2A3 Amp

[Nick]

Out of interest you could reduce the 2nd stage bypass cap to 6uF for 1hz.

Unless I am missing something 6uf and 27k is 1Hz -3dB, but the 27k is in parallel with the cathode resistance of the valve. So I make 6uf would be closer to 18Hz for the -3dB assuming Rk is something like 1k6. 22uf would be 5Hz.

[Cressy Snr]

Finally managed to track down my 6H 150mA Collins potted choke.
Got it fitted and now the circuit is almost identical to the
Sun Audio SV 2A3, which uses a 5H choke in its power supply.
But 6H is as near as dammit.

Collinschoke.jpeg (67.58 KiB) Viewed 16463 times

[Paul Barker]

Nice.

[Andrew Ivimey]

Bit shoddy tho'

[Cressy Snr]

OK,
I had one NOS 6SN7, Fivre brand and thought, how can I use it in the amp?
So I fooked about over three days after buying a couple of Pinnacle 6J5s off Langrex.
Anyway, I ended up with three valves at the front end, a 6SN7 in the middle flanked by the 6J5s.
When I fired it up, the Fivre 6SN7 turned out to be worn out.
I put in a Sovtek 6SN7 courtesy of Mr I.
I does sound nice as it always has and both halves of the 6SN7 are at 70Va DC coupled and both 6J5s are at 76V.
Waste of bloody time though.

2A3AmpVers3.jpeg (121.23 KiB) Viewed 9509 times

[Cressy Snr]

Good news,
I found another NOS Fivre 6SN7 and this one is fine.
Sounds great.

[Cressy Snr]

Schematic showing both channels:
Those Pinnacle 6J5GT are excellent and good value for money.

SunAudio2A3with6J5.png (75.4 KiB) Viewed 8276 times

[Cressy Snr]

I had a couple of 1k5 resistors in parallel to give 750R for the 2A3s.
I've now tried a couple of Ken-Rad 45s in the amp. All it entailed was
sorting the cathode resistors so that there was only one 1k5 resistor
and moving the wire on the James OPT to 5k.

Also, I needed to increase the resistance in the power supply so that there was 275V
across the 45s. It's got 2/3s the power but it sounds really rather good.
Haven't used the 45s for ages.

I got a pair of Mission 734s off our Ant last week and they are 90dB efficient.
Excellent little speakers. It's only a tiny study so it's not bad at all for filling the room.

[Cressy Snr]

Pic:

IMG_20260424_162403097_HDR.jpeg (88.89 KiB) Viewed 1719 times

[steve s]

Looks great steve. I should get on and build something new, ive still so many old valves to try..

[Andrew Ivimey]

Marvelous! Is that a mini Mac I see.?

[Cressy Snr]

Marvelous! Is that a mini Mac I see.?

Yep!

[Cressy Snr]

Looks great steve. I should get on and build something new, ive still so many old valves to try.

I remember you telling me at Owston to get on with something.

[simon]

Got to love 45s

[steve s]

Got to love 45s

Its one of the valves ive not built with, ive a pair of 2 volt mazda px 4 equivalents that I would like another try out.. my 'office' has turned into a storeroom for the grandkids toys, theres a mofo half built in there, from 3 years ago, and a phono stage kit thats been waiting even longer