It was mentioned here, so I decided to chip in with my previous experience from back in 2013. Here is what I tried myself:
The 4P1L pair in class A2 can deliver up to 6W with a 3K2 Zaa output transformer. I didn’t push the 4P1L hard and a 250V/35mA operating point was performing well.
One (of the many) advantages of the 4P1L compared to other DHTs (like 2A3, 300B, etc.) is that the grid drive requirements aren’t as demanding. The maximum power is achieved with 20Vrms. This means that with a simple driver stage of a gain of x10 or more, we can drive the output stage to full tilt.
I’m using a gain of 20 today with the EML-20A mesh DHT. However, back then I used the 6e5p driver which provides a higher gain but is a fantastic driver, albeit not DHT. It does sound really nice, it’s one of my preferred ones.
If I were to build this amplifier again today, I’d do the following:
The driver is the 6e5p (or better 6e6p-dr) which is very linear and works well with a bit of degeneration in the cathode. As the stage is a hybrid mu-follower (i.e. gyrator load) the reflected cathode resistor in the anode isn’t impacting the output impedance. The follower is a classic MOSFET (high GM, low Crss) follower using a symmetric +/-80V to provide enough headroom. The HT is a bit marginal for the driver but provides good headroom for the 4P1L.
The rest of the circuit is self explanatory
As I don’t need the A2 power, I end up with the 4P1L PSE with filament bias I have now. Probably will rebuild this version as the follower drives well the PSE pair. I’d go for 3 valves next time.
Hope this helps.
