Push-pull 520B DHT amplifiers
I've had these amplifiers for a while, and they are my "main" system amps. They use push-pull Emission Labs 520B directly heated triode tubes - more or less 50 watt 300B tubes. The operating point is 500V, fixed bias, running about 90mA per tube, into a 6k:8 OPT. Output power at this point is 45-50 watts, mostly class A put pushing into AB. There is no NFB.
The driver uses push-pull triode connected D3a tubes. Transformers are used at the input and interstage. All the audio transformers are Lundahl - the power transformer (mounted on top under a cover) is a custom wound toroid.
I designed several custom PC boards for this amp. So far I have not offered them for sale as they are sorta specialized, and I've not really documented them for others to build. But I will post here at least schematics and PCB layout screenshots so you can see what's in here.
Also, a bunch of photos of the inside. If you are a skilled amp builder you could re-create something similar.
The inside. Power supplies are mounted on the sides of the chassis. The LL1620 OPT in the center, a PCB holding the LL1660 interstage and a couple of bypass caps, then the driver PCB with the d3A tubes and an LL1676 input transformer. Grid stoppers for the 520B and a B+ bypass cap (the yellow one) are "floating" between the tubes and PCBs.
The LV DC supply, which is posted on its own page here:
Note thermal pads on the Vicor modules to heatsink to the chassis:
HV power supply:
Current sense PCBs, mounted to one side of the LL15290 OPT. One PCB for each tube, they sense the actual plate current using a shunt resistor, isolated instrumentation amp, and an isolated power supply.
The driver PCB, with the d3A tubes and input transformer:
Picture of the rear panel:
