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Old 20th June 2007, 04:52 PM   #1
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Default 2A3s in push pull; class A

I was thinking about an amplifier using two 2A3 tubes in push pull at the output, when I coincidently stumbled across such a circuit in the 1934 thordarson transformer manual that claims a total of 2.5% harmonic distortion. I will post a scan of the article and schematic later today.

But basically the amplifier design employs 3 stages. With a single 56, 2 56s in PP, followed by the 2A3s. The design calls for the interstage transformer for the output stage to have, instead of a single center-tapped winding, two independent windings with a wirewound potentiometer on each for precise bias matching.

I'm not rushing out to build this amp, but was curious as to peoples' opinions on the subject; using 2A3s in push pull. The majority of designs I have seen that use 2A3s operate as SE amplifiers. I assume it is because of the high Gm of the 2A3 tube that keeps people from operating them in such a way, as such a design would call for razor-thin tolerances in the area of bias matching(?) Has anyone hear ever heard 2A3a in push pull?

In any case, it's a subject I've been pondering as of late. In general I like push pull over single ended; but I do like the way my friend's 2A3 SE amp sounds, enough to sway me in either direction.

-Trevor
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Old 20th June 2007, 07:52 PM   #2
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2A3 in push pull can be very nice. Look at Magnequest.com, the Seth amps should be very nice and not to difficult build. I built something similar, same layout different iron. Sounded great with power to drive real world speakers.
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Old 20th June 2007, 07:52 PM   #3
JimW is offline JimW  United States
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2a3's work very well in pp amps, there are a number of designs floating around including the "Seth" design on the magnequest website, Pete Millet has a design that can be easily adapted to 2a3 on his websit, as well as a 2a3 version of Lynn Olsen's "Amity". I have built three versions of pp 2a3 amps, at present have a 2a3 Amity. They were all very good amps, with the Amity being my favorite.
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Old 20th June 2007, 11:09 PM   #4
2004ex is offline 2004ex  United States
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Brook 12A (22A) and 10C are both 2A3 PP amps. Never heard one, but they are all highly regarded in the asia audiophile circle. Also read that customized versions had been shipped with Klipsch horns at one point.
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Old 20th June 2007, 11:20 PM   #5
arnoldc is offline arnoldc  Philippines
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There is a PP 2A3 design here, made by frank de grove (where is he?)
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Old 20th June 2007, 11:35 PM   #6
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Trevor,

"Uncle" Ned Carlson has a way to set up a Dyna ST70 so that both Sovtek 6B4Gs and EL34s work. Since the 6B4G and 2A3 are electrically equivalent, except for basing and filament voltage, it's safe to say that the 2A3 is well suited to PP circuitry. AAMOF, the lower filament voltage means less trouble with residual hum, when AC heating is employed

FWIW, I have some thoughts about the direction you might take. Include a common cathode 12AT7 section in the front end circuitry. PP "finals" cancel internally generated even order HD. Adding a 'T7 to the mix makes the NET distortion spectrum come out as an ear pleasing "waterfall". MOSFET source followers DC coupled to the 2A3 grids eliminate blocking distortion and allow for as much A2 operation as you are comfortable with. DIY Audio member Tubelab offers PCBs for the FET circuitry. A general description of MOSFETs DC coupled to power O/P grids can be found in MOSFET Follies.
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Old 21st June 2007, 05:54 AM   #7
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I am building guitar amplifiers with push/pull 2A3's and love the tone. They make a wonderful sounding guitar amplifier.
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Old 21st June 2007, 08:10 PM   #8
zacster is offline zacster  United States
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I have my ST70 set up partially as Eli suggests, with 6B4G tubes. It sounds very nice. I don't think I can put the EL34 back in though without swapping around a few wires, although I never thought about whether it would work as is. Thinking about it now while at work, I wouldn't have negative bias for EL34 as the cathode bias is via the heater with 6b4g. I never could find the Ned Carlson method spec'ed out anywhere.
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Old 22nd June 2007, 01:29 AM   #9
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Quote:
I never could find the Ned Carlson method spec'ed out anywhere.

Ned Carlson terminated his relationship with Triode Electronics some time ago. Ned's current venture is Tube Zone.
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Old 22nd June 2007, 09:34 AM   #10
gerd is offline gerd  Germany
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Default 2A3 PP

hello,

here is a link to my project 2A3 PP

the first stage is not necessary if a preamp is used. Then the input needs 1,5Veff, with a source impedance up to 7 kiloohms.

Gerd
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