[Ian Iveson]

Edward Morris wondered:

I have a McIntosh C22 preamp. I'd like to replace the two
diode B+
selenium rectifiers. One lead from the transformer
attaches to the + and -
leads together in the middle of the rectifiers and the
other lead attaches
to a 40mfd capacitor. One outside rectifier leg attaches
to the bridge
rectifier that powers the filaments and the other leg ties
into a common tie
with a number of resistors that tie into a number of
capacitors. Does
anyone have a ballpark guess what value of resistors I
need for the voltage
drop

Just been trying to find out about selenium rectifiers. It
seems they can have several cells in series, and each cell
can vary in several ways. The voltage drop can be measured,
or could have been estimated from an appropriate data sheet
which is unlikely to be available now. If you can measure
the drop across and current through each one, you can get an
idea of what resistor you would need in series with each,
using ohm's law. The waveforms aren't sinusoidal, though, so
you would need to measure the two final voltages and adjust
to suit, depending on how exact they need to be. You would
also need to be careful with power ratings, for the same
reason.

A much easier way is to measure the resistance and
open-circuit voltage of the transformer secondaries, and
then simulate the circuit with SPICE. Especially if you
don't happen to have a stock of real resistors to try.

If you can measure but are unable to simulate, someone here
might do that for you.

Or perhaps someone has already made a near-enough guess?

There is a question of strategy. Would it be better to add
resistance to the heater circuit after the feed to the HT,
and to add no resistance to that feed, thus raising the
floor of the HT circuit. Then add resistance to the HT after
the doubler?

It would be a bit interesting to find the optimum
arrangement.

I wonder how you should dispose of the old rectifiers?
Authorities here are, quite rightly I suppose, getting very
touchy about what goes in the bin. I noticed, passing a so
called "health food" shop the other day, that selenium is
essential for fitness, so perhaps you could grind them up
and eat them?

Ian