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Old 13th October 2008, 10:34 AM   #1
Elias is offline Elias  Finland
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Question I'm having grid1 current in EL34

Hello,

With my proto SE amplifier with Elektro Harmonix EL34 there is Grid 1 current going when the input signal is high level and V(gate-kathode) approaches zero. The current starts very abruptly and it seems that EL34 grid 1 will eat all the current I feed to it! How much it can handle? Is this anyway normal??

The problem is that the ECC83 driver stage I was planning to use cannot provide the neccessary current. Now I can only get 4W out from EL34

However when I feed EL34 with my solid state amp I can get much more (obviously it can push all the neccessary current). But I'm a little bit concerned of grid 1 current, because any of the data sheet I have do not mention anything about it's specification.

- Elias
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Old 13th October 2008, 12:33 PM   #2
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What you describe is called Class A2. It is possible, but requires a driver stage which can source (potentially significant) current. As you have discovered, your ECC83 cannot.
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Old 13th October 2008, 01:33 PM   #3
Elias is offline Elias  Finland
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Hello,

Yes. How much is the max grid 1 dissipation of EL34? Datasheets do not talk about such numbers.

How much will the expected life time be shortened with A2 operation?

- Elias
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Old 13th October 2008, 05:03 PM   #4
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AFAIK, the EL34 is not rated for a positive grid current regime. THE multi-grid power O/P type to use under such conditions is the 6L6GC.

The 12AX7/ECC81 is a fine voltage amplifier, but it can't provide any sort of significant drive, let alone x2 operating O/P tubes. You can't have coupling caps. feeding x2 operating "finals". Either you drive them with interstage trafos or voltage followers that are DC coupled. Tubelab's Power Drive shows how it's done. The IRFBC20 is very well suited to the task. Unfortunately, the 'X7 is such a wimp that driving the gate of a IRFBC20 could overwhelm it. A solution to the problem is a cascade of FET source followers. DC couple a ZVN0545A's gate to the 'X7 plate. Cap. couple the little FET's source to the gate of the big FET.
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Old 13th October 2008, 05:08 PM   #5
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Did a little search, and found some interesting articles:

Introduction to Class A2 Amplifiers:
http://members.aol.com/sbench/a2part1.html

Thread discussion of push/pull EL34 running class AB2:
EL34 in class AB2 triode mode push-pull?
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Old 13th October 2008, 05:42 PM   #6
Jeb-D. is offline Jeb-D.  United States
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Quote:
How much is the max grid 1 dissipation of EL34? Datasheets do not talk about such numbers.
Don't worry about the el34. It's grid can handle more than the ECC83 can throw at it.


If your running triode wired, 4W output isn't that bad. You may be able to squeeze another watt or two out by messing with your operating points, but that's it. If your pentode or UL wired, that's quite underpowered and the design will have to be altered. Pentode wired should give you at least 10W out before the grid starts to draw current.
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Old 14th October 2008, 05:36 PM   #7
Elias is offline Elias  Finland
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Hello,

Quote:
Originally posted by Eli Duttman
AFAIK, the EL34 is not rated for a positive grid current regime.
...
Tubelab's Power Drive shows how it's done. The IRFBC20 is very well suited to the task. Unfortunately, the 'X7 is such a wimp that driving the gate of a IRFBC20 could overwhelm it. A solution to the problem is a cascade of FET source followers. DC couple a ZVN0545A's gate to the 'X7 plate. Cap. couple the little FET's source to the gate of the big FET.
Ok, so probably it's a risk on my own to try A2 with EL34 because it's out of it's specification.

Yes, Tubelabs power drive is one solution. I was just thinking simple FET source follower at first.


Quote:
Originally posted by Jeb-D.
If your running triode wired, 4W output isn't that bad. You may be able to squeeze another watt or two out by messing with your operating points, but that's it.
My EL34 is in SE triode mode, and I should get at least 8W out of it with reasonable distortion because that's what I get when I drive it with a separate solid state amp. So far it's a driver issue.


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Old 14th October 2008, 05:58 PM   #8
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Default Re: I'm having grid1 current in EL34

Quote:
Originally posted by Elias
...it seems that EL34 grid 1 will eat all the current I feed to it! How much it can handle?
My reasonably is almost certainly flawed, or at the very least questionable. Regardless, the data sheet for EL34 states that grid 2 can handle 8 watts dissipation. I'd suspect grid 1 can probably handle a similar amount of power. The construction of the control and screen grids can't be all that different from each other, can they? How far positive do you need to drive it to get the output you want?
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Old 15th October 2008, 12:12 AM   #9
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Default Re: Re: I'm having grid1 current in EL34

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Originally posted by Ty_Bower


My reasonably is almost certainly flawed, or at the very least questionable. Regardless, the data sheet for EL34 states that grid 2 can handle 8 watts dissipation. I'd suspect grid 1 can probably handle a similar amount of power. The construction of the control and screen grids can't be all that different from each other, can they? How far positive do you need to drive it to get the output you want?

You'd best believe there's a difference in construction between g1 and g1. Something has to be done with the heat generated, when power is dissipated. If you get a chance, look at the construction of an EI KT90 or 6CA7. The screen grid posts have radiator fins attached to them.
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Old 15th October 2008, 11:10 AM   #10
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I was the perpetrator of the second thread that Ty-Bower quoted - the one about EL34s in AB2. In the years since then, I haven't learned much more about the EL34's grid-current capability but I suspect it isn't great.

The screen (g2) is rugged enough to tolerate 8w dissipaton and is sufficiently spaced from g1 and g3 that a small amount of physical distortion due to occasional overheating will probably not cause it to crash into the other electrodes.

G1 is not at all similar to g2. G1 is made of much finer wire, is very small and is very close to the cathode. I believe that overheating of the EL34's fragile g1 could easily occur and the resultant distortion of its structure would most probably wreck the tube.

I feel sure that the EL34 was NOT designed with grid current in mind and could easily fail if used for that purpose. I've never been game to try it. It's horses for courses and other output tubes exist (like 6L6, 807 and 6550) that seem to be far better candidates for Class A2 or AB2 operation..
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