Frequency response vs. supply voltage

I have made some tests on a small cathode follower circuit. It uses half a 2C51 (5670, 6385, WE396A, 6N3P etc). I have tested various data with two different supply voltages (125 and 150V).

On the low voltage the -1dB point is at about 350kHz, and on the high voltage at about 430kHz. Why does the frequency respone change with the supply voltage?

The maximum output and distortion also changes, which I do understand, but what about the frequency response?

Best regards,

Mikkel C. Simonsen

Just a guess:

The higher supply voltage increases plate current, possibly causing the the tube's plate resistance(output impedance) to drop. This might reduce the high frequency loading caused by capacitive elements in the circuit at the tube's output.

Dean

I'm with Dean. The tube's transconductance is going up, and since the output impedance of the cathode follower is approximately the reciprocal of the transcoductance (1/gm), the output impedance is lowered. The output impedance of your cathode follower is working into the capacitance of your probes as well as that of whatever load impedance is connected to it.

Thanks for your replies. And yes, of course that's what's happening - why didn't I think of that myself...

I'm not used to being able to measure these things, so now when I have working test equipment I measure a lot of things...

Best regards,

Mikkel C. Simonsen