More than 4 years ago I ran a lovely 71a preamp which sounded amazing. I used it for some time and enjoy its sound up until I continued with my exploration around DHT preamps. Recently I was asked about how to implement this lovely valve again.
The CX371a / 71a valve is a great candidate for a line stage with its low mu and anode resistance. In my experience you have to run it above 20mA and over 100V to get the best out of this valve:
The implementation of this preamp is dead simple and a few components are needed on top of the gyrator PCB:
I haven’t starved the filaments as I found this valve not to be microphonic. If you have an 01a preamp you can modify it slightly. The interesting thing is that you can run it with just 180V. Even 150V should work and you need 25mA on each channel. A J310 or BF862 lower JFET device will work fine and you will need a heatsink for the top device (e.g. DN2540). Filament resistor is anything close to 50Ω. I used some 51Ω Russian NOS wire wound resistors, but any combination will be fine.
Enjoy
Ale
Do you think inductive vs. non-inductive resistors matter? I have a ton of old enamel wire wounds and plan to use those for filament bias.
A wirewound resistor has no excess noise and only Johnson noise. They tend to have very low inductance (especially when values are low) and this isn’t a problem in a low impedance circuit like the cathode of the DHT. Not needed in my view to have a non inductive one. Different story in a Phono stage though.
Reading your posts is always interesting Ale! Your have that talent for mixing old and new stuff .
Thank you Pierre, glad you like them. Feedback is always welcome