"=K=5=D=H=" wrote in message
...
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.


One of the better receivers ( Racal RA-17) also has a solid
state power supply with no means of delay.




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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"sparky" wrote


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.


One of the better receivers ( Racal RA-17) also has a solid
state power supply with no means of delay.



**** OK I'm nitpicking but the Racal RA17 was designed for the British Navy
and was fitted with a GZ34/5AR4 vacuum tube recifier as were a number of
other variants such as the L etc.The model designed specifically for the US
market was the RA17C and its later variants,the RA17C-12 being the most
common.These used solid state power supplies as you wrote,and did not have a
B+ delay fitted.

Any picking of my nit will be appreciated!!! :-)

Brian Goldsmith.

"Antonio Vernucci" wrote



If you anyway wish to replace the vacuum rectifier with a solid state one,
using a voltage dropping resistor is a silly way to go, as it would impair
the power supply regulation.

My suggestion is: after measuring the voltage increase you have with the
solid state rectifier, just put an appropriate zener diode in series with
the DC line. The zener diode causes a fixed drop and does not impair
regulation.

If, for your application, a high-power (and hence expensive) zener diode
would be needed, then use the well-known zener diode emulation circuit (a
power transistor and a small zener).

***In the vast majority of receivers that I have come across,the power
supply loading is more or less constant,the main drain being by the audio
output stage which is almost universally a class A amplifier.Hence power
supply regulation is
of no consequence as the load is static.Fitting a resistor will drop a
constant number of Volts,try it and see.

Brian Goldsmith.

What you say is specifically true for receivers; it is obvious, no =
reason to try and see it as you suggest. But that is not true in the =
general case (e.g. transceivers).

Moreover, using a resistor, voltage will only drop only once tubes will =
have warmed up and drain current. With a zener instead voltage will =
immediately drop (e.g. any minimal load caused by an high-value resistor =
anywhere in the circuit will be sufficient to have the zener dropping =
voltage).=20

In some cases it is important to NEVER reach a voltage higher than =
specified. I had a bad experience at that regard with 6146Ws fed by a =
Collins power supply where the vacuum rectifier had been substituted by =
a solid state one. Two new 6146Ws immediately flashed and were damaged. =
A zener diode solved the problem.

73

Tony, I0JX=20

***In the vast majority of receivers that I have come across,the power =

supply loading is more or less constant,the main drain being by the =
audio=20
output stage which is almost universally a class A amplifier.Hence =
power=20
supply regulation is
of no consequence as the load is static.Fitting a resistor will drop a =

constant number of Volts,try it and see.
=20
Brian Goldsmith.=20
=20

"Antonio Vernucci" wrote

What you say is specifically true for receivers; it is obvious, no reason to
try and see it as you suggest. But that is not true in the general case
(e.g. transceivers).


**** In the case of a transceiver using a vacuum tube rectifier I would
expect the B+ to drop as the forward Voltage drop across the diodes
increases with load.

Brian Goldsmith.