JCA22H bias problem

  • Thread starter Thread starter ledvedder
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ledvedder

ledvedder

Well-known member
Can somebody help me out here? I'm trying to bias my 22H. B+ to ground is reading 356V. This would mean around 23mA for bias. Well, when reading from B+ to both the brown and green wires, it's reading 0.4mA. If I turn the bias adjustment wheel, it goes from 0.0 to 0.7.
IMG_20201216_195200_copy_1024x768.jpg
IMG_20201216_195902_copy_1024x768.jpg
Something is wrong here.
 
I'm not exactly sure what you're measuring, but it ain't the bias.
In order to measure the bias correctly in a 22h, you have to cut traces and install 1 ohm resistors.
Then you measure voltage drop with your meter set to volts.
You can not measure current as shown in the pics.
 
fusedbrain said:
I'm not exactly sure what you're measuring, but it ain't the bias.
In order to measure the bias correctly in a 22h, you have to cut traces and install 1 ohm resistors.
Then you measure voltage drop with your meter set to volts.
You can not measure current as shown in the pics.
Click to expand...
This isn't correct?
 
Jesus, I don't know what to say.....
You can not measure current in parallel.
The meter has to be in series with the load to get a correct reading.
Or..
You measure a voltage drop, with your meter set to VOLTS, and use ohms law to calculate the current.
I'm not trying to be a dick, but if you don't understand what I'm talking about here, you probably shouldn't be trying to bias this amp.
 
fusedbrain said:
Jesus, I don't know what to say.....
You can not measure current in parallel.
The meter has to be in series with the load to get a correct reading.
Or..
You measure a voltage drop, with your meter set to VOLTS, and use ohms law to calculate the current.
I'm not trying to be a dick, but if you don't understand what I'm talking about here, you probably shouldn't be trying to bias this amp.
Click to expand...
I wasnt even looking at his pics. I assumed what he was doing was correct and it was a tube.
 
psychodave said:
I wasnt even looking at his pics. I assumed what he was doing was correct and it was a tube.
Click to expand...
I was just following the procedure in that video. The same procedure is also written a few other places.
 
ledvedder said:
This isn't correct?
Click to expand...


This is called the output transformer shunt method, it can be done this way but should be avoided. Just be careful as the current is passing through the meter and could cause arcing and don't short any of the leads to ground while current through one probe or you could toast your output transformer
 
bwgintegra said:
This is called the output transformer shunt method, it can be done this way but should be avoided. Just be careful as the current is passing through the meter and could cause arcing and don't short any of the leads to ground while current through one probe or you could toast your output transformer
Click to expand...
I use this method for all of my amps, it fast and reliable... of course its dangerous, but I live life on the edge. :ROFLMAO:
 
Well, regardless if it's right or wrong... For whatever reason, it's not working for me.
 
Maybe my multimeter isn't reading mA correctly? What would be the easiest way to test it?
 
ledvedder said:
Maybe my multimeter isn't reading mA correctly? What would be the easiest way to test it?
Click to expand...
I don’t think you need to move the leads on your meter, check the manual but I see mA on the far right
 
I'm not the smartest guy posting in this thread so take what I say with a grain of salt. I don't think you should be doing the shunt method. I suggest a bias probe. I also suggest a 2nd meter to double check with. And I have had on several occasions a weak battery in the MM cause funky readings. Doesn't look like your meter has an AUTO feature so your next one get that - well worth it. I guess you are set at 200 mA for your bias but it is hard to read from here.
 
Last edited:
1. You really need to invest in a quality multimeter

2. If you use clips of possible instead of probes, one small slip is expensive

3. 0.4ma would produce very low output. I led it sounds okay and you didn’t modify the bias, I’d suspect the multimeter could be reading it with a decimal point off and it’s between 0-70ma. I dont see with an “A” or on “mA” on the display.
 
scottosan said:
1. You really need to invest in a quality multimeter

2. If you use clips of possible instead of probes, one small slip is expensive

3. 0.4ma would produce very low output. I led it sounds okay and you didn’t modify the bias, I’d suspect the multimeter could be reading it with a decimal point off and it’s between 0-70ma. I dont see with an “A” or on “mA” on the display.
Click to expand...
What multimeter would you recommend?
 
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