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.

"mc" wrote in message
. ..
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.

Theoretically, these should both be fine -- in a PS filter, the more
capacitance the better -- but does actual experience agree with theory?

MC

With lots of experience I can safely recommend the following.

Working Voltage plus any AC voltage times 2 and the next voltage
rating up will be the way to go. For the microfarads go at least the
original.

Bob AZ

"mc" wrote in message
. ..
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.


Silicon rectifiers are a good substitute for vacuum-tube ones. You can
limit the inrush current with a suitable NTC thermistor.

Jim Hawkins

"mc" wrote in message
. ..
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.


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.

Dave S.