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biasing help - strange reading on multimeter

Blue Strat

Member
Messages
30,243
Do the Tweaker instructions indicate you should read 65mV? The reason I ask is that it's possible to use a 10 ohm, rather than a 1 ohm, resistor for current sensing in which case you would read 6.5 versus 65.

What happens when you tweak the bias pot?
 

oldhousescott

Gold Supporting Member
Messages
2,884
According to a post by Bruce on that other board, it should be set for 65mVDC between test points 1 and 2, for 6L6s, 5881s, and EL34s.

Make sure the battery in your DMM is good. Try the 2VDC scale, it might give you 3 decimal places of accuracy, enough to measure .065 VDC.
 

afterburner

Silver Supporting Member
Messages
1,551
The instructions say 65mV.

The initial reading with the stock tubes were 07.3 or so. I got the amp used so I don't know what (if any) biasing was done. The bias pot is really sensitive to small adjustments, but I didn't try to see what the min/max I could get it to be was.
 

afterburner

Silver Supporting Member
Messages
1,551
According to a post by Bruce on that other board, it should be set for 65mVDC between test points 1 and 2, for 6L6s, 5881s, and EL34s.

Make sure the battery in your DMM is good. Try the 2VDC scale, it might give you 3 decimal places of accuracy, enough to measure .065 VDC.

Would the 200 mV setting be wrong? I will try the 2VDC later.
 

afterburner

Silver Supporting Member
Messages
1,551
It is labeled 10 MegOhm input impedance on the DC V settings part. Does that have anything to do with it?

Going to pull the chassis and try to take the reading again now...
 

afterburner

Silver Supporting Member
Messages
1,551
Didn't check the battery yet...

BUT I did finally get it working. I turned the bias adjustment knob all the way down and gradually turned it up. It started showing readings in the proper range... and I got it to 65mV.

Will have to check the battery in the DMM.

Thanks everyone!
 

Blue Strat

Member
Messages
30,243
Didn't check the battery yet...

BUT I did finally get it working. I turned the bias adjustment knob all the way down and gradually turned it up. It started showing readings in the proper range... and I got it to 65mV.

Will have to check the battery in the DMM.

Thanks everyone!
Congrats! Try another battery just in case though. It would really suck if you were really running them at 650mV. :D
 






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