[Amps] cathode driven/grounded grid magic revealed.

[Gary Schafer]

It is well known that a grounded grid amplifier places a varying load on its
driver stage. It is recommended that the driver stage be heavily swamped to
avoid such load variations.
A driver stage driving a 50 ohm dummy load and the final should be well
regulated. 
By not having the driver swamped the varying load from the input of the
final amplifier can cause the driver IM performance to deteriorate.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Herzog
> Sent: Tuesday, July 21, 2009 7:34 PM
> To: Ham-amps
> Subject: [Amps] cathode driven/grounded grid magic revealed.
> 
> Quote:   The measurement of this improvement, however, would be
> difficult, because the tuned input would improve the IMD of the
> transmitter driving the amp (or, to be more specific, the lack of a
> tuned input could decrease the IMD of the transmitter).
> 
> Herzog insists that The above is not true.
> 
>       I have two points;
> 1.  Using a tuned circuit, or deleting it and saving a million dollars,
> left me lots of time to study it's effects.
> IMD was the same, and the AN/FRT 84 transmitters, and it'
>   successors prove something, 600 Ten Kw transmitters driven by standard
> 1 KW transmitter, through 15 feet of coax.
> 
> 2.   Driving a tube, (I only used cathode driven stuff) with transistor
> drivers gives  MAGIC IMD performance.  a 25 db IMD transistor driver,
> driven to same power as used by the tube, but into a 50 ohm load.
> Then a pair of paralleled tubes which gave also 25 db IMD distortion
> when driven by a really clean lab transmitter.
> But when these were coupled, the 45 db IMD distortion spec was met
> consistently.  They were Eimac triodes, Beryllium block cooled, and gave
> 100 watts out.  We had to get Eimac to promise NOT to change anything
> about the tube, for fear it wouldn't correct the transistors distortion
> so well.  There was no tuned circuits between. This was a AM???? Marine
> transmitter, with over 600 units produced.
> 
> Similar lab test using a stock 500 watt transistorized transmitter to
> drive the AN/FRT 84 final yielded IMD that was below 50 or 60 db,
> couldn't be seen.  Unfortunately The management said there was no market
> for it, as this only held true up to 24 MHz. Then the driver distortion
> changed.
> 
> This all dropped when I was retired in 1986, as Collins won the next
> Military transmitter round.
> K 2 L B  Wil Herzog, professional engineer.
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