100w OT DC resistance of primary?

[andresound]

Help please.
A freind has brought me a 100w Mk2 Master model for repair. I suspect the OT may have shorted windings on the primary. What sort of DC resistance should I be seeing?

[martin manning]

Hammond's 100W replacement OT is 32 ohms end to end.

[andresound]

Thank you. To be clear.. approx 15 ohms from center tap to each side? Because if true, I am getting similar readings.

I am getting .4 ohms/.5/.6ohms on the secondary (4/8/16 taps). Is this right?

[sluckey]

Low resistance on the secondary is normal. Here's a simple test to check for shorted turns...

[martin manning]

Thank you. To be clear.. approx 15 ohms from center tap to each side? Because if true, I am getting similar readings.

I am getting .4 ohms/.5/.6ohms on the secondary (4/8/16 taps). Is this right?

That all sounds good.

[andresound]

Thank you Martin. I was supplied the amp with the transformer un-attached and given the “spare xfmr” to fit and get the amp running again. Just making sure.

[martin manning]

The original OT could have just a few shorted turns so the DC resistance appears normal. The test mentioned by Steve above is a good way to test for that.

[Stevem]

If you do not want to build that tester then another way to prove out the OT is to just install a 1 ohm 2 watt 1% resistor in series to ground off of pin 8 of one output tube and then strap a D.C. Volt meter across that resistor .

It will read milliamps direct and if when you Crank signal thru the amp you see the current go up , if you get it to read 80 ma yet have little sound output to the speaker it will be certain that the OT has short somewhere and is toast!