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[AMPS] Neutralising tetrode HF amplifiers

To: <amps@contesting.com>
Subject: [AMPS] Neutralising tetrode HF amplifiers
From: km1h@juno.com (km1h@juno.com)
Date: Sat, 10 Oct 1998 11:34:00 -0400

On Sat, 10 Oct 1998 10:10:23 +0100 Chris W Tran <chris.tran@zetnet.co.uk>
writes:
>
>Saturday 10th October 1998
>
>Hello all
>
>I enjoyed a visit to Rich Measures AG6K web-site - thanks for
>the interesting info. Rich. (I also enjoy this reflector when
>you guys can't agree on something!)


There are often several paths to the same results. I just like to have
fun with those who have a fixed mind-set.



>Thinking about neutralising a tetrode for 1.8-30MHz use, I
>noted the method proposed by Rich (disconnect the tank cct.
>and terminate with non-inductive R equal to the load impedance,etc)
>- I was wondering if the simpler method (which I've only used
>once with a 2 x 4CX250B amp. for 144MHz) would give as accurate
>a setting ?  
>The method is to feed 2-3W of, say, 21MHz RF, INTO the 'RF output' 
>connector (with the HV and G2 supplies off, but filament and G1 bias 
>'on' and the T/R coax relays activated), then adjust the 
>'Neutralising' variable capacitor for minimum RF voltage developed 
>across the input (grid) tuned circuit ?
>(I'm not sure if the G1 bias should be 'off' or 'on')
>
>Any thoughts ?

Either way will get you in the ballpark but final touch-up will require
HV and is best done at the highest frequency used.
I prefer to get close with screen voltage removed, HV applied and
sufficient drive that does not exceed G1 ratings. 
A bit of caution is required but maximum neutralization is pretty much
certaim.

When running swamped grid input, neutralization should not be necessary.
What tube are you contemplating?

73  Carl   KM1H


>
>73    Chris GM3WOJ/GM8V
><chris.tran@zetnet.co.uk>
>
>
>
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