[Amps] 144 MHz GS35B input circuit

[Steve Thompson]

That sounds totally wrong to me. The rf needs to go onto the 
common point of the cathode and filament (or heater as I prefer to 
call it where there's a separate cathode).

As long as the heater supply has a decent rf choke close to the 
tube I think a capacitor across the two connections isn't vital, 
but I can't see it does ay harm either.

Inside, the GS35 is a big planar triode. The cathode is a slightly 
domed disc about 1" diameter and I assume the heater is spiral 
wound against the underside so there's likely to be enough 
capacitance between them to matter at VHF - but I doubt it's the 
quality of capacitor you'd want to use to couple rf onto the cathode.

Steve

> Just looking for a reality check here. I am trying to get an
> amp running. It was not built by me and had apparently been
> sitting for quite a while.
>
>
> Everything seemed to be working but the input SWR wouldn't tune
> below 4:1 with about 25W drive. I understand it will change
> some at full power but I don't think this circuit is working as
> desired and am reluctant to put my SS driver PA into it.
>
>
> I pulled the bottom cover to look at the input circuit. It's
> the usual T with two coils (not the usual dimensions) and shunt
> capacitor between them. BUT, it is connected to the bottom
> connection of the tube. I think that is the filament connection
> that is NOT common to the cathode. There is NOT a capacitor
> across the two filament/cathode connections. All of the
> circuits I could find online this afternoon show the RF input
> to the next ring up on the tube (filament/cathode common). Some
> show a capacitor across the filament connections, some don't.
> Comments?
>
>
> I am inclined to rip this out and rebuild. The 10 AWG hard
> silver plated wire the coils are made from makes it impossible
> to spread or compress turns to experiment with the match.
>
>
> Thanks, Paul