Updates and Changes

Raising CW Plate- and Screen-Voltages

Collins' reduction in High Voltage and Screen Voltage when using CW mode held back the 4CX1500B gain, and total-power. I was almost at the point of using the SSB setting for CW (as many do), when Chet VE3CFK pointed out...

That feeding both the primary wires from the front-panel-switch to the SSB connections on the transformer would give me a constant 3kV plate-voltage, and the elevated screen-voltage, no matter the mode. In fact, doing this means the only difference between SSB and CW is the negative grid-bias, and the resulting Class of operation.

The realization of Chet's suggestion: simply moving the CW-primary-power wire from Term #1 to Term #2, and similarly moving Term #6 to Term #5:

Now I have good gain and total power! Thanks, Chet! Here's a view of comfortable operating conditions now, for CW:

Thermal Overload Switch K102

My thermal sensor kicked my amp offline a couple of times, at only modest power-levels. Perhaps decades of time have taken their toll on the sensor; perhaps tube-changes have un-calibrated it; certainly my blower-motor-change will have altered it's response. The purpose of the thermal sensor: to open the HV-enable 12V circuit and K203 (primary AC power) if the tube gets too hot. The tube can get too hot with either excess dissipation, or lack of cooling-airflow.

I changed this protection to accurately focus on protecting the tube against overtemperature and also against loss of cooling airflow.

How and Why I Changed It

• Thermal and Pressure switches to replace Collins Overtemp K102
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  Rear View of the RF Compartment, Showing New Thermal-Safety

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  Top View of the RF Compartment, Again Showing Thermal-Safety

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  Air-Pressure sensing tube simply stuck up between blower nozzle, and flexible coupling-boot