[Central-Electronics] Solid State Power Supply for 100V

[Radra]

I thought there might be some power supply expertise out there, I was right.

My purpose for replacing the tube supply with a solid-state version is
primarily related to reliability.  Electronics suffers from the three deadly
sins: electrical stress, thermal stress and mechanical stress.  If we
eliminate the three power supply tubes, V23 - V25, then we eliminate the
heat their filaments generate.  According to voltage and current numbers on
my schematic, that amounts to a whopping 37.68 Watts of heat!  Not only does
that raise my utility bills, it places thermal stress on the rectifier tubes
themselves and on anything else the heat reaches including those expensive
large electrolytic caps I am about to replace.  The net result is that
things fail prematurely.

Heat is also generated by the rectifiers (diodes) themselves because they
have a relatively large forward conduction voltage.  I am not a tube guy and
haven't looked it up, but I suspect the drop is on the order of tens of
volts, maybe 50V.  Multiply that by their current to determine how much
power is converted into heat by the rectification action of V23, V24 and
V25.

The straightforward approach is to simply do a one-for-one replacement of
each tube with an appropriately rated diode.  This approach may work but,
and it is a big but, the resulting B+ voltages will be higher than the 100V
designers had in mind.  This occurs because the solid state diodes have a
forward voltage drop of a volt or less compared to the tubes which have tens
of volts.  The higher B+ will increase electrical stress on the components
to which it is applied and, more importantly, it will alter the tube
operating points.  If we assume the 100V designers knew their stuff, then
all tubes are properly biased with the +600 Vdc and +340 Vdc the power
supply was designed to provide.  If you change the B+, then you really
should also re-bias the tubes to accommodate the change.  That may not be an
easy task because bias is an important design/performance parameter.
Depending on the specific circuit, you may want to bias for optimum
linearity or possibly for a specific degree of compression (non-linearity).
[Just as an aside, I see the 200V supply design technique will yield a
two-to-one ratio of the high and low B+ voltages, i.e. 680 Vdc and 340 Vdc
because they use the same transformer winding taps.]

So at first blush, my approach would be to retain the B+ voltages the 100V
was designed to deliver so I don't have to mess around with re-biasing
tubes.

A second goal is to have the solid state B+ increase slowly when the 100V is
first turned on in a fashion similar to the way it does with the tube
supply.   If full B+ is applied before the tube cathode has become hot,
plate stripping can occur resulting in shortened tube life.  Solid-state
diodes will apply B+ much more quickly than their tube counterparts because
solid-state does not need to warm up for it to function.  Thus some
circuitry must be added to slow the initial rise of B+ in a solid-state
design.

The solid-state conversion now has two goals: retain original B+ voltages
and control initial rise time of the B+.

The existing tube power supply will have a significant amount of sag (poor
load regulation) as it attempts to respond to varying current loads.
Whereas the solid-state design will likely have less sag.  I wonder if that
would be a problem.  If desired, the amount of sag can be greatly reduced by
using a solid-state rectifier circuit followed by a solid-state voltage
regulator.  The entire circuit could be housed in one of the Weber Copper
Cap housings and installed in the existing tube sockets with only a minimal
amount of wiring changes.  I like their housing because it plugs in and has
a nice copper heat sink.; besides, it looks cool.

A third goal might be to reduce the amount of sag (improve load regulation)
but I think that may just be overkill.

No doubt power supply conversion was a hot topic years ago; so I am
convinced there is a proven, elegant solution just waiting to be
re-discovered.  Whatever it is, it won't be very complicated.  I am hoping
the Weber Copper Cap products, or something similar, will provide a quick
plug-and-play solution to the conversion.

Lyman K7LH