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Old 26th February 2004, 07:58 PM   #1
Prune is offline Prune  Canada
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Default Tubes for direct-driving ESL

Need to handle 4kV+. Someone recommended 4CX250, but I read on a website that they have high IMD and the focused cathode 3CX800A7 should be used. However, the measurements were in RF applications. My question is whether the 3CX800A7 would also perform better in the audio band, and whether there is a HV tube as good or better for the application but lower power (the 3CX800A7 is over 700 Watts and costs half a grand; two channels with a pair for output and pair for current source ends up at four grand just for the tubes).
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Old 26th February 2004, 08:07 PM   #2
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That's rather a lot of voltage! Most ESLs expect about 1kVRMS of audio to be applied to each stator, and 5kV of bias on the diaphragm. One valve that you might like to consider is 6HS5 or 6HV5A. It's dead cheap, but Va(max) = 5.5kV and Pa(max) = 30W. Even if you used forty of them for a stereo amplifier, it wouldn't be expensive!
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Old 26th February 2004, 08:22 PM   #3
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I've been looking at these 8' full range beasts:
http://www.ele.tut.fi/~artoko/audio/...range_esl.html
They are using a 1:60 transformer that can take 100V on the primary...
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Old 26th February 2004, 08:23 PM   #4
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The 6HV5 is a good suggestion. Avoid the 6HB5; it's quite squirrely.

I haven't tried the 572B (American spec, not Svetlana) in audio service, but in RF, I've run them at 3kV with no problem. I'd wager they'll handle 4kV OK. They're about $35, and have a plate dissipation of 125-160W, depending on the manufacturer.
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Old 26th February 2004, 09:08 PM   #5
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Over at head-fi.org there's a discussion about electrostatic headphone amplifier such as this one by Kevin Gilmore:
http://www.cs.ubc.ca/~trifonov/blueHawaii.png
I asked about direct driving speakers, and Kevin said that the above can be used by raising the positive rail to 4kV and substituting 4CX250 for the output tubes and current source. When researching those tubes, I came across the recommendation for 3CX800A7 instead.
That would need one hell of a power supply
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Old 26th February 2004, 11:34 PM   #6
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For sanity's sake, I think you need to use an output choke (like a standard push-pull output transformer, but without a secondary). This allows each anode to swing to 2HT in theory, but a little less in practice. Rule of thumb is that you can swing (VRMS) about 0.9 x Vht between the anodes. Thus, if you needed 2kV between the anodes, a 2.2kV HT would be fine. Just very dangerous.

SY: What do you mean by "squirrely"? Does it run very fast up trees and hide nuts?
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Old 27th February 2004, 12:25 AM   #7
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The only reason i would use 4cx250's is that i have a bunch
of them in the drawer with the appropriate sockets.

The 4kv is not out of line, That was the B+ on both the
beveridge and accoustats.
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Old 27th February 2004, 12:51 AM   #8
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Indeed, Kevin is right.
The Acoustat direct drive amp runs at 4.8kV !
At this moment I'm repairing one, and no it's not the audiotubes that have gone bad, but some electrolytics have dried out.
The tube used in this design is the 6HB5, and like Sy said it can be flaky but that's depending on the manufacturer used.
At this moment I have reliable results with the National brand.

Another tube that has been used with good succes at these B+ voltages is the PL519. (Like the 6HB5 this is also a TV sweep tube)
This tube is very rugged and can be punished without dying.

In both cases the output amp is an SRPP, so it uses 2 tubes for driving one stator.

Depending on the ESL used, most of them need more then 1kVrms to drive them to loud levels.

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Old 27th February 2004, 01:09 AM   #9
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Quote:
Originally posted by EC8010
For sanity's sake, I think you need to use an output choke (like a standard push-pull output transformer, but without a secondary). This allows each anode to swing to 2HT in theory, but a little less in practice. Rule of thumb is that you can swing (VRMS) about 0.9 x Vht between the anodes. Thus, if you needed 2kV between the anodes, a 2.2kV HT would be fine. Just very dangerous.

SY: What do you mean by "squirrely"? Does it run very fast up trees and hide nuts?
In a manner of speaking, yes. They tend toward random oscillations and runaways.

I'd agree with you on the former points. The prototype I've been itching to scale up (currently, it's at about a kV, but really needs to be closer to 4-5kV in my application) uses just the secondary of an ESL stepup transformer as a centertapped choke- this allows the tubes to run close to class B. And the now-disconnected primary can be shunted with various things to provide damping or compensation. It's also a convenient feedback take-off point, since the voltage swings there are more easily managed.
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Old 27th February 2004, 05:34 AM   #10
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Quote:
Originally posted by kevin gilmore
The only reason i would use 4cx250's is that i have a bunch
of them in the drawer with the appropriate sockets.

Dude, if you don't have any better use for those bad boys, put 'em up on e-Bay and get some cash-ola for 'em! HAMs need them! Think of the children! (Oh wait...)

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