[Milsurplus] DY-105/GRC-9X Relay K3

[Richard Brunner]

Re:  "I appear to be having problems with K3 in my DY-105." 

Relay K3 serves several purposes.  As stated in the book, it protects the filaments against grossly high input voltage, and it also may save tubes, especially the 3A4's, if one of them gets an open filament, allowing the voltage from the ballast tubes and dropping resistor to rise.  The problem with the original design is that key-down, the 6.3 volt line voltage rises because the current pulled in the 2E22 looks like a resistor connected from B+ to the 6.3 volt filament line.  As you may have noticed, this introduces significant chirp.  The ballast tubes are no help because they are thermal, and take many minutes to stabilize at a new operating temperature when the input voltage (or current) changes.  I think ballast tubes are an interesting idea that doesn't work very well in practice.  In the transmitter the 3A4's are fed by a dropping resistor from the 6.3 volt line, and regulated by a selenium rectifier.  After 60 years the selenium rectifier is probably non-conductive in both directions, giving no regulation.  The solution is to put the ballast tubes in a safe place, and provide regulated voltages to the 2E22 and 3A4 tubes.

Richard, AA1P
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