Specifications

DIY Tube Stereo 35 Board - TubeZone Assembled -Instructions - Page 5
Initial Settings: Feedback and Bias
WARNING: Do these adjustments with the unit off and unplugged, as well as having had one hour to bleed
voltage from the filter caps. Note: Most amps will just use a fixed resistor here, but the diytube circuit uses a pot
for experimentations sake thus this extra step.
➔ 1) Don't have your negative feedback connected yet, that is your output secondary or else it will be a
real low impedance.
➔ 2) Probe your DMM from J4 pin 3 (right channel) to ground and adjust R30 until the resistance is 20.3K
(set to 28.3K if using the 16 ohm tap as feedback, as in the original Z565).
➔ 3) Probe your DMM from J3 pin 3 (left channel) to ground and adjust R31 until the resistance is 20.3K
(set to 28.3K if using the 16 ohm tap as feedback, as in the original Z565).
➔ 4) Your feedback is set.
Adjusting Bias
➔ 1) With the unit off, probe your DMM from V3 pin 3 (the cathode) to ground. Adjust R22 until you
measure 400 ohms to ground.
➔ 2) Then probe your DMM from V4 pin 3 (the cathode) to ground. Adjust R19 until you measure 400
ohms to ground.
➔ 3) Then probe your DMM from V5 pin 3 (the cathode) to ground. Adjust R28 until you measure 400
ohms to ground.
➔ 4) Then probe your DMM from V6 pin 3 (the cathode) to ground. Adjust R25 until you measure 400
ohms to ground.
WARNING: The following is done while the unit is ON and IDLE.
Take care and follow proper high voltage safety rules.
➔ 5) With all your connections in place, power up. Your initial bias has been set - this makes sure that you
don't accidentally have a tube aggressively biased, and they should all end up near the same range on
power up.
➔ 6) Probe your DMM (set to DC volts) from the test point which is sandwiched between the R19 & R22 to
ground. V3 is adjusted with the 'inside' test point. Adjust R22 until the voltage is .350V or 350mV.
➔ 7) Probe your DMM (set to DC volts) from the test point which is sandwiched between the R19 & R22 to
ground. V4 is adjusted with the 'outside' test point. Adjust R19 until the voltage is .350V or 350mV.
➔ 8 ) Probe your DMM (set to DC volts) from the test point which is sandwiched between the R25 & R28 to
ground. V5 is adjusted with the 'inside' test point. Adjust R28 until the voltage is .350V or 350mV.
➔ 9) Probe your DMM (set to DC volts) from the test point which is sandwiched between the R25 & R28 to
ground. V6 is adjusted with the 'outside' test point. Adjust R25 until the voltage is .350V or 350mV.
Voltages
These are no-signal, nominal measurements in VDC, unless otherwise noted. They are all referenced to ground.