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[AMPS] AB2 Amps

To: <amps@contesting.com>
Subject: [AMPS] AB2 Amps
From: km1h@juno.com (km1h @ juno.com)
Date: Sat, 26 Jul 1997 11:59:33 EDT
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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