Ian, I guess you know this already, so it's for information.
"only -19dB and quite irresponsible by implying that level of IMD is fit to
be put on the air".
I may agree, but it depends.
You can not only look at the 3rd.IMD product but should measure how the IMD
products fall off.
There is a big difference in how products fall off. If it falls off quickly
it is of no big concern.
An excample is the TS-2000 tranceiver that has got bad reputation (?) in the
UK based on the bad 3rd. and 5th products presented in Radcom.
However looking at the IMD spectrum in QST, it falls of quickly and on air
it sounds acceptable to me.
Using a speech processor, you will typically measure bad 3rd and 5th, but it
sounds OK off frequency.
About the 4CX250B, that I have used in a W2GN Arcos type 144MHz amp and
replaced with 4CX400A, the latter one
is clearly better off frequency. It can produce more power.
If the operator is greedy for more power the 400 is also capable for a lot
of IMD in SSB.
4C250B can sound terrible locally, using a typical tranceiver that reduces
power from abt 50W to 5W with ALC curcuit.
A typical ALC circuit will easily give a lot of overshoot and overdrive the
tube at speech transients.
This is most often the reason to the splatter you hear from local stations
using small tetrodes.
A solution to this is to check the modulation envelope with a oscilloscope.
If it overshoots, compared to
whistling a tone, use a power attenuator between the tranceiver and amp.
Adjust anttenuator so that transmitting full power will give full power from
the amp.
73 de LA6MV
Helge
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
Behalf Of Ian White, G3SEK
Sent: 3. november 2003 17:49
To: amps@contesting.com
Subject: Re: [Amps] About single 4CX250b output power
Mike Tubby wrote:
>
>Page 65 of Eimac's "Care and feeding of power grid tubes" shows a
>selection guide for SSB service for which the 4CX250B is listed as
>being capable of 348W "useful power output" in AB1 with a 3rd order
>distortion product of -25dB (doesn't say whether its relative to
>carrier power or one tone of two tones).
>
Relative to PEP, ie only -19dB relative to either tone.
Eimac were being quite irresponsible by implying that level of IMD is
fit to be put on the air, and that data sheet has given the 4CX250B an
undeserved reputation for poor linearity.
If you're less greedy than Eimac, you can get 250W PEP from a single
tube with excellent linearity.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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