I'm getting ready to overhaul some old tube-type equipment and was wondering if either of the following would cause a problem:
(1) Using appreciably larger capacitors for power supply filters, e.g., 50 or 100 uF in place of 15 uF;
(2) Using 450-volt capacitors in circuits that will only charge them to 150 or 200 V.
Theoretically, these should both be fine -- in a PS filter, the more capacitance the better -- but does actual experience agree with theory?
I'm thinking of circuits with vacuum-tube rectifiers, whose warmup helps to limit startup inrush. Obviously, with a silicon or selenium rectifier, a bigger capacitor would mean a bigger inrush.
In most cases this will be just fine. I try to keep within +100% of the original voltage and capacitance rating, but often you can go much higher and it will work fine.
If the equipment has a vacuum tube rectifier, then DO NOT exceed the maximum rating of the first capacitor. It's value can be found in the tube data sheet and compared to your average SS filter cap is generally tiny, maybe 20 or 40 uF. You can have a look at duncanamps.com in their online tube database. A larger capacitor has a stronger inrush pull to charge up and will drastically lessen the lifespan of a rectifier tube. In a CRC or CLC pi filter, feel free to increase the SECOND capacitor as much as you want, which will have the desired effect of increased smoothing and ripple reduction without sacrificing your pricy tube rectifier.
In equipment with a selenium rectifier, swap it out for silicon. Selenium rectifiers have a bad history of failing catastrophically, and new silicon diodes are cheap cheap cheap insurance.
If you decide to change from tube rectification to solid state realize that tubes drop up to 50V vs SS which typically drop a volt or two. You may need to compensate with an added power dropper resistor. Also, power tubes like the slow turn-on afforded by tube rectification vs. the sudden SLAM of full voltage given by solid state rectifiers. You can install inline inrush limiters to compensate.
It won't hurt a thing to use overrated caps, i.e. 450V caps in a 150V application. They're more expensive, that's the only drawback.
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