23% bias difference between tubes not being cause by tubes

[LPSGME]

I must not have explained things clearly.
Here's a diagram to clarify.

If the voltage on the 1 ohm resistor, where it attaches to the tube [at the red star] reads 40mv, then the voltage at the red test jack, that is attached to the same point [i.e. red star] should also read 40mv.

But my jack was reading 50mv.

In any case all I was saying is that if the red test jack had a cold solder joint [to the wire that attaches it to the 1 ohm resistor] then I would expect a lower and not higher reading - as a cold joint would add resistance between the probe and the 'red star' test point.

For example: A 2 meg resistor put in between the 1 ohm resistor and probe would read lower nearer to 37 or 38 mv.

Does anyone still think the voltage would be higher in that scenario?

[IMG:504:360]http://i275.photobucket.com/albums/jj30 ... G/Bias.jpg[/img]

[martin manning]

What if the bad connection is between the 1-ohm resistor and the socket?

If the resistor's ground connection is good, then the current draw is as indicated by your measurement at the resistor lead, 40 mA. You could check the voltage across this resistor directly at its leads to be sure.

If this is true, then there must be an additional 0.25 ohms between the resistor lead and the socket to get the same 40 mA when you measure from the socket or test jack and get 50 mV.

Re the 3 mV drop across the 2-Meg, it's caused by your meter drawing a very small current through it. From this you can see that small resistances in the connections to the test jacks and in the meter leads will not cause significant error.

MPM

[Structo]

The 1 ohm resistor should be between the red test jack and the black test jack, with the black test jack going to ground.

[LPSGME]

What if the bad connection is between the 1-ohm resistor and the socket?

If the resistor's ground connection is good, then the current draw is as indicated by your measurement at the resistor lead, 40 mA. You could check the voltage across this resistor directly at its leads to be sure.

If this is true, then there must be an additional 0.25 ohms between the resistor lead and the socket to get the same 40 mA when you measure from the socket or test jack and get 50 mV.

Re the 3 mV drop across the 2-Meg, it's caused by your meter drawing a very small current through it. From this you can see that small resistances in the connections to the test jacks and in the meter leads will not cause significant error.

MPM

The 1 ohm is wired as per my diagram and not between the tests jacks as in the diagram Structo provided above.

I think that the way it is wired is OK because, as you note "small resistances in the connections to the test jacks and in the meter leads will not cause significant error". Although, as I believe as you also suggest, even a small resistance between the 1 ohm and ground or the tube would.

I was puzzled because when I first measured the voltage at resistor/tube junction, it read 50mv just like at the jack. But then something changed leaving the resistor/socket junction to measure 40mv while the jack was still at 50mv. Then, after resoldering the jack and wire that leads back to the resistor, the voltage at the jack and resistor were the same 40mv.

In any case both tubes are now within 1.5 mv of each other, but the .5 AMP fast blow FUSE is still blowing. A 1 amp fuse will hold, but not the .5 amp that was originally in the amp and working.

None of the other voltage reading different between the two tubes, so I guess it's time to just swap in some new ones.