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Old 9th August 2004, 09:50 AM   #1
Prune is offline Prune  Canada
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Default HV tubes

What tubes will handle 2 or better yet 3 kV with good performance at audio frequencies, and have good power (at least tens of watts)?
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Old 9th August 2004, 10:40 AM   #2
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Forgot to mention it should be suitable for class A operation. I need to produce output that looks something similar to the attachmed graph.
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Old 9th August 2004, 11:54 AM   #3
SY is offline SY  United States
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572B/T160L. If you just need the AC part and don't need to have enough offset to swing to 3kV, you could get by with an 811- it's good for 2-2.5kV swing, peak to peak.
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Old 9th August 2004, 12:05 PM   #4
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What about the 833a ? This cheap (~100 $US) DHT is good
for about 40w single ended. But if you only need 20 watts you
might run it at a little less than the recomended 10v ,10a cathode current to extend tube life,it might even sound better....
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Old 9th August 2004, 02:14 PM   #5
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I may try to find some of those Eimac ceramic power tubes used somewhere. It'll be fun figuring out how to do the forced air cooling
Actually, the graph I posted above is mistaken, as I need current output...
40W are not enough, as only a fraction of the tube's output will be audio; this is biased a lot as the average power goes into maintaining the plasma; essentially it is a current source modulated by audio.
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Old 9th August 2004, 02:16 PM   #6
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Default 810

The 810 should do the job nicely. This is like an 805 on steroids and has a grid cap on the side as well as a plate cap on the top. Graphite plate good for 175 watts PD and 2500 volts on the plate is the CCS DC rating IIRC.

As suggested earlier the 833A would also work but would be a huge overkill for your application needs. With 450 watts of PD it can procuce ~120 watts in SE, not just 40.

Never starve the filament of a thoriated tungsten power tube as this will not lengthen tube life but rather shorten it! The RCA transmitting tube manual, and they ought to know, is very specific on the importance of proper filament voltage for such tubes. Starved filament operation, something thought up somewhat recently by audio experimenters is a technique best left only to low power, signal path tubes when using DH cathodes. It's purpose here is not longer tube life but reduced distortion and noise.
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Old 9th August 2004, 02:19 PM   #7
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So will these tubes work well for single ended class A transconductance amplifier? I'm driving plasma, and the load in a SPICE sim essentially behaves as two antiparallel zeners in series, with 2-3 kV breakdown voltage.

One more question. I've been looking at the Eimac 3CX800A7 because it uses beam forming to shoot the electrons between the grid wires, and gets improved IMD figures according to a site I came across. However, the tests were for RF, and I'm wondering if this would also show improvements at AF. Another thing is, I read a comment (http://www.eham.net/reviews/detail/2933) that the Svetlana 4CX800A is a substitute, and much cheaper instead of the $400+ Eimac focused cathode tube. However, I wonder if how close the distortion performance would be. And when it says on Eimac's site that it's designed for class B, does that mean that it will merely not be as efficient in class A, or blow up or what?
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Old 9th August 2004, 08:36 PM   #8
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Quote:
Originally posted by Prune
So will these tubes work well for single ended class A transconductance amplifier? I'm driving plasma, and the load in a SPICE sim essentially behaves as two antiparallel zeners in series, with 2-3 kV breakdown voltage.
I'm not sure what you are trying to do, but that load, both the plasma and the zeners, sounds to me like an awfully low load impedance. Could you perhaps post a circuit, please?

Quote:
One more question. I've been looking at the Eimac 3CX800A7 because it uses beam forming to shoot the electrons between the grid wires, and gets improved IMD figures according to a site I came across. However, the tests were for RF, and I'm wondering if this would also show improvements at AF.
I will have to pass here, I have never attempted to use these tube types for anything resembling audio, nor do I recall seeing datasheets specifying operating parameters for class A, audio or otherwise.

Quote:
Another thing is, I read a comment (http://www.eham.net/reviews/detail/2933) that the Svetlana 4CX800A is a substitute, and much cheaper instead of the $400+ Eimac focused cathode tube. However, I wonder if how close the distortion performance would be. And when it says on Eimac's site that it's designed for class B, does that mean that it will merely not be as efficient in class A, or blow up or what?
Either may blow up if you try to run them in class A, but not for the reasons you may expect.

The Svetlana 4CX800A, a tetrode, is not a direct substitute for the Eimac 3CX800A7, a triode. They will not even fit the same socket. They are only replacements insofar as each, given the right operating parameters, can provide around 16dB of amplification at a power output of around 800W of RF (class AB).

In your shoes I might not attempt to work with these ceramic power tubes in the first place, they are cranky beasts to say the least.

First, due to their fairly high maximum frequency of operation of several hundred MHz, they are quite prone to parasitic oscillations. They are not suitable for breadboarding and kitchen desktop operation. You definitely need the proper socket for the type in question, the one made by the manufacturer with a screen bypass capacitor good to well into the UHF region built in. Also your physical layout needs to provide adequate isolation between the grid and the anode circuit, even more so if you make an audio circuit with unknown parasitic resornances in the 100-300MHz region.

If you do get a parasitic oscillation, you should count yourself lucky if you only burn out the grids in the tube, these are made from a very fine wire mesh. If unlucky, then something will blow up, literally.

Secondly cooling these monsters is not as easy as that. Finding a centrifugal blower, which can provide adequate air volume at the needed back pressure, is not easy, and in particularly not noiseless if you want to continuously dissipate close to the maximum allowed on the anode. A friend of mine use a large 2' welding ventilation blower for cooling an amp I designed, though that was for a cheramic Eimac tube, which is just 'slightly' larger than the 3CX800A7.

For the cooling noise alone I would not use this type of tube in a Hi-Fi project in an otherwise quiet room.

I would perhaps look into one of the radiation cooled graphite anode/glass envelope transmitting type triodes or tetrodes, like the 4-125, 3-500Z or the 4-250. Run one (or a pair) below maximum dissipation, and you may get away with using a simple, low velocity fan or two to cool the tube envelope and the socket pins. With high quality fans that could be made nearly noiseless.

Side benefits: They are specified for audio use if memory serves. Additionally they are very rugged tubes for the experimenter with grids made to be run literally at white hot temperatures. They are nearly indestructible and looks cool (actually, hot!) to boot. 16 or 17 years ago I built and ran a 300W+ amp for the 28MHz amateur band using a single 4-250 @ 2.8KV in ground grid configuration. A friend of mine still uses that amp with the original tube in place.

Frank.
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Old 9th August 2004, 11:58 PM   #9
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I just got this reply at rec.radio.amateur.homebrew:
"The 3CX800A7 has been used sucessfully in class-A single-ended audiophile amps for a while now."
Funny, I didn't come across any during my Googling.

Anyway, you can see the I-V characteristics of my load in the attachment. It will be in the region F-H. Sound compression wave generated is approximately proportional to Power input, but in the F-G region this means essentially current input. Class A is efficient driving this since the bias goes towards maintaining plasma power and is not wasted. This is single ended since the rarefaction wave depends on the thermal relaxation constant of the plasma and not the amplifier.

What I said about the zeners is because I have seen region F-G simulated in SPICE using zeners.

I don't know whether I'm going to have an F-G or G-H type discharge, because I don't have a finalized electrode configuration. Is it possible to have an amplifier that will drive both well?
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Old 10th August 2004, 12:04 AM   #10
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WTF, the attachment didn't load...
Here's again:
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