On Jul 21, 2006, at 11:44 PM, Joe Subich, W4TV wrote:
>
> Rich writes:
>
>>> Rich, where is K-202?
>> On the 374 Plug In Board CB2 Schematic, to the right of Q203, D213
>> and Q204. K202 is labeled "Grid" so I assume it has something to do
>> with protecting the grid.
>
> If you would follow the schematic ... (or read the manual at page 5e)
> you will find it disables the PTT line if grid current is excessive.
>
> How odd ... Alpha does the same thing as G3SEK, WD7S, and any number
> of other builders ... they ground the grid solidly, use an effective
> and resettable grid current trip, an anode overcurrent trip and a
> properly sized "glitch" resistor. Seems to me that's the norm for
> commercial transmitters, induction heating and MRI amplifiers too.
> None of them use a fuse between the grid and ground.
>
>>> Now, what amplifier is it that uses a transistor to open a 50 A
>>> grid circuit?
>>
>> There aren't any, which was my point.
>
> Then why were you saying there was one?
Mr. Subich --- I did not say that there was an Alpha amplifier that
used a 50A-rated transistor to protect the grid (of an 8877), I said
that due to the 8877's extraordinary emissive capability, one would
need a grid-protect disconnect transistor that could interrupt the
flow of >50A to protect the grid's gold plating.
I also said that I had seen a 374 with a gold sputtered 8874 that
had a shorted transistor in the 374's grid protect circuit. As I see
it, grid-protect circuits of the past were somewhat less than adequate.
Cheers
>
> 73,
>
> ... Joe, W4TV
>
>
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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