The ST-2A3 is Finished
I put the finishing touches on it tonight. I did the usual voltage checks and everything was as expected. This was after I smelled something and noticed the filament transformer was real hot. I had mismatched the phases on the primary.
I managed not to fry the xfrmr and placed all the tubes in their sockets and hooked it up to the sacrificial BOSE bookshelf speakers.
It sounds good considering the speakers. It is every bit as quiet as the first version I buit on the hammond chassis. It does have a lot cleaner wiring layout than before, especially considering I had less space to work with.
I will have to make some corner supports to raise it off the table. The dual filament xfrmr is taller than the depth of the chassis. One of the filter caps sticks up a little too much as well.
Here is the schematic diagram for the circuit used in this amp. It uses the Single Rail Push Pull topology in the driver portion of the circuit, courtesy of Joseph Esmilla.
Here is my version which combines portions from another of Joseph’s 2A3 designs:
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The power supply for the above amp:
I am currently swapping between a solid state Copper Cap rectifier that emulates a 5U4 vacuum rectifier. I am currently trying a real 5U4, and a 5R4 vacuum rectifier.
The 2A3 tubes dissipate 16.7 watts at idle. This is 1.7 watts more than an NOS 2A3 can safely handle. The new production tubes are built to be stronger and can take the added wattage. With the output transformer’s 5000 ohm impedance, the amp produces 3.66 watts per channel, so says SECad software.
Here is a picture with the new Transcendar output transformers. Their frequency response is a full 20hz to 20khz. This really improves the bass response compared to the budget Hammond 125ESE trnsformers.