Dave Burson wrote:

I still don't understand the need for 2 caps of such different values and
voltage ratings.

Has to do with the caps themselves. The large cap is for the 60 cycles
(actually 50 to 133 cycles); while the smaller cap is for higher
frequencies. I didn't see the power supply schematic - but dimes to
donuts it's full wave- so the ripple frequency is double the AC line
(110 to 120). That's likely to be phase-shifted a bit before reaching
this circuit. The lamp runs on line frequency - and in combination with
the ripple (riding on the B+) it'd be easy to generate some rather
complex waveforms - with some pretty high harmonics. The large cap
"eats" the lower frequncies - however - it's construction limits it's
usefulness at higher frequencies - so there is the smaller one to deal
with those. Look at most any power supply - you'll find smaller value
caps by-passing the main filters.

The voltage is insignificant (within reason). I'm sure the 25V was
overkill (likely the highest voltage across the primary was 10V); but
25V was "common" back then (often found as the output tube cathode
bypass cap). Since a .1 is seldom seen in lower than 150V - that value
was probably stocked on the shelf as well. Even today - most .1 - even
in solid state stuff - are seldom less than 50V. Just a matter of what
was already on hand (big quantities of a common value are cheaper than a
few "special" values even if those special values could be smaller).


best regards...
--
randy guttery

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