[AMRadio] Re: WRL Globe King 500C rebuild

Jim Candela jcandela at prodigy.net
Fri Jul 7 11:39:29 EDT 2006


   I once used a GK 500B modulator deck as modulator
for a H.B. KW RF Final. I used 2250 volts, and used 4
type 808 in push pull parallel that produced a very
clean 600 watts of audio. The iron was not WRL iron,
so my story isn't too relevant to the discussion other
than stating how well those 808's did in that
application. I would not run 811's any higher than
2000 volts like what Johnson does with the 500. The
572B can take that and more with safety margin. As for
808's, fair radio used to sell these for 75 cents
each. Ozona Bob, W5PYT bought out their remaining
stock for a big HB AM rig that he never finished. If
you have a pile of them, they are a good choice. If
not go for 572B.


--- Donald Chester <k4kyv at hotmail.com> wrote:

> In the stock transmitter, the final amplifier plate
> voltage is 1800 volts, 
> so the 2000 volts shouldn't cause too much of a
> problem.  The modulator 
> originally ran only 1200 volts on the plates of a
> pair of 811A's, and the 
> modulation transformer was just barely step-down,
> with a turns ratio of 
> something like 1.15:1.  Even so,  the modulator
> would just barely make about 
> 85% modulation in most transmitters, if you were
> lucky.
> Running 2 kv on both with a common power supply
> should give better 
> modulation, but the final tank cap might arc over on
> modulation peaks.  This 
> is assuming that the G-C driver stage is capable of
> fully driving the 
> modulator at the increased plate voltage.  With a
> common power supply, the 
> modulation transformer should have turns ratio of
> about 1.4 or 1.5 to 1.  
> The 572B's MIGHT work OK with the stock audio 
> driver and mod transformer.
> Don k4kyv
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