[BC] 4CX20000A vs 4CX20000C

[RichardBJohnson at comcast.net]

There was a question about two tubes, both 4CX20,000 tetrodes. I can explain the difference between the two tubes.

The 4CX20000A/8990 has a rated plate voltage of 10 kV.
The 4CX20000C has a rated plate voltage of 12.5 kV.

The two tubes have slightly different inter-electrode capacities. This is an artifact of the measurement method. The 4CX20000C was designed for FM transmitter service at frequencies even higher than the FM band. Its capacities were measured using a special test fixture that grounds the screen electrode ring.

The 4CX20000A/8990 should have identical construction. However, it was adapted (read de-rated) for use in plate-modulated AM broadcast transmitters. The special number was assigned to keep track of any tube life differences for warranty purposes. Its inter-electrode capacities were measured using the standard Eimac socket.

When I was designing the world's first four-tube 50 kW broadcast transmitter, I wanted to use the, then brand new, 4CX20000C tube. I tested it with a HIPOT tester and it did not arc at the peak voltages expected at the crest of 150% modulation. Furthermore, no published voltages or currents would be exceeded in the proposed design.

Eimac said NO, "They wouldn't hold up under warranty when plate modulated!" I think it was a political decision because they feared the loss of sales of their most expensive and profitable 4CX35000 used in many 50 kW transmitters. They had designed the 4CX20000 for the expanding FM broadcast market and did not want it to replace any existing moneymakers.

Therefore, I proposed that the McMartin Industries, the company wherein I made the first 50 kW design, simply purchase these tubes and handle any warranty issues themselves. When Eimac management realized I was serious, they decided that they could just re-label and de-rate the existing vacuum-tube design for plate-modulated service. They de-rated it to 10 kV and called it an 8990.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/