[Tubesforme]

   

This chassis has been fully restored with all of the original value components. It works quite well, however the second you turn it on you get a hum that builds slightly as the tubes heat up. Once the tubes reach operating temperature, the hum remains and it can't be varied in intensity. In normal room volume it will drive you crazy. All tubes have been changed, several times. All resistors and capacitors shown in the picture have been changed, the ones in the B lines twice. The only capacitor that hasn't been changed is part 16, the 250uuf mica capacitor in the 24 plate circuit. Unless indicated, following voltages taken from PT CT. Meter being used is a new Fluke DMM. Voltages taken with volume full on and no antenna. I can't ground the chassis. The interstage transformer was replaced with one from AES which is indicated to be equivalent to Stancor A53-C.

#24 detector plate reads 117V
#24 detector screen reads 17V
#24 detector cathode 8V

#27 audio grid reads -7V common point cathode.
#27 plate reads 130V
#27 cathode reads 7.5V 

The voltage divider resistor measures correct resistance between all sections. Point F reads +19VDC and Point B reads -7V.


The grid voltage for the PP 71's read -52V

The hum can only be eliminated by grounding the grid of the 27 audio tube. Grounding the detector grid or pulling it out of its socket makes no change to the hum. The primary filter has been rebuilt twice, with original value 630V capacitors.


Any idea? Thanks