4 WATT + 4 WATT SINGLE ENDED
TRIODE STEREO AMP.

This stereo amp was made to power horn speakers.
I was able to get 4 watts from each channel in SET.
The large transformers behind the 2A3 are Hammond 1627SEA
OPT for 2.5k : 4,8,16 ohms.
I found they gave exemplary technical performance and the amps
gave the best 4 Watts I have ever heard from a small tube even
with low sensitivity speakers.
Of course with horns with sensitivity at 100dB/W/M at least so
there is no need for more than 4 Watts.


The two transformers nearest the output terminals,
left side of the pic are a Hammond choke and Hammond B+ transformer.
There is another transformer under the chassis for the heaters for the 2A3s.

I have a schematic which I have not drawn up neatly for the web just yet.
It is very simple with a 1/2 a 12AU7 input triode powering a paralleled 6SN7
driver input for the 2A3 with approx 10dB global NFB to reduce Rout from
2.5ohms with zero global NFB to 1ohm when using the 8ohm output tap
connection. Nothing is technically extraordinary, but there is beautiful sound
with horns or sensitive speakers such as Tannoy dual concentrics.

Someone asked me "Don't you reckon those OPTs are a bit too big?"
"Nah, not for the big sound from the tubes...." I replied.

When in doubt, ALWAYS buy bigger iron than you need; size matters.

The chassis is a rectangle of 50mm x 25mm x 3mm wall thickness
anodized aluminium channel mitre cut at the corners
then filed to fit nicely and held together with internal angles fitted and
siliconed into each corner. The Al chassis top plate and bottom cover are
well screwed to the channel and the chassis is quite sturdy.
An Alps gain pot, 'on' switch and blue led is at the front,
IEC mains input, fuse holder, output terminals and input terminals
at the rear.

The owner placed the Hammond trannies and 2A3s on my bench
and bribed me suitably to use them. This is the only occasion where
I have not wound my own OPT and PT and choke.

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