[Collins] 30S-1

[Alan Chandler]

The 30S-1 is one of the lowest IMD amplifiers ever produced for amateur use. 
It uses a grounded screen grid and operates the cathode at about -200 volts 
as the screen supply.  The RF drive is supplied to the cathode so the RF 
adds about 100 volts to the cathode which results in a total screen supply 
of about 300 volts when fully driven.  The Eimac 4CX1000A maximum screen 
voltage is 400 so adding 125 volts to the screen supply will exceed the maximum rating under RF drive.  Typical AB1 operation for the 4CX1000A is with 325 volts on the screen which is about where the 30S-1 actually operates.  Increasing the screen voltage does increase the gain of the tube, requiring less drive power but it reduces the available plate voltage swing before the screen current rises to the level that exceeds the screen dissipation (35ma at 325v is just under 12 watts, 
the maximum screen dissipation.)  and the tube is in saturation. The net result can be more output power but with a substantial increase in IMD.  You will typically get about 1 to 2 dB increase in power (0.2 to .04 S units) and about 10 dB higher IMD (2 S units).  Be kind to operators on adjacent frequencies and leave the screen supply where it was designed to be operated.
    I have just about finished setting up a variable screen supply and will 
make power and IMD measurements with a 4CX1000A and 4CX1500B in a 30S-1 later in the fall.  I started the project in the Spring but good weather forced yard work to take precedence.  When I complete this, the results will be sent to The Signal.

73, Alan - K6RFK