[AMPS] AB1 vs AB2

[km1h@juno.com (km1h@juno.com)]

I have been following this discussion with extreme interest and wish to
thank all the contributors. 

What still has me confused is the low impedence vs high impedence of the
power supplies. Is a gas regulator tube high or low?  What about a series
regulated tube supply such as is in handbooks for years....using a triode
for the pass tube?

In a bias supply how about a gas tube regulator feeding a bias adjust
pot; the value to keep ~20ma flowing ?

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. Since the Svetlana 4CX400A is a virtual
clone (comparing E and I ratings, etc) this might warrant some further
discussion.

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?

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....???


73...Carl   KM1H

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