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December 06, 2022, 10:47:42 AM

Author Topic: Bias and Meter Reading Slightly Off  (Read 208 times)

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Offline RdDgWest

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Bias and Meter Reading Slightly Off
« on: July 27, 2022, 07:16:36 AM »
I'm at the Bias and Meter Test stage of the build.
Good news is both meters work and perform virtually identically. 

When the switch is in the 50mA position the meter goes to the first mark to the left of the 40mA indication on the meter.

When the switch is in the 60mA position the meter goes to the first first mark before the 60mA indication on the meter. 

Just not sure what could cause both meters to be off this way and not aligning with the build manual.  Open to ideas to check. 

Offline Paul Birkeland

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Re: Bias and Meter Reading Slightly Off
« Reply #1 on: July 27, 2022, 07:21:01 AM »
You'd want to report the DC voltage on the center lug of the hum pot in both positions to know with absolute certainty.  There is also a slotted screw on the front of the meters that can be used to fine tune the meter to the actual current reported, as there will be some variations in the 9.09 resistors soldered across the meter.

Although there have been many reported issues with the meters, the last batch I got back all tested fine and went into my own projects, so a proper debugging seems like a good idea.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline RdDgWest

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Re: Bias and Meter Reading Slightly Off
« Reply #2 on: July 27, 2022, 07:35:01 AM »
Thank you Paul
Measurements from one of the amps (right)

50mA switch position - Center pin of hum pot = 64.1VDC
60mA switch position - Center pin of hum pot = 59.3 VDC

Offline Paul Birkeland

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Re: Bias and Meter Reading Slightly Off
« Reply #3 on: July 27, 2022, 11:08:01 AM »
Can you also do a DC resistance measurement between ground and the center lug of the hum pot in both positions?  That would enable me to give you the most accurate information.  It will take your meter a few moments to stabilize in the presence of the cathode bypass capacitor, but it should still provide an accurate reading.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline RdDgWest

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Re: Bias and Meter Reading Slightly Off
« Reply #4 on: July 27, 2022, 01:12:01 PM »
Thank you Paul

50mA switch position is 1.18K - Current would be 64.1V / 1.18K = 0.054A
60mA switch position is 0.910K - Current would be 59.3V / 0.910K = 0.065A

I have this correct?
« Last Edit: July 27, 2022, 01:19:41 PM by RdDgWest »

Offline Paul Birkeland

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Re: Bias and Meter Reading Slightly Off
« Reply #5 on: July 27, 2022, 01:55:55 PM »
I would just adjust the needle up a hair on the meter and be sure things are well connected there. 
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline RdDgWest

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Re: Bias and Meter Reading Slightly Off
« Reply #6 on: July 27, 2022, 03:25:32 PM »
Thank you Paul,
I rechecked the connections on the meter.  Also between the blue and black wires and the respective connection points.

I can only get the meter to slightly increase.  Just under the second notch past 40mA.  When I flip the switch for 60mA the meter still comes under the 60mA reading. 

Is this close enough or should the meter be hitting the 50mA and 60mA marks on the meter with ease?

Offline Paul Birkeland

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Re: Bias and Meter Reading Slightly Off
« Reply #7 on: July 28, 2022, 05:00:18 AM »
I would consider it close enough and move onto listening to some music.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline RdDgWest

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Re: Bias and Meter Reading Slightly Off
« Reply #8 on: July 28, 2022, 05:32:46 AM »
Thank you, Paul
Awesome.
What I suspected but didn't want to continue with the build and mask or create other problems. 
Let the build continue.  :D


Offline RdDgWest

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Re: Bias and Meter Reading Slightly Off
« Reply #9 on: August 01, 2022, 03:59:39 AM »
Instead of starting a new thread I thought I would keep this one going.

After the build was complete I went with the meters in 60mA switch position.  When the amps turn on the meters both start at the first notch below 60mA.  After some hours of playing the meter reading in both amps lower to virtually the identical reading.

Is this normal or a result of something I might have done during the build?


Offline Paul Birkeland

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Re: Bias and Meter Reading Slightly Off
« Reply #10 on: August 01, 2022, 05:19:05 AM »
That is likely normal.  You can adjust the needle positions a little bit with a small flathead screwdriver.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline RdDgWest

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Re: Bias and Meter Reading Slightly Off
« Reply #11 on: August 01, 2022, 05:56:36 AM »
I did that during the initial build.  The adjustment on the meter only moves it up a few millimeters.
I listened for 1 hour this morning and the meters did not move downward much or at all.
I noticed this after 5 hours of continues use over the past two days.