On Tue, 27 Jan 2015, gareth wrote:

The problems associated with replacing (or even obtaining) the HT reservoir
capacitors in valve projects might be replace with a series / parallel
arrangement of those dinky 100uF 35V SMD ones?

Probably could be rolled up and placed inside the original aluminium can,
too!

You didn't specify what voltage you need.

For "average" high voltage, there are all those line side capacitors in
switching supplies. I thought they were about 450volts, but it's been a
while since I checked. So one can extract from a scrap computer supply,
or likely buy those as replacement parts. If they aren't high enough
voltage, then they are a better start for combining than 35v ones.

I was actually surprised 20 years ago when I discovered ripple in my
Tektronix scope. I found which of the many supplies was the problem by
using the scope itself, touching each of the high voltage lines until I
found one where the ripple increased. The needed capacitor wsa much larger
than "average" value in the days of electrolytics, I figured it would be
hard to find. Yet I checked the bins at the local surplus store, and
easily found a replacement, high enough voltage, and some large value like
100uF (which is large for tube power supplies).

If you're talking about high voltage for power amplifiers, I thought a lot
of those oil-filled capacitors from the old days were still viable. They
were sealed and hence don't dry out. The relatively small capacitance
ones but which have ceramic insulators for the leads to come through. They
were useful fifty years ago, they are apparently still good now. All you
have to do is dig deep enough to find them under more recent items.

If this is for a receiver, run it at a lower voltage. I gather the
Collins 75S line of receivers used "relatively low" voltage on the plates,
not real low but low relative to what other companies put on the plates.
I remember an article in CQ by Fred Brown where he updated his HRO for
subminiature tubes, and he followed this line about lower plate voltage,
though I can't remember the benefits he mentioned.

Michael