10-05-2014, 06:33 PM
Do you have the shield plate installed on top of the chassis between the rectifier 80 tube and the 27 audio amp? Without this shield the 27 will pick up hum from the large AC voltage on the plates of the 80. Later models may have a regular tube shield on the 27 instead of the metal plate.
You can increase the capacitance of the second filter cap after the choke to a higher value like 20 or 30 uF. Do not increase the first filter cap right after the rectifier as it will raise the DC output voltage.
Keep in mind that there will always be some residual hum due to the use of AC to heat the filament of the 47 output tube. You might try another 47 which may have better match between the two halves of its filament, better cancelling the filament induced hum
You can increase the capacitance of the second filter cap after the choke to a higher value like 20 or 30 uF. Do not increase the first filter cap right after the rectifier as it will raise the DC output voltage.
Keep in mind that there will always be some residual hum due to the use of AC to heat the filament of the 47 output tube. You might try another 47 which may have better match between the two halves of its filament, better cancelling the filament induced hum
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