6P21S triode curves

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

6P21S triode curves

And here is a SPICE model for any volunteer to test:

 

Finally, here is a THD plot for the 6P21S showing how linear this valve can be. The following quiescent point was found to be very good for this valve:

  1. Ia=40 mA
  2. Va=200V
  3. Vg=-11V

 

Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

13 thoughts on “6P21S triode curves”

    1. Well, you’re not the only one. Paul Leclerq, Andy Evans and I are planning to build a PP amp with this valve.

      Many interesting ideas flying around design, so we may need to open a thread shortly 🙂

      Cheers
      Ale

      Reply

  3. I have read that these valves were designed fast heating in battery powered transmitters. To conserve battery power the filament supply only came on with the ‘push to talk’ being pressed on the microphone. This type of valve can have a short filament life, reportedly typically 500 hours.

    Reply

    1. Martin,
      Very interesting, the datasheet I posted is the only I have and doesn’t report any lifetime figures.
      Would be nice to have a more detailed one or some other feedback
      Thanks for your input!
      cheers,
      Ale

      Reply

        1. Interesting. Hopefully with filament bias and regulators lifespan may increase? Haven’t seen the original data sheet though.

          Is a lovely DHT which I’m sure we can build a great amp with…

          Reply

  4. Hey Ale,
    NICE!
    Have you also traced some pentode curves? I see this tube as a candidate for a SE Schade at 400V/250V. I also note you only go to 200V, why not higher? Would like to see the possibility to run it at 2A3 working point.

    Reply

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