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

To: "John T. M. Lyles" <jtml@lanl.gov>
Subject: Re: [Amps] 3-500Z grid driven -- TSPA
From: R.Measures <r@somis.org>
Date: Thu, 2 Sep 2004 08:10:24 -0700
List-post: <mailto:amps@contesting.com>

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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