SCOTT 16 6K7 FIRST AUDIO AMPLIFIER CIRCUIT QUESTION

[Dave McClellan]

The schematics I have (Steve Johnson and Riders) shows that the 6K7 cathode is grounded and the bias is supplied through a 1meg resistor from the -10 volt bias divider resistor string. My radio is quite different: the 6K7 cathode (and suppressor) go through an unbypassed 680 ohm resistor to ground, and the 1meg grid resistor is grounded. The resistor (5%) does not look original.

In addition, in my radio the 6K7 first audio amp gets its screen supply through a 100K resistor. The schematics say 15K. The question is: should I leave the circuit as is (the radio works), or restore the circuit to the schematic? I am concerned that the changes may reflect a production change or improvement.

[Kent King]

The Sixteen was a fairly short-lived model that did not see much in the way of change. Since you noted the resistor was a 5%, I'm guessing (without pics) this may be a more modern replacement. Do the solder joints look "original"? If it was me, I'd probably put the set back to its factory diagram in this case. However - as you not, the set is working well, so there is no need to do this. I'd look closely at the joints to see if it appears to be changed.

Kent

[Dave McClellan]

The connections are definitely NOT original. After further reading on the Forum, hum originating in the 6K7 1st AF amp stage was discussed. In the original circuit, the 6K7 grid bias comes a tap on the bias divider resistor string, sourced from the power transformer high voltage center tap. There will be some small amount of 120Hz ripple on this source, filtered only by a 30mfd capacitor and further by a 1meg resistor and 0.5mfd capacitor. Any small amount of ripple here would be greatly amplified by the 6K7 1st AF amp and could possibly result in audible hum independent of the volume control setting. Switching to cathode bias would eliminate this possibility. Since my radio works with no audible hum, I will leave it as is. The screen dropping resistor change from 15K to 100K still worries me. I plan on measuring the screen voltage as is and with a 15K dropping resistor. The change may not be significant, or if it is may be part of the cathode bias change.