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6AS7GA as push pull output?
6AS7GA as push pull output?
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Old 25th August 2017, 12:23 AM   #21
mr2racer is offline mr2racer  United States
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Hey All, thanks again for all the help! It's made the early planning stage much easier. The biggest change is a lateral shift in output tube. This morning I bought four Winged 'C' 6N13S tubes. They were also billed as 6AS7's but they aren't really the same. They seem to be an improvement. The grid voltage is only -30 volts so they should be easier to drive. And the cathode resistor is only listed as 250 ohms. I don't have an audio data sheet for this tube so if anyone has one it would be much appreciated.

I did find a schematic that listed a 2500 ohm output transformer primary with a rated power output of 10.5 watts. With four tubes I intend to run the outputs in push pull parallel. If I'm correct that would drop the plate to plate to 1250 or so and give a power output of around twenty watts.

The purpose of this amp is to build an all triode 20 watt amplifier with little or no feedback. Since triodes produce mostly even order distortion and push pull tends to cancel even order distortion that might work out well. Another target is to make the amplifier as simple as possible. Cathode bias and AC filaments, old school.

Power supply will be solid state. This puppy will have a B+ supply of almost an amp so there would need to be too many rectifiers to use tubes. And I don't want to complicate the power transformer with a five volt winding. Also, I'm going to need a big choke.

As for the front end? Basic layout will be a differential amp. I've used Edcor line matchers as input transformers with good effect, and they're cheap! With only a 60 or so volt swing needed to drive 6N13S a couple driver tubes come to mind, 12AU7, 6SN7/6CG7 or maybe 6N6P? Other suggestions are welcome.

With a B+ of 250 volts I won't have to isolate the drivers from the output tubes. So I'm inching out on a limb here and thinking of making the stages direct coupled?

And rather than handles I might put some wheels on it.
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Old 25th August 2017, 01:07 AM   #22
Shef is offline Shef
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6080 (6AS7, 6N13) is a really screwed choice if you are such a beginner, read again what Shoog commented on it.
Search for the topics on his and others' experiences with it. E.g.
Help designing a tube amp...
Ideas for a simple 6080/6AS7 Push-Pull amplifier
And, of course, this one:
Idea for a 2 tube 6080 PP amp.
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Old 25th August 2017, 08:13 AM   #23
Shoog is offline Shoog
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6N13S is functionally identical to a 6080 apart from its dissipation.
I would go with something like a ECC99 as a driver. With the horribly low gain of a practical 6080 valve you can expect to need to go into positive grid territory to get your target 10Watts and that will need a strong driver with a reasonably high gain, not a combination easily found in triodes. Better yet a pentode driver would suit better. A more realistics expectation is 7watts from a pair.

I always found (and others agreed) that these valves sound best when driven hot at 100mA and just 100V of plate to cathode voltage.

Shoog
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Old 25th August 2017, 10:20 AM   #24
mr2racer is offline mr2racer  United States
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Thanks Shoog, I have a bunch of 6N6P/ECC99's. Any reason I can't parallel both halves?
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Old 25th August 2017, 11:45 AM   #25
Bigun is offline Bigun  Canada
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6AS7GA as push pull output?
The 6AS7 is not that linear. If you've a bunch of ECC99's then I'd use them instead, drop the 6AS7 altogether.
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Old 25th August 2017, 12:52 PM   #26
jdarg is offline jdarg  United States
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I've also found the two sections in a single tube hideously mismatched almost without exception and I have dozens of them. Individual bias adjustment plus at least roughly matching them into at least ballpark quads/pairs is a requirement IMHO.
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Old 25th August 2017, 02:22 PM   #27
artosalo is offline artosalo  Finland
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Quote:
I always found (and others agreed) that these valves sound best when driven hot at 100mA and just 100V of plate to cathode voltage.
With these numbers you can get some 2 W (as PP) from one 6AS7G and then the output transformer should some 500 to 1k.
With planned 2.5 OPT hardly 1.5 W is possible.

So, to get the required 10 W, just 5 tubes should be put parallel.

I would like to know what is wrong with the well tested and fully functioning
construction with 12 W output power, 375 V supply voltage, 5k OPT and cathode bias, which I suggested??

Last edited by artosalo; 25th August 2017 at 02:33 PM.
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Old 26th August 2017, 12:25 AM   #28
PRR is offline PRR  United States
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> [i]6N13S .... grid voltage is only -30 volts

At 90V. With nearly no swing around that point.

You really want the supply to be 200V-250V. Mu is near 2 (same-as 6080), so bias and grid swing will be ~~100V (same-as 6080).
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Old 26th August 2017, 12:35 AM   #29
audiowize is offline audiowize  United States
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I know you said triode, but starting with the Mullard 20W amp may be a more solid choice that will yield more pleasurable results.

If you have to use the 6AS7, then you might want to start there.

Last edited by audiowize; 26th August 2017 at 12:39 AM.
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Old 26th August 2017, 12:49 AM   #30
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He also contributes here as well on occasion.
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