Fender speaker jack wiring?

[MikeH]

Hello all - first post here (I have finally come to terms with my amplifier addiction and decided I can commit myself to yet another guitar electronics forum.)

Anyway, here's what I'm wondering:

Almost every old fender head I have looked at (maybe some new ones too, I don't recall) have 2 parallel jacks for speakers. One "Speaker" and one "Ext. Speaker". The thing I don't get is why the "Speaker" jack uses a shorting jack which grounds out the OT when nothing is plugged in. Obviously, the problem is if you're not paying attention you could accidentally plug the speaker into the "Ext" jack which would leave the speaker jack closed and the OT unloaded. Which would be really bad, right?

So why are the jacks wired this way?

[g1]

Hi Mike: Sorry but you will have to reverse your concept of load. No load condition would be when there is nothing connected to the output, infinite resistance. A piece of wire (short) is an extreme load, or zero ohms resistance. The shorting jack is there so you DO have a load, even if you have forgotten to plug in a speaker or if the speaker is accidentally unplugged.
If you should plug your speaker into the ext. speaker jack you will not hear anything because of the short across the main speaker jack.
In contrast to solid state amps, tube amps do not have as big a problem with such an extreme load as a short, although if you kept playing it like that it would strain the power tubes.
However, a short across the speaker jack of a solid state amp usually blows the amp up unless it has good protection circuitry, but SS amps can usually run happily with no load.
To sum it up, the lower your speaker impedance is, the greater the load. We want to always have a load for the OT, the shorting jack is a quick and simple solution.

P.S. Most Fender amps use shorting jacks for the speaker outs, most older Marshall's do not. Guess which are more prone to OT failure (though I'm not saying that's the only cause).

[MikeH]

Ok that makes sense. I knew that having no load was bad news, but I also thought that a shorted load was bad too. I suppose even if it is, it isn't as bad as having no load. Thanks for the explanation!

[Ronsonic]

You've got it. A short is bad, but it just causes heat in tubes and trannies and wear on the tubes. A no load can cause arcing and terribly expensive burning smells in tube sockets and output transformers.