vintage diodes

[embotone]

I picked up an old Bogen receiver I am going to rebuild.  It uses silicon diodes in the bridge recitifier for the power supply and to provide dc for the preamp heaters.  It uses a selenium diode for the bias supply and has several pairs of matched germainium diodes in the tuner section.
Why would they use different types of diodes.  I know I  gotta get that selenium diode out of there, and will replace the power suuply diodes with FREDs, but why germainium in the tuner?  Are the germainium diodes prone to failure like selenium?

[sluckey]

Germainium diodes have better properties for low signal detection and are still common in modern circuits as audio detectors, AGC detectors, and rf mixer circuits. Selenium diodes are not really prone to failure especially in a low voltage/current situation such as a bias rectifier. I'd leave it in place if it is working.

[PRR]

Silicon can do 2+A or 400+V but used to cost a lot.

Selenium was much cheaper for 10mA 100V such as bias.

Both are slow, largely because "power" devices must be BIG. Radio circuits need fast low-power devices. Cheap is good and at the time Ge radio diodes were very cheap.

> Why would they use different types of diodes.

Same as sticks of wood. I use 12"x24" beams to build my concert hall. When leading the orchestra I wave a 1/8" stick. Power supply has big bones, small radio signals need small bones. True, I could glue-up roof beams from hundreds of batons, or a 400V 500mA power rectifier from hundreds of 40V 10mA small diodes, but that's awkward.

Do not mess with the tuner diodes. They don't (hardly-never) fail, and there is no clearly superior replacement. Neither you nor I know what parameters they need to be; probably not fussy but we don't know. The factory aligned with those diodes in place; changing them will probably upset alignnment and you do not want to go there.