Also, the Symmetry trim when calibrated for even amounts of clipping on the top and bottom peaks of the tube
will be a setting that affords maximum signal throughput. Make this setting using a static waveform input and the
Blend fully post or 100% tube. Monitor the output signal and adjust the input to the unit so the Drive adjusts the
signal in and out of overdrive. Adjust Drive to the onset of overdrive or clipping of one extreme of the wave and
then adjust symmetry to shift this signal up or down out of overdrive. Repeat until no further Drive adjust
produces clipping that can be adjusted out with the Symmetry trim. This is the setting that will provide even
amounts of clipping on the top and bottom of the wave (and approximately when the Drive LED lights).
Then, with this optimum setting of signal throughput, the Output (gain) control won't have to approach the
amount that ground noise becomes trouble.
--0-0-0-0-0
DC V (volt) and mV (milliVolt) readings at circuit points (referenced to ground/circuit-common):
K/C15- = 0 to 0.4 mV
C15+/R10 = 18 to 20 V
R10/tube1 = 19 to 22 V
tube6/R16 = 22 to 24 V
R16/C17+ = 21 to 22 V
C17/R22 = -60 to -90 mV
tube2/R22 = -60 to -90mV
R22/R29 = -12 to -16 mV
tube3/ R33 = 115 to 145 mV
tube 8/ R32 = 90 to 110 mV
tube 7/C20/R24 = -110 to -160 mV
C20/R28 = 0.3 to 0.8 mV
R28/H = 0.3 to 0.8 mV
R11/R12 = 35 to 40 V
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Scott's Tube MicPreAmp mods.
1) modify the wire length for wire A so instead of running from the switch to the board along between the circuit
board and the front edge of the case, it runs back from the switch to the case rear and along the back edge and
up through the middle area of the case to wiring point A (twisted with the transformer wire to the switch to the
mid area, then with the other transformer wire as they both go from this rear mid area up to wiring points A and
G). Also, modify the routing of the three wires to the second channel from the 'front' area of the board to be;
from A/G/SG at a diagonal towards the tube of the second channel, under it, then up towards the top, and then
along with and arcing up and over to the second channels A/G/SG points.
The wire A has the transformer ac voltage on it and it being near the mic input wires and the area for the circuits
attached with IC3A can introduce hum into the output signal.
2) Substitute a greater value capacitor with a 50V or more rating for capacitor C14.
