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High voltage "800v" capable, budget el34?

Anode Melter

Anode Melter

Guest
Feb 19, 2017
32
2
#1

Is there any wallet friendly quad el34 out there that will hold on at close to 800v?

So I have this amp that I am building that is around the dynacord OT & mains... 800v plate voltage quad el34 push pull is what i'm currently running.

Tests with quad GT EL34LS at 800v idling at 22w per tube gave 163w rms at 200hz and held on at 800v.. thing is there are not a matched set and even if there were, a new quad is something i would like anyhow..

Tried a quad Marshall EL34 WK06/09 (relabelled shuguang el34-b tubes?)... Today the mains was up a few volts (peak idle of 810v B+) and one of them started arcing and died taking a fuse with it. I have since lowered B+ to 766v. Are there any wallet friendly octal 25w to 30w tubes that can handle 766v? OT is a 4k plate to plate beast that should handle up to 200w easily.

Annexed you should find the melted el34.

IMG_7357.JPG IMG_7354.JPGIMG_7360.JPG IMG_7362.JPG IMG_7365.JPG IMG_7366.JPG IMG_7368.JPG
 
#3
BassmanPaul said:
IIRC the P to P load for a pair of EL34s in fixed bias push pull is 3500Ω. That would require 1750Ω four four tubes. 4K is way high.
Click to expand...

Hey Paul! It is the original OT from a dynacord b1001 that runs quad el34 originally.. wouldn't running close to 800v put that usual 1.75k impedance way up..?
IMG_7369.jpg

I know its a 6ca7 in class b but it says 11k p to p
 
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#5
I don't have any datasheets handy... but I seem to remember that even in the Elder Days before the secrets of making good 6550s were lost they were rated for 660V design max on the plate.

A KT-88 might be what you're looking for - rated for 800V on the plate. Try JJ, I think they're about $40 each from eurotubes.com.

Edit: grabbed a few datasheets from Eurotubes - EL34 is good for 800V and a lot less pricey than KT-88, around $65 for a matched quad.

(I'm not affiliated with Eurotubes, just a happy customer.)
 
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#6
I'm not certain the mains transformer will handle quad kt88.. But if it did, there would still be the inicial question of a wallet friendly 800v capable quad... both the kt88 and the el34 are rated for 800v plate voltage.. but not all new production tubes seem to be made with the same standards..
 
#7
Are you using the lower rail on the Dynacord PT for your screen supply? I think the Dynacord trick to not killing EL34s was to keep the screen voltage down at around 410v and set the bias conservatively. I have JJ El34 in my Eminent II and they haven't died yet, though it's only had light use.
 
#8
Martin Beer said:
Are you using the lower rail on the Dynacord PT for your screen supply? I think the Dynacord trick to not killing EL34s was to keep the screen voltage down at around 410v and set the bias conservatively. I have JJ El34 in my Eminent II and they haven't died yet, though it's only had light use.
Click to expand...

Hey Martin! That's true, I had forgotten you had a dynacord amp!! Grid 2 was at 421v when it melted the tube.. I've dropped it to 394v now. I have had my eyes on some JJ tubes, should i go el34l or 6ca7? (flatter character and more bass say the people, vs normal el34)
 
#10
Anode Melter said:
Hey Martin! That's true, I had forgotten you had a dynacord amp!! Grid 2 was at 421v when it melted the tube.. I've dropped it to 394v now. I have had my eyes on some JJ tubes, should i go el34l or 6ca7? (flatter character and more bass say the people, vs normal el34)
Click to expand...

Mine are the JJ E34L. I don't have any experience with their 6CA7. I have mine biased a little cooler than is specified on the schematic, though I can't find the note I made of the exact setting I used - that might help too.
 
#17
BassmanPaul said:
IIRC the P to P load for a pair of EL34s in fixed bias push pull is 3500Ω. That would require 1750Ω for four tubes. 4K is way high.
Click to expand...
The load impedance of tubes strongly depends on the anode voltage. The higher the voltage, the larger the impedance. At 750-800 V on the anodes of a PAIR, a load of 8-10kOhms is needed, otherwise currents will become too large. Hence the 4 kOhms for 4 EL34 in PP are ok. You might have a look at the example section of the EL34 spec sheet by Telefunken, where the 800-V design has originally been published.

When i restored the Gigant of our former keyboarder, i was using 4 new Svetlanas, but i was careful and kept the anode voltage on the safe side (voltage selector 240V for 230 V from the wall).

My G-2000 and the Eminents still use old tubes; my Eminents with their slightly lower anode voltage design are run in the 220 V position, but in the G-2000 i am not as brave and stay with the safer 240-V setting.
 
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#18
Martin Beer said:
I think the Dynacord trick to not killing EL34s was to keep the screen voltage down at around 410v and set the bias conservatively. .
Click to expand...

Yes, the screen voltage is kept well within the specs of the EL34, which permits not more than 420 V (the KT77 should go higher because it is designed to be used in UL settings).

The biasing is not as conservative as it might look on first glance, especially not in the early versions of the Eminent and the Gigant. These are actually biased pretty close to the permissible limit of the anode currents at these large voltages.

In later versions of these power stages (G-2000, Imperator, i.e. from the 70s) the voltages have been a lot more conservative - smaller screen voltage (Imperator), and colder bias (G-2000). And occasionally screen resistors enlarged by 50%. This is necessary to limit the screen current when the power tubes are overdriven.
 
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#19
beate_r said:
In later versions of these power stages (G-2000, Imperator, i.e. from the 70s) the voltages have been a lot more conservative - smaller screen voltage (Imperator), and colder bias (G-2000). And occasionally screen resistors enlarged by 50%. This is necessary to limit the screen current when the power tubes are overdriven.
Click to expand...

That's interesting, as my Eminent II appears to be from close to the end of the production run (it is stamped 1978 on a label inside). While the Eminent II does not use individual screen resistors, the resistor in the screen supply rail is 2.2k in my amp, where it is shown as 1K in some versions of the schematic, like this one; Link Removed
At idle, the plate voltage in my amp is 775V and the screen is 420V, though that was measured in a building where the mains supply was over 240V so I think it will be lower in most circumstances.
 
#20
That concept has been used in many of the younger power stages - Imperator, HiFi-Favorit 2 as well. Electically this should be equivalent to enlarged screen resistors of 1.1k per tube.

The enlarged screen resistors have been used in the large power stages, and there sometimes only as a service recommendation.
 

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