro New Year's!
Could take a road trip to Dayton Ohio, about 4 plus hours!

Wayne
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Old 28th December 2004, 02:56 PM   #4
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I've had good luck with the EH EL34's in my ST-70.
Not so great luck with the Sovtek 5AR4,Had a couple start arcing at the bottom insulator,from the cathode to plate,in the same spot. (it's a pretty small gap down there.) Been using a 5U4GB since the last one arced over.
I'm sure it's not the filter caps,I checked the leakage of the stock can (still performs quite well!) but replaced it with the SDS labs cap board just to be sure...Still arcs.
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Old 28th December 2004, 04:14 PM   #5
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hey digital junkie,
what is the input filter caps value? is it above 47uF? many people "mod" the power supply by adding capacitance or new caps (which are really better than old ones) only to make it a rectifier killer. if you have any doubt about what i am saying, please read (or reread the chapter on power supplies in any RCA tube manual... a good little reminder if only that) if so... almost any rectifier you put in there will have a rough time because the inrush current is exceeding the peak current of the rectifier (that is why it arcs, not because it is a piece of ****). try putting a 50 or 100 ohm 5W resistor in between the cathode (pin 8) and the first cap...
happy new year!
jc
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Old 28th December 2004, 04:25 PM   #6
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Hi Everyone

I'm looking to order a quad in the next 2 hours in order to guarantee delievery by Friday, New Years Eve. So I'm in a little, tiny hurry hehe... Has anyone had any experience with the Electro-Harmonix Fat Bulb 6CA7? What I'm looking for besides good CRAFTMANSHIP (remember those days?) is the ability to handle a good high plate voltage, 800V would be beautiful, but will settle for 600V. If I could get my PS up to 475V, which I can very easy (taps on my PT) except for locating good 220µf - 470µf 350WV snap-in caps, I'd be in tube heaven! Can't seem to find the caps I need that will fit without to much fuss, 30mm max in diameter without having to resort to using clamps.
And BTW that trip to Dayton would be 9-10 hours round-trip! When I was a kid you could go to almost every drug store on the corner and buy tubes!

So I've narrowed it down to 4:
EH Fat Bulb 6CA7
EH EL34
S.E.D. EL34
Sovtek EL34WXT

Thanks
Wayne
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Old 28th December 2004, 04:44 PM   #7
nanana is offline nanana  Sweden
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i hate to be a party pooper or rain on anyone's parade but the problem is not the plate volts... almost any EL-34 can take 800 or 900 volts on the plate... (that was the way they were run in class C) providing there's only 250 volts on the SCREEN! you CAN"T PUT JUST ANY OLD B+ ON THE SCREEN AND EXPECT TUBES TO SURVIVE. all those UL amps with 590 mains are tube killers. it makes me crazy to see just how nutty people are when it comes to remembering the most basic stuff about tetrodes and pentodes. please look at the tube manual. 6CA7 is even less suited to high volts on the screen although everyone does it. it doesn't even have a shadow grid... so it lights up like a christmas tree once the screen supply goes anything above 400. thats actually OK for awhile... it was made that way, but above 450, it starts to get to the temperature where it begins to stretch... no matter how skilled or careful the tensioning is, there is a limit to the geometry. the welds don't melt but the wire tends to break just above the weld. BOOM! they do sound nice before that happens though... the 6CA7 is a tetrode, not a pentode (sarnoff wouldn't pay philips the license money) so its performance is different from the EL-34. both are nice, just different.
ranting ranting
jc
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Old 29th December 2004, 09:01 AM   #8
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Could there be paek cathode currents causing it to open up like a fuse?
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Old 29th December 2004, 10:42 AM   #9
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Quote:
Originally posted by SY
That's just wrong. This is the sort of stuff that any self-respecting manufacturer would not let out the door. I've been using winged C's, not happy with the consistency unit-to-unit, but none of them have died on me.
Umm.....not sure about that one....EI let me down with a batch of EI KT90's used in UL mode, which when tapped were short- circuiting internally from G2 to G3.... Luckily I had a fuse in the O/P tranny primary.....So far no problems with clear tube JJ88's nor Svet 6550B. Due to the expense of the o/p tranny I can recommend putting a fuse in the centre tap lead. The sonic difference is nil.

