2A3 Push-Pull amplifier

A Directly Heated Triode PP amp can combine the harmonic naturalness of triode amplification, with the lower output impedance and 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 …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.  

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.  

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.)

You can see that over 10 Watts output is available with less than 0.4V input (with 76 cathode bypassed) – this amp is sensitive: I already had some suitable Partridge 6k6 PP 25W Output Transformers: I wanted wood-style, mono cases, with all components inside, except tubes. 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

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

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

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.

Frequency Response at 1 Watt output into 8 ohms load… not bad, some HF roll-off but excellent LF performance (20 – 20kHz. -0.4dB, -1.6dB, ref. 1kHz) :

( Re-measured Sept 2018 )

This amplifier sounds very good indeed… not quite the ‘see-through’ transparency and beautiful treble tone of a Single-Ended amplifier but… very good bass and 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!