Rich Measures wrote:
>5. The 4CX1000A has a maximum grid dissipation rating of 0W!!. With the
>anode cooler removed, the 4CX1000A's grid wires are so delicate they are
>virtually impossible to see without magnification.
Funny that Eimac allow 5mA on peaks, if you read the whole data sheet.
>To limit grid
>current, I would use a grid supply whose output R was about 100k ohms.
Only if you can promise never, never, never to run any grid current at
all! With an output resistance that high, the slightest trace of grid
current puts you in IMD hell. Correction: it puts everybody else in IMD
hell.
Most modern exciters considerably under-run their output transistors to
obtain better IMD, so there is every risk of transient power spikes.
You simply cannot promise that you will never, never, never exceed a
certain drive level. Instead you need a solution that will handle
overdrive gracefully rather than disastrously.
>The naysayer-recommended "stiff" grid voltage supply could make a stiff
>blow to your wallet.
If you use a 100k output resistance, you'll probably receive several
stiff blows from your neighbors on the air.
This isn't an either/or situation as Rich portrays it. As thousands of
amplifier users know, you can have good IMD performance at normal drive
levels, that doesn't degrade disastrously with the occasional spike of
overdrive, and still avoid damaging the tube.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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