Monday, April 6, 2020

"MWH" - 1920s UX-171A tube preamp







I was offered the challenge of building a DHT triode preamp and offered a schematic. But there was way too much sand in the design with an active load supplied by a FET. There was also a CCS reference in the cathode cct. Any delicate suppleties in the DHT tube were to be lost and buried in all that sand. Not for me. I was shaken and a little stirred and felt a Bond to the UX-171A tube which in the past I had built a power amp around ("Intermezzo"). I have a number of 71A pairs and now the opportunity to use a pair.

Fortunately an American DIY buddy, Bruce of oddwatt fame, came up with a simple single stage designe. Using close to century old tubes I felt a Hammond retro wood cheek Al chassis was the way to go. Add to that a Hammertone blue poxy enamel paint job, a big gold retro knob and ceramic sockets and the look is there. The tubes are RCA and they use nickel plates. 

Keeping old school the output cap is a Russian military PIO 1uf metal jaket. Never done it before but this time earthed the metal (which is actually copper) jacket. Not sure any improvement but like to think it can't hurt. Due to hum issues I moved the 24 step stepper pot to after the output cap. 

There are two selectable inputs and the preamp uses my standard external PS which works with any of my 6SN7 preamps of which I am currently using two. The sound is extremely transparent and captivating. Pleased with the build, both the way it looks and sounds.




1 comment:

Pete B. said...

Looks great- would you please share your schematic?