[Mark Aitchison]

west wrote:
I built a typical phono amp with 3 12AX7s... CF... 100k step resistor volume control
,,,My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem

You're using a 100k 'pot' effectively to drive a cable...
With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz !

Your answer is what I suspected but not having much experience, I asked this
NG. I was looking for a Tube Basic 101 generic answer that I did not find in
my reference books. I don't understand where Patrick is coming from....

(1) the limited bandwidth problem being only there is presumably due
to the high output impedance (100k step resistors not the same as 100k
potentiometer, which is not the same as a 100k output impedance though -
you might not even need 100pF to get a serious high frequency cut). The
test is to measure (on a scope or whatever, with a low capacitance
probe) directly at the output socket, with no cable connected, what is
going on with the frequency response. The trouble with this test is that
you said it is a phono preamp, so you have to rig up a reverse RIAA
circuit for your signal generator's output. Another, easier, test would
be to shove a 22k resistor or so across the output and then see if the
valve amps are then happy. Presumably the solid state amp's low input
impedance is reducing the effects of the RC filter you make with the
interconnect cable. Or the valve amps have lots of capacitance/Miller
effect at their input and want a lower input impedance (not so likely to
the a significant factor I guess). If the oscilloscope test shows a
high frequency cut is still going on, maybe the RIAA values are wrong
and maybe the SS amp has some slight high frequency lift (some nfb
circuits do this) that is compensating - two wrongs making a right??
(pretty unlikely).

(2) I get the feeling Patrick sometimes growls about the questioners
as a sign of friendship.

Mark A