[Amps] 3CX800A7 Grid Current Problem

Wed Jan 25 14:06:40 PST 2012

The HF-1250 SHOULD have a grid trip circuit.  Check your manual.   Most 
ceramic tube amps running 3CX800 and 8877 tubes will present very high  grid 
current out of tune.  The grid current rises VERY quickly as drive is  
increased but drops like a rock once the loading is increased.  Therefore,  one 
must load up in small increments and be quick to advance the loading once  the 
key is down.  

In a message dated 1/25/2012 5:00:43 P.M. Eastern Standard Time,  
paab at valornet.com writes:

Thanks  for the replies.  I, too, at first thought it would involve a  
padder
in the output tank circuit.  Something I should have out in my  first post:
there are no padders in the output circuit.  The schematic  I have shows an
output circuit that is a Pi network (the inductance is an  air core coil in
series with a toroid inductor) into a   toroid  transformer (I am guessing 
it
transforms about 200 ohms down to 50  ohms).

The input SWR is 1.2:1 on 160 m.

The grid  current will also take off on the other bands if the loading is 
way
too  low. I have a lot less trouble if I always start with the  factory
settings.  On any band I always start the tune up with very  low power, 
about
10 watts, and tune for max power out.  I then unkey  the amp, increase drive
power to about 18 watts, and key the amp while  watching the grid current
meter to be sure it doesn't take off.  I  then quickly swing the tuning and
loading caps slightly above and below  where they are tuned to be sure the
grid current doesn't take off when they  are changed, then retune for max
output.  I repeat this procedure  three or four times to get to the power
level I want.  As I said,  there is no grid over current trip out on this
amp.

I need  all the power I can get on 160 m to bust thru the local QRN and
pile-up on  HK0NA.

72 Paul W5DM  

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