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FDP Forum / Fender Amps: Vintage (before 1985) / Multi tap OT question

roadhog96

USA / Sesame St.

Jan 31st, 2017 03:11 PM   Edit   Profile   Print Topic   Search Topic

Can someone explain how a multi tap OT wiring would work on a Fender amp. The stock OT is 8 ohm, the multi tap has 4-8-16. Do you pick the leads for the ohm your looking for and wire it in place of the original single ohm OT? What do you do with the other leads install more jacks so that they can be used if needed with a different speaker configuration. I've never seen one before and what little I know about them seems like it might be a better option installing one of these than buying new speakers to get the impedance correct. I know you can have a 100% mis match with the speaker impedance and it won't harm the OT but what I noticed is the amp is much louder with the one 8 ohm than with two 8 ohm which is actually 4 ohm. The 2 x 12 cab sounds better, it's fuller and warmer just doesn't have the same volume as the single speaker. I've read you can loose as much as 40% of the amps power with a mis match. So if I could have a 4 ohm output the 8 ohm should be close to the same volume level as the one 8 ohm speaker.

(This message was last edited by roadhog96 at 07:38 PM, Jan 31st, 2017)

amphead4

Cincinnati, USA

Jan 31st, 2017 09:15 PM   Edit   Profile   Print Topic   Search Topic

One way to use it is with a switch and two jacks in parallel, the main jack should be the shorting type and the extension jack should be open.

If you use a single 8 ohm cabinet, set the switch to the 8 ohm tap. If you use a 4 ohm cabinet, set the switch to the 4 ohm tap.

If you use two 8 ohm cabs with both jacks, set the switch to the 4 ohm tap.

I'm not big on setting a mismatch on purpose, particularly running a higher ohm cabinet than the tap wants to see. It reflects that higher impedance onto the output tubes which is hard on them and the transformer itself.

Roly
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Whitehorse Canada

I don't get out much
Feb 1st, 2017 12:40 AM   Edit   Profile   Print Topic   Search Topic

Buy a switch which selects the desired load, and install it in the courtesy outlet hole.
So.....you have an output transformer with 4, 8, and 16 ohm secondarys...connect each of the secondary outputs to the corresponding connector of the switch....send the switch output to the + connector of the output jack....done.

I do this all the time when I build amps with Hammond multi tap output transformers.
The switch is not a drop in part but there isn't a lot of metal work necessary.
The extra jack thing is a bad choice for many reasons.

cheers

impedance switch

Hammond101
Contributing Member
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So. Cal. USA

Feb 1st, 2017 09:36 AM   Edit   Profile   Print Topic   Search Topic

The switch Roly found is quite nice and the acc. A/C outlet hole is a perfect place to mount it.

In my black face Vibroverb build I used a rotary switch from Weber. It's set up for 4-8-16. In my other builds I used the ground switch and just used the 4-8 ohm taps. I used a push/pull switch pot (volume) once in a customer amp where there was no option for other switches. I use Hammond or Heyboer transformers mostly.

Any unused taps I cut to usable length bend the end in a 180 and insulate with shrink tubing. Then then get tucked neatly in the chassis.

I would make every effort to not drill extra holes in your chassis.

guitarcapo

U.S.A.

Feb 1st, 2017 10:35 AM   Edit   Profile   Print Topic   Search Topic

I like the idea of a 3-way switch if you can find a nice place for it. Then whatever external speaker you plug into the jack you can accommodate or get close to in terms of matching the load to the o.t.

Peegoo
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The spotlight

looks like a prison break
Feb 1st, 2017 11:05 AM   Edit   Profile   Print Topic   Search Topic

That is how I do it: an impedance selector switch to a single jack.

If the OT is going into a combo amp and you'll not be using an extension speaker, run the outs required for the on-board speaker directly to the speaker jack.

The others you can leave long and 'capped off' with heat shrink. Bundle them insidde the chassis and secure with a little zip tie. If you ever decide to modify the thing, the leads are there for you to bring into the circuit.

(This message was last edited by Peegoo at 01:10 PM, Feb 1st, 2017)

FDP Forum / Fender Amps: Vintage (before 1985) / Multi tap OT question




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