>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.
Peter Chadwick said that was how much grid current he ran.
>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?
There are two flavors of Class AB2 grid-driven operation. Non-linear,
and linear.
>
>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.
Only with the RF-NFB config. listed on the RCA spec. sheet. The NCL-2000
does not use it.
...snip...
>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.
>
To make a 4-250A work at its emissive capability in Class AB1, SSB-mode,
I would use 800v on the screen and 4kV on the anode. I'm guessing that
pep output would be about 900W.
>IMO, there is nothing wrong with AB2 as long as the stringent criteria
>are met. It is not a no brainer for a beginner.
>
It's harder to build a dirty AB2 amplifier than a cleaner AB1 amplifier.
With AB2, you need a regulated grid bias supply. With AB1, the bias
supply furnishes zero current so regulation is not needed.
Rich---
R. L. Measures, 805-386-3734, AG6K
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