[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Karl-Arne Markström]

Neither the 4CX250 nor the 4CX10000 have any DC grid returns,
which would lead to unstable DC operating points to say the least...

To me this looks like drawing errors, maybe in combination with schematic simplifications to enhance
"readability"

A suitable point to attach a bias feed network could be at the "cold" ends of the roller coils
in the 4CX250 and 4CX10000 grid circuits.

Also, be prepared to go for some "parasitic hunting" in an amplifier circuit like this,
as there are no parasitic suppressors or grid loading resistors, 
and the screen bypass of the 4CX10000 is shown as a single 0.005 uF capacitor.

It is not incidental that the Collins 208U-10 and HF-8022 used directly grounded screens in their
amplifier circuits, as it usually makes high-gain class A and AB amplifier chains very much more "docile".

Early Philips and Telefunken PA's used bypassed screens and filaments quite similar to this circuit
and could sometimes be quite temperamental.

73/

Karl-Arne
SM0AOM



----- Original Message ----- 
From: "Tom Cathey" <K1JJ at comcast.net>
To: <wc6w at juno.com>; <amps at contesting.com>
Sent: Saturday, December 17, 2005 5:45 PM
Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback


> Marv,
> 
> Just as a note on your Hughes sample amplifier URL below....
> 
> I see no negative DC bias on either the 4CX-250 nor the 4X10. Strange that 
> it's not there. Bias is OK on the 6146. Would appropriate bias be introduced 
> via an RF choke at the grid of each tube?  Should the 4CX-350 run deep class 
> A like the 6146?  Also, the schematic shows the 4X10 with a grounded 
> cathode, though it uses a directly heated filament. Would you bypass the 
> filaments with .01's for the equivalent circuit and still have the 
> feedback/neutralization work OK?  Just want to double check...
> 
> I'm trying to scale this down for a 4CX-3000 with the same driver chain.
> 
> http://wc6w.50webs.com/wc6wamps/popp.html?h10a1.jpg
> 
> As a note to Bill.....   The 6db manufacturers peak IMD test is very 
> interesting. I see it used all the time in literature...  :-0   I never 
> realized what they were doing.
> Much like the old F-B antenna specs.
> 
> Yes, I will put together a two tone set up and get some real numbers, though 
> I am convinced that the FT-1000D 10mW tap is going to be usable at this 
> point.
> 
> 73,
> Tom, K1JJ
> 
> 
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