Regulated variable power supply, it WORKS!

[Richard Post]

Last week I queried the list on a Chatham Electronics E-50 Reg power
supply. In the absence of a schematic, checked voltages in the control
tube circuit and found that the negative bias supply was faulty.  I
replaced a bad cap and other parts which were keeping the negative
supply from doing its job.   What threw me was how well this thing
performed even without the negative supply.  The big variable supply
now happily goes to zero and the unmarked pot that I thought should
be the zero-set for the supply is now indeed doing just that.

In tracing through the circuit, the voltage is controlled in two stacked
stages.  The cathodes of the two 6AS7 tubes feed the plates of the
triode-connected 6L6 pass tubes.  I am guessing this is because the 8
paralleled 6L6s cannot take all of the drop by themselves.  The supply goes
to 500 volts.  The pots for the 6SJ7 control tube for the 6AS7 adjust
points are on the rear of the chassis and are intended to remain in fixed
position.

In testing the supply after repairs, it happily brought my 250 volt 20 watt
lamp from zero to max with no sweat (80 mils).  I also fed three 40 watt
candelabra lamps wired in series.  This combo drew about 200 mils at a
relatively low voltage point  (making the pass tubes take the brunt).
Going to 225 mils causes one of the 6AS7 triode section plates to glow
orange.  Will have to go back and check the 1000 ohm grid resistors and the
100 ohm plate resistors to make sure the four paralleled 6AS7 triode
sections are balanced.   The one glowing plate says the load is not evenly
distributed.

Without specs on the supply, I am guessing that it should handle the full
300 mil load based simply on the number of 6L6 tubes (8) and the specs on
the commercial grade UTC power transformer (rated at 360 mils).  300 mils
is the max on the meter, which does not have any max warning such as on the
Heath PS-4 current meter which goes to 120 mils but has a 100 ma warning
right on the meter.  Maybe 250 mils is a safer bet.

(HEATH tie-in) What I like about this supply is the way it is built
(ruggedly).
Because of the appropriate bleeder resistors, it also does not have the
"kick-back"
effect common in the PS-4/ IP-32 and IP-17 Heath supplies when DC is removed
(when switched back to stand-by)  This kickback is caused by the three sets
of electrolytics decaying to zero at different rates.  I have been able to
largely fix that by adding a small bleeder to the Heath's 6L6 screen grid
supply.

73,
Rich   KB8TAD  "Keeps Big Tube Active Devices"
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