Re: [Amps] High C out

[Mike <k4gmh@arrl.net>]

Hello,

I seem to remember an article in Ham Radio from the 80's regarding an amp. 
using three 813s built by Bob, W4MB, where he did the calculation for the 
inductance that went from the plates to the blocking capacitor.  He was 
interested in reducing the affect of the plate capacitance so he could tune 
10 meters.  As best I remember he was converting the parallel capacitance 
to an equivalent series capacitance and then inserting an equal inductive 
reactance at 30 MHz to compensate for the series capacitance.

Anybody remember the article?

At 05:10 PM 1/21/05, Dennis12Amplify@aol.com wrote:
> In a message dated 1/21/05 1:49:18 PM Central Standard Time,
> wlfuqu00@uky.edu writes:
>
> For an L  you need the capacitance on the high-z side.
> *** In this case the tube side would be the high-Z side.  It would be the
> plate capacitance of the tube itself.
>
> The  capacitance
> should be from  plate to ground
> ***Plate capacitance IS from plate to ground!
>
> and the  L in series with the 50 ohm
> load.
>  ***In the case I was describing it is series with the original  tank 
> circuit.
>
>  ***It is basically a 2 stage impedance matching  filter.  Or you can
> consider it a pair of back to back PI networks where  the output C of the 
> first PI
> network and the input C of the second PI network  are added together as a 
> single
> C value.
>
> Q is the  square root of the impedance ratio. You need adjustable L
> and C to match  on all frequencies.
> **Correct in theory but not necessarily in practice!
>
> ***But if he only had that tuning problem on the highest frequency, the
> solution provided by adding that small amount of inductance may work  fine 
> because
> as he switched to lower bands the added inductance would have less  and less
> effect on the impedance transformation ratio. Thus it would have  less effect
> on his ability to tune and match impedances to the new lower  frequency
> settings.
>
> Oddly enough I have seen it used but when I tried it with my  4-1000 I could
> not get it to work.
>
>  ***The only times I have had 'significant' success with it was  when the
> plate to ground capacitance of the output stage was so high that I  needed to
> up-convert impedance to get a reasonable Q and reduce the  circulating 
> currents
> in the tuning circuitry. I doubt that you had that type of  a problem with a
> single 4-1000.
>
> I will revisit it some time or another. I had hoped to
> use a L output  on my amp and then follow with switched low pass filters
> which could be use  with any of my amplifiers but simplify the amp itself.
>
> *** Then I suggest that you find yourself one of those chain driven  rotary
> inductors like they used to use in the old Henry amplifiers because you  will
> probably need one.
>
> ***Regards,
>
> ***Dennis O.
>
>
> 73
> Bill wa4lav
>
>
>
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        73,
        Mike, K4GMH 


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