Replacing selenium rectifier with diode

[Richard Crowley]

It is trivial to over-spec the voltage and current ratings. It literally costs only pennies.

You did not identify WHAT machine you are working on, but if it uses tubes, beware that a modern solid-state diode is much more efficient and could over-stress the circuits. Research replacing selenium rectifiers with silicon diodes. I suspect you will find that inserting a series resistor is recommended to emulate the voltage drop across the original selenium rectifier and save the filter capacitors from damage, and the circuit from over-voltage.

Suggested reading: http://w3hwj.com/index_files/RBSelenium2.pdf

[mschaeffer]

Yes I've read about using a resistor. I think I've read that document before. I'll read it again. I am working on an Ampex A122 circa 1957. It uses tubes.

[brianroth]

I don't have schemos handy for an A122, but typically Ampex used the selenium rectifiers for the filament voltages. Regardless, you almost ALWAYS have to add a series resistor when replacing a selenium with a modern silicon rectifier.

Bri

[mschaeffer]

Here is the service manual with schematic: https://docs.google.com/viewer?a=v&q...QThtHjEP6jJU-g

The document rcrowley linked to is useful but I still am unsure about what kind of diode to get. The rectifier has 3 terminals, should I buy a 3 pin diode? The schematic says 1.5 amps, should I get a 2 amp diode? And what about forward current and surge current? From the schematic it looks like the selenium rect goes to a rectifier tube.

[Matt Syson]

Hi
The Selenium type is obvioulsly a 'pair' of diodes in one assembly. These are available in the 'silicon' world too with both common cathode and common anode versions available.
You can always go up in voltage rating, and unless you are making thousands of units, the cost of increased voltage and current rating is only a matter of a Dollar which is worth far more as 'insurance'. A 600 Volt 8 amp dual diode (probably in a TO220 case) would be practically bullet proof for a replacement of a selenium rated at around 1 Amp.
You WILL need either 1 or 2 resistors to 'bulk out' the inherent 'loss' exhibited by the selenium type and this (these) can get pretty warm so a wirewound part possibly bolted to the chassis could be good here.
You may also need to consider 'snubber' capacitors (typically 0.05uF or thereabouts) across each diode as 'silicon' diodes 'switch' much faster than Selenium and could introduce some noise into the audio circuitry. Being relatively low current and having resistors which will act to lower current peaks these may not be necessary.
Matt S

[Richard Crowley]

As Mr. Roth said, the selenium rectifier appears to be used to create DC voltage for the tube filaments. If you look at the schematic on page 20, you see that the rectifier looks like two diodes drawn together with a common cathode. And then a series resistor (R60). So you can replace the rectifier with two diodes with the anode end connected to the blue wires from the transformer and the cathode ends (the end with the line) connected together to the R60 resistor. Since the silicon diodes are more efficient, you will probably need to increase the value of R60. The exact value should be chosen to retain the rated 6.3 volt filament/heater voltage.

The schematic says "6.3VAC" across "X" and "Y", but that is clearly a misprint since they have used M1, R60 and C2 to produce DC.

You could also replace the 6X4 rectifier tube with silicon diodes to save some heat (and power) there as well.

[brianroth]

If replacing the tooob rectifier with silicon diodes, you will also need to add a series dropping resistor so the B+ doesn't shoot up through the roof.

Bri

[mschaeffer]

Isn't that 6.3 VAC marking for the fuse at M2?

[Richard Crowley]

Originally Posted by mschaeffer

Isn't that 6.3 VAC marking for the fuse at M2?

Not at all. The fuse at M2 is clearly at 120V (the power mains voltage).