Mark51 Posted October 19, 2018 Share Posted October 19, 2018 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? Quote Link to comment Share on other sites More sharing options...
Danieln Posted October 20, 2018 Share Posted October 20, 2018 My guess is that since the 300B heater current is quite high, there will be residual AC that can be cancelled out using the pot. Dan Quote Link to comment Share on other sites More sharing options...
WMcD Posted October 21, 2018 Share Posted October 21, 2018 Maybe follow up starting at the article. http://diyaudioprojects.com/Schematics/300B-SE-Tube-Amp-Schematic.htm 1 Quote Link to comment Share on other sites More sharing options...
WMcD Posted October 21, 2018 Share Posted October 21, 2018 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 1 Quote Link to comment Share on other sites More sharing options...
NOSValves Posted October 21, 2018 Share Posted October 21, 2018 9 hours ago, WMcD said: I too can't figure out how the hum balance pot would help the d.c. situation. WMcD I'll third that. Quote Link to comment Share on other sites More sharing options...
Mark51 Posted October 24, 2018 Author Share Posted October 24, 2018 I asked the same question in the DIY forum. They say that with only a capacitor in the filter there will still be some AC and with Klipsch speakers I would be glad there's a pot to balance it out. Makes sense. Quote Link to comment Share on other sites More sharing options...
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