[Boatanchors] Replacing rect tubes with diodes question

[Robert Nickels]

Rodger wrote:
> I always build the SS as a plug in and disconnect the 5 volt heater 
> leads from the socket to remove the B+ from the 5 volt winding of the 
> transformer since this is a fairly common failure mechanism in these 
> older transformers.

I agree with Rodger's suggestions, although I've done both plug-in and 
permanent conversions, having never come up with a good reason why I'd 
ever want to go back and "un-convert" one.  If you wire the solid state 
rectifiers in permanently, you can make use of the chassis as a heatsink 
to dissipate the power that is converted to heat in the resistor - just 
check the Mouser catalog for both wirewound and thin film power 
resistors that are inexpensive and will do the job nicely.  Be sure to 
label the old socket so no one (including you!) will try to fill it with 
a tube at some future time.  This is the cheapest, quickest, and in my 
opinion, "best" option, but it will annoy purists ;-)  (In my defense, 
if Bill Halligan could have bought 1N4007s for a nickel you can bet he 
would have used them!)

Or if you prefer to make a plug-in module, since you'll be relying on 
convection cooling to get rid of the heat,  consider using a number 
(typically 6 to 10) resistors in parallel.  A recent article in Electric 
Radio showed the same method I've used in the past, with eight 2.7K 2 
watt resistors in parallel, mounted in a cylinder between two pieces of 
PC board or a donut-shaped piece of thin metal that provides electrical 
connection and acts as a heat sink.   A little experimentation will let 
you determine the right value of resistor needed to develop the original 
B+ level, and Ohms Law will do the rest.

Or, if you just want to buy something ready-to-go that also incorporates 
inrush current limiting:  http://www.webervst.com/ccap.html    Just 
remember to remove the 5 volt heater wires to help safeguard your power 
transformer. 

73, Bob W9RAN