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FDP Forum / Amp Mods, Repairs, and Projects / External speaker wiring and impedance

guitarcapo

U.S.A.

Jan 5th, 2017 10:47 AM   Edit   Profile   Print Topic   Search Topic

Say you have an amplifier that has an external speaker jack.

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.

This is done all the time I know with no problems.

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?

I'm searching online and not finding any wiring diagrams.

amphead4

Cincinnati, USA

Jan 5th, 2017 11:12 AM   Edit   Profile   Print Topic   Search Topic

You can do a lot with the right jacks.

Switchcraft jack schematics

guitarcapo

U.S.A.

Jan 5th, 2017 11:21 AM   Edit   Profile   Print Topic   Search Topic

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.

I'm thinking jack VI is what I'm after ("transfer circuit J5"

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.

Why didn't Fender use these?

(This message was last edited by guitarcapo at 02:23 PM, Jan 5th, 2017)

Te 52

Laws of Physics

strictly enforced
Jan 5th, 2017 03:07 PM   Edit   Profile   Print Topic   Search Topic

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.

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.

(This message was last edited by Te 52 at 05:10 PM, Jan 5th, 2017)

guitarcapo

U.S.A.

Jan 5th, 2017 03:28 PM   Edit   Profile   Print Topic   Search Topic

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

(This message was last edited by guitarcapo at 05:34 PM, Jan 5th, 2017)

pdf64

UK

Jan 6th, 2017 01:57 AM   Edit   Profile   Print Topic   Search Topic

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.

External link

ECS-3
Contributing Member
**********
*

USA / Virginia

Jan 6th, 2017 04:25 AM   Edit   Profile   Print Topic   Search Topic

"Why didn't Fender use these?"

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.

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   Edit   Profile   Print Topic   Search Topic

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   Edit   Profile   Print Topic   Search Topic

(duplicate post??)

(This message was last edited by Steve Dallman at 09:45 AM, Jan 21st, 2017)

guitarcapo

U.S.A.

Jan 6th, 2017 09:02 AM   Edit   Profile   Print Topic   Search Topic

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   Edit   Profile   Print Topic   Search Topic

"...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..."

Take another look at my post (#4 in the thread).

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.

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   Edit   Profile   Print Topic   Search Topic

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.

(This message was last edited by guitarcapo at 01:55 PM, Jan 6th, 2017)

FDP Forum / Amp Mods, Repairs, and Projects / External speaker wiring and impedance




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