>Rich says:
>
>>In a typical 4CX1500B Class AB1 amplifier, the grid-bias supply has
>>enough series R to limit grid-diss. to a safe value in the event that the
>>operator fails to adjust the grid bias to a level that allows no grid
>>current with max. drive applied.
>
>Shouldn't the bias be set for correct zero signal plate current?
? ZSAC can be set quite nicely with E-screen. The primary consideration
is to set the dc grid potential so that virtually zero grid current flows
with max. drive.
>Then limit the drive.
? a sticky wicket when all transceivers have a natural limit. .
>There's also a maximum for grid circuit resistance.
? Agreed, however, I could not find one in the 4CX1500B specs. My guess
is that 30k-ohm would suffice.
>
>I was thinking of a 'last resort' ALC output for my 4CX1000 provided when
>the grid voltage gets more positive than -5 or -6 volts. I'm not a great
>lover of ALC for preventing overload - it's a 'shutting the stable door
>after the horse has bolted' approach to my mind
? indeed, Peter, indeed.
>- but as a last resort,
>followed by a grid current trip (or maybe a PIN diode attenuator) when the
>grid current starts (4CX1000s are rated at zero watts grid dissipation), it
>may be useful. Comments?
>
? A possible workaround. The simple solution is to choose a grid-term.
R that is the most appropriate for use with the exciter/tube combo. The
practical choices for a wide-coverage amplifier are 12.5-ohms, 50-ohms,
200-ohms, and 450-ohms (discussion on my Web site). If it is found that
the exciter delivers a bit too much RF v-p, add some cathode RF-NFB
resistance to raise the drive level required to produce grid-current. As
I recall, Eimac recommends 11-ohms for the 4CX600.
>
- cheers, Peter
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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