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[AMPS] AB2 Amps

To: <amps@contesting.com>
Subject: [AMPS] AB2 Amps
From: measures@vc.net (Rich Measures)
Date: Fri, 25 Jul 97 21:42:09 -0700
>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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