On Sep 2, 2004, at 7:51 AM, John T. M. Lyles wrote:
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?
Because tuned inputs for g-g have a narrow bandwidth and a roller-coil
tuning the grid is adjustable over a wide bandwidth. -- ref.:
http://www.somis.org/D_a_05.GIF
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?
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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