2A3 Push-Pull amplifier

A Directly Heated Triode PP amp can combine the harmonic naturalness of triode amplification, with the lower output impedance & good LF control of Push-Pull amplification.  

“I think an amplifier builder should consult with himself and his amplifier before asking the reference books and authorities.  It is only after much cut and try that I find my best matching point for audio. The tube manual is like a telephone book. It gives perfect numbers. It is useful to make it possible to speak with a girl but we cannot see her beautiful face from the mere telephone number.  To get the best sound from a tube, even if the proper match is 5k, we will try 7k. And we try these “mismatched” values with input transformers and other parts. When you finally find the best operating point for the tube, you get the girls address.” 

~ Susumu Sakuma

First, some history…

In 1996, I built an Audio Note Kit One 300B amplifier.

This was a 5687 triode input stage with 6SN7 SRPP-style driver for 300B Single Ended output of 7-8 Watts.  300B filaments were SS regulated DC.  Coming from Audio Research D70 (6550 Push Pull) amplification, this 300B amp was a revelation.

Low power but transparent and open.  Wide dynamic range.  Single Ended DHT but not exactly a ‘vintage’ sound.

AN Kit One - Kondo-inspired styling

AN Kit One …my Kondo-inspired styling

In 2002, I built another all-triode SE amplifier.  (Together with my pal Mark aka ‘Winding Man’ ).

This amp was inspired by Ciro Marzio’s ‘Katelelo’ 300B design (publ. Costruire HIFI).  In our build, we used a 76 triode input stage with 71A (or 46) DHT power tube driving a 2A3 (or 300B).  I used a 83 Mercury Vapour rectifier… low impedance, more impact, wider bandwidth.  (Note: 3-stage amp inverts phase.)

The all-triode topology gives excellent resolution, big, warm tone.  Lacking only a little dynamic ‘attack’ on some music.

I learnt that AC Heating sounds cleaner, less harsh, more dynamic.  Fleaxmp-schematic

In 2003… a 2-stage SET amp.

This amp is based on the Joe Roberts (Sound Practices) interpretation of  Western Electric’s classic Model 91 300B circuit… a pentode in combination with a 300B.   The WE310A pentode is replaced with a 6SJ7 or 6J7 (or EF37A).  I used a 2A3 output triode.  You can use other rectifiers, eg. 5Z3 or 5U4.

As others have found, there is pentode-triode synergy here… dynamic, agile and musical.  The 6J7 has high gain & the amp is very sensitive.  RRA 2A3

This brings us to… a Triode (2A3) Push-Pull amplifier.

I liked the sound of the 2A3, more neutral balance than a 300B, with solid low frequencies.

A Directly Heated Triode PP amp can combine the harmonic naturalness of triode amplification, with the lower output impedance & good LF control of Push-Pull amplification.  Additionally… 3-4 times power output of a SE amp.

I had been told that this 2A3 Push-Pull design, by Japanese builder Valves World, was very good.

Notes:

  • Negative Feedback loop deleted, for the time being.
  • PSU values changed to suit mono PSUs.
  • Separate filament & heater transformers used.
  • 3-stage amp inverts phase  (Additional comment Dec 2015: Check the ‘absolute phase’ of the output signal, if you can, eg. using an oscilloscope – and wire the OPT accordingly.  In my case, the OPT secondary connections needed to be wired opposite polarity, for the ouput signal to match phase with the input signal.)F-2A3PP OY

You can see that over 10 Watts output is available with less than 0.4V input (with 76 cathode bypassed) – this amp is sensitive:F2A3PP.I-O I already had some suitable Partridge 6k6 PP 25W Output Transformers:IMG_8709b I wanted wood-style, mono cases, with all components inside, except tubes.2A3PP-chasses-1 DSC01925 DSC01930 DSC01931 DSC01946 I used ‘good quality’ but not costly parts for the initial build:

  • Obbligato metallised polypropylene ‘Copper case’ coupling caps
  • Mills wirewound  resistors (PSU)
  • Takman metal film resistors
  • 50W non-inductive, aluminium-cased power resistors (2A3 cathode)
  • Elna Silmic II cathode bypass capacitor (2A3 cathode)
  • F & T high voltage electrolytic PSU filter capacitor
  • JJ high voltage electrolytic PSU filter capacitor
  • Panasonic TSUP electrolytic B+ decoupling capacitors

IMG_6893a With a wood chassis, careful placement & orientation of transformers & chokes is necessary to avoid EM coupling (hum). DSC04560 DSC01945 The tubes were all New Old stock:

  • RCA or Ken Rad 2A3 (bi-plate)
  • RCA 76 & 6A6
  • RCA 5Z3 rectifier

IMG_0051 The additional power switch is for switching HT centre-tap for optional 83 (mercury vapour) rectifier pre-warm up. Also handy if you wish to pre-heat a directly-heated rectifier like 5Z3/5U4.IMG_7468b

Frequency Response at 6.4 Watts output into 10 ohms load… not bad, some HF roll-off but excellent LF performance (20-20kHz. -0.25dB, -1.25dB, ref. 1kHz) :Freq-Resp-270411

This amplifier sounds very good indeed… not quite the ‘see-through’ transparency & beautiful treble tone of a Single-Ended amplifier but… very good bass & dynamic headroom, low-level resolution, harmonic detail & attractive tone.

Overall, a combination of the beauty of triodes, together with the rhythm and power of push-pull output.

See 2016 Upgrades:  This design has proved to be excellent, so I will upgrade some parts.

Any comments welcome!

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5 thoughts on “2A3 Push-Pull amplifier

  1. I so want to build. I love tubes!!. I would like to try them with an electromagnet speakers. Like rebuilt Zennith add on 12,s. About 1939. Also want to rebuild these Charlie Chapman beauties with rubber surrounds. Beef up the voice coil capacity..(How) Maybe you , add circut for the power source for electromagnet?

    • Hi Robert – I have used field coil drivers recently (see my Tractrix Midrange Horn post) .Yes a simple tube-rectified DC PSU for your field coil-electromagnet speakers would be good, using a suitable power transformer depending on your required volts.

      • I recently saved a early 60,s 6BQ5x4 stereo amp. Knight kit 735. Love it . I made a seprarate circut for some early field coils. 12inch deep set voice coil, say mid forties. What an exciting sound. along with the conventional tweeters. I am truly amazed at the quiet crispness of this amp. Tons of good undistorted power. I would like to take it one stage further. I have two mono blocks with 4×6550,s I know these babies could take it.

      • Many people say that it’s hard to find a bad 6BQ5 (EL84) amp. Great tube. The British Leak Stereo 20 from the 1960s was a highlight in my experience – EL84PP for 11wpc. Cheers, Owen.

  2. Pingback: 2A3PP amplifier – upgrades | D a r k L a n t e r n

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