[Top] [All Lists]

[AMPS] Another angle on cathode-matching ckts

To: <>
Subject: [AMPS] Another angle on cathode-matching ckts
From: (
Date: Sat, 13 Feb 1999 13:01:47 -0500

On Fri, 12 Feb 1999 18:09:12 Scott Townley <> writes:
>I've followed the thread on cathode-driven input networks using 
>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 
>"pigtail" the coax connectorization inside the amp at all.  
>would seem to me (and I've measured it as well) that the shield 
>is poison to achieving good input matches at 21 MHz and above.  
>the introduced inductance would be that much worse at 3rd and 5th 

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

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 
>input, or is it the lack of proper coax termination?  Would using 
>bulkhead/panel fittings allow for remoting the input network the
>traditional 6-12" from the tube filaments?
>Just curious...
>Scott Townley          
>Gilbert, AZ
>Collector of:
>       Stoddard Aircraft EMI/RFI receivers and accessories
>       Big Parts for that Big Linear Amp 
>       70's era RF test equipment HP/GR/Tek
>       Radio-related technical reference material 1940+
> (someday might have useful 
>FAQ on WWW:     
>Administrative requests:

You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at
or call Juno at (800) 654-JUNO [654-5866]

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>