Tying to build an ammeter just for bias measurements...

[Tonegeek]

i have an old busted VTVM with a nice, big, working meter on it that I want to convert to a passive ammeter just for measuring bias current using the transformer shunt method. The coil measures 1704 ohms and 340mv applied will give a full scale reading. This works out to be .1955ma of current through the meter. I was planning on using a 1ohm resistor as the shunt across the output tranny and setting the meter up to give full scale when there was 100mv across the 1ohm resistor. This should cover most biasing situations for a 50 watt amp. Then I would have a X 2 switch to do 100 watters. However it looks like i need a larger resistor because it appears the meter will only register about 1/3 the way up with the 1ohm one at 100mv. I am concerned that larger values will throw my reading off due to the fact that some current will still flow through the tranny on the side I am measuring. I guess what I need to know is how large a resistor I can use before this becomes a problem and pollutes my measurement?

thanks

[mhuss]

The DC resistance of a typical 50 or 100 watt OT is in the tens of ohms, so you don't have a lot of room to play with before the readings start to get inaccurate.

I like the idea of a dedicated shunt measuring tool, I've fried more than one meter fuse by forgetting to move the test lead to the "A" jack.

I now generally just measure the DCR of the OT and then measure the voltage instead, safer for absent-minded people!

--mark

[Tonegeek]

I now generally just measure the DCR of the OT and then measure the voltage instead, safer for absent-minded people!

--mark

The ammeter conversion is a bit overkill considering the alternatives, but I needed a little project this weekend and it forces me to "do the math" to keep the ole noggin workin. I figured out the margin of error is a few ma using a 3.3 ohm resistor which I just happen to have and which just happens to give me full scale when the current is 100ma (hey that math stuff works!). I double up with another 3.3 ohm and get x2 for 200ma max (and a bonus of slightly more accuracy). 200ma is more than enough to cover any amp bias adjustment(unless something shorts!) I will be putting a fuse in it for my first mod, but until then here's a picture of the end result: