On Fri, 25 Jul 97 21:42:09 -0700 Rich Measures <measures@vc.net> writes:
>>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.
You are wrong Rich. It is More Linear and Less 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.
What is a RF-NFB? If you mean a cathode resistor, the NCL does not use
one. According to the RCA sheet, adding a 20 Ohm resistor will decrease
IMD by an ADDITIONAL 5 dB.
I also made a slight error...in AB2 the 8122 is -28 / -32dB for 3rd/5th
orders. The tube in AB2 has a peak Ig of 3ma and a peak drive power of
only one half watt. This is at its maximum suggested ratings also, not at
some substantially lower voltages.
>
>...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.
Sounds like an excellent candidate for screen meltdown IMO.
>>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.
I wasn't aware that a regulated bias supply was that complex and
difficult.......
I would also suggest one for AB1 just in case you develop a bit too much
drive while tuning, debugging, etc.
73...Carl KM1H
>Rich---
>
>
>R. L. Measures, 805-386-3734, AG6K
>
>
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