.......
>What still has me confused is the low impedence vs high impedence of the
>power supplies. Is a gas regulator tube high or low?
Fairly low
>What about a series
>regulated tube supply such as is in handbooks for years....using a triode
>for the pass tube?
Mu=2 triodes such as the 6080 and 6336 can be used to make low R
supplies. The Rout depends on the gain of the comparator. High gain
equals low Rout. However, too much comparitor gain can lead to
instability.
>
>In a bias supply how about a gas tube regulator feeding a bias adjust
>pot; the value to keep ~20ma flowing ?
It works.
>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.
As I recall, they used RF-NFB in Class AB2.
I can run my 8169 amplifier in Class AB2 with 50uA of grid current, and
virtually no one can tell the IMDifference between running it in Class
AB1. However, with 1mA of grid current, this is not the case. In Class
AB2 the grid currents can be quite different.
>Since the Svetlana 4CX400A is a virtual
>clone (comparing E and I ratings, etc) this might warrant some further
>discussion.
The screens appear to have different characteristics.
>
>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?
Beats me. There are a plethora of variables. However, run 2500v on the
anode, 400v on the screen, 10 ohms of cathode feedback R, and one 8122
will can deliver more than 600 clean SSB watts with ZERO grid current.
>
>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....???
A low-L resistor helps. The 20 ohm cathode RF-NFB feedback resistance we
supply consists of 5, 100 ohm Matsushita MOF units. These resistors have
an intrinsic-L of about 10nH each. The 5 resistors should be distributed
among the cathode pins and directly grounded to the chassis. No bypass C
should be used. // NCL-2000 owners report that low Rp VHF suppressors
appear to improve stability. Except for those who have a free lifetime
unlimited supply of 8122s, adding a shunt zener series-string 400v screen
regulator is a must.
Rich---
R. L. Measures, 805-386-3734, AG6K
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