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6AS7GA as push pull output?
6AS7GA as push pull output?
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Old 22nd August 2017, 03:36 AM   #1
mr2racer is offline mr2racer  United States
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6AS7GA as push pull output?
Default 6AS7GA as push pull output?

Hey Anyone,

Has anyone ever heard of an amplifier using 6AS7GA output tubes? I first saw them in Bruce Rozenblit's OTL power amp. They were intended for use as DC amplifiers not as audio tubes. But in looking at their specs they are really interesting. Its a low mu twin triode, mu=2, with and extremely low plate resistance of 280 ohms. I'm assuming that if you parallel a single tube the plate resistance would be 140 ohms and then if used in push pull the plate to plate resistance would drop to 70 ohms?

If the 70ohm plate to plate is correct, (My math is always suspect.) in push The output transformer would need a turns ratio of 2.95 to 1 to drive an 8 ohm load. With such a low turns ratio the drawbacks of an output transformer would be minimized. It might even be possible to use a toroid?

But the real question? Has anyone ever heard this tube? Would its sound be worth building an amp with it?
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Old 22nd August 2017, 03:52 AM   #2
PRR is online now PRR  United States
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The tube is 250V 100mA. A good-power load would be 2.5K per plate, not the matched load you have figured. You can go a long way either way from 2.5K/plate, but not matched if you want serious output.

You will basically be looking at 6L6/EL34 iron.

> mu=2

Which means HUGE drive voltage. For audio, it tends to need a driver supply B+ higher than the power stage B+, an awkward combination. Clever design and large cathode resistor drop can lead to a fine audio amp. (Note that in DC regulator service we can accept less gain than in audio, because distortion is less of a problem and we can load-off the long tail of the cut-off.)
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Old 22nd August 2017, 04:44 AM   #3
planet10 is online now planet10  Canada
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6AS7GA as push pull output?
I have a pair of 6AS7 PP mono blocs somewhere.

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Old 22nd August 2017, 04:54 AM   #4
Wavebourn is offline Wavebourn  United States
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6AS7GA as push pull output?
6S19P push-pull.
Do you see an additional transformer?
It is to add voltage for the driver, QQE03/12 tetrodes.

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Old 22nd August 2017, 05:00 AM   #5
nerdorama is offline nerdorama  United States
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http://www.triodeguy.com/Triodeguy%2...20PP%20Amp.pdf

Never heard it but it seems like a good design.
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Old 22nd August 2017, 05:01 AM   #6
BinaryMike is offline BinaryMike  United States
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I breadboarded a hybrid PPP 6AS7 amp using a 1K output transformer. It was a successful design that delivered 25~30W, but I never moved forward to chassis construction after I got it working to my satisfaction. Here's the schematic: http://www.diyaudio.com/forums/attac...breadboard-pdf
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Old 22nd August 2017, 05:57 AM   #7
artosalo is offline artosalo  Finland
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This is my version. About a dozen of these have been built by Finnish hobbyists.
This produce some 10...12 W. If you want a fair priced class A triode PP-amplifier, this is one candidate.

http://kuva.termiitti.com/image/43328.GIF
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File Type: gif 6N13S_6SN7_6SL7_PP_koko.GIF (59.7 KB, 754 views)
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Old 22nd August 2017, 07:21 AM   #8
Shoog is offline Shoog
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Built a few but abandoned the valve in the end because they had self resonance issues which was very noticeable with my 100db/w speakers. Produced a high pitched wine which would come and go at random. Others have reported similar experiences in headphone amps using these valves.

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Old 22nd August 2017, 07:58 AM   #9
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by PRR View Post
. . .
Which means HUGE drive voltage. For audio, it tends to need a driver supply B+ higher than the power stage B+, an awkward combination.
. . .
Yeap !

Click the image to open in full size.

Yves.
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Old 22nd August 2017, 08:27 AM   #10
artosalo is offline artosalo  Finland
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Yves, why only 140 V at the anode ? How much is the output power with these values ?
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