2A3 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 my own low sensitivity speakers
of about 88dB/W/M.
Of course with horns with sensitivity at least 100dB/W/M, there is no need for more
than 4 Watts.

2A3 stereo amp
The two "transformers" nearest the output terminals on left side of picture are
a Hammond choke and Hammond power transformer for HT for B+ and 6.3V
for input triode heaters. There is another transformer under the chassis for the
2.5Vac heaters for the 2A3s.

I had a schematic which I had drawn up neatly somewhere but can't find it now.
Design your own! Nobody would want to build my idea anyway.
It is very simple. Each channel has 1/2 a 12AU7 input triode feeding a paralleled 6SN7
driver for the 2A3 with approx 10dB global NFB from output to 12AU7 cathode 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 made as follows :- A length of 50mm x 25mm x 3mm wall thickness
anodized aluminium channel is mitre cut at the corners with hack saw, then filed
carefully to make mitre joins nearly invisible. I forget overall plan size, but about
500mm long x 250mm wide. Accuracy of the cut and joined corners is important,
and the 4 sides must be tried together while clamped down onto a template
and holding blocks on a bench so that when all 4 sides are together, the two
diagonal measurements are each equal, within 1mm. The chassis top and bottom
plates are 1.6mm thick natural aluminium and made to sit over top and bottoms of
channels but kept back 3mm from channel edges. All sharp exposed plate edges
were filed neatly and sanded with 400 grit paper. Top plate was kept free of scratches
while drilling holes for sockets, ventilation holes around power tubes, bias pots,
bolts etc.
The chassis parts were all well held together with miles of masking tape
while drilling machine screw "pilot" holes through top and bottom plates with 3mm drill for
4mm machine screws, and at not more than about 120mm apart. Use too many screws,
and never ever use too few!
The tape was removed from plates only, but left around outside of channels so the
relative positions of screw holes are not disturbed.

Pilot holes for screws were tapped for machine screws.
The plates were "linished" with 600 wet and dry sand paper using turps as lubricant,
and keeping paper washed clean, while sanding in ONE direction only.
After all holes were drilled and de-burred and or linished and looking good,
screws were all tried to check positions. Tape around the rectangle of channel
was removed, and when I was happy there were no horrid gaps and all looked well,
the bottom plate only was removed, and 4 lengths of 25mm x 25mm Alum angle
40mm long were glued into each internal channel corner using Selleys
401 industrial strength silicone. Then the bottom plate was screwed back on using
4 screws, and chassis left to cure a day before the next lot of work. The angles in
internal corners will hold the channels together if both top and bottom plates are ever
removed, but while the top and bottom plates remain fixed, the chassis is strong.

You NEED the channel wall thickness to be 3mm to prevent screws stripping
threads and to give the chassis enough strength to resist bending if dropped
with heavy transformers bolted to it. Shit happens!

This amount of metal work
is going to challenge those who have never done anything much with
any tools, and have no tools or workshop, and or any patience.
The above is how I did the chassis in the pictures. It looked very neat, and just
like the work of the dedicated expert.

An easier way to make a chassis :-
My customer who ordered me to make the amp placed the Hammond trannies and 2A3s on my bench
and bribed me suitably to use them. This was the rare occasion where I have not wound my own
OPTs and PT and choke, but it all worked out fine.

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