2. amps-preamps
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Can't get proper tube bias per manual

Hi All,

Regarding a MCIntosh MA-230,

How important is it to set the bias according to the manual?

Mac states set idle voltage to .7 v. Best I can get is .3 to .4

Considering it's age the amp sounds very exceptional, Does the amp have a problem that needs to be urgently addressed or am I just being overly cautious?

Looking at the schematics, I could drop the values on a set of 4 resistors to change the levels, but need to check with the experienced on this.

thanks..
toddbailey
3 responses Add your response
newbee
4,767 posts
 
Being unable to bring tubes up to recommended the bias range often is due to tube aging. If those are old tubes it may just be time to change them.
bdgregory
1,714 posts
 
I agree with Newbee . . . you need to determine if the problem is in the amp, or the tubes. I have had brand new tubes that wouldn't bias properly. Once you rule out tubes as the problem, you will want to have the amp looked at. It's important to have proper bias.
hifigeek1
770 posts
 
Newbee is correct. I would also check the actual bias supply voltage. It's possible that it's low to begin with in which case either resistors or a resistor have changed value. It's possible you have a bad filter capacitor causing the lower than spec. bias supply voltage, or you have a rectifier diode or diodes that are leaky and need to be replaced. Happy hunting. G
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