Not very linear, is it? Is this indicative of most tetrode-based designs?
I had always thought that in addition to higher gain, tetrodes offered
better IMD performance over GG triodes, all other factors being equal.
I went back into the QST archives and noted the IMD specs for several other
amps. Of the best noted, the Ameritron AL-800H (-49/-55 dB; 3rd/5th) and
Alpha 87A (-50/-46 dB) produced some of the best IMD numbers. Both are
triode designs. The QRO HF-2500DX with 4CX800 tubes produced -43/-40 dB
numbers so it seems that the tetrodes are indeed capable of good
performance. In the case of the Titan II, could it be a matter improper
bias or low plate E?
Also, it seems that until the mid '80s, QST was not measuring amplifier IMD
in their performance reviews. I would think that should have been a
fundamental test for any linear amplifier product review. On page 53 of the
July, 1992 issue, the ARRL's IMD testing methodology is explained. It's
found mid-way through the Ameritron AL-82 product review.
-Paul, W9AC
----- Original Message -----
From: "Tom Rauch" <w8ji@akorn.net>
To: <amps@contesting.com>
Sent: Wednesday, August 15, 2001 5:39 PM
Subject: [AMPS] Tetrode poor IMD performance
>
> Hi All,
>
> Does anyone have any idea why tetrode amps like the one just
> reviewed in QST seem to have such poor IMD performance?
>
> Is it the tube, the circuitry, or a testing error? I mean, -29dB ala
> ARRL is really -23dB using commercial standards. That's pretty
> nasty.
>
> I'd hate like hell to have a bunch of them near me, or was
> something else going on??
>
> I was bothered about 25kHz away by a guy in Florida using one a
> few weeks ago (he was on 1850 SSB, I was on 1823 CW) but I
> blamed it on his TS-2000. Now I'm wondering which was actually at
> fault.
> 73, Tom W8JI
> W8JI@contesting.com
>
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