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
W5FFK
Don,
I think you would be making a mistake to do that modification. The grid
impedance of the 833A's varies widely over the audio cycle, and the original
Gates class-B driver tansformer is wound specially to have extremly close
coupling from primary to secondary and thus very low leakage reactance, in
order to minimise distortion generated by the varying load impedance. You
want a source of audio driving power with not only low distortion, but also
negligible internal resistance. This cannot be accomplished with the vast
majority of audio transformers not specifically designed for class-B driver
service.
Each grid presents an average of about 500 ohms load on the driver, which
means that the input transformer winding to the grid would need to be rated
for about 2000 ohms total impedance, and the other winding would need to
match the output of your PA amp.
Hams have often accomplished this by feeding the output of a PA or hi-fi
amplifier into an output transformer wired in reverse, but these audio
output transformers are rarely wound to the exacting specifications of the
Gates class-B driver transformer because a speaker does not need such tight
coupling, and such a transformer is expensive to manufacture. Therefore,
with such a set-up the odds are overwhelming that the distortion would be
substantially worse than with the stock circuit. They main reason hams have
used this lash-up is because they could not find a satisfactory class-B
driver transformer.
I assume your PA amplifier is solid state. If it is tube type, that would
further exacerbate the situation, because the audio drive would be coupled
through TWO transformers, each contributing its share of distortion.
I am curious, what do you find unsatisfactory about the original 845 class-B
driver?
A more satisfactory upgrade would be the solid state class-B tube driver
described some years ago by WA1QIX. It presents the nearly ideal class-B
driver with extremely low internal resistance. Check out his website; if the
circuit is not there, e-mail him. I'm sure he would be glad to share the
circuit.
If you really insist on changing out the original driver, I'd be interested
in your old driver transformer. I have an audio output transformer rated to
match a quad of four 807's, which might work with your PA amp, but I will
have to check it out because I don't remember if the secondary winding
impedance is 500 ohms, or for a low impedance voice-coil. If it would work,
I'd be willing to trade, but I still say you would be better to keep the
stock audio driver.
73, Don K4KYV
