[Amps] About single 4CX250b output power

Ian White, G3SEK G3SEK at ifwtech.co.uk
Tue Nov 4 18:20:21 EST 2003


Helge wrote:

>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.
>
I'd agree with that - higher-order IMD covers much more of the spectrum.

But the fact remains that a single 4CX250B, driven to the 380W PEP 
output level that Eimac recommends, is a potent source of both low and 
high order IMD. The best exciter and power supplies in the world won't 
change that - it's the tube that is responsible... or rather, the greed 
for too much output power.

>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 it has a bigger HV supply - 2.5kV on-load at 400mA per tube. (A 
4CX250B will often go well at the higher voltage, but not at the higher 
current too.)

>
>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.

Yes, all agreed... there are so many ways to get something wrong, and so 
few to get everything right.


-- 
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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