COLIN LAMB wrote:

Regarding the conversion of vacuum tube rectifiers to solid state, I believe
there are serious issues that need to be addressed.

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.

I won't dispute your findings but if the caps are bad (leaky) they
should be replaced as a matter of course in refurbing. The higher
voltage wouldn't appear to *make* them "become leaky". There's better
ways to check them rather than "smoke test".
The adages from the old days that "mica caps don't go bad" are falling
to the wayside these days. They may not be as circumspect as a paper
cap but the failure rate nowadays is becoming more than negligible. I
don't trust them anymore in a 50-60 yr old radio but I haven't reached
the point of shot-gunning them like paper caps. I suspect that the time
will soon come where that is an efficient way of dealing with them. In
the meantime, Q-challenged circuits will simply have to deliver poor
performance due to leakage until the hapless 'restorer' sees some real
visible sparks flying to believe there is an issue with these old boys.

A second problem is that high voltage is applied to the tubes and current is
drawn before the tube is warmed up. We do not do that to indirectly heated
transmitter tubes, and for long life we should not do it to receivers,
either.

A CL-90 inrush protector can go a long way where this is a concern.

-Bill M