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

Donald Chester k4kyv at hotmail.com
Tue Jan 4 19:21:29 EST 2005

>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


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 

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 

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 

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

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