Re: Selenium rectifier question
- From: "Jimmie D" <GFENDER@xxxxxxxxxxxxxxx>
- Date: Sun, 13 Apr 2008 16:48:51 -0400
"Arfa Daily" <arfa.daily@xxxxxxxxxxxx> wrote in message
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"Jerry G." <jerryg50@xxxxxxxxxxxxxxxxx> wrote in message
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The diodes are probably breaking down. You need to have them rated at
2.8X of the voltage for the peak to peak from the AC. I would use
diodes rated to at least 3000 V to 4000 V minimum in this case because
the no load voltage is over 900 V. You can use 4 of 600 V in series to
get the proper voltage rating. I would also put caps at about 0.005 uF
/ 5 kV rated across each diode. This would be for HF noise suppression.
You should find out the required current needed for the supply load. The
diodes should be rated to at least 3X the required current at minimum.
This will allow for the inrush when the power supply is started up. The
electrolytic filter caps have to get charged up.
In series with each leg of diodes in series, I would put a 10 ohm 5 Watt
resistor in an average tube power amp supply.
If you use 1 Amp rated diodes, you can easily find diodes rated to 1000
Volts. This would allow for 900 Watts maximum load at 1000 Volts. Using
3 in series on each leg of the rectification path would be very adequate
for a 1000 Volt no load source.
I would not re-use the present diodes at this time. Most likely they may
be damaged even if they read okay.
Before going to all this trouble, make sure you find the cause of the
original rectifier failure. It may be age, but a short or something
pulling too much current should not be ruled out.
If you want to go more authentic and you have the space, get an 8 pin
octal socket, and a 5U4-GT tube. Knock out the holes in the chassis and
install the tube rectifier assembly. You will need a high voltage
isolated floating 5 Volt 2 Amp supply just for the heater. The heater is
tied hot to the cathode of this tube. You can then feed the AC 900 Volts
plate to plate, and the rectified plus source would be on the cathode.
--
Jerry G.
I'm not sure that 10 ohms is going to do too much to limit the inrush
current at these sorts of voltages, and will produce little useable
running voltage drop to compensate for the higher DC that will be produced
by the replacement silicon diodes over the original selenium stack.
I agree, 100 ohms is probably a more reasonable value.
When working on old equipment like this I try to add enough resistance to
keep the B+ at least 10% below the rating of the filer caps
If the filter caps were rated at 450 I would add enough resistors after
replacing selenium diodes to keep the voltage around 410 or so.
This is just kind of a best guess way of doing things when a minimum amount
of information is available.
Jimmie
.
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