Chris Hornbeck wrote:

On Thu, 28 Apr 2005 16:54:49 -0500, DougC
wrote:

chapter 23, page 925, halfway down the page they have a list of
variables and what they signify. They have one capital-R
subscript-?something? for "valve input resistance at working frequency".
I can't see what this is, in either the CD or my copy, in either the
useage in the equation or the definitions. IS this an "i" or a "t"? Or
smoething else? The only relevant entry I can find is in chapter 38,
page 1366, under (B) Valve and circuit, which lists a lowercase-r
subscript lowercase-i for "input resistance".... ?:|

This is a Greek letter that I have long forgotten, and am too
lazy to look up. It stands for the equivalent input resistance
caused by cathode circuit inductance (I **** thee not), and
doesn't matter at audio frequencies.

Unless one uses a grounded grid circuit in a phono amp
where you have a highish gm j-fet driving the low Rin of a grounded grid
triode. This type of circuit has the tendency to become a
good oscillator at 100 MHz, so one has to treat it as an RF circuit
and carefully bypass things to prevent such oscillations.

Sould anyone try to drive a pentode connected grounded grid tube
with a j-fet, I wish then the very best of luck, since it is the land of
absurd outcomes ands wierd oscillation effects.

In such instances the gain of the fet between gate and source is less than
unity,
and the fet acts into a very low value load of the tube's cathode Rin,
so a small amount of inductance may well have a big significance.

Where the input device in cacode circuit has a high source or plate
impedance,
such as a j-fet, gain between the input and cascode tube load is very nearly
gm x RL, regardless of the
tube used.

So a 2sk147 with gm = 40mA/V with a 6DJ8 triode in cascode
powering a 20k load will produce an overall gain of 0.04 x 20,000 = 800.
If a 12AU7 or 12AT7 is used instead, gain stays the same.

The beauty of cascode circuits is that the input device acts will
very little gain in itself, so miller Cin is lower than if you try to use
the device for high gain to start with.
The grounded grid or grounded gate device following the input
device then also has very low Cin, circuit R is low, so
F reponse is very good.
But at the output of the second device, the outout resustance is very
high, usually a pure current source even if the output device is a
triode with low Ra.
So the load connected determines the Rout.
So if the load has inductance, and there is stray C, an LC resonant circuit
exists which may encourage RF oscillation.


Patrick Turner.




Good fortune,

Chris Hornbeck
"Don't panic."