Using a big sound reinforcement amp to modulate an AM transmitter

[r***@hotmail.com]

A friend of mine has just got hold of a Gates harris BC-1G
transmitter with a duff mod transformer. Since one will have to be
wound anyhow....insteaad of the stock arrangement of two 833s in push
pull why not just use a modern PA amp to modulate it directly?

The question is how to minimize the impedance ratio of the mod
transformer....what's the highest impedance a reasonably priced such
amp could put, say, 600 watts into? Since the transmitter is a 1 kW
unit, 500 watts is going to be required at a minimum 80-85% duty cycle
to fully modulate the finals. Any ideas?

[Phil Allison]

Post by r***@hotmail.com
A friend of mine has just got hold of a Gates harris BC-1G
transmitter with a duff mod transformer. Since one will have to be
wound anyhow....insteaad of the stock arrangement of two 833s in push
pull why not just use a modern PA amp to modulate it directly?
The question is how to minimize the impedance ratio of the mod
transformer....what's the highest impedance a reasonably priced such
amp could put, say, 600 watts into? Since the transmitter is a 1 kW
unit, 500 watts is going to be required at a minimum 80-85% duty cycle
to fully modulate the finals. Any ideas?

** Any PA amp rated for 300W at 8 ohms per channel can also do 600W at 16
ohms in bridge mode.

Also, PA amp rated at 500W per channel can do 600W into 26 ohms the same
way.

However, one needs to be VERY careful when driving large transformers that
the low frequency saturation limit of the tranny is NEVER exceed by the
driving amplifier. This last requirement means fitting amplitude limiting
PLUS a sharp roll of high pass filter *inside* the amp that together
eliminate the possibility.

Failing to do this or getting it wrong will leave the amp vulnerable to
instant destruction if a large low frequency signal or transient comes
along.


.... Phil