[Boatanchors] Replacing rect tubes with diodes question

[Rodger]

Hi Gene,

You also need a dropping resistor to reduce the voltage to close to what 
would normally be supplied by the tube rectifier.  You can get a decent 
idea of what the resistor should be by looking at what the typical tube 
voltage drop is in normal operation, this information is available in 
the tube data books or the NJ7P online data base is handy:  
http://www.nj7p.org/Tube.php  I would imagine a 270 ohm 20 watt resistor 
connected in series with the common cathode connection would get you 
close enough.  Of course you could also use separate resistors for each 
replacement diode, in this case the resistor value would be doubled and 
the power rating would be halved.  I do replace a fair number of my tube 
rectifiers with SS rectifiers and some of the Halli power transformers 
definitely benefit from this replacement.  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.

A couple of other considerations:  The B+ will come up instantly and 
will be higher than usual until the remaining tubes warm up and begin 
drawing current so if the filter caps are marginal this extra voltage 
will probably cause their rapid demise.  So replace any doubtful caps 
first.  By the way, even with a tube rectifier the voltage is much 
higher just after turn on as the directly heated rectifier cathode heats 
up almost instantly in the existing tube type rectifier.  I would also 
consider adding an inrush current limiter to your set to further reduce 
the stress on the transformer and tube filaments, suitable ones are 
available from Mouser or Digikey for around a couple of bucks and they 
are installed after the fuse and in series with the transformer primary.

73, Rodger WQ9E

WØQFC wrote:
> I want to replace the # 80 in my S-40A with diodes so I can reduce 
> heat by less draw on the tranny.
>
> Can I just use two diodes that will handle the current, or is there 
> something else I should do ~ additional parts, etc?
>
> Also, I would like to replace the 6H6 for the same reason, but what 
> diodes should I use?
>
> I know that these items are available but in the interest of saving 
> money, I would like to do it myself. . . .
>
> Comments?
>
> 73
>
> Gene, WØQFC
>
> www.w0qfc.com
>
> _______________________________________________
>
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