[Amps] 3-500Z grid driven -- TSPA

[John T. M. Lyles]

There are instances where 3-500Zs are used in 
grid driven RF and audio applications.For 
instance, the Continental 314R1 broadcast 
transmitter (was Rockwell-Collins 828C1) uses a 
pair for class C RF finals, grid driven, and a 
single as the switchtube PDM modulator, again 
grid driven. Of course, these are not you typical 
amateur applications. The beauty of the 500Z (and 
the 400 before it) was and still is that it can 
run zero bias with a reasonably low idling ZSAC, 
and it is quite stable if the grid is properly 
grounded for RF. Neutralization is unnecessary 
for 2-30 MHz in this arrangement.

Rich is also on target with his comment about low 
and high mu triodes. In grid driven operation, 
the 3-500Z requires neutralization, and it is no 
more 'pain' than any neutralization scheme. In 
the 314R1 it is taken off the anode with a string 
of doorknob capacitors, and fed to the opposite 
end of the input transformer, standard stuff. 
However, this is class C anode modulated as I 
mentioned before, not linear...

Input impedance of grid (when driven) would be 
much higher than cathode drive. This means 
building a tuned or broadband transformer 
circuit, preferrably with some Q to it, for 
matching.

Why try and do it more complicated when you can 
build a bifilar filament choke, drive the cathode 
(at low impedance) and ground the grid?

73
John
K5PRO

>On Sep 1, 2004, at 1:52 PM, Kim Elmore wrote:
>
>>  While idly thinking the other day, I began to wonder why no one builds
>>  grid-drive 3-500Z amps. I know the tube was designed for GG service,
>>  but that doesn't preclude grid-drive service.
>
>For non-linear operation, a high-Mu triode may be grid driven. 
>However, for linear operation with a grid driven triode, only a low Mu
>will do.  The reason why can be seen by looking at the characteristic
>curves for a 3cx3000A1 and a 3CX3000A7.
>   --  note --  virtually zero grid current can be allowed to flow in
>grid driven linear service.
>
>>   Eimac even specs grid-drive service on the data sheet. Is
>>  neutralization a big pain for them?  Or, is the result not worth the
>  > effort given the variety of high-quality tetrodes available?