46 DHT driver final tests


Having built the 4P1L filament bias driver stage in a breadboard, I now have the sufficient voltage swing to drive the 46 to maximum sweep. In my 4-65a SE amp, a maximum of 200Vpp is required to drive the amp into class A2.

The following tests conditions were used:

  • 4P1L first stage:
    • DN2540 gyrator in mu follower output
    • 220nF/450V Capacitor coupled into 46 driver
    • Filament bias: 15 ohms, Vgk=-10V
    • Vsupply=355V and Va0=210V
    • Output set to about 30-32Vpp to drive 46 at 200Vpp
  • 46 driver stage:
    • IXYS 01N100 gyrator in mu follower output
    • Load impedance is 100K (Pete Millett’s interface)
    • Filament bias: 10 ohm / 100W Vgk=-17V
    • Vsupply=355V and Va0=204-208V
    • Output set to 200Vpp

I tested 28 valves. Just a few of my lot are NOS. The average THD was about 0.4-0.5% but a good selection of 8 valves (mainly Sylvania NOS) provided a consistent 0.18% THD:

4P1L into 46 driver test2Happy now with the initial tests and selection of 46 pairs for the amplifier, I can now continue with the build…