Hiwatt DR103 project Output transformer wiring question

[Lifto]

Hi all,

Last couple of weeks I have been building a Hiwatt DR103 copy using the information from hiwatt.org (love that site). However I have one question about the wiring of the output transformer.

I'm using this layout
http://hiwatt.org/Layouts/DR103hiwattlayout.pdf
And the schematics on the website:
http://hiwatt.org/Schematics/DR_100wOutput.pdf

For my build I used an outputtransformer from tube-town:
https://www.tube-town.net/info/daten...t-dr103-ot.pdf

After I wired the output jacks I got confused. I wired the jacks correctly (and tested this with a speaker to see if the switches etc. were installed correctly). So when I plugged in a 8 ohm speaker the resistance between the tip and sleeve was approximately 8 ohm. However, after I hooked up the output transformer (black from secondary to sleeve (grounded), 4/8/16 ohm to selector switch, selector switch common to tip/switch jack), I don't have a resistance reading between the tip and the sleeve anymore.
Is this normal? Ik double checked everything and it seems I connected everything correctly. I'm confusing myself if this is something that is supposed to be or not.

[frus]

Hi all,
I don't have a resistance reading between the tip and the sleeve anymore.

you mean the resistance is zero?

[Lifto]

Yes that's it. With an 8ohm speaker the resistance reads zero. It might that it is supposed to be like this but i'm starting to doubt myself and would like to know for sure.

[frus]

well the OT secondary's resistance is supposed to be around zero (or better, an order of magnitude lower than the speaker), to minimize the losses and deliver most of the power to the speaker. So when you connect the OT secondary parallel to the speaker, the resistance should be almost the same as the secondary alone. Our meters will read around zero.

[Lifto]

Yes I thought it would be something simple like that but the more I kept thinking about it the more confused I got. Thank you for clearing this up. Now I can continue my build and start wiring the power supply and maybe in a few days I will be ready to start the amp up. I will post pictures when it's finished.

Cheers!

[Gingertube]

An 8 Ohm speaker will generally measure about 6.5 Ohms DC Resistance using your multimeter.
The series resistance of a typical 8 Ohm secondary winding of an output transformer is about 0.2 Ohms (up to say 0.5 Ohms in low power output trannies)

Once you wire the speaker to the tranny (or plug the speaker into the wired jack) you will no longer see the speaker resistance but instead see that low 0.2 Ohm transformer secondary wiring resistance which is "in parallel" with it.

Cheers,
Ian