Bias question

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Sixtiesvibe

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I am getting a pretty good amount of hum (home, gigs and studios) that really sounds like a Bias problem in my Mark III black dot. I looked at the schematics and there is no adjustment that I see.

Is the way to adjust the bias that I am missing?

Thanks in advance for any info and help on this.

8)
 
Bias is fixed on Boogies. Make sure you're using Mesa Power Tubes or equivalent as their bias is within required tolerance. Also check bias caps if they've never been changed. Worth doing filter caps as well as general maintenance.

Apart from that, could be a bad preamp tube. I normally tap them with something rigid and plastic like a chopstick and see if the noise gets worse, or swap with known good valves one by one.

If not, take the chassis out if you're confident to look for any obvious component that is burnt. If in doubt, get an amp tech to look at it.

Normal things to go are the 470 Ohm carbon resistors on the Power Tubes and the 680 Ohm R119 which should be replaced with a higher W version anyway as a precaution (2W or higher recommended)
 
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