COLIN LAMB wrote:


I have repaired a number of Collins 75A4 receivers that had leaky mica
coupling caps after a solid state rectifier is used. The reason is that the
solid state rectifier puts out the full voltage prior to the vacuum tubes
warming up. Thus, there is very little load on the power supply and the
voltage soars to something around 500 volts, which causes the mica coupling
caps in the if stage to become leaky, causing the grids to go positive and
reducing sensitivity.

Most rectifier tubes have directly heated cathodes and warm up in a
matter of seconds...way before the other tubes with indirectly heated
cathodes warm up enough to draw current. Other than a reduction in
inrush current during the rectifier warmup, you will have the same
problem with tube rectifiers. A possible solution is the use of a
NTC thermistor in the AC line.

73, Roger

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