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Post subject: SF Twin wiring/extension cab
Posted: Sun Apr 27, 2014 10:54 pm
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Hello!
I recently got a 73 Twin and ideally i would like to use in conjunction with a 1X15 cabinet. From my internet research, it appears that either i can only use the 2x12 (8 ohm) speakers, or one of the 12s with a 1x15 cab (8 ohms). Im curious however if i were to get 2 12s at 4 ohms and wire them in series (8ohm load) and plug that into the main jack,would i be able to then plug a 1x15 (8ohm) cab into the extension jack? Or is series wiring in a SFTR something i should avoid?


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Post subject: Re: SF Twin wiring/extension cab
Posted: Mon Apr 28, 2014 12:06 am
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Rewiring your amp's internal speakers in series for a 16-ohm configuration is possible. Used thus in conjunction with an 8-ohm extension enclosure yields roughly 5.33 ohms for the output tranny -- well within the parameters for matching source to load in a tube platform such as a Twin Reverb.

However, you're likely perfectly safe leaving your amp's wiring alone and merely plugging your 1 x 15 cab into the extension jack. The total system impedance load will drop to 2.33 ohms but still within the acceptable margin of safety. I've often played my TR in this condition, wired to a Showman cab.

Arjay

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Post subject: Re: SF Twin wiring/extension cab
Posted: Mon Apr 28, 2014 12:18 am
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amarion831, welcome to the Forum! :D

So you want to run the speakers in the amp AND external cab, as well. You like Motor City Madman, Ted Nugent style of guitar, eh? :lol:

The stock speakers in the TR are wired for 4-ohms. The external jack is for a 4-ohm minimum load.

To run both internal & cab together, you'd need the primary speakers to have a total of 8-ohms and the external cab = 8 ohms. As Arjay stated above 16-ohms per speaker, each pair wired in parallel. Total load of all speakers = 4 ohms as the two speaker jacks are in parallel.

But, if you ever use the amp with just the two onboard speakers, the output tranny is going to now see 8-ohms, instead of 4-ohms.

If the two external speakers are 8-ohms and wired in series, final impedance = 16 ohms. And are added to the stock speakers in the amp:

1/4 + 1/16 = 0.3125 = 1/R, total

R total = 3.2 ohms. Pretty close to the nominal amount for that output tranny. That's prolly what I'd do. Then, you won't need to get new speakers for the amp, just the cab.

However, 16-ohm cab won't work that well with any other amp. Just a thought.


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Post subject: Re: SF Twin wiring/extension cab
Posted: Mon Apr 28, 2014 6:21 pm
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Thanks for the responses yall, im stoked to finally have a vintage fender amp and sure as hell dont want to ruin it! Im excited to get my amp back from getting it serviced to try this out :D
Another question: if i did hypothetically get 2 4 ohm speakers for the internal cab and wired them in series would i be able to take out v1 and v4 (half power mod) and still be able to use the twin without the extra cab?


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Post subject: Re: SF Twin wiring/extension cab
Posted: Mon Apr 28, 2014 6:28 pm
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All that removing V1 and V4 will accomplish is to dead-line the amp. V1 provides the first two gain stages for the "normal" channel and V4 is the reverb recovery tube. Yank them both and you'll have no signal whatsoever going to the power amp.

Arjay

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Post subject: Re: SF Twin wiring/extension cab
Posted: Mon Apr 28, 2014 10:53 pm
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Retroverbial wrote:
All that removing V1 and V4 will accomplish is to dead-line the amp. V1 provides the first two gain stages for the "normal" channel and V4 is the reverb recovery tube. Yank them both and you'll have no signal whatsoever going to the power amp.

Arjay

oops brain fart on my part, i meant to say v7 and v10 (as in two of the 6l6's) :P
i thought for some reason the numbers ran left to right


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Post subject: Re: SF Twin wiring/extension cab
Posted: Tue Apr 29, 2014 3:20 am
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Removing V7 and V10 (or conversely, V8 and V9) will drop the maximum available power output by fifty percent. To give your output tranny half a chance at survival in this mode, the impedance load should be doubled (ie: to eight ohms). Check your idle bias and adjust if necessary after pulling the two output bottles.

Arjay

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