MagneQuest/Peerless Forum: Hank Murrow's 2A3 Amp *pics* by Caucasian Blackplate

This is the last (for now) piece of Hank Murrow's system that I have built.

Each channel is fed by a fully shunt regulated power supply, with a Bottlehead PT-5 power transformer at work. The filament supplies for the 2A3's are full-wave bridge rectified, with Mikey's filament chokes and Panasonic CMC's. No capacitors are used in this supply, just lots of iron.

The high voltage supply is a full-wave doubler with Cree diodes, then a parallel C4S current source in to the PJ/Broskie hybrid shunt regulator. I used a 6S4 for each channel as the "current burning" triode, as the 6S4 maintains low cathode voltages to protect the TL-431 from excessive input voltage.

The audio circuit owes much of its design to Danlaudionut. It is a CCS'd 6GK5 cap coupled to a 2A3. Hank could not resist the new Copper Foil and Teflon capacitors from V-cap, so we ordered the largest value they had. I must say, these are some very special caps! The 2A3 is cathode biased through high wattage Kiwame resistors bypassed by one giant Solen Polypropylene. Output iron is a parallel feed combo of the TFA-2004JR-Ni/Co with an EXO-003 plate choke and Pyramid 6uf vintage PIO capacitors. There are also cobalt grid chokes on the grids of the 2A3's, and MQ15335 input transformers wired as grid chokes at the input.

At Paul Joppa's suggestion, I added an external supply to bias up the LED string on each of the primary CCS's that feed the whole circuit. This allowed a bit more current headroom, and cooler iron temperatures. I also installed a 12v fan (running at 6.4v) under the power supply section of the chassis to keep things very cool. I also installed an LED indicator in each channel to indicate failure of a 2A3.

So far this amp is extremely quiet on my AKG-K1000 headphones, I'm sure Hank will really enjoy it! I will post some photos of the inside later, despite the size of the chassis, it is very packed inside.