Berkshire Power Supply / Amp

Comment

Comment by Tom Jardine on June 20, 2016 at 3:31pm

Good information, Norman. My seleniums are long gone. I've learned a lot since 1978 when I first tackled this set. At that time, I did just enough repairs to get the set to operate reasonably, waiting until 2013 to get serious about it. -Tom

Comment by Ken Carr on June 20, 2016 at 3:30pm

Norman, I totally agree with leaving the rectifier in place even if I bypass it. When I get to the point of working on this you will see me here asking questions again.

Comment by Norman S Braithwaite on June 20, 2016 at 3:14pm

FYI, I like to leave the selenium rectifier in place for posterity and correct visual impact.  Generally, the failure mode on the selenium rectifiers is a reduction in capacity (unable to conduct needed current) resulting in low voltages.  On occasion they will short with internal sparks and external smoke.  The smoke is toxic but so noxious that it is unlikely anyone is going to get a serious dose without ignoring the smell.  Placing silicon diodes across the selenium rectifier using the very convenient solder lugs integral to the selenium rectifier will take the load off the selenium rectifier but voltage will remain.  It is better to isolate silicon diodes from the selenium rectifier.

Comment by Norman S Braithwaite on June 20, 2016 at 3:07pm

There is a clever fix for the voltage increase problem.  The supply consists of a main transformer and a booster transformer.  If the primary of the booster transformer is hooked as if the set were being supplied by 220v rather than 110v and the secondary is wired to buck the voltage from the primary transformer, no high power voltage dropping resistor will be required when replacing the selenium rectifier with silicone diodes.  However, if this is to be done, the amplifier must be the self bias version.  If not, conversion of the fixed bias version to self bias is not difficult.

Norman

Comment by Ken Carr on June 20, 2016 at 12:03pm

Thanks for that information, Tom. When I get to that point I will most likely contact you for more specifics. I have the old rectifier (seen in one of the photos). It is massive! I'm told that they are a potential hazard also since they can release poison gas. As if getting electrocuted wasn't enough (LOL).

Comment by Tom Jardine on June 20, 2016 at 10:22am

When replacing the selenium rectifiers with SS devices, which are more efficient, the DC filament voltage will increase as well as the 28 (I think)DC volts for the motors which will have to be reduced somehow.