Modern HV EL34s?

[mhuss]

Can anyone recommend a currently manufactured EL34 that actually meets the original spec (800v plate, 500v screen)?

I'm working on a HIWATT DR201 which runs at 750v or so, and the JJs that the owner put in are flashing and arcing like old-fashioned strobes as soon as you take it off standby.

[Leo_Gnardo]

In a word, no.

Whattaya got these days? JJ, Shuguang (under various brands Ruby, Valve Art, Tube Doctor etc), and all the so-called "varieties" from New Sensor. Some may hold up to 550V but beyond that it's fireworks time.

I got an old hi-voltage prize from Laney to behave itself by constructing a separate supply to run screen grids at half the B+, then adjusting bias to an appropriate level. Lost some power in the translation BUT the amp sounds terrific and owner's happy. So far so good. I can see how you might not want to sling extra parts into a highly collectible Hiwatt. This old rag bag Laney though was begging for it. Same customer had a Marshall Major where I was about to install a similar fix, but he wisely sold it on for some serious cashburger I might add. "Let it be somebody else's problem." for him and me!

[mhuss]

In a 201, the screens are run at a little over 1/2 B+ (see http://hiwatt.org/Schematics/DR_200wPS.gif). But it's still 750/450 with modern mains voltages,

Which tubes did you use in the Laney?

[Leo_Gnardo]

In a 201, the screens are run at a little over 1/2 B+ (see http://hiwatt.org/Schematics/DR_200wPS.gif). But it's still 750/450 with modern mains voltages,

Which tubes did you use in the Laney?

JJ KT77, what the customer supplied. I've used them before to good effect. Not magic, not super, maybe a pinch better than the rest. Or maybe just lucky.

[mhuss]

Thanks. I guess I'd expect a beam tube to be a little more resistant to arcing (as opposed to having a ground potential grid between the screen and plate in the tight confines of a skinny EL34 tube).

[Justin Thomas]

Anybody ever tried a Sovtek 5881WXT in one? They seem to do okay @600V or so in Ampegs. I mean, at least we might figure out the actual limits of the tube...
If you're willing to try a beam tetrode...

Justin

[nsubulysses]

I always say it, and have not tried it myself at that high of a voltage, but what about JJ 6CA7. Seems pretty rugged compared to JJ EL34.


5881 is cool but you always see old fenders with a quad of those and they only put out 85W or so. Put in a new quad of 6L6 and it jumps to 100 or 105W like it should be. For a 200W amp you would have twice the power loss, probably about 30-40W. I guess it's worth it if the amp works, but I think if this person owns a 200W Hiwatt and had a Marshall Major, they must want it to be really loud.

I have not gone beyond 585V on plate and 560V on screen with JJ 6CA7 but they seem to hold up good and don't explode (screen supply drops to about 400-420V or so when you crank the amp loudly, it is not a stiff screen supply).

[mhuss]

I'm a bit nervous, because a sextet of anything will make for a very expensive (and non-chargable) experiment!

[nsubulysses]

With JJ 6CA7 sextet you can get 200W with the same plate and screen voltages I posted above. 4 power tubes does 140W or so

I thought having a high screen voltage derived from B+ that sags a lot to not destroy the tube during loud playing (a la Ampeg V4/Traynor Custom Special) was a klunky way of going about getting high power compared to the hiwatt/SVT/sound city 200/etc. method with VERY high plate voltage and screen voltage at about 1/2 the plate voltage.

Maybe THAT way has it's own klunky aspects too becasue even though you're not putting so much high voltage on the screen, you are putting even higher voltage on the plate and causing an arcing hazard.

I am not suggesting you change the whole amp's power supply around. Just thinking out loud

hmmmm.......

[mhuss]

Lower screen voltage is a non-optional feature on really high volt amps, because the screens are only rated for 500v (even on the original Mullard and Siemens units).

The JJ KT77s seem a little better, ratings-wise (800/600v, same as their KT88s.)

The original tube used in these was a beam tube (4x GEC KT88). I think they switched to 6xEL34s when KT88s got harder to source. The 70s Mullards were pretty rugged tubes. I wonder if 4xKT88 would be a good idea. 3 EL34s have a slightly lower output impedance than 2 KT88s (~20%), and I don't know if they switched OTs when they switched tube complement. And God only knows what the plate impedance really is at 750 volts, you won't find that on any data sheet, lol.

My only hesitation is that the KT88 has some specific ventilation requirements, and the KT88 models had slot cut in the chassis on each side of the tube sockets to accommodate this. I burned up a (JJ) KT88 in my Fender PS300 after only a little use, so they're not bulletproof either. I need a time machine...

[frus]

And God only knows what the plate impedance really is at 750 volts, you won't find that on any data sheet, lol.

What do you mean, it's on all data sheets
It's 11k

https://frank.pocnet.net/sheets/128/e/EL34.pdf
https://frank.pocnet.net/sheets/010/e/EL34.pdf

[mhuss]

Good find - thanks. It's interesting that the A-A resistance is about 3.2x higher than when running at normal voltages.

Now to find equivalent KT88 data...

[tubeswell]

The ST 6CA7/EL34B is pretty reliable up to 500V.

I think the problem with most modern production tubes is finding a set that match to within 1mA of each other. I fixed up a RI handwired 1959 a couple of months ago where the OT had blown because of mismatched tubes. Believe it or not, I bought about 2-dozen Sovtek EL34G from the UK (for about $4 each incl shipping) and mixed them up till I got a stable matching set. The amp sounded quite good and (touch wood) hasn't come back to haunt me yet.

[g1]

Is ST Shug Treasure? I've never seen a 6CA7 or EL34 in that bottle style before.

[nosaj]

Is ST Shug Treasure? I've never seen a 6CA7 or EL34 in that bottle style before.

Here's some real pretty ones in blue.
https://positive-feedback.com/audio-...-vacuum-tubes/
nosaj