Quick question - speaker socket polarity?

[mcgruff]

Have a WF-55 Tweed Champ clone from ampmaker which I'm about to power up for the first time. To test it, I'm going to plug it into a 50W 8ohm hi-fi speaker (no guitar cab).

The speaker colours (AFAIK) are positive terminal red and negative terminal black.

I don't know the polarity of the output. Here's a picture of the output transformer, if that helps:



The green wire 8ohm and the orange wire 0 are connected to the output jack. Green positive, orange negative?

[Jazz P Bass]

As the ouput voltage is Vac, as such there is no polarity.
It is when you hook up another speaker that the details start to matter.
Anyway, using the 8 ohm tap as positive is the way to do it.

[g1]

Yes, you are correct. But in cases where you have a single speaker it is pretty much irrelevant, the polarity (phasing) only becomes an issue when multiple speakers are involved. Then they must all be connected the same way, otherwise there will be phase cancellations.
In your case with a single speaker, either way would work. Conventionally, the "0" terminal is negative or ground.

edit: JPB, "coke"

[mcgruff]

Cheers - thanks guys

[Enzo]

Um, fellas...

I see a NFB line in the amp. so the speaker polarity won;t matter, but the phase of the primary terminals will.

[g1]

Yes, if you get a massive howling when you turn it on, you will need to reverse the primary connections. This is due to the NFB loop as Enzo mentioned.
As there is no way of knowing which way the primary winding should go, it can happen no matter how the secondary is wired. It is strictly trial and error.
Wiring the primary one way will give negative feedback (correct), while wiring it the "wrong" way will result in positive feedback (howling).

[mcgruff]

Fired it up and... success!

This is the first amp I've made and I enjoyed it tremendously. I'd recommend this kit to other toob noobs because the instructions are very good and Barry from ampmaker is very helpful.

Sounds great. Lots of champy character: fast attack, overdrive like sawing through a wooden hive full of very angry bees, but nice clean too. Too much high end with the hi-fi speaker though, as you'd expect (tweeter...).

Next step is to build a reactive load and, eventually, a proper cabinet. I got this for practicing and recording and I think they're incredible value for £160 (head only). Cheap but classic tone from one of the simplest circuits ever.

[loudthud]

It's easy to fry a tweeter in a 100W HiFi speaker with a 5W guitar amp. That angry bees sound really stresses the tweeter. Turn the tweeter down or off, or disconnect it.

[mcgruff]

Thanks for the tip loudthud - I didn't know that. Just looked at the 50W rating and thought yeah plenty...

PS: like you say in your sig, anyone inspired to try out a DIY amp should make sure they know how to work safely. Always unplug before poking around inside and give the caps plenty time to drain.