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Help me out with this 300b schematic


Mark51

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Now that I have a pair of Fortes I want to build a 300B amp.  The schematic here is exactly what I built about 20 years ago but sold.  My question concerns the 100 ohm pot in the filament of the 300b.  My recollection is that this was for balancing hum when using 5 volts AC.  But this schematic shows the 5 volt supply rectified.  So why a hum balancing pot?   

6SN7-300B-Single-Ended-Tube-Amp-Schematic.jpg

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I followed the diy audio thread and found http://diyaudioprojects.com/Tubes/300B-SET-Amplifier/

 

It seems the builder was experimenting with several regulated d.c. supplies for all the tubes.  It is possible that the schematic was never updated from the 300B with a.c. to a 300B with a d.c. supply.  I too can't figure out how the hum balance pot would help the d.c. situation.

 

WMcD

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