I'm fixing a Radiogram with a valve amplifier.
It has a selenium rectifier, ITT SenterCel type C2D.
Does anyone have any spec?
I'm thinking of replacing it by a 1N4007 and a series resistor.
The smoothing capacitor is 300V and the transformer output is 265V AC, so it needs some voltage drop!

 

Hi I,
Have you seen this link, it may help.
E
https://www.vintage-radio.net/forum/showthread.php?t=127531

 

Hi I,
Have you seen this link, it may help.
E
https://www.vintage-radio.net/forum/showthread.php?t=127531

Thanks. I'm trying to get some idea of the voltage drop.
The original capacitor still measures the correct values so I don't want to ruin it, because it is one of those "three capacitors in one case" devices.

 

hi I,

The "one volt per plate" is simply a rough rule of thumb. It tells us that a 5 plate rectifier is going to have a lot more drop than a 1N4007!

 

Hi
dispose off properly.When selenium burns, poisonous selenium compounds are formed

 

Yes, I ran into that issue when I was studying electrical engineering in college and knew just enough to be dangerous.
I had a little B&W TV that had a failed selenium rectifier, so I thought, "why not just replace it with a silicon rectifier?".
Well that worked, but the higher DC voltage affected the CRT sweep circuits, causing a reduction in the horizontal sweep size which generated a white vertical line on the right side of the display.

In general, a selenium rectifier drops about 1V per cell, so if you count the number of cells (plates) and multiply by 1V, that should give a reasonable idea of the total drop.
You, of course, would then have to determine the current draw to select an appropriate value of series resistor to drop that much voltage (or add a series Zener diode of the appropriate voltage and power).

 

Hi
dispose off properly.When selenium burns, poisonous selenium compounds are formed

Yes, when they fail and overheat, they can generate an obnoxious odor, something like rotten eggs, so it was easy to determine when one failed.

 

This is the offending item. It rectifies the B+ supply for an ECL86.
As an ECL86 runs with an anode current of 36mA and a 6mA G2 current, it is only rectifying about 50mA, and for it to need a substantial heatsink it must drop a lot of voltage.

 

If the DC voltage is about 250V, and since a single junction of selenium has a reverse voltage rating of 25V, I would expect about 10 junctions in series, giving a forward drop of about 10V.
Thus using a silicon diode would require an added series resistor of about 10V / 50mA = 200Ω, 1W rating.

 

That makes sense.
That package attached to a heatsink would probably dissipate about 1W, given that its maximum operating temperature will be quite low.

 

Hi I,
One method we used was to not cut the diode leads shorter, but to coil the surplus wire around a small screwdriver, then bend and solder the diode ends into a piece of Vero board.
The coiled diode wire made an effective heat sink for the diode.
Also, stand the diode a little above the surface of Vero.

Mounting the Vero vertically also helps the cooling.

E