Directly heated triode amplifier
Following the success of my KT88 SE amp, I decided to explore directly heated triode (DHT) territory. Again, I asked Kenji for advice and he kindly provided a special circuit diagram tailored to my goals. I wanted to try both the 300B series, high output valve and also lower power but perhaps more "musical" ones like the 2A3. Both are readily available new from several manufacturers and over a wide price range, as well as less readily, used or NOS antiques (with commensurately higher prices...). A pair of new Psvane 2A3 costs about $100 plus shipping.
Layout
Like the KT88 amp, the most important hardware design task is to minimise hum pickup by the OPT from the power transformer. Tests with the James SE OPT and VT4C mains gave the same layout choice I used before- mains on one side and OPT together on the other side of the chassis. To make better use of the remaining space and keep the input and mains separate, the triodes and 5Z3P ended up in a row. This leaves space for preamp and driver valves.
Heater supply
I also wanted to use regulated DC heater supplies for the DHT valves and spent some time designing this section before I ordered the specially wound mains transformer from VT4C. This has 4V and 8V windings for the 2.5V and 5V heater types respectively. With hindsight (or more experience) I'd have used 7V for the 300B heaters as I had to add a dropper resistor to help keep the regulator ICs cool. A switch changes the regulator divider resistor value and input (winding) for the different heater voltages. Although the heater supply works well, it ended up larger than anticipated and this area is a bit cramped for space.
Point to point wiring
The amp is point to point wired with the ground return using heavy copper tube and the remainder of the wiring between the valve sockets and glassfibre tagstrips. I used a 10H choke and C input pi filter for the B+ supply.
Finished item
I am quite pleased with the result- it sounds as good as it looks and the "WAF" (wife acceptance factor) is also high.
I have listened to it with Guiguang 4300B, 2A3, Psvane 2A3 and (with appropriate cathode bias) a pair of old type 45's from the 1950's, which are extremely nice but low output tubes (a watt or so). But, as you rapidly learn from amps like this, the first watt is the only important one: mostly at home you only use it and the other 10 or 100 or whatever are largely irrelevances. At 10W the speakers are contributing more distortion than the amp....
I have listened to it with Guiguang 4300B, 2A3, Psvane 2A3 and (with appropriate cathode bias) a pair of old type 45's from the 1950's, which are extremely nice but low output tubes (a watt or so). But, as you rapidly learn from amps like this, the first watt is the only important one: mostly at home you only use it and the other 10 or 100 or whatever are largely irrelevances. At 10W the speakers are contributing more distortion than the amp....