Shorting output jacks

[Ron Worley]

I unintentionally hijacked Gearhead's thread.. sorry!

So I will start a new one.

In that thread Allyn said something about the output shorting jacks being incorrect (if I understand). I looked all the pics I could, and it appears that both the input and output jacks are shorting.

Can anyone enlighten me on this?

Ron

[gearhead]

Sorry, my rudeness poked out.

The jacks I am using aren't quite shorting jacks. There are four lugs; two each for hot/signal (tip) and ground. The two closest to the jack input are ground and the others are signal. The process of inserting a jack disengages two of them; one each hot and ground. The way I had it wired up was ok (I had figured this out before but forgot I had done so).

If you want to use it as a shorting jack, you have to conciously do so as per how the layout indicates for the input jack. Connect between the ground lugs and the soon-to-be-disengaged hot lug and without the jack inserted, everything is grounded (and quiet). Insert the jack and only the non-grounded hot lug is still engaged.

Edited again to add: Well, maybe there is a good reason to do so for single speaker jack?

[Ron Worley]

No worries... My bad....

I seem to recall from some fuzzy recess of my brain that Fender used shorting jacks to prevent total tranny failure if the amp was running without a speaker load...

I may have this completely wrong....

I just checked a Fender I have opened up, and yup- it has a shorting jack on the primary speaker jack, normal mono on the extra / external jack. It has them on both input jacks too, BTW.

Any and all insights welcome, please!

Ron

[skyboltone]

No worries... My bad....

I seem to recall from some fuzzy recess of my brain that Fender used shorting jacks to prevent total tranny failure if the amp was running without a speaker load...

I may have this completely wrong....

I just checked a Fender I have opened up, and yup- it has a shorting jack on the primary speaker jack, normal mono on the extra / external jack. It has them on both input jacks too, BTW.

Any and all insights welcome, please!

Ron

Hmmmmmph. Curious. So we have a reflected load of 0 ohms into a plate running at several hundred volts AC on top of the DC voltage. I suppose you'ld be ok as long as no signal was applied. Pity the poor fool who plugs into an amp running without a speaker though with a shorting jack. Then again, with an open secondary things aren't much better. If I had my druthers I think I'd just as soon not short the secondary of any OT. Just my finicky nature I guess. By the way Ron, you ever spend any time in the San Lorenzo Valley, near Felton California.

[Firestorm]

I can't remember where I read this -- probably somewhere in KO'C, but I can't find it at the moment -- but supposedly it's much better to run into a short than into an open load. Fender generally knew what they were doing -- I can think of very few out and out mistakes -- so there must be some benefit to using the shorting jack or they wouldn't have spent the extra few cents.

[Allynmey]

FWIW KF used switchcraft jacks in the TW. He didn't use shorting jacks in the output of any amps I've seen except Ginger with the line out.

[Ron Worley]

Allyn-

I'm confused- here's the pic from franny.. Aren't these shorting jacks?

Ron

[Allynmey]

Sorry Ron, I should have clarified. I should have said He used switchcraft jacks in his builds not Cliffs and he didn't use the shorting connection on outputs with the exception of the line out on Ginger.

[geetarpicker]

Yes those are shorting jacks and my original also has the same thing. However, they are not wired up as shorting jacks and the extra switch contacts aren't even hooked up. I would bet Ken got a good deal on a bunch of them.

[novosibir]

I seem to recall from some fuzzy recess of my brain that Fender used shorting jacks to prevent total tranny failure if the amp was running without a speaker load...

I can't remember where I read this -- probably somewhere in KO'C, but I can't find it at the moment -- but supposedly it's much better to run into a short than into an open load.

It's at least 1,000 times safer for the OT running into shorted secondary than running with an open secondary.

Theoretically it should be even possible to dime an amp into shorted output jacks w/o the danger of a OT failure, since the OT is reflecting the secondary's load to the primary - and zero ohms at the secondary reflected to the primary is zero again, so the output tubes can't produce any power in this case - but I've never tried it out in reality.

And what's killing an OT is almost always a toasted insulation between the primary & secondary, what only may happen due to hi-voltage punch through, what only may occur with an 'open' secondary.

If you're using insulated Cliff jacks or similar, you may wire the two output jacks the way, I'm doing it in my DINO amps like on this photo seen left:

[img:800:533]http://www.larry-amplification.de/dino9 ... 001-01.jpg[/img]

Plugging into any of the two output jacks breaks the tip-to-ground connection.

Larry

[Ron Worley]

Sometimes my lack of observation is astounding..... the middle lug for the grounding function is not wired..... ergo, it is functioning as a regular jack....

DOH!

Ron