Jack Schmidling wrote:
Most often mis-modification:
Reducing output power when driving a linear amplifier through the use of
a T connection and a dummy load and/or reducing loading or screen
Voltage. This situation wastes one of this rig's better resources namely
modulator headroom. When altering the R.F. final's impedance through
reduced plate current, the modulation transformer no longer sees the
designed load.
The correct way to reduce output power is to use an external plate
supply with a Variac on the primary or switch the low B+ Voltage to the
final (removing the high B+) through the accessory jack. Either way the
high B+ is left on the modulators. This gives an immediate improvement
to the audio as the modulator requirements are now cut way down without
forcing the modulator into an unknown load. With the accessory low power
mod, the rig will put out abour 12-14 Watts carrier which is a good
level for driving quitea few linear amplifiers. Every available effort
to clean up audio distortion should be considered when runnung QRO AM!
................
Whom do I believe?
js
Believe the above; they speak the truth.
While I have no doubt others have used the PA screen voltage and have
been satisfied with the results, it is not the best way to go, for the
reasons outlined above.
Using the low voltage B+ requires no modification except to the
accessory plug jumpers; use an external 100mA (optimally) or 200mA
full-scale meter for plate current, and you're done. At 300V B+, you'll
want to load the plate current to about 60mA.
Modify the accessory plug as follows (or make a new one if you can find
a spare): remove the jumper between pins 2&6. External Ip meter (+)
terminal goes to pin 4; meter (-) terminal to pin 2. The jumper between
pins 3 & 5 stays as-is.
Almost took as long to describe it as it takes to do it...
73/GL,
-Larry/NE1S
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