Collins 32S-3 Power Reduction

By Chris Codella, W2PA

March 2009

Since modern amplifiers require comparably low drive, in the 50W range, the 32S-3 will overdrive them as is.  A simple solution to the problem is to reduce the power output by installing a resistor in the screen grid supply line for the final amplifier stage.

Collins actually designed this in for the tune position of the emission switch.  In "tune", R103, a 150k ohm resistor is inserted in the line, which reduces output to a low level.

By experimenting, I found that a resistance somewhere between 7k and 10k ohms reduces the output level to around 35 watts, which is about right to drive my Alpha 89 to a little over 1kW.  Since it varies by band, I set it to a value that would keep it from overdriving on the band with the most drive.  For me, this was 80m and the resistance I use is in the neighborhood of 10k ohms - but I use a small potentiometer with an adjustment screw so I can change it if necessary.

The easiest way to insert a resistance in the screen grid line is to use the "PA Disable" connectors on the back of the transmitter.  These two phono (RCA) jacks are normally shorted together at the factory.  This shorting wire supplies normal screen bias to the final amplifier tubes.  If you need to disable the final PA, for example, to use a transverter, you snip the shorting wire inside the rig and connect a switch to those two connectors (sometimes supplied by the transverter).  Without the shorting wire, the final PA is disabled.

But shorting and opening the circuit aren't the only options.  You can insert a resistance and control the power level. 

I chose to mount the resistance in a small box outside the rig and plug it into the PA disable jacks.  This way there is no internal modification and I can also have a switch on the box to short the line so the transmitter can be used normally without an amplifier, or simply decrease the potentiometer to zero.

I've noticed no affect on signal linearity on a simple monitor scope, nor any on-air comments that indicate anything other than a clean signal.  But I can't categorically state anything about this until I do some real measurements.  I'd be interested in any comments - please email me.

The figures below illustrate the portion of the schematic showing the shorting line, and the rear of the transmitter where the PA disable jacks, J5 and J9, are located.