Silly output jack question

[Gibsonman63]

I understand the input jack being a shorting jack. I believe that the output jacks I ordered were the same as the BOM, but the schematic does not depict them as shorting jacks. It seems to me that you would not want to short out the jack, but maybe I am looking at this wrong. Can someone set me straight on this>

[Bruciep07]

There should always be a speaker plugged into the speaker jack (load on the OT), with a pair of shorted jacks, if you plug just one jack in, the second jack will still provide a path to ground for the output signal which, among other things, will cut the output signal to the speaker completely. a non shorting jack will be floating and the only path will be to the speaker... I probably don't have to tell you that it's a bad thing to ground the outputs of an OT but I'll mention it anyway.

[Gibsonman63]

Thanks for the sanity check. I was aware that you never want to run your amp with no load and it seemed to me that parallel speaker outputs with shorting jacks wasn't a good thing either.

[Structo]

If using two speaker jacks only the first one can be a switching jack.
Otherwise when you plug one speaker in Jack 2, Jack 1 is still shorted.

I think the reasoning behind using a shorting jack is because some feel it is is just slightly better to short the secondary than to leave it open on a tube amp.

Some builders have wired it so a resistor kicks in if no speaker is connected.
That probably is the best but the resistor would have to be pretty big in size to handle the current or it could be a fire hazard.

I just did some work on an old Danelectro DM-25 guitar amp.
It was one of those piggyback amps where the head can be stowed below the speaker in the back so it becomes one unit to carry.

The speaker cable which looked like regular zip cord, had an octal plug on the end.
This plugged into an octal socket on the back of the amp.
What was interesting is that this plug had the power switch circuit go to this socket. On the plug was a jumper between two pins of the socket so that when plugged in, it completed the primary circuit of the power transformer and allowed it to be switched on.
So if you forgot to plug in the speaker, the amp would not turn on.

Pretty clever and most likely not UL approved but I thought it was a good method of protecting the amp from damage.

You can see the little yellow jumper wire in the plug in this photo.

[paulster]

Going way off topic but Ampeg did a similar thing too. They had a 4-pin speaker connector on the amp and cab, and completed the B+ circuit by plugging in the cable at both the amp and cab ends, so you couldn't get it out of standby unless there was a cab connected too.

Fine if you like having proprietary speaker cables, connectors and cabs. Oh, and B+ hanging around outside your chassis, but the thought behind it was sound!

[RJ Guitars]

Fender blessed us with this technology way back when. A short is not the ideal load on an output transformer but it beats the no load alternative. Meanwhile it shouldn't take all day to figure out you need to do something different if you are getting no sound... thank you Leo for saving me from blowing up my output transformer while I was dazed and confused!

You can use the same jack in both output holes (at least for the typical switchcraft design). In the 2nd hole just don't wire the jack to short out when it's unplugged.

rj

[Gibsonman63]

I am not looking at a schematic, but I seem to remember that on my Blackface Fenders the external speaker won't work unless the main speaker connection has something plugged into it, which tells me that the main speaker connection is a shorting jack and the external speaker jack is just a normal jack. My Marshalls, don't care which output you use, which would tell me that they are both normal jacks.

I just changed both speaker jacks to normal jacks on my Express build and called it a day.

[paulster]

My Marshalls, don't care which output you use, which would tell me that they are both normal jacks.

Not necessarily. With the switching arrangement on Cliff jacks you can wire them so that the outputs are shorted unless you plug into either speaker jack, which is quite neat.

That's how Hiwatt do it, although Marshall don't bother.