Replacement part for 50yo bridge rectifier?

Stabby

Member
2010-09-07 1:56 am
I'm working on the pre-amp of a 50-year-old electric piano and will probably have to replace the bridge rectifier. It's pictured in this photo next to the transformer:

http://i.imgur.com/rH3KW.jpg

I have no idea what to look for though, since I can't interpret the information printed on the rectifier:

B250 075
KC068 22/8

Also, the rectifier is connected to two 350/395V capacitors.

Could anyone help me out?
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Hard to make out...
I assume it's silicon and not selenium... did selenium work at those voltages ?

You need to measure the AC voltage across the bridge and the DC across any reservoir cap. Does it follow the Vdc = 1.414 times Vac formula or not. Selenium has a larger volt drop. Just with you saying it was 50 years old.

Silicon... loads of choice but measure the voltages first and see what it runs at.
 
You can replace selenium rectifiers with silicon, but sometimes you have to add series resistance at the appropriate wattage and voltage rating to keep the load voltage from getting too high. Note 1/4 W resistors are no longer rated at 400V because they are too short. 1 W resistors are usually rated above 400 V but check your supplier tables. Phenolic headers are handy for mounting discrete components to chassis at these high voltages, attaching to the metal with machine screws. tubesandmore.com sells them, calling them "terminal strips". I can't find headers at mouser, farnell, any of the usual international sources. With phenolic headers, you can also make a bridge out of 4 each 1n4007 etc. the way most vendors did in that era.
 
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Are you sure it's not a B250 C75 bridge? If it is there is some information on replacing the B250 C75 selenium rectifer about two thirds down the page on this site:-

1958-1959 Grundig "2066 PX" AM/FM/SW Radio Restoration

You could use a small silicon bridge rectifier (about 1A 1000V) or four 1N4007 diodes in bridge formation, and a 100 ohm 10W resistor in series with the output to compensate for the voltage drop.

Mooly, 30KV Selenium diodes.
http://www.nteinc.com/specs/500to599/pdf/nte504.pdf
 

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