This is a single ended 6s4 amplifier with a 6dj8 SRPP driver stage. The design is by Gary Kaufman who has built many cool amps with unusual tubes. The 6s4 is a small 9pin indirectly heated triode.

If you look closely you can see the Digital Equipment Corporation logo on this surplus chassis.

The circuit is a fairly straightforward SRPP driver into a conventional single ended output. I am using the Hammond 125BSE output transformer which is really far too small but I had a pair left over from a guitar amp project I never built so what the hell.

Gary did not specify a power supply but suggested a B+ of about 200V. I used a 5y3 rectifier into a CRC filter for each channel with a 15K 25W dropping resistor to hit 200V. This large resistor makes this a very quiet power supply, and is necessary because the power transformer I used puts out about 280V per leg unloaded. The driver stage supply is decoupled from the main B+ with a 22K resistor and 16uF capacitor (10uF shown in the schematic).

Wiring was very easy in this large chassis. The gold heatsinked resistors in the power supply are rated for 25W and are dissipating about 2W so they don't really even heat up.

Detail of the 5y3 socket. Love those cloth insulated transformer leads :)

Detail of the power supply caps. I like to hot glue them together to make everything a little more solid and also a little easier to work with. You can also see one of the chassis ground lugs on the left. I have a lug for each channel and then connect them both to the ground wire from the wall plug. Seems to be plenty quiet.

Detail of the 6s4 (left) and 6dj8 (right) sockets. Those 9pin sockets get pretty crowded. The big yellow coupling cap is from a vintage oscilloscope module made by Hughes Aircraft.

I would characterize the sound as being very neutral with exceptional imaging. Bass and smoothness seems to be limited by the very small output transformer. Even just a few hours of breakin have helped the sound considerably, and it will continue to get smoother I bet. I would put the power output somewhere around 2W. At maximum volume I swear I can start to really hear the output transformer, so I think I will have to break down and get the Hamond 125ESE or something.

Update [2002-3-5]:

Gary Kaufman sent me a couple images of his final revision of this amplifier. It looks like the only major change is the cathode bypass capacitor on the 6s4. This new schematic also shows a 6n1p instead of a 6dj8. My understanding is that the 6n1p can be substituted for 6dj8 types if the higher filament current of the 6n1p can be supported. I have not tried a 6n1p, but I have heard good things about them.

Update [2002-3-14]:

Here is the load line at this operating point. Yes it really is that low.

(image is linked to the full size image)