FDP Forum / External speaker wiring and impedance/ 12 messages in thread.

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guitarcapo



U.S.A.

Jan 5th, 2017 10:47 AM        

Say you have an amplifier that has an external speaker jack. <br /> <br /> It's wired like a lot of Fenders where plugging in an additional speaker simply adds the speaker in parallel to the internal speaker. So for example if you plugged in an 8 ohm external speaker (and the internal speaker was also 8 ohms) The output transformer would be dealing with a 4 ohm load...even though it's using the secondaries meant for 8 ohms.<br /> <br /> This is done all the time I know with no problems.<br /> <br /> But if you had an output transformer with both 4 ohm and 8 ohm secondary taps available, is there a way you could wire things so that plugging in the external speaker would disconnect the 8 ohm tap and use a 4 ohm tap WITHOUT adding an impedance selector switch?<br /> <br /> I'm searching online and not finding any wiring diagrams.



amphead4



Cincinnati, USA

Jan 5th, 2017 11:12 AM        

You can do a lot with the right jacks.



guitarcapo



U.S.A.

Jan 5th, 2017 11:21 AM        

That's what I was thinking. A jack that disconnects the 8 ohm secondary and connects the 4 ohm when the jack is external speaker is plugged in. <br /> <br /> I'm thinking jack VI is what I'm after ("transfer circuit J5"<br /> <br /> Both the 8 and 4 ohm lead go to this jack. When an external speaker is plugged in, the 4 ohm lead is used. When unplugged, the 8 ohm is used.<br /> <br /> Why didn't Fender use these?



Te 52



Laws of Physics

strictly enforced
Jan 5th, 2017 03:07 PM        

Yes, the type VI Transfer Circuit (J5) jack would do what you want. You'd install it as the external speaker jack. The internal speaker jack would stay a standard Type I Single Open Circuit (J1) jack, and would have no direct connections to the OT.<br /> <br /> As shown on the Switchcraft document, the upper lug on the type VI would go to the 4 ohm tap on the OT, the next lug down would go to the hot terminal on the internal speaker jack, the next one to the 8 ohm tap, and the bottom lug would be wired to ground, as would the ground terminal on the internal speaker jack.



guitarcapo



U.S.A.

Jan 5th, 2017 03:28 PM        

I'm confused as to how the internal speaker doesn't get disconnected though



pdf64



UK

Jan 6th, 2017 01:57 AM        

Fender have used these switching jack sockets, the 135 TR even had an arrangement to put the ext speakers in series with the cab speakers, changing the OT tap from 4 to 8 ohms when a jack was inserted.



ECS-3

Contributing Member
**********
*

USA / Virginia

Jan 6th, 2017 04:25 AM        

"Why didn't Fender use these?"<br /> <br /> If you are asking about vintage Fender amps, they didn't use a multitap speaker jack because their output transformers were not wound with multiple outputs. They only had a single output lead.<br /> <br /> Multiple output increases the costs of a transformer somewhat, so by having a single output you can save some costs.



Steve Dallman

Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Jan 6th, 2017 06:46 AM        

Fender uses such switching in the Hot Rod and Blues Deluxe/Deville amps.



Steve Dallman

Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Jan 6th, 2017 06:46 AM        

(duplicate post??)



guitarcapo



U.S.A.

Jan 6th, 2017 09:02 AM        

I'm working this out on paper and it doesn't seem to work with a J5 jack because I can't get around how you can wire this up so that plugging the external speaker in doesn't disconnect the 8 ohm internal speaker.



Te 52



Laws of Physics

strictly enforced
Jan 6th, 2017 10:20 AM        

"...I can't get around how you can wire this up so that plugging the external speaker in doesn't disconnect the 8 ohm internal speaker..."<br /> <br /> Take another look at my post (#4 in the thread). <br /> <br /> The "hot" contacts (springy prongs on the jack) are wired in parallel. Inserting a plug into the external speaker jack just moves the prong from the 8 ohm output to the 4 ohm output.<br /> <br /> If that's still not clear, come back and I'll provide a diagram that shows the signal flow under both conditions.



guitarcapo



U.S.A.

Jan 6th, 2017 11:38 AM        

I drew it out on paper and can see it now. I'm horrible mapping out stuff. What I was missing is that the hot lug of both speakers connect at that secong lug. Not just the hot lug of the internal speaker.



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