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Original Message

RE: A triode driver example

Posted by Tre' on June 26, 2012 at 20:01:05:
"Note that the OP asked about the first stage of an amplifier. If that stage happens to be a 6SL7 or 12AX7, Miller rears its ugly head in a way that can be difficult to deal with."
Yes. I looked at his system and I see that he uses a 4kn8 choke loaded line stage.
I couldn't find a data sheet for that tube but I did read that the transconductance is higher than a 6dj8.
If I assume that the plate resistance is the same as the 6dj8 or no higher then the output impedance will be the plate resistance with the plate choke in there (at least through the midband and lower everywhere else).
If I assume 3k for the output impedance I don't think I'm too far off.
The Miller of a 12ax7, assuming a in circuit mu of 65, will be about 112pf. Let's call it 120pf.
3k (assuming a very high value grid resistor on the 12ax7) driving 120pf has a -3db point of 442kHz.
If the 12ax7 is actively loaded and the in circuit mu is the mu of the tube then we have 1.7pf (grid to plate) X 101 + 1.8pf (grid to cathode capacitance) for a total of 173.5pf
The -3db point would now be 305kHz.
I don't see any reason to load down the preamp tube with a low value grid resistor on the first stage of the power amp (even if it's a 12ax7) when you have a 3k drive impedance to start with.
Unless there is a lot of cable capacitance HF roll off shouldn't be a problem.
Tre'