> What still has me confused is the low impedance vs high impedence of the
> power supplies. Is a gas regulator tube high or low?
You could measure it by doing a load pull. Change the load and
measure the voltage change, that would give you an equiv ESR.
> What about a series
> regulated tube supply such as is in handbooks for years....using a triode
> for the pass tube?
Series regulators generally are more complex and difficult to
build, but do dissipate less power. Unfortunately when you add a
bleeder resistor to absorb reverse screen current, the heat advantage
usually disappears. Series regulators also require you add some form
of over-current protection.
Shunt regulators with low ESR are better. If the supply voltage is
two times the screens operating voltage, it is impossible to over
dissipate the screen grid. Screen dissipation is limited to the
standing dissipation of the regulator.
> In a bias supply how about a gas tube regulator feeding a bias adjust
> pot; the value to keep ~20ma flowing ?
A better way is to use a HV NPN transistor, with the collector
grounded. The base goes to a pots wiper, the hot end of the pot goes
to a regulated source higher than the grid requires. The emitter to
the grid return (through a low dc resistance choke) and also through
a large resistance to a negative supply higher than the grid
requires.
Whenever the emitter (grid) becomes too negative, the transistor turns
on and clamps the voltage. You would have to pay attention to
transistor beta and available base current to be sure the supply was
stable.
> I finally dug out a full RCA spec sheet for the 8122 tetrode as used in
> the NCL-2000 and was amazed to see that they list AB1 and AB2 operation
> with virtually no change in IMD.
Right. Many or most tubes are that way. Grid current has little to do
with IMD, UNLESS you let the grid swing around. Most people agree on
that.
> The 8122 is listed in "typical RF Linear Operation" at 2000VDC for AB1
> and 2500VDC for AB2. The 3rd/5th IMD are 29/32 and 28/32 respectively.
> Grid currents are .05 and 3ma respectively and power outputs are 380 vs
> 570W. The power difference makes AB2 attractive but what am I missing
> here?
Nothing at all. That's generally how it works. Some PA's actually
have BETTER IMD performance in AB2.
> The RCA sheet also suggests a 20 Ohm cathode resistor to obtain an
> additional 5 dB IMD improvement. I have tried that in a few stock
> NCL-2000's years ago but always wound up with instability....???
Why? Did you neutralize the tube?
If you do do that, be SURE you do not bypass the resistor. Bypass the
resistor with too much C and it will ADD IMD.
73, Tom W8JI
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