Power Supply Considerations

Alan W. Fremmer AWFremmer at AOL.COM
Fri Apr 11 08:49:06 EDT 2008

Fellow Restorers/Collectors:
I'm in need of your expertise once again.  I have a Harvey-Wells T-90  in 
need of a power supply.  I decided to try and use what's available,  namely a 
Heathkit HP-23B.  The T-90 requires HV B+ of 500-600 volts and LV  B+ of 300 
volts.  The power supply provides 820 volts, no load and 700  volts at 250 ma.  It 
also provides the low voltage B+.
Using a back of the envelope Ohm's Law calculation (I don't recall  what 
parameters I chose) I came up with the idea of putting approximately a 500  ohm, 
25 watt resistor in series with the HV line.  The closest value I  had at hand 
was 750 ohms.  Well, everything seems to be working but I need  your 
assurance.  Here are my measurements: IR drop across resistor, 127  volts, plate 
current at resonance about 100 ma, total current through the  resistor, 167 ma, 
watts out 60.  The resistor gets VERY hot.  I'm  calculating 20 watts dissipation.
Would very much appreciate any suggestions as to how to improve or modify  
the setup, including getting a power supply with the voltages closer to those  
actually needed.
Thanks in advance for your past help and for whatever you can tell me about  
the present circumstance.
73, Alan - KB2HEI  

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