[AMRadio] Matching a 100 watt PA am to drive 833'S

Jim Candela jcandela at prodigy.net
Tue Jan 4 18:52:09 EST 2005


   If the PA amp is tube type, just take the audio off
the plates of the audio output tubes, and interface
the the amp to the grids of the 833's with high
voltage transistor emitter followers. This is what
John WA5BXO created, and W5OMR does well with his
Titanic. I think John can give you the link to the
circuit. It is pretty simple, and high performance.

   If the audio amp is solid state, then you need a
step up transformer with a secondary center tapped
winding of suitable turns ratio. Load the secondary so
that the amp sees the proper load impedance. This
arrangement will work with a tube amp as well, but I
personally don't like cascading transformers.

   In either case if the PA amp has 4, 8, and 16 ohm
outputs (some solid state amps do) AC ground the 4 ohm
tap, and you have 180 degree opposing audio at the
common, and 16 ohm taps. The problem here is that
833's need a hell of a lot of grid swing, and this
scenario when driven to maximum level (16 ohm swamping
resistor across transformer entire secondary winding)
will only provide about 40 volts RMS, or 56 volts peak
grid to grid (E^2/R=W; so at 100 watts into 16 ohms
you get 40 volts rms). This might do for a pair of
high mu triodes AB2, but not 833's.

Jim Candela

--- Don Moore R Moore <wa5ffk at juno.com> wrote:

> I have a older BC 1F series gates transmitter.  I
> would like to do away
> with the 845 driver tubes and drive the 833's with a
> 100 watt PA amp. 
> Does anybody have any Ideas what I would need to
> match the amp to the
> driver transformer of the 833's.
> Don Moore
> AMRadio mailing list
> Home:
> http://mailman.qth.net/mailman/listinfo/amradio
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