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In Reply to: RE: Question on biasing MA-1 with DMM posted by MLin on December 9, 2023 at 21:07:24:
All MK3s have automatic bias so this is an older amp.
A DC Offset is generated when you depress the Bias Test switch. That switch disconnects one of the two power supplies in the Circlotron output section. This causes half of the amp to push DC though the load (in this case, the meter). Typically, the meter is such a vastly lower resistance than the DCR of the loudspeaker that you can leave the speaker in place while this is being done, and the DC isn't harmful to any speaker we've encountered. Older amps put the meter in series with the speaker and we found that there was a tendency to set the bias a little high.
So with a DVM set to DC Amps, you put it across the speaker terminals and then depress the Bias Test switch. If a 12-tube MA-1 you set for 650mA initially; after one hour it should nut exceed 700mA.
With a 14-tube version you set the bias to 700mA and do not exceed 820mA after one hour.
You can set the bias lower with less heat if you want. You may notice a loss of authority if you set it below about 80% of the values I've indicated.
Regarding the DC Offset adjustment: Do that first. The meter is connected the same way and all you're doing is setting the DC Offset control for as close to a zero reading as you can get. If there is a gross mismatch between the tube banks in the amp you won't have enough range and may have to juggle a couple of power tube back and forth until you're within the range of the control. The tube banks are either right hand or left hand on all MA-1s.
After you set the bias, allow the amp to sit for about a minute at idle and then check the DC Offset again. The two settings interact, so you might have to go back and forth a little to get things right.
When the amp is warmed up for an hour, usually you can just check the DC Offset and then set the Bias since the movements on the controls will be slight.
If you find the DC Offset drifts a bit, its an indication the driver tube (not the voltage amplifier tubes) is weak. On the 12-tube MA-1s this might be a 6CG7; if updated and on all the later amps its a 6SN7. Contact me if you don't know which tube this might be in your amp.
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Follow Ups
- RE: How to set the bias with a DMM - Ralph 11:15:45 12/14/23 (1)
- RE: How to set the bias with a DMM - MLin 18:35:33 12/14/23 (0)
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