"Scott Dorsey" wrote in message
...
In article .com,
wrote:

Or you could use a different tube for the output stage.

do you think the 6072 makes a poor follower? it'll just be driving the
5 or 10k load of an AD converter, and i don't plan on using long cables
between the two. do i need a higher current tube, like a 12AU7 or
something? since i'm modding, i'm trying to minimize the
pain-in-the-ass factor, and also not bludgeon the PCB too much. the
preamp used 12ax7s before.

I don't know, I never did the math on the 6072. But if you look up the
follower section in the Radiotron Book, there's a method for calculating
basic linearity from the load lines, and there's a formula for figuring
the effective output impedance with a given cathode resistor. Your whole
goal is to balance voltage swing with output impedance and get the best
combination for the device you're using. My personal feeling is you
probably want the load to be at least four or five times as high as the
output impedance of the stage.

Not really -- you want it to be a good deal higher than that. Typically a
cathode follower circuit has an output impedance of some 300-600 ohms, but
hang 3k on its output and suddenly it sucks, because you've eaten up most of
its open-loop gain.

Sean's wanting to drive a soundcard input. If it's 5k input impedance I'd go
for something with some real muscle, like a 12FQ7 (same tube as a 6SN7 but a
different filament setup, and it's a 9-pin). If it's 10k a 12AU7 will do.
The goals are linearity and good current delivery, and those two tubes are
good in both respects. The 6072 is basically a mil-spec version of the 12AY7
which has low microphonics, but only so-so linearity and current delivery.

Peace,
Paul