Somewhere along the line Rich came up with a value of 25 ma grid current
for a pair of 4-250A's in AB2 grid driven service. Now, I dont see that
number in any literature here; the closest is in a 1964 ARRL Handbook
where they rate 10ma PEAK at 3KV Ep operation.
But didnt Rich also say that only the 4CX1500B and 4CX600J were rated for
AB2? What gives here anyway?
Note that the Ig rating is peak....which means what you see on key down
A0 tuneup. Not what the damn meter is going to bounce to.
I run a NCL-2000 on 6M in AB2 which is what it was designed for. It uses
a pair of RCA 8122 tetrodes in swamped grid driven service. At key down
I see 25-30ma Ig for the pair of 8122's. On SSB the grid meter barely
wiggles on voice peaks which is exactly what the manual says to do. The
output is a good 1200W PEP and RCA spec sheet claims -29dB IMD in this
configuration. The locals say it is very clean...much better than brick
amps at a tenth the power. I did regulate the screen supply; the bias was
already regulated.
A pair of 4-250A's or 4-400A's would show somewhat equivalent meter
readings in AB2 IMO. Maybe 20-30ma key down Ig for a pair and a wiggle on
SSB. The difference between AB1 and AB2 is that wiggle. That wiggle means
the grid is consuming power and is just not seeing a voltage swing across
an equivalent open circuit.
According to RCA, way back in Dec 1954 Ham Tips, is that a 4-250A in
AB2 requires 4.5w of peak drive. Yet at the same Ep the output is 520W
vs 370W in AB1. I would think that a 41% power increase would rate a
little wiggle...dont you? On top of that the Es requirement drops from
500V to 300V.
IMO, there is nothing wrong with AB2 as long as the stringent criteria
are met. It is not a no brainer for a beginner.
73....Carl KM1H
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