6P21S

This is a great unknown russian DHT. Just like his brother 4P1L :). An output beam tetrode with direct heated cathode filament. This 18W valve is a great candidate for an output stage, with its standard filament voltage of 6.3V and octal socket, many ones out there will be really attracted by this valve as it can easily be adapted in many classic output stages.

I did some initial tests about a year ago and this valve proved to be very linear and capable DHT. Was planning to use it as a pre-amp valve with filament starvation as THD results were more than promising.

Paul Leclerq, Andy Evans and I now want to test this one as an output valve. With 18W+3.5W=21.5W of anode power capability, it’s a great DHT for output stages clearly.

So I went back to the workshop to trace the curves first.

Initial tests were done with If=700mA / Vf=6.3V