Another vote for Moth, especially if you have Craig Uthus, the original designer, now at Eddie Current, do a power supply mod to it. I talked about this in SET Asylum but I'll recap it here.
Craig created an outboard power supply connected via military-grade umbilical to the amp, mounted the 5AR4 rectifier on top of the PS box, and converted the amp from DC to ultra-high-frequency, 40kHz AC on the heaters. The power supply is regulated and much stiffer now, and the amp runs far cooler. It should last pretty much forever.
Here's some of the technical info Craig sent me:
The power oscillator board is comprised of a discrete 18 watt power amplifier designed to run continuous sine wave power. The amplifier has current limiting, and provides a soft start for the DHT's when they are cold. On the amp is a plug in surface mount crystal controlled 40kHz oscillator board with output level adjustment. A single twisted pair brings the high frequency sine wave up to the amplifier chassis. Inside the amplifier at the tube sockets are a pair of ferrite core transformers, which provide isolation from the power oscillator and drive the 2A3 heaters. The transformers are wound 2:1 so 5V in is 2.5 volts out. The high frequency is calibrated to exactly 2.5V at the tube socket with the tubes operating.
I'm not a tech guy; I'm a listener. How does it sound? Fantastic. It's immensely improved, and it was really good to begin with. The sound is much more liquid and organic without losing any detail, and the sound stage has opened way up. The bass is tighter and more authoritative, and the amp is more dynamic and seems more powerful than before when driving either my 99 db Galante Silverdales or 96 db Omega Grande 8R speakers. Plus, it's absolutely *dead* quiet, which the original was not. I mean, with the attenuator wide open and my ear to the horn, I hear no sound at all. Amazing.
Craig thinks this is now the most refined DHT amp he's ever heard, and he's heard just about all of them. He might offer a production version through Eddie Current. It won't look as cool as the Moth but it'll be reasonably priced at something like $2,800. It's really quite a mesmerizing amp.
In the mean time, if you own or find a Moth S2A3, I strongly suggest you contact him about getting this PS mod. It's really something special and takes this already outstanding 2A3 amp to an entirely new plane.
