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Most odd grid choke behaviour.

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Have measured capacitance from the 640H grid choke terminals to ground,using an old BK capacitance tester.

Looking at the chokes,the LH terminal = 426pF on one,432pF on the other,and the RH terminal = 368pF on the first,370pF on the second. Barely significant differences?

RDF's idea of isolating/insulating appeals,thanks for that,it will be tried for curiosity,as I'm content with the performance I have now. -3dB@50K,-6dB@82K,-12dB@99Khz.

The CCS in question has been around since 2004 at least. Lengthy threads on AA describe it's development,some also wrestling with the question of accurate measurement of output impedance from the mu output.
Re: Ping Gary Pimm - CCS question - Gary P - Tube DIY Asylum
impedence of CCS mu output - bobbyj - Tube DIY Asylum are illustrative.

Or have I committed a mortal sin, by mentioning another tube site?
 
I didn't read the whole thread, but they seemed to be discussing how to calculate the output impedance rather than measuring it. I suspect that the top FET is messing up the circuit, in a way which improves it as a plain CCS but ruins it as a 'top-follower'. Decent measurements would confirm this. The reason people haven't seen this clearly is that they have used this circuit in situations where it doesn't matter e.g. low following capacitance, or with an added build-out resistor anyway. Of course, some may have seen the problem but not recognised it.

My thinking goes as follows. The top FET helps to isolate the lower FET from the supply rail, by acting as another follower so the drain of the bottom FET follows the output set by the source of the bottom FET. This increases CCS impedance, so is a benefit under normal operation.

To calculate output impedance in the 'mu' connection, you need to divide open-circuit voltage gain by short-circuit transconductance. Applying a short to the 'mu' output stops the upper FET acting as a follower - it is more like just a resistor added to the drain circuit of the lower FET. However, now it acts to reduce the required current flow. As the lower FET tries to draw some current the resistor in its drain restricts it. The lower FET is running with only a small source-drain voltage anyway, so it can't pull much current.

So fine as a CCS. No good for 'mu' connection.
 
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