JCA22H bias problem

  • Thread starter Thread starter ledvedder
  • Start date Start date
My first bias probe was from Eurotubes and it blew out pretty quickly. The second one I got was from Weber and believe it had a built in resistor to help prevent from blowing but yeah - in your case I think you would be wise to get any kind of probe and then make sure your MM's have good batteries and good fuses. Switching back and forth between volts and milliAmps is easy enough. Never Ever pull a probe off of an amp that is turned on.
 
This is an EL84 fixed bias amp, a normal octal bias probe doesn't work with these tubes. In this situation I would also do the shunt method but I think it's important for people to understand the method before performing it.
 
bwgintegra said:
This is an EL84 fixed bias amp, a normal octal bias probe doesn't work with these tubes. In this situation I would also do the shunt method but I think it's important for people to understand the method before performing it.
Click to expand...
I understand the method.

1. Measure voltage from ground to the center tap of the output transformer. I'm getting 356V.

2. Measure mA from the center tap to pin 3 of each power tube. Either the blue or brown wire. Here is where I'm not getting a correct reading. With my MM set to 200mA, it's reading 0.4. I should be getting something in the range of 10-20.
 
I would suggest you do this:
1) Cut the traces for the Cathode grounds.
2) Solder in 2 x 1 OHM resistors
3) Measure voltage drop across resistors. Because the resistors are 1 OHM, mV = mA

Having the full B+ running through your meter is a recipe for disaster. You may not get a second chance.........

EDIT: It may not be clear in the second pic, but the 2 resistors are connected to the center terminal on the terminal strip, which grounds the cathodes to the chassis. Sand the paint off the chassis under the foot of the terminal strip to ensure a good ground connection, or run a ground wire to a preferred location.

JCA22H cut traces.jpg
JCA22H cathode resistors.jpg
 
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fusedbrain said:
I would suggest you do this:
1) Cut the traces for the Cathode grounds.
2) Solder in 2 x 1 OHM resistors
3) Measure voltage drop across resistors. Because the resistors are 1 OHM, mV = mA

Having the full B+ running through your meter is a recipe for disaster. You may not get a second chance.........

EDIT: It may not be clear in the second pic, but the 2 resistors are connected to the center terminal on the terminal strip, which grounds the cathodes to the chassis. Sand the paint off the chassis under the foot of the terminal strip to ensure a good ground connection, or run a ground wire to a preferred location.

View attachment 66086View attachment 66087
Click to expand...
Where can I get that 3 tabbed piece that you have the resistors soldered to?
 
fusedbrain said:
You should be able to buy terminal strips anywhere you would purchase resistors and capacitors.
For example:
https://tubedepot.com/t/diy-central/terminal-stripshttps://www.amplifiedparts.com/products/terminal-strip-3-lug-2nd-lug-common-horizontal
I would also recommend 1 watt or 2 watt 1 OHM resistors, as the larger size and thicker leads makes things easier.
Metal Film usually has the closest tolerance. Buy a few and use the 2 that measure closest to exactly 1 OHM. They're cheap.
Click to expand...
Will these work? I don't see metal film ones. https://tubedepot.com/products/2-watt-metal-oxide-power-resistor
 
Is it easy to mount test points on the back of the chassis from these 2 resistors?
 
Are you supposed to be plugged into the 10A jack on the meter when set to 200mA? EDIT: Sorry I was only reading page 2.

I pretty much always do what fusedbrain suggested. Just remember that your screen current is also going thru that 1 ohm cathode resistor.
 
ledvedder said:
I understand the method.

1. Measure voltage from ground to the center tap of the output transformer. I'm getting 356V.

2. Measure mA from the center tap to pin 3 of each power tube. Either the blue or brown wire. Here is where I'm not getting a correct reading. With my MM set to 200mA, it's reading 0.4. I should be getting something in the range of 10-20.
Click to expand...
pin 3 would be applicable to EL34/6L6 variants. I believe it would be pins 7 on EL84,
 
SpiderWars said:
Are you supposed to be plugged into the 10A jack on the meter when set to 200mA? EDIT: Sorry I was only reading page 2.

I pretty much always do what fusedbrain suggested. Just remember that your screen current is also going thru that 1 ohm cathode resistor.
Click to expand...
I just noticed this. If set for 200ma it should be in the other jack.
 
ledvedder said:
Is it easy to mount test points on the back of the chassis from these 2 resistors?
Click to expand...
Those banana plug test points are nice to have...as long as you already know the plate voltage or target idle current. But you'd have to drill for them, I wouldn't but I'm a lazy fuck.
 
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