[AMPS] Another angle on cathode-matching ckts

[km1h@juno.com]

On Fri, 12 Feb 1999 18:09:12 Scott Townley <nx7u@primenet.com> writes:
>
>I've followed the thread on cathode-driven input networks using 
>toroids,
>and the general advise that coax between the "tank" and tube filaments 
>is a
>bad thing.
>Although I have not seen a great many amps in my day, the few I have 
>always
>"pigtail" the coax runs...no connectorization inside the amp at all.  
>It
>would seem to me (and I've measured it as well) that the shield 
>"pigtail"
>is poison to achieving good input matches at 21 MHz and above.  
>Obviously
>the introduced inductance would be that much worse at 3rd and 5th 
>harmonic.

I wonder if the harmonic content has any measurable effect. 
A modern xcvr has >40 dB harmonic attenuation. A good LP filter has 70+
dB attenuation at the 3rd harmonic. So to paraphrase  " Where's the beef?
" 

I dont know about others but I run a good LP filter between the rig and
the amp as well as one at the amp output.  I have had no problem with
efficiency on 10M or any other band with an LK-500, LK-780, PT-2500A or
any other amp with long RG-58 runs to the tube.

Might the problem in certain amps be unbalanced currents on the outside
of the coax shield?  Perhaps a ferrite sleeve balun might offer an
improvement.

Are there specific amp models that are more prone than others?

73  Carl  KM1H



>My question:  is it really the coax itself that causes such pain in 
>the
>input, or is it the lack of proper coax termination?  Would using 
>quality
>bulkhead/panel fittings allow for remoting the input network the
>traditional 6-12" from the tube filaments?
>Just curious...
>------------
>Scott Townley		
>nx7u@primenet.com
>Gilbert, AZ
>------------
>Collector of:
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>	70's era RF test equipment HP/GR/Tek
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>	http://www.primenet.com/~nx7u (someday might have useful 
>info!)
>
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