On Tue, 3 Mar 1998 21:57:16 +0000 "Ian White, G3SEK"
<G3SEK@ifwtech.demon.co.uk> writes:
>David L. Thompson wrote:
>>
>>The 4CX400 is an excellent replacement for the 8873, 8875, and 8122
>in the
>>NCL-2000. They average about $135 each retail amateur net.
The NCL-2000 mod is not a Plug n Play. The sockets must be changed and
the Svetlana sockets make for poor screen bypassing and instability on
the higher bands. I suggest ...at least... adding Johnson or equivalent
screen ring capacitors. The Svetlana socket also requires a bit of sheet
metal work in that steel National chassis.
An ideal solution would be to use Eimac sockets mounted in an above deck
aluminum sub chassis. Removing the 8122 sockets will assure plenty of air
flow and access to leads, etc.
The final step would be to use silicone rubber chimneys right to the
cabinet lid; the NCL requires all the help it can get to remove heat.
While you are at it modify the screen voltage circuit to Ian's regulator
otherwise you will still have poor IMD performance.
So the bottom line for the cheapest way out is $270-280 for tubes, $10
for sockets, $50 for screen bypasses, $20 or so for chimneys and a lot of
work.
OTOH, 8122 pulls are beginning to show as well as new surplus as military
and broadcast gear is modernized. I just picked up several Eimac 8122W's
for the price of one 4CX400A.
Ive done the mod and it does work but it is not a 1 hour job.
73 Carl KM1H Ex-National Radio and on the NCL-2000 design team.
>
>It's also an excellent replacement for the 4CX250/B/R, but it needs
>power supply modifications to realize the full potential.
>
>With 2500V on the anodes and 375-400V on the screens, a pair of
>4CX400As
>will give 1000-1100W output on 6m or 2m in the classic W2GN
>amplifiers.
>Operating condx are exactly as specified by Svetlana, allowing 1-2mA
>grid current per tube. In strict class AB1 (zero grid current) the
>power
>ouput is an even kilowatt.
>
>Grid drive requirements are very low indeed - for example 10W on 2m
>with
>120 ohms grid damping, or 2.5W on 6m with 250 ohms damping.
>
>Apart from larger anode contact rings, chimneys and exhaust holes, no
>other RF-deck modifications seem to be required.
>
>
>73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.demon.co.uk/g3sek
>
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>
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