Stuart Polansky wrote:
Learned, forgotten, then learned again.
Assembling a stereo push-pull amplifier with significant output stage (center tap) capacitance. Power supply to the output stages is a voltage doubler (Fisher 500C power transformer) using Hexfred diodes, 600uF & photoflash caps then a 420uF cap. This is followed by a time-delay relay, shunted with a 3 watt 100k resistor. After that is a pi network for each channel. The pi network is made up of a Mills 5 watt 100R resistor and a parallel pair of caps: 420uF and 10uF film.
The first time one was built like this, the Mills resistors failed. Not fried, on fire, exploded or anything like that. They look fine, like new. They just went open. Thinking something else went wrong, I double checked all, slowed down relay timing, etc., then replaced the resistors. Once again, after testing, installed in the system, they just opened.
When building the second stereo amp, you'd think that would be a memory not forgotten. Wrong. Did it again!
The answer is a parallel pair of 200R 10 watt Dale wirewound resistors (not the ceramic kind, mind you; decent devices). These have been just fine.
It would seem that the Mills resistors use too fine of a wire to allow the inrush current to charge the caps; the cathode current in the output tubes still ramps up relatively slowly.
There suckers ain't cheap, so beware!
Stuart
I have a rule-of-thumb regarding the use of resistors in power circuits or even cathode bias -- Required current X 5 = power rating needed
In power circuits you have "wiggle room" for surges and in cathode circuits you limit hysteresis effects.