by erichayes » Thu Sep 23, 2004 9:44 pm
Hi All,
It's been known for quite a while that allowing a tube to run at nominal (normal) heater voltage with no B+ applied causes the cathode to "bake", thus reducing its emissive capability. This is particularly true of power tubes such as those used in the Dyna and Eiclones.
A nifty way to prevent this from happening is to use a double pole standby switch. Wire a 6 amp 100PIV rectifier across the second set of poles--polarity doesn't matter--and wire one side of the heater source and load in parallel with the rectifier.
When the switch is in the "standby" position, the high voltage is disconnected, and the heater voltage flows through the unfiltered half wave rectifier, dropping the voltage to roughly half normal. This keeps the cathodes warm enough to emit electrons when the switch is thrown to the "operate" position, but not hot enough to cause the baking process to take place.
Obviously, when the switch is in the Operate position, the diode is shorted out, allowing full heater voltage to be applied to the tubes, and the other set of poles completes the B+ circuit.
I've been using this gimmick since the late 60s and still marvel at the increase in useful tube life it provides.
Eric in the Jefferson State