The amplifier is a normally configured 2A3, i.e 320v anode, 48v bias, approx 60ma idling, driven by a bog standard srpp using 6SN7, with Hammond transformers and chassis. The output transformers are believed to be Tamura 2.5k, rescued from a sadly neglected Chinese amplifier. The resulting sound is very detailed and dynamic but a little shy in the volume area. The circuit shown will produce only modest volume levels when driving speakers with less than 95db efficiency.
The amplifier was run-in for a few days with the ariel 6 speakers and the volume was always ‘flat-out’ in a 15×15′ room. This was a bit of a surprise as the ariel was assumed to be a good match for a 2A3 until this point. Moving the amp to a larger room with vofo speakers was a revelation, however. In this situation, with the volume at 12 o’clock(half way) the level started to become uncomfortable. The difference between 92/93 db speakers and 95-96 db efficiency is profound to say the least.
It would be prudent to point out that the circuit as shown does not run the 2a3 to anywhere near its limit and a lot more could be squeezed out. However at this unstressed and conservative point the amp does sound very beguiling and gives more than a touch of the magic that is associated with the 2a3.
SRPP – Other issues
Valve experts will hopefully forgive the naive and amateur discourse that follows:
As the 6SN7 had proved to be a bit timid in the volume area it was decided to try a 6SL7 instead. At around the same time an article was discovered on the interweb regarding balanced srpp. The original article can no longer be found, although an article by Merlin Blencowe is a good read on the subject. It would seem that srpp might only be optimum when driving a load specific to the valve. Using the formula in the balanced article it was discovered that some published circuits were not using a load anywhere near that described by the formula, so some experimentation followed.
With the 6sl7 as a guinea pig it was discovered that using a load (470k)common to a few published circuits the clipping was asymmetric, and at the same time the voltage gain was not consistent. This led to experiments with various loads on the same srpp set-up, the results of which are shown here. The upshot of this experiment showed that using a load pretty close to that prescribed by the balanced article formula, did result in a circuit which clipped symmetrically and also showed a consistent gain over the full range of input voltages.
The optimum load did cause the amplifier to sound much more crystalline and expansive. However, it is yet to be vetted by more discerning ears.
At this stage it is uncertain if this result will be similar or repeatable with other valves, but more experimentation is planned. From other sources on the internet it has become apparent that asymmetric clipping is highly sought after in guitar amp distortion and is an ingredient in some guitar stomp boxes. That being the case, a serious attempt will be made to exclude it from these hifi amps…mmmmm.