Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: texwest on April 01, 2021, 04:58:23 pm

Title: Measuring output trasnformer secondaries
Post by: texwest on April 01, 2021, 04:58:23 pm

Hey guys, since Classictone went under I'm looking at different options.

I bought at Hammond deluxe reverb OT.  Whenever they come in a just test them to make sure nothing is wrong.  I can test from the blue to red and the brown to red and get about 200ohms on both, but if I measure from blue to brown I get an OL with a - sign.  I tried this with the classictone and I can get the ohms from blue to brown.  I also tried it with a heyboer and can measure from blue to brown.

Is this a problem?  Should I send it back before I even try loading it into an amp?

Thanks

Title: Re: Measuring output trasnformer secondaries
Post by: pdf64 on April 01, 2021, 05:43:45 pm

Autoranging meter?

Title: Re: Measuring output trasnformer secondaries
Post by: texwest on April 01, 2021, 06:04:26 pm

Yes it's auto ranging.  It's funny that it works on the classictone which measures nearly the same. Around 200ohm for each push-pull side.

Title: Re: Measuring output trasnformer secondaries
Post by: texwest on April 01, 2021, 06:11:37 pm

Haha showing my ignorance here.  When I shifted to manual range it worked.  I've never had to move my meter over to manual ranging before.  If you care to enlighten me why it wouldn't work, I would be appreciative.

Title: Re: Measuring output trasnformer secondaries
Post by: pdf64 on April 01, 2021, 06:14:22 pm

It’s due to the inductive kick back, when the meter switches range.

Title: Re: Measuring output trasnformer secondaries
Post by: texwest on April 01, 2021, 08:09:44 pm

Thanks PDF!  You respond to so many of these threads.  I want to thank you for your extensive knowledge and sharing it.

Title: Re: Measuring output trasnformer secondaries
Post by: PRR on April 01, 2021, 10:39:39 pm

Short the secondary to kill the inductance which is (as said) confusing the autorange.

I do think that 10 or 20 Henries is about the limit for these auto-meters, and the inductance at VERY low current (such as meter-test) is very different from working inductance at normal current, so it does not even hint at which iron may be "better".