OT Secondary

[strato17]

Right Now, I have my OT primary wires hooked up on my express(2 to the plates, CT to standby switch).

However, when using my DMM, I observed that there is virutally no resistance between each seconday wire and the ground(chassis), and there is also no resistance between any two OT secondary wires.

Is this normal?

[Fischerman]

Right Now, I have my OT primary wires hooked up on my express(2 two heaters, CT to standby switch).

The primary wires should be to the plates of the power tubes. CT to B+ (SB switch is fine).

However, when using my DMM, I observed that there is virutally no resistance between each seconday wire and the ground(chassis), and there is also no resistance between any two OT secondary wires.

Is this normal?

Think about how it's connected. The speaker jack (and thus the OT secondary 'common' wire) is grounded to the chassis. The secondary is just a coil of fat wire...so between each secondary tap and ground you just have a coil of fat wire (fatter wire has less resistance than thinner wire). And between any two secondary wires you also just have a coil of fat wire (unless it's a JTM45 Radio Spares type OT which has isolated, seperate secondary windings...but most guitar amp OTs just have one 'tapped' secondary winding). You're just measuring the very low resistance of the secondary coil (or a portion of it). It's totally normal.

[strato17]

Right Now, I have my OT primary wires hooked up on my express(2 two heaters, CT to standby switch).

The primary wires should be to the plates of the power tubes. CT to B+ (SB switch is fine).

However, when using my DMM, I observed that there is virutally no resistance between each seconday wire and the ground(chassis), and there is also no resistance between any two OT secondary wires.

Is this normal?

Think about how it's connected. The speaker jack (and thus the OT secondary 'common' wire) is grounded to the chassis. The secondary is just a coil of fat wire...so between each secondary tap and ground you just have a coil of fat wire (fatter wire has less resistance than thinner wire). And between any two secondary wires you also just have a coil of fat wire (unless it's a JTM45 Radio Spares type OT which has isolated, seperate secondary windings...but most guitar amp OTs just have one 'tapped' secondary winding). You're just measuring the very low resistance of the secondary coil (or a portion of it). It's totally normal.

OK thats what I was thinking. What made me nervous was the fact that when I soldered the ot secondaries to the switch, I always read no resistance on every wired lug with the output wire on the speaker jacks, no matter which position the switch was in.

I guess that is normal? Complete noob here

[Fischerman]

Look at the hook-up diagram for your OT, and also at a schematic of your amp and try to figure out why that is normal. The secondary is just a coil of wire (tapped in a few places for different speaker impedances)...which is a lot of wire so there will be some resistance...but there are less turns in the secondary and the wire is fatter. The primary has more turns and the wire is thinner so it has more resistance (but it's still kinda low-ish).

[strato17]

Look at the hook-up diagram for your OT, and also at a schematic of your amp and try to figure out why that is normal. The secondary is just a coil of wire (tapped in a few places for different speaker impedances)...which is a lot of wire so there will be some resistance...but there are less turns in the secondary and the wire is fatter. The primary has more turns and the wire is thinner so it has more resistance (but it's still kinda low-ish).

I guess I need to clarify better. By low resistance, i mean less than one ohm, almost zero(probably is zero).

Can you just confirm for me that the impedance switch should only let the signal through from the selected impedance secondary wire, not all of them at once?

[strato17]

And yes, in the first post I meant the plates, not heaters. I corrected it.

[Fischerman]

When you play the amp there will be signal on all of the impedance taps. There will be more signal on the 16 ohm tap, then less on the 8, then less on the 4. The impedance selector just connects the selected tap to the speaker jack. But if you measure with your DMM you'll get just a fraction of an ohm...and since it's all one winding it won't matter much which taps you measure. You're just measuring one long wire at a few different points along the wire...and one end of that wire is connected to the chassis.

[strato17]

OK I got it all figured out now. Makes much more sense now that I realize that the impedance just come from different spots on the same wire that is wound in the tranny.

Thanks for the help and patience!

[gearhead]

I went down this same road not long ago. Only a small portion of the OT output is resistance, ie ohms. The rest is AC impedence.