I found the following information to be very interesting regarding alleged
tetrode performance. What they're saying is that a current-regulated screen
grid ensures linearity regardless of drive power. Is this true? I found
the quote below from the CTI web page. Note that they do not specify an IMD
figure in their technical specifications.
http://cticomtec.com/CTI/pla800.html
"Triodes are Good, Tetrodes are Better....
Unlike triode amplifiers, the PLA800 uses tetrode tubes, which have a
control screen grid. The screen grid not only adds to the amplification
factor of the tube, but also controls the electron flow to the plate. When
current regulation is used for the screen grid, as with the PLA800, plate
and screen currents are limited to their maximum value. As a result you will
not overdrive the PLA800 creating sideband distortion or splatter.
Increasing the drive power beyond 50 watts, does not increase the power
output. Because of this self-limiting feature, an ALC circuit is not needed.
The amplified audio, as in SSB service, is clear and clean, void of
sibilance and distortion."
----- Original Message -----
From: "2" <2@vc.net>
To: "Steve Thompson" <g8gsq@qsl.net>; "AMPS" <amps@contesting.com>
Sent: Wednesday, August 22, 2001 12:17 AM
Subject: Re: [AMPS] Tetrode poor IMD performance
>
> >
> >
> >-----Original Message-----
> >From: Lamb <k7fm@teleport.com>
> >To: W8JI@contesting.com <W8JI@contesting.com>; amps@contesting.com
> ><amps@contesting.com>
To: <amps@contesting.com>
> >Date: 16 August 2001 06:20
> >Subject: Re: [AMPS] Tetrode poor IMD performance
> >
> >
> >>
> >>While not claiming to know the answer to Tom's question, I would like to
> >add
> >>some information using a similar tetrode.
> >>
> >>The 4CX1000 was originally not designed for linear amplifier service.
> >Eimac
> >>specifies the 3rd order distortion as -23 db and 5th order distortion is
> >>specified as -26 db. The Collins 30S-1 used that tube, but used a
cathode
> >>drive configuration (which has inherent feedback) and applied additional
> >>feedback to arrive at a clean amplifier.
> >>
> >>Without feedback, though, a 4CX1000 with a resistive grid and perfectly
> >>stabilized voltages ought to yield no better than a -23 db 3rd order
> >>distortion.
> >There was a 2m amp described in VHF Comminications some years ago using
> >4CX1000A. I don't remember any 2 tone measurements, but there were
spectrum
> >photos under voice mod. I know voice mod is not a controlled test, but
the
> >output looked a hell of a lot better than I've seen while bench testing
amps
> >that did have mid 20's 3rd order products.
> >
> Amen, Steve
>
> cheers
>
> - R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
> end
>
>
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