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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Sun 27, 2019 4:33 pm 
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Joined: Aug Thu 23, 2018 1:11 pm
Posts: 28
The fact that '... holding a small screwdriver near the driver transistor.." increases the noise implies that the noise is being radiated into the driver transistor and then amplified by the high gain of the driver stage. The C21/R22 network should roll the gain off at frequencies above about 700 Hz, but since the stage gain is so high noise may still receive enough gain to be audible at the loudspeaker. To test this theory, try putting a LARGE capacitor (~1uF) across resistor R21, which will all but kill the stage gain at audio frequencies. If the noise goes away the hypothesis is confirmed. A smaller capacitor (~100 pF) across R21 should roll the gain off further at frequencies above ~10kHz and provide a fix.

I find it interesting to see all the "heroic measures" the design engineers tried as they made the transition from vacuum tubes to transistors mid-century. I would imagine that in large companies such as GE most of the engineers were older and were schooled in vacuum tube design techniques but self-taught in the nuances of designing with transistors. A lot of the issues which drive solid state designs, for example achieving thermal stability, were not factors in the vacuum tube world and were "terra incognita" to the designers of the day.

Again, good luck!

John


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