When one orders tubes, insist the "thump test" be done before they leave the door.

rich
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Old 29th December 2004, 11:59 AM   #10
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Unhappy being scolded???

Hi jc and everyone

I lost you guys for hours yesterday, Site not found! So I'm a little late in posting a response. Am I being scolded jc?

I know all about the diffs betwixed an EL34 and 6CA7. 6CA7 is really a beam tetrode with beam forming plates, like a KT88, KT66, 6L6, and one of my faves the 7591. No one (me) here is talking about an amp with 800V anode voltage. Class B would be design max 800V anode, 400V screen of a 6CA7 tetrode, Push-Pull "Pentode Mode", 100W for a pair! (published rating hehe!) A friend of my Father built one around 1960 or so. Don't remember how well it worked tho! It was probably always on the verge of melting down! Of course you could tweak down the screen voltage to keep the tube from self-destructing! Then you might as well go for Horizontal Output tubes. Much higher plate dissipation.

What I'm asking about is quality/headroom! I've run 6CA7/EL34's with 475V Class AB1 Ultralinear PP for years back in the 1960's and 70's. Westinghouse, Mullard, Sylvania etc. They lasted for years with no problem at all! I tried a set of Rat Shack EL34's, continental slimmies as they were known back then, cherry at 430V with cathode or aka self bias. Got in a big argument with the salesman! Lifetime guarantee plus gold plated pins ya know! Got tired of it (and him) and put in a set of Westinghouse EL34's, no problem! Same operating point, using a 5V3A rectifier. Did try to use the Rat Shack ones at a later date, with fixed bias @475V. Had to bias them way down near cut-off to keep them from self destructing. I threw them away! Wish I kept them tho for posterity!

My JJ's didn't fry! They were actually operated well within spec! About 15 watts plate dissipation. The first JJ to exhibit the broken connection at the cathode sleeve was about TWO days after I installed it! Operated with 440V plate voltage @35mA cathode current, fixed bias, class AB1 ultralinear PP 5.5k OPT. Tapping the tube caused it to make/break the connection! Same thing happened to the other two except a year or so later, this past week for one and two months ago for the other. There was no so-called melting! I hate to pee on this "parade" but I'm no toobe-noobe!

I was thinking of running them close to a very well known operating point. For example:
Data taken from Amperex data sheet.

Two Tubes Push-Pull Class AB1 Ultralinear Connection

Vbb: 500V
Grid #2: taken from tap @ 43% of plate winding
Grid #1: -44.5V
a-a: 7k
Plate plus screen current (no signal): 2x57mA
Plate plus Screen current (Max Signal): 2x112mA
Input Signal (RMS): 32V
Power Output: 60W
THD: 2.5%

That's the about the max you can go with an EL34 UL PP! About 24.5W plate dissipation. I do know for a fact there were/are tubes being made that you could not do this with! I want to operate them close to 475V (not even 500V!) at around 33mA of standing current, @16.7 watts plate dissipation to get a little extra power. May need to increase the screen grid stoppers from 100R to 470R - 1k. IIRC David Manley (opened a can of worms around here!) of VTL did so in his amplifiers (500V Vbb) as did many others. So jc, I don't get it?
Believe me I understand what you are saying. I have no intention of going anywhere near (gulp) 800V! They don't make them like that anymore. Even then it was pushing it! And yes there were EL34's made that could not go anywhere near 800V no matter what screen voltage was applied. My experience with the 6CA7 is very limited and from long ago! My thought of possibly using an EH 6CA7 fat bottle is merely out of curiosity and nostalgia!

There is a certain make of an EL84 that can only handle 250V anode (Svetlana?), as apposed to 300V or can be pushed a little higher, i.e. certain makes of a 6BQ5/EL84, watching your cathode current and screen voltage/current of course. You can do this by using the ultralinear mode of operation. And lets not forget the size of the grid leak resistor, for stablity.

That 800V was a wish! More volts, more power. I wouldn't even consider doing so! Anyway class B is not my cup of tea! It's just that there are some EL34's that can handle a higher plate voltage along with the high-ish screen voltage "supposedly" better than others. Better built in other-words. Did I even say I wanted to "stick" 600V to my toobes anodes? Bad day jc?

Cheers jc
Wayne
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