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

[R.Measures]

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