How to set, check and adjust the bias on a Dynaco 70  

For amplifiers with Standard Bias Only

A voltmeter/multimeter that can display DC voltage is needed to perform the bias check and make proper adjustment.

Before plugging the amplifier in

Take the cover off, remove the cardboard shipping spacers and ALL packing and wrapping from the amplifier and from between the transformers.

Install the tubes

Be sure to align the "keyed" areas of the tube with the "keyway" in each tube socket. Apply gentle steady pressure, slightly and gently rock the tube back and forth while pushing down until the tube is well seated into the socket. Be sure the tubes are straight, upright and completely seated against the socket.

Hook up speakers

Never power on any tube amplifier without speakers attached, there needs to be a load on the output terminals. Failure to load the outputs properly can result in damage to the transformers. Double check your speaker wiring for correct and proper connections!

Set the Bias before using the amplifier: (set before first use!)

Bias is related to the current flowing through the power tubes. The bias will change over time as hours of use (age) affect the tubes and other components. Although the amplifier ships with the bias already set, the bias needs checked and /adjusted as necessary before first use. A voltmeter that can display DC voltage is needed to perform the bias check and make proper adjustment.

Bias will be set with the amplifier powered on and warmed up but do not play music through the amplifier during this process. If your preamp is connected to the amplifier, simply do not power it on during the bias procedure. Again be reminded, you must have a proper load (speakers) attached to the amplifier, if it is to be powered on.

• Before powering up the amplifier, turn both chassis mounted bias adjusters counter-clockwise to their full stop positions

• Before powering up the amplifier, be sure you have all tubes installed and in their correct location.

• Be sure you have speakers correctly attached to the output terminals.

• Set your voltmeter to DC volts, ground the black lead to the chassis.

• Power on the amplifier, insert the red lead into the designated hole in the front mounted octal socket for the left channel.

• Once the meter reading settles (1-2 minutes) rotate the left channel adjuster clockwise to reach a reading of about 1.5 DC volts.

• Move the Red lead to the right channels front socket and use the right bias adjuster to achieve a reading of about 1.5 DC volts.

• Move the Red lead back to the left channel, make an adjustment to reach the 1.5 volts DC.

• Move the Red lead back to the right channel, adjust again to reach 1.5 volts DC.

• Continue working back and forth until both channels settle at 1.5 to 1.56 DC volts.

• Let the amplifier continue to idle for about 15 minutes, check the readings again and adjust as necessary.

• Bias is now set, the amplifier is ready to use.

It is suggested to keep the bias just slightly below the factory suggested setting of 1.56 DC volts. A setting of 1.45VDC to 1.5VDC (DC volts) is friendlier to your tubes and allows for some drift or changes in the AC supplied to the mains.

Helpful hint: when making bias adjustments, since all the power is coming from a single transformer, increasing bias on one channel will lower it on the opposite channel and visa-versa. You should work back and forth, left channel then right, until very close or identical voltage readings are reached for each side. Once you have an acceptable setting, leave the unit on for about 15 to 30 more minutes and check it again. This makes sure the voltages are stable.

The symbol for DC Voltage is seen below.

Check the bias again at about 40 – 50 hours of use, then again at about every 100 – 120 hour intervals thereafter. 

Keep the bias slightly below 1.56 DC volts for best results, never higher though. A setting of 1.45VDC to 1.5VDC (DC volts) is friendlier to your tubes and allows for some drift or changes in the AC supplied to the mains.

When or if the bias adjustment becomes “maxed out”, meaning there is no more adjustment possible, it is usually time to replace the power tubes or possibly the rectifier tube. You should have the tubes tested to be certain. If new tubes do not resolve the issue, do not continue to operate the unit, seek qualified service for the amplifier.

The rectifier tube will eventually wear out and need replaced, but usually the rectifier (5AR4/GZ34) will last much longer than the output/power tubes. Test the rectifier for strength when in question. Most guitar shops or shops that repair vintage equipment can test vacuum tubes.