I was commissioned to build an amp based on 100 year old UX-171A (71A) tubes. Fortunately I had a few pairs and was able to match two. The driver tubes - 6SN7 - are Ozzy RCAs which I also had. The rectifier tube is an"80". Along with the power tranni, base and top plate, some caps, choke, ceramic sockets and OPTs (Edcor) I needed just a few minor components.
The schematic I had only found a few days before the request to build the amp. The recipient was happy with all vintage tubes though I suggested, what I felt might be better, other driver tubes. Suited me, I had all the above parts and tubes for years and was looking for an excuse to use them.
Colour is Ocean Blue Hammertone epoxy enamel. Base is Australian Spotted Gum and heavy. Filter is double Pi - CLCRC - with further RC feeding the driver stage which is SRPP. 71As are DHT and the amp is a DHT SET producing about 1/2W. That's low power but the amp will be used with very high efficiency speakers. On my 92db speakers it played plenty loud enough for me. The "80" rec. tube, double filtering and 5H Hammond choke provide a dead quiet robust PS with 260uf of caps. Total current drain is low at 40mA. 1.5mV of noise measured in my system with all gear on. Nothing to hear at the speakers.
The Hammons power tranni is way over spec but I had it on hand and time to put it to use. Inter-stage caps are Russian PIO copper jacket types which always sound good. The jacket was earthed. I used solder from the 60s and single strand copper wire to ensure a retro sound which the client was after. The top plate is fused and earthed. Sound is silky smooth (think low power 45 or 2A3) with exceptional bass, creamy smooth mids and clear highs. Highs are not over stated and I would say the amp is a little on the dark side providing a very relaxed presentation.
