On Wed, 23 Jul 97 19:15:00 -0700 Rich Measures <measures@vc.net> writes:
>...snip...
>>>2. Low Mu triodes are not appropriate to use in Class AB2
>>>cathode-driven
>>>linear amplifiers because they exhibit low gain.
>>
>>So what do you consider low gain Rich?
>
>3 in 7 out. (3cx3000F1/A1 in g-g)
>
>>What is your example of a good AB2
>>grid driven triode?
>
>810, 1000T, 450TL, et cetera---but not for linear service.
Back before the modern triodes became available some of the most popular
AB2 grid driven triodes were the 304TL, 450TL and 6C21. Of course these
were the old PP design with plug-in coils, cross neutralization, etc.
I built quite a few one and two 4-1000A amps in AB2 that were quite
clean. The key was regulated bias and screen supplies.
The only
>tubes I know of that are usuable in grid-driven Class AB2 linear
>service
>are the 4CX600J and the 4CX1500B. However, the permitted grid
>currents
>are less than 1 mA.
>
>>Why do you continue to make such a big deal over driving voltages?
>
>The grid has C. Developing 1000V or so across the typical grid's C at
>
>28MHz is not a piece of cake. Another complication is that when you
>are
>trying to drive a triode's grid in the positive-going direction, the
>anode is headed in the opposite direction. Since the anode/grid C in
>a
>3cx3000 is about 20pF, it is a big deal at 28MHz. Have you ever seen
>a
>3CX3000A1 in grid-driven linear service at HF? I have not. The
>4CX3000A
>and the 5CX3000A, yes.
>
>>...... The final system
>>complexity is not much different from a 3CX3000A7 that requires
>400-500W
>>of drive in GG or a A3 version in grid driven service that wants
>1000V+
>>of grid swing.
>
>IMO, the A3 would not be suitable for grid-driven linear service.
>Eimac
>does not recommend it. The A1 would be suitable for grid-driven
>linear
>service up to a few hundred KHz.
>Rich---
This goes back to Mike's comment. Just because Eimac, etc. do not list a
particular class of operation does not mean it wont work. The big tubes
are mainly sold for AM/FM or similar commercial service.
I notice that Eimac lists the 3CX3000A7 as an AB2 grid driven linear amp
in their "Quick Reference Catalog".
Of course I would not use a A1 version but the A3 holds promise. What
would the distortion difference be between AB1 and AB2? I do not know.
73....Carl KM1H
>R. L. Measures, 805-386-3734, AG6K
>
>
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