PA-10 Bias Setup

Technical Notes for Tube Rolling

For professional users only.

Trends Audio has already pre-set the jumpers and fine-tuned the DC bias voltage of each PA-10 with the packaged vacuum tube before shipping. However, changing different tubes to taste different sound is great fun for many audiophiles and also a great feature of PA-10. There are some important notes to be aware of before changing tubes.

Trends PA-10 supports two major tube series: (1) 6DJ8, 6922, 6N11, 6H23n, ECC88, E88CC, etc. and (2) 12AU7, 5963, 5814, ECC82, etc. If you change the tube within the same series, you need to re-tune the DC bias voltages only. If you change the tube to another series, you need both to re-tune the DC bias voltages and reset the jumpers. Otherwise, the tube may be burnt out.

1.1 Jumper Setting for Different Tube Series

You can configure the amplifier to support the two different tube series by resetting jumpers J3, J4 and J5 on the PCB as follows. Remember to power OFF the machine before setting.

J3, J4 and J5 jumper settings for 6DJ8 and 12AU7 tube series, with hollow circles and clear connecting lines

Inside view	6DJ8 series	12AU7 series
Inside view: PA-10
Inside view: PA-10.1D

Click an inside-view picture to open the full-size image.

1.2 DC Bias Voltage Tuning

Since each tube may vary a bit in parameters, especially between different series, the DC bias voltages need to be re-tuned to ensure the machine is working at its best operating point and performing with the best sound quality.

There are two blue precision adjustable potentiometers, PA-10 R3/R13 (L/R channel); PA-10.1D R5/R6 (L/R channel), for tuning the DC bias voltage of each channel. The DC bias has been tuned at the factory. After running in for a period or changing the tube, it may deviate and need fine-tuning.

PA-10 6DJ8 series	PA-10.1D 6DJ8 series

Before tuning, you need a screwdriver to remove the case, a small screwdriver to turn the potentiometer screw, and a digital multimeter to measure the DC bias voltage.

Bias setup meter example from the original guide

For example: Use a digital multimeter to adjust the DC bias voltage.

Multimeter example from the original guide

Use a screwdriver to remove the two upper-level screws in the front panel and another two screws in the back panel. Then open the upper case.
Power ON the machine with no audio signal input.
Wait at least 5 minutes for the machine to run in.
Use the multimeter to measure the DC voltage between test point A(L) [red] on the PCB and GND [black], for example the ring of the power switch or the outer ring of the RCA sockets, for the LEFT channel.
Turn the screw on top of the R3 of PA-10 (or R5 of PA-10.1D) potentiometer with a small screwdriver clockwise or anticlockwise until the voltage is as close as possible to +16.5 V.
Repeat steps 4 and 5 for the RIGHT channel, changing the test point to A(R) [red] and turning the screw on top of the R13 of PA-10 (or R6 of PA-10.1D) potentiometer.
Power OFF the amplifier.
Replace the upper case and tighten the screws.