2A3PP amplifier – Upgrades

Update 21 Jun 2020:

This 2A3 Push-pull amplifier, built in 2009, has proven itself to be an excellent performer.  Compared with Single-Ended amps,  I am happy about the additional watts and improved bass control.

The design of an amplifier is more important than parts quality – don’t waste $$ on expensive parts in a poor design.

However, I think it is worthwhile to spend some more time and $$ on this amp.


Firstly, we know that the Power Supply is very audible in amplifiers, especially Directly Heated Triode amplifiers.

1)  Power Supply:

The first pi-filter capacitor (C1) has a large effect on the sound of an amp.

I was keen to change the existing electrolytic cap  (F&T 47uF 500v) to an ‘aluminium film in oil’ type (33uF Obbligato Film Oil).  This is a much larger capacitor however, see below…


The original C1 – F&T capacitor – 47uF/500v electrolytic


C1 – Obbligato Film Oil cap – 33uF/630v (22uF used temporarily)

The sound – not unexpectedly, was ‘bigger’, more ‘expansive’, with warmer-tone – but also with more resolution, more natural, less ‘distortion’.  On the other hand, not as sharp-edged.

However, the ripple passing through the CLC filter increased – from 37mV to 58mV pk-pk.  With my high sensitivity horn loudspeakers (~105dB/W), the ‘hum’ is audible.

So, after modelling using PSUD2 software, the 2nd filter cap (C2 – JJ 100uF) was increased to 330uF (F&T) – this should reduce ripple to ~16mV pk-pk…..PC230122-s


OK, the PSU/amp schematic now looks like this…..

F-2A3PP OY 301215

( Correction: 76 tube cathode bypass cap has been restored )

2)  Signal Capacitors:

The coupling caps C5 , C6, and the C7 phase-splitter ground decoupling cap, were originally Obbligato Copper caps (metallised polypropylene)…..


( Original 2009 construction – showing Obbligato Copper coupling caps )

I have previously liked the Ampohm aluminium foil and paper-in-oil caps (used in Phi-42 phonostage and JEL Linestage) – but sadly, the Ampohms are now out-of-production.

I had some Ampohm Tin Foil paper-in-oil versions.  This type of cap is much larger than the metallised plastic film Obbligatos…..


(Note how the Ampohms are labelled with a line, for Outer Foil orientation – see also below)

The new coupling caps installed…PB284885

The sound – significantly better in all respects.  Much more ‘presence’ and clarity, upper-midrange/treble detailing.  Voice presence is very ‘alive’.  ‘Soundstage’ scale seems increased, with ample ‘ambient decay’.   Percussion is ‘meaty’, weighty and dynamic.  More ‘power’ top to bottom.

Next time:

The plate decoupling caps for the 6A6 phase-splitter and 76 input stages (C3 and C4) are Panasonic TSUP and ED types.

The Panasonics are good, but in the Linestage, I found the Panasonic TSUP to be more ‘mellow’, less hard-edged, less ‘rhythmic’ than the F&T caps.

I will try some more F&T electrolytics.

For me, dynamic ‘freedom’, un-restrained dynamics, is very important in order to sound like real, live music.

This is also one reason too, why I favour horn loudspeakers.

But that’s another story of course….  🙂


Update – 21 June 2020:

Input load resistor 510kΩ – this traditional value is too high for a modern amplifier. In combination with the input interconnect cable capacitance (~50pF), the HF roll-off is much too low (-3dB @ 6,250 Hz).

This has now been changed to 91kΩ (see corrected schematic below) which will improve HF bandwidth (-3dB @ 35,000Hz).

Sound quality – the result is much more HF detail, tonal detail, more instrumental ‘shine’, faster ‘transients’, more dynamic ‘headroom’ and dynamic ‘openness’. Bass tome ‘resonance’ (LF harmonics) seems also improved. A significant change, a significant improvement.

11 thoughts on “2A3PP amplifier – Upgrades

  1. Pingback: 2A3 Push-Pull amplifier | D a r k L a n t e r n

  2. Hi Owen, came to your blog from a link you posted in Stereo.net.au and it is very enjoyable. I am currently re-building my 6b4g amp and am always eager to soak up more information about 2A3/6B4G implementations. One thing I find intriguing about your amp is how hard you run the 2A3s. Around 21 watts plate dissipation which is impressively higher than the normal 15 watts! What type of valves are you using, do they have a higher max. dissipation or are you just a push-it-to-the-limit kind of guy? 🙂

    • Hi Rod – thanks for looking in & a good question – Tossie’s original design used Sovtek 2A3 tubes i believe, which can handle 27-28W plate dissip.

      My amp had been set originally for ~21W dissip, but has been reconfigured for slightly lower dissip now – 15-17W. (Will post an update.)

      Yup, the RCA 2A3 max dissip is 15W – but I had read (Glass Audio vol 6/6) that this was originally for RCA monoplate 2A3s & is conservative for later bi-plates. No doubt, the tubes would last a bit longer run cooler. And there’s debate about sound quality at the different operating pts. – Cheers, Owen

      • So many 2A3/6B4G designs seem to exceed either the max plate dissipation or current. The data sheets are not particularly helpful as most just list typical operating conditions, not max values. I will be using a nice quad of NOS Sylvania 6B4G so I want to get the most out of them without the risk of a big (or even little) bang.

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  6. Hi Owen,
    Very nice amp. I’m in the process of building a similar one. However I do not understand such hefty current you can run through the 2A3s. According to the schematic, It seems that the 2A3 tube is running at 70ma in static condition? is that 21W plate dissipation vs 15W in the 2A3 datasheet? Can you explain?

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