21-09-2017, 04:44 PM
(21-09-2017, 11:36 AM)boater sam Wrote: Is the metal pot body grounded?
It is via the neutral mains switch tag. The wire link has now been cut off and mains neutral is returned to chassis along with negative tag of the 16 + 16mfd smoothing can. The earthy end of the volume control is returned to chassis by means of the securing nut. The result of this work was a useful reduction of hum but it was still present at high volume settings, just not as bad as it was before. Bias for the 6Q7G is by grid current. The 0.002mfd coupling capacitor between the slider of the volume control and grid of the valve was found to be leaky. Replacing it resulted in a slight increase of hum so we're almost back to the same level of hum again.
Today the correct 25A6G output valve arrived from an eBay seller. So out with the temporary 25L6GT and in with the 25A6G. An improvement in sound quality because of the correct matching of the valve to the output transformer, but intriguing is the fact that the hum from the audio amplifier stage has gone. It's ear to the loudspeaker to hear any hum now.
The 6Q7G DDT was never available in metallised form whereas the European EBC33 always was. However, French valve manufactures made their own version of the American octal series of valves and I'm sure a 6Q7MG was made. I do have in my stock the 6K7MG and 6L7MG.
From the Radiomuseum: the Mazda France 6K7MG. http://www.radiomuseum.org/tubes/tube_6k7mg.html
Geordie McBoyne.
