[Amps] GS-35B efficiency

[Bill, W6WRT]

IGINAL MESSAGE:

On Fri, 18 Dec 2009 22:07:48 -0700, "Jim  Garland" <4cx250b at muohio.edu> wrote:

>
>If builders find that a GS-35B amplifier is significantly less efficient
>than, say, a 3-500Z amp, then could someone explain why that is so? I always
>thought that the efficiency is a function of the conduction angle (i.e.,
>whether the tube biased to class A, B, C, etc.), and whether the tube is
>loaded properly with its plate impedance matched to the tank circuit. I also
>know that the input network needs to properly matched, and that stray
>inductances at the cathode can affect efficiency on the higher bands. But
>none of these characteristics pertain to the tube itself. So what's going on
>with the GS-35B? 

REPLY:

I suspect the difference is probably in the grid structure. This tube was
originally designed as a radar pulse tube and is no doubt optimized for that
service, not for linear amplifier service. I have not taken one apart, but I
would bet the grid does not have the fine control over emitted electrons that an
8877 does, for example. Some of the electrons slip by, so to speak, and do not
effectively contribute to the RF output. Thus the lower efficiency.

Old timers will recall the receiving tubes from years ago that were designed as
"variable mu" or "remote cut-off". Their grid structure was make of wires with
uneven spacing so the gain changed with different bias levels. Something similar
may exist in the design of the GS-35b.

This is just a guess and should be taken with a grain of salt until the real
facts are known.

73, Bill W6WRT