Ohms reading for output transformer

Steve Waltz · Post by **Steve Waltz** » 16 Jun 2005 4:48 pm

i just changed my output transformer on a 59 RI fender bassman. I put a super/concert O.T. in it because it has three taps for 4/8/16 and they weren't labeled. So I took a guess and used the solid green wire. I used my cheap GB instruments mutimeter and tried to get an Ohm reading to see if I got the 8 ohm tap. I can't get an ohm reading! it goes to 0. Maybe my meter is bad because when I set the multi-meter to the RX1k or RX10 and attach the leads to my speaker I get a 0 reading just as I do when I attached them to the speaker outs on the amp.

What am I doing wrong?

Thanks,

Steve

jim milewski · Post by **jim milewski** » 16 Jun 2005 5:49 pm

get a digital meter and set it to it's lowest range, most lowest are around 200 ohms

Vern Wall · Post by **Vern Wall** » 16 Jun 2005 9:32 pm

Resistance is a DC measurement. A coil also has reactance which is an AC measurement. X = 2*pi*f*L where L is measured in henries. Impedance is the complex sum of the two, Z = sqrt(x^2 + R^2). An output winding has only a few turns of heavy wire, so it's expected that the DC resistance would be near zero.

If you care to spend about $20, you can get an RLC meter that will measure resistance, inductance, and capacitance. If you do, be sure you get one with a range to measure your filter capacitors.

John Daugherty · Post by **John Daugherty** » 17 Jun 2005 4:52 am

Steve, the "Rx100" means the first mark on the meter scale is 100 ohms. In that case, you could not see a reading of less than 100 ohms. If you use an analog meter, it will need a Rx1 scale in order to see one ohm. If you use a meter that will display a reading that is above zero, the 4 ohm tap will be the lowest resistance. The 16 ohm will be the highest resistance. The resistance readings are DC resistance and do not represent impedance.

Michael Whitley · Post by **Michael Whitley** » 17 Jun 2005 10:15 am

Also, Steve, just because you want the 8 ohm tap doesn't mean that the TRANSFORMER should read 8 ohms - it means the SPEAKER should read 8 ohms (nominal impedence, more or less, as explained above). I don't know what the transformer's secondaries should read, but, once you get your meter sorted out, you can measure all three wires, and the 8 ohm tap would be the middle reading.