6P21s 6n21c tube, True Direct Heater?

[50AE]

Hey Fuling, thanks for sharing!

I'm wondering how would this tube perform in pentode mode + CFB.. or schaded?

 

[Fuling]

Pentode mode and Schade feedback is to be tested before the test rig is disassembled. I tested UL with CFB but there was no real increase in output power (with grid driven positive).

 

[Wavebourn]

Conclusions so far: 6P21S seems to like A2 operation and it doesn´t seem to require too much of a driver stage to do get there. The question mark is why it still seems to work fine with a 500R cathode resistor in place? My best guess would be that this tube draws an unusually small amount of G1 current.

Bias definitely changes, but you don't see it because it gets to the stationary state when you observe it on the oscilloscope. Try switching the input signal level abruptly and see the result at the moment of switching. It would be audible though, dynamic distortions do not sound good.

 

[Fuling]

Good point there Wavebourn, I knew it seemed too good to be true

I witnessed something similar in my earlier experiments with 6P21S driven into grid current by a 2,25:1 interstage transformer. The cold side of the secondary winding had to be connected to a negative supply rail with a very low impedance , otherwise the bias would drift for a second or two after each peak of grid current.