by Bob Latino Sat Jan 23, 2021 10:34 am
How to bias a VTA ST-120 amp.
Get your multitester and set the range for 0 to 2 volts DC scale. CONNECT THE SPEAKERS. Place the 5AR4 rectifier tube, the three 12AU7 driver tubes and the four 6550/KT88/KT120 output tubes in their sockets. Turn on the amp. Allow the amp to warm up for a minute and check to see that the 6550/KT88 tubes light up properly. Measure the bias on the LEFT output tubes by placing the BLACK NEGATIVE probe anywhere on the chassis and the RED POSITIVE probe in the hole marked “Biaset 1.56 v”. With a small screwdriver adjust the FRONT bias adjuster on the driver board closest to the front left tube. Turn the adjuster CLOCKWISE TO INCREASE BIAS. COUNTERCLOCKWISE WILL DECREASES BIAS VOLTAGE. Set the bias for .500 volts for the FRONT LEFT 6550/KT88/KT120 tube. Now place the RED POSITIVE probe in pin 4 (which is on the opposite side of the power take off socket) and use the BACK LEFT bias adjuster to measure bias on the BACK LEFT output tube. Repeat this procedure on the two right output tubes.
Facts about bias below ..
Since all tubes share the bias current > as you adjust one tube UP, the other tubes will go DOWN very slightly. Go back and forth between the four output tubes until all are between .490 and .510 for the VTA ST-120. That is plenty close enough. You will never all four output tubes to stay at .500 exactly. (Note > for the VTA ST-70 the bias recommendation is .400 VDC for each output tube)
As the amp warms up from a cold start, the bias will rise very slightly over the first half hour. After 1/2 hour recheck the bias and reset the output tubes so that all are between .490 and .510 VDC.
If all your tubes have a wandering bias UP or DOWN over time, then probably your line voltage is wandering UP and DOWN.
If just ONE tube just cannot be biased at .500, then that tube is probably bad or going bad OR possibly the 10 ohm 2 watt resistor that goes from pins 1/8 to chassis ground is bad and does not measure 10 ohms anymore.
If ALL tubes cannot be biased, then probably your rectifier has a problem.
Check your bias about once a month. As the output tubes wear in, the bias will change slightly.
Bob
