Bias question

Mark I's, II's, III's and the almight IV

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Sixtiesvibe
Mark I
Posts: 37
Joined: Wed Apr 27, 2016 9:46 pm

Bias question

Post by Sixtiesvibe » Sun Apr 15, 2018 1:10 pm

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)
'58 VOS Gibson Les Paul || 1984 Custom Stratocaster || 2014 Gibson Firebird
Vai Legacy 1 Head || Mesa 2x12 Cab || Modded Blues Jr || Boogie Mk III ||
TC electronic Delay/Reverb || Plimsoul - Xotic Wah - EP booster - CS Deja Vibe

Rider1260
Mark I
Posts: 37
Joined: Sun Aug 21, 2016 10:00 pm

Re: Bias question

Post by Rider1260 » Sun Apr 15, 2018 7:40 pm

No Bias on any Boogie I know of , could be a bad tube I would start there
Mesa MKIIb ( my first in 82 )
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jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Bias question

Post by jrb32 » Mon Apr 16, 2018 8:23 am

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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