How to safely bias Cayin A88T Tube amp

The way to do it safely is to connect dummy load resistor to the speaker binding posts of your amp such as 10Ohms/50W available at partsexpress.com. Watch your multimeter not to go way over or under the tolerant bias voltages. Ask the manufacturer or use the bias manuals to do so. Giving it to someone else might be safe but it's just another way of giving out easy money for no work.

Peaman, in what part of the country are you located? If you are local to me, I'll do it for you.

It is easy to bias the Cayin A88T. You do not need to pay anyone. Buy a multimeter at Radio Shack or elsewhere for $20 or less.

Put the amp on its side and take off the metal bottom (lots of screws, do not take off the feet). The voltages can be lethal, so do not touch any of the wires and stay with the amp while it warms up, so no one will come in and touch anything. Turn the amp on and let it run for at least 20 minutes. Before that the bias will not be accurate as it keeps changing in the first 20 minutes of warming up.

You will see inside underneath around the periphery round ceramic sockets wired for the power tubes. Near them is a blue board with a .5 inch tall adjustment screw that is slotted on top for a screw driver. You can carefully adjust with your hands being careful not to touch anything, or use a slotted screwdriver with a naturally insolating plastic end (be damn sure you are not touching the metal in case the end of the screw driver accidently touches something else).

The four round tube holders underneath the amp have wires soldered to the pins of the ceramic tube holder. The positive contact point for your multimeter (red multimeter probe) on one side is the blue wire where it is bare and soldered to the tube socket pin and on the other side of the amp orange wires are the positive. A group of bare thick silver wire in the center of the amp is the negative for all adjustments.

I set my multimeter at 20V. Use your red multimeter probe and touch the positive contact point (blue or orange wire bare contact point to the ceramic tube socket) and the black to touch the negative contact (thick bare silver wire set in middle of amp) then adjust the blue adjustment screw on the corresponding blue board to match your desired bias setting. .4 is the factory recommended bias. You can turn the adjustment knob on the blue board to match. I set mine at .35 so the tubes will last longer and the amp will sound slightly warmer.

If you are using new tubes, after the tubes have broken in the bias may change significantly. After about 200 hours of use with new tubes, bias the amp again, then once a year is probably fine.

Plangco, very detailed explanation, thank you!

Plangco:

Thank you for your write up.  I just successfully biased my recently acquired unit.  Being comfortable with electronics, it could not have been simpler to do. In my case, the tubes were far out of balance and it now is much quieter with a much more organized presentation.