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
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
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.
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.
"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.
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.
Ned Carlson terminated his relationship with Triode Electronics some time ago. Ned's current venture is Tube Zone.
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
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
my first ever DIY tube amp I built back in '93ish was a PP 6B4G amplifier - it was straight from an Acrosound design... used 6SL7 / 6SN7 and an odd phase splitter design. Not a Class A amplifier though - and I cut out the feedback loop too. It eventually got torn down and built into something else. It wasn't exactly 'ultra-fi' but certainly had that 'tube sound' in spades driving a pair of Stephens 12" coaxes.
http://www.triodeel.com/acrotrio.gif
These days I would certainly go a different route!
http://www.triodeel.com/acrotrio.gif
These days I would certainly go a different route!
My current amp is totally balanced PP2a3. I have just enough gain in three stages. I have 1G4GTs in the input going into 112A as drivers. I'm pretty happy with this combination. All DHT gives great clarity and detail and a delicate top end. Right now I'm transformer coupled with Lundahl iron, but I want to take out the first transformer and direct couple instead. Hope this will be cleaner, not that it isn't very clean already. Andy
How is the low end of the spectrum? My current amp is PP with 7189s(a Scott 222)- I've had it ever simce I was 16(5 years now) I love the sound; but one thing that I've felt is lacking since the first time I heard it is a deficiency in the bass of the music. I love my bass.andyjevans said:
PS- thanks to everyone for the abundance of helpful input.
2A3 PP
Hi,
I'm new to the group but have done alot of research, follow this link: http://www.magnequest.com/diy_lessard_2a3pp.htm, there is a complete parts list and sep by step build manual thats just perfect for a novice like me....
Am still debating between the above and the Bottlehead paramour II 2A3, any input....
Also fount this site wile surfing : http://www.worldtubeaudio.com/directory/categories/kategorie_8.htm for all sorts of kits fron around the world...
Keep the joints good...
Al
Hi,
I'm new to the group but have done alot of research, follow this link: http://www.magnequest.com/diy_lessard_2a3pp.htm, there is a complete parts list and sep by step build manual thats just perfect for a novice like me....
Am still debating between the above and the Bottlehead paramour II 2A3, any input....
Also fount this site wile surfing : http://www.worldtubeaudio.com/directory/categories/kategorie_8.htm for all sorts of kits fron around the world...
Keep the joints good...
Al
A very good 8 watt PP 2a3 class "A" kit or build, called Joplin, by DIYHIFISUPPLY.
www.diyhifisupply.com
www.diyhifisupply.com
In answer to Sir Trefor's worries about bass, I can't really comment - my OPTs are fairly small and a bit bass light and my speakers are 5 inch full range, so bass is just about there. I leave this to others to comment, since I can't really give a proper answer as to what the 2a3s contribute.
Sir Trefor said:
The low end (under 100Hz) holds less musical information than say 500-2500Hz. Perculiar how people start looking for lacks in this erea first. It might be an indication of mainly listening to house or other synthesized sounds...
In time 'impressive' sounding systems get boring or worse. That's where the real good systems enter the arena without listening fatique.
No offense, but I rather listen to music than to a reproduction system. Being a well designed PP or a simple looking SE on large or small speakers. Some systems manage to get out of the way of the music, but most print their own 'style' on reproduction. Excessive dynamics, too loud deep bass notes, agressive highs for example contribute to an unnatural but impressive reproduction. The slight coloration of SE is more like natural distortion inherent to mechanical reproduction of music.
But, ok, this is just the opinion of an individu.
andyjevans said:
Hello,
totally balanced is great..
If you will remove the transformer, try out a plate choke and coupling capacitor. This is better than resistor ang gives the same headroom like transformer, without parasitic effects of an interstage transformer. There are some types avaliable with 2 symmetric windings for balanced stages.
Gerd
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