• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

55 WPC 1625 Mono Block Amps


2010-04-24 3:08 am



These are 1625 mono block amps that were just finished by my good friend.

They sound excellent, and will sound much better as they break in. The output power is around 55 WPC. I had to special order a pair of power transformers for the voltage taps. There are separate power supplies for the 12SN7 and 1625 tubes.

The soundstage is very wide and deep, and a much more noticeable 3D effect than other amps I have on hand. The theory as to the depth of the soundstage is use of 1 watt PRP resistors vs. ½ watt units. I put in a pair of 12SX7 input tubes in the 12SN7 slots, and 1940's RCA Army Signal Corps output tubes. It also sports NOS ceramic sockets from the late 40's or early 50's, along with NOS chokes from Chicago Transformers from early 50's, mil-spec 10 turn bias pots, 1 watt PRP resistors, about 3 times the required reserve cap storage, Mundorf Silver/Gold coupling caps, Cardas copper binding posts, and Vampire RCA jacks. They are dead quiet, as quiet as or quieter than most solid state amps.

If one is going to use tubes, an amp like this makes a lot of sense. The output tubes are cheap and plentiful, long lasting, and MUCH better made than any current production tubes. The 1625 is a 12V 807 tube. My buddy says the war was won with these tubes. The 1625 tube sound a lot like NOS 6L6, but better (IMHO), as one can run the plate at higher voltage, due to the plate cap.

So, I'm pretty happy with them so far. Also, a note of thanks to Miles Prower is in order. Researching the threads uncovered an issue with oscillation that was easily corrected with the plate and grid stoppers. The posts by Miles helped me sort this out quickly.
Last edited:


2010-04-24 3:08 am

As far as the schematic goes, the exact circuit values are proprietary, so regretfully, cannot share them. My buddy got the schematic from a well know industry participant many of you know or are familiar with. He provides them to my buddy as a personal favor.

Look for the Radiotronics No. 124 on the web, which is a 45 watt push/pull design. It is similar to this: