Hi all,

I've heard and read a lot of discussion on the "problem" of the
B+ being applied before the tubes can warm up. However, plenty
of tube-type HF rigs came from the factory with solid-state power
supplies: Drake, Swan, Heath, National, Galaxy, et al. Did they
do something to make their gear more tolerant? Just curious.

73,
Dean K5DH


In article . net,
says...

Regarding the conversion of vacuum tube rectifiers to solid state, I bel
ieve
there are serious issues that need to be addressed.

I have repaired a number of Collins 75A4 receivers that had leaky mica
coupling caps after a solid state rectifier is used. The reason is that
the
solid state rectifier puts out the full voltage prior to the vacuum tube
s
warming up. Thus, there is very little load on the power supply and the
voltage soars to something around 500 volts, which causes the mica coupl
ing
caps in the if stage to become leaky, causing the grids to go positive a
nd
reducing sensitivity.

A second problem is that high voltage is applied to the tubes and curren
t is
drawn before the tube is warmed up. We do not do that to indirectly hea
ted
transmitter tubes, and for long life we should not do it to receivers,
either.

The higher voltage is a secondary issue, although that can be easily sol
ved.

I built a solid state supply for my KWM-2A. It has one minute delay pri
or
to applying high voltage and all the voltages are within original specs.
I
have not changed tubes since about 1980, when I made the power supply, e
ven
though it was on daily for over 10 years.

DC on the filaments can be another long topic. There is some evidence t
hat
dc on the filaments will shorten the filament life of tubes, because the
electron emission is off one end of the filament. Regular changing of
polarity would reduce that problem. As to what that practical effect is
, I
do not know. A pilot light manufacturer rated some pilot lights for so
many
hours of use for military lighting. After most failed to come close to
the
life specifications, the manufacturer studied the problem and realized a
ll
the tests had been on ac rather than dc. When run on dc, the life was
diminished.

I do not know of any studies that apply the same effect to vacuum tubes,
but
it probably does. However, very few tubes fail because their filaments
fail, so maybe it is not a big issue.

However, a number of things can be done to reduce the hum, when using ac
-
such as applying a dc bias to the filaments, which are otherwise isolate
d
from ground, or grounding only through a ct transformer and have both si
des
of the filament above ground.

73, Colin K7FM


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