Selenium Rectifier Voltage Drop

[analogman]

I am restoring an older tube short wave radio that contains a selenium
rectifier for the high voltage power supply. I've replaced it with a
silicon diode, however the lower voltage drop of the diode requires an
additional dropping resistor to obtainthe correct voltage.

The problem I'm having now is that when the radio turns on, no current
is drawn from this power supply until the tubes warm up and the output
voltage is initially as high as 160 volts. The audio output tube,
50C5, has a max plate voltage of 150. Does anyone have an idea of how
I can limit the voltage? I've tried 5W zener diodes but the seem to
get quite warm even though are disipating about 2.5 watts.

It's been a while since I worked on this radio and I don't remember
what prompted me to replace the origional selenium rectifier. In any
case, I'm not all that sure it is bad now. I experimented with it and
found it has a voltage drop of about 20v. Does anyone know what a
typical voltage drop should be for these things?

Thanks in advance,
analogman

 

[Watson A.Name - 'Watt Sun']

Use a thermistor.

Do you meant a NTC varistor? A thermistor is for sensing temperature.

Problem with the NTC varistor is that it will probably warm up much
faster than the filaments, therefore it will minimize the inrush
current, but the voltage will still be excessive while the filaments
are warming up. It takes something like 11 seconds for the filaments
to get warm.


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[Al]

I am restoring an older tube short wave radio that contains a selenium
rectifier for the high voltage power supply. I've replaced it with a
silicon diode, however the lower voltage drop of the diode requires an
additional dropping resistor to obtainthe correct voltage.

The problem I'm having now is that when the radio turns on, no current
is drawn from this power supply until the tubes warm up and the output
voltage is initially as high as 160 volts. The audio output tube,
50C5, has a max plate voltage of 150. Does anyone have an idea of how
I can limit the voltage? I've tried 5W zener diodes but the seem to
get quite warm even though are disipating about 2.5 watts.

It's been a while since I worked on this radio and I don't remember
what prompted me to replace the origional selenium rectifier. In any
case, I'm not all that sure it is bad now. I experimented with it and
found it has a voltage drop of about 20v. Does anyone know what a
typical voltage drop should be for these things?

Thanks in advance,
analogman

Since you're not doing a "true" restoration, put a load resistor across
the supply that simulates the warmed up load. Use a time delay relay
that opens just before the filaments warm up. Or you could do nothing.
The maximum ratings are specified maximums. There probably is a safe
margin above that rating.

BTW, many years ago I replaced a rectifier tube with a diode like you
did. I had no problems with the tube radio. I admit I never checked the
voltage afterward. It worked, so that was the end of that.

Al

 

[Dbowey]

in the mid 50's this changeout was very common. Typically, a 300 Ohm resistor
was put in series with the silicon diode to make it look more like the selenium
stack.

Don
---------
dant posted:
<< I am restoring an older tube short wave radio that contains a selenium
rectifier for the high voltage power supply. I've replaced it with a
silicon diode, however the lower voltage drop of the diode requires an
additional dropping resistor to obtainthe correct voltage.

The problem I'm having now is that when the radio turns on, no current
is drawn from this power supply until the tubes warm up and the output
voltage is initially as high as 160 volts. The audio output tube,
50C5, has a max plate voltage of 150. Does anyone have an idea of how
I can limit the voltage? I've tried 5W zener diodes but the seem to
get quite warm even though are disipating about 2.5 watts.

It's been a while since I worked on this radio and I don't remember
what prompted me to replace the origional selenium rectifier. In any
case, I'm not all that sure it is bad now. I experimented with it and
found it has a voltage drop of about 20v. Does anyone know what a
typical voltage drop should be for these things?

 

[analogman]

Sorry I can't help with forward voltage, although it must have been several
volts or they wouldn't have needed all those fins to dissipate heat. This
means that whatever you replace it with will have to dissipate the same
amount (unless you introduce a sswitched mode psu).

How about making a simple resistive voltage divider with an emitter
follower, bolted to a heat-sink, to drop the voltage to say 140 V.


Graham

I thought about the emitter follower circuit but I really didn't want
to go through adding a heat sink and all. That may be a last resort.
The zener diode approach was actually 3 - 43v 5W zeners in series
totaling 129 volts. The normal running supply voltage is 130v. With 43
volts drop across each zener and about 60ma current flowing through
them results in ~2.6W. They get a bit hot to the touch. I'm not sure
if this is safe for them or not. (My power dissipation and heat
sinking experience is on the weak side.) But these diodes are not made
for heat sinking. They look more like a standard 1N4001 diode but
about twice the size. I have them connected as a shunt to ground.
There was a resistor already in series with the PS to produce the 60ma
current limit through the diodes.

 

[Dave Platt]

I think that worrying about plate voltage when there's no plate current is
a bit of a red herring. Obviously bad things happen if you go way over
the rated voltage... but the capacitors are likely to give out far before
the tube does.

