Easy Bias check without looking at the inside?

[guitarpsych]

I see there is a place that says "bias set" on the unused tube-socket looking things on the front of the ST70. Is there an easy way to adjust the bias using this? If so, where does one touch each end of the voltmeter, and what should the reading be? (obviously I am a beginner with these amps... please pardon my basic lack of knowledge).

[TomMcNally]

You measure from pin 8 of the octal sockets on the front to ground and look for and adjust for 1.56 volts. Check out this manual (and print it to have handy) It tells you all about it.



http://www.curcioaudio.com/st7_mnl.pdf

[mesherm]

Connect the black probe to the speaker GND terminal (with speakers connected) and insert the red probe into pin #8 of either the left or right side. If it is stock, then the resistor is still 15.6 ohms. Adjust the left or right bias pot until the meter reads 1.56 volts DC. Repeat with the other side . Do this back and forth a few times as adjusting one side will slightly change the other.
The 1.56 volts is 100 ma (50 ma each pair) across the 15.6 ohm resistor. Adjust to 1.25 volts if you only want 80 ma (40 ma each pair).

[EWBrown]

The 1.56 volts is derived from it's being the voltage of a brand new carbon zinc D cell battery. In those days, digital voltmeters for the masses didn't exist, and even the most humble cheap VOM still can be very accurate as an analog comparator, so the process was, measure a brand new D cell, take note of the relative reading, and then use that value to set the bias (100 mA for a pair of tubes, hopefully pretty well matched).

For uppgrades or new builds, it's easier to use a 10 ohm ersistor (and set for a 1.00 volt reading) , and if you set it up for individual 50 mA for each tube biasing, so much the better. IIRC the MK III was set for 140 mA trotal, 70 mA for each of the 6550s or KT88s.

/ed B in NH