My understanding is that the opposite is true - bringing up the plate
voltage while the cathodes are still cold can lead to "cathode
stripping", which damages the electron-emitting coating on the
cathodes. If I recall correctly, this can lead to substantially
reduced tube lifetime, and (in some high-power applications) can
actually trigger arcing within the tube.

In devices which use solid-state rectifiers, it's not a bad idea at
all to add a time-delay circuit to the plate-voltage supply, so that
the cathodes have time to heat up before the juice comes on.

 

[Watson A.Name - 'Watt Sun']

in the mid 50's this changeout was very common. Typically, a 300 Ohm resistor
was put in series with the silicon diode to make it look more like the selenium
stack.

So far in this discussion I've yet to see anyone state that the
resistor must be a power resistor. We're talking about several watts
here at least.

Don

[snip]


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Just when you thought you had all this figured out, the gov't
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[Watson A.Name - 'Watt Sun']

It won't help in the pre-warmup condition, where there's zero current flowing
anyway.

I think that worrying about plate voltage when there's no plate current is
a bit of a red herring. Obviously bad things happen if you go way over
the rated voltage... but the capacitors are likely to give out far before
the tube does.

The "right" way to do this is to rewind the plate transformer to give
slightly less voltage. There may already be a tap on the transformer
winding that you could use instead.

Umm, he hasn't been talking about a radio with a transformer. Radios
with 50C5s use series string filaments and no transformer. Nada.
Zilch. B+ is directly rectified AC line voltage.

Tim.

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My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@

 

[Watson A.Name - 'Watt Sun']

Sorry I can't help with forward voltage, although it must have been several
volts or they wouldn't have needed all those fins to dissipate heat. This
means that whatever you replace it with will have to dissipate the same
amount (unless you introduce a sswitched mode psu).

How about making a simple resistive voltage divider with an emitter
follower, bolted to a heat-sink, to drop the voltage to say 140 V.

The supply voltage _still_ has to be filtered before it goes to the
emitter follower, which means that filter cap still has the same
excessive voltage problem. He's not trying to make a silk purse out
of a sow's ear, he's just trying to make a few accommodations to
prevent a failure in the future. Let's keep it simple, and not turn
it into a major project.

Graham

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###Got a Question about ELECTRONICS? Check HERE First:###
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My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@

 

[Graham]

I am restoring an older tube short wave radio that contains a selenium
rectifier for the high voltage power supply. I've replaced it with a
silicon diode, however the lower voltage drop of the diode requires an
additional dropping resistor to obtainthe correct voltage.

The problem I'm having now is that when the radio turns on, no current
is drawn from this power supply until the tubes warm up and the output
voltage is initially as high as 160 volts. The audio output tube,
50C5, has a max plate voltage of 150. Does anyone have an idea of how
I can limit the voltage? I've tried 5W zener diodes but the seem to
get quite warm even though are disipating about 2.5 watts.

It's been a while since I worked on this radio and I don't remember
what prompted me to replace the origional selenium rectifier. In any
case, I'm not all that sure it is bad now. I experimented with it and
found it has a voltage drop of about 20v. Does anyone know what a
typical voltage drop should be for these things?

Sorry I can't help with forward voltage, although it must have been several
volts or they wouldn't have needed all those fins to dissipate heat. This
means that whatever you replace it with will have to dissipate the same
amount (unless you introduce a sswitched mode psu).

How about making a simple resistive voltage divider with an emitter
follower, bolted to a heat-sink, to drop the voltage to say 140 V.


Graham

 

[Terry King]

Sorry, I don't see this as a big problem..

The original selenium rectifier had 'instant-on'
and almost zero drop at startup, so the capacitors
originally were rated to withstand the well-known
high startup voltage. Many, many pieces of equipment
from that era had this behavior.

Only some high-power amplifiers and transmitters had
time-delay relays to delay application of high voltage.

If you're concerned about the warmed-up runtime voltage
being 10 or 20 volts high, then use a series resistor
to bring it back to 'nominal' if you know what that
is/was.

 

[Pieter Hoeben]

Typically, a 300 Ohm resistor was put in series with the silicon diode

So far in this discussion I've yet to see anyone state that the
resistor must be a power resistor. We're talking about several watts
here at least.

Depends on the current, when for example the max. current is 100 mA,
then I*I*R = 0.1 * 0.1 * 300 = 3 Watts. The current of old radio's is
not very high, just measure it with volume high.

Regards,

Pieter Hoeben

 

[Mark Zenier]

Sorry, I don't see this as a big problem..

The original selenium rectifier had 'instant-on'
and almost zero drop at startup, so the capacitors
originally were rated to withstand the well-known
high startup voltage. Many, many pieces of equipment
from that era had this behavior.
Agreed.

Only some high-power amplifiers and transmitters had
time-delay relays to delay application of high voltage.

If you're concerned about the warmed-up runtime voltage
being 10 or 20 volts high, then use a series resistor
to bring it back to 'nominal' if you know what that
is/was.

My suggestion is that the OP could use a microwave oven rectifier,
which is a stack of silicon diodes with a voltage drop more like
a selenium stack, without the smell ;-).

Mark Zenier [email protected] Washington State resident