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Author Topic:  Running different Ohm speakers..
Dave Meis


From:
Olympic Peninsula, Washington, USA
Post  Posted 3 Aug 2017 10:51 am    
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Fender Twin. 4 Ohms at 'speaker out'. If I run 2x16 Ohm 'Hot-Spots', is the calculation: 4 (main) + 8 (2x16 Ohm ext.) / 2 for 6 Ohms, or is it 4+16+16 / 3 for 9 Ohms? Looking for a way to spread my sound to the other end of the stage using what I have..(I have ext. cabs, but the Hot Spots have attenuators). Another question would be: how hard (expensive) would it be to 'attenuate' a speaker cab? Thanks!
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Randy Schneider


From:
SW New Mexico, USA
Post  Posted 3 Aug 2017 11:34 am    
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If the speakers all connect in parallel, you're already at 4 ohms with the main and probably can't go lower.

Parallel resistances/impedances add in inverse:

The calculation would be:
1/R = 1/4 + 1/16 + 1/16
1/R = .25 + .0625 + .0625 = .375

R = 1/.375 = 2.67 ohms

Not too many amps can drive a load less than 4 ohms...

If the speakers connect in series, then the resistances/impedances just add.

So, it's important to know how the speakers would be connected.
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Dave Meis


From:
Olympic Peninsula, Washington, USA
Post  Posted 3 Aug 2017 11:55 am    
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The speaker out is rated 4 Ohms (min.)and has a 4 Ohm load.. (2x8 Ohm speakers). The ext. speaker out is also rated 4 Ohms min. The Hot Spots are 16 Ohm, and when 2 are connected, it reads 8.2 with a meter. Is the transformer seeing 4+8 / 2 at this point, or two separate loads, 1 at 4 and 1 at 8? Thanks!
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Randy Schneider


From:
SW New Mexico, USA
Post  Posted 3 Aug 2017 12:04 pm    
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The two hot spots, if connected in parallel, would make an 8 ohm load by themselves: 1/16 + 1/16 = 1/8.

I do not know whether the Fender amp jacks connect in parallel or not -- or whether the main jack AND the external jack can support 4 ohm loads at the same time, as the labeling would suggest. It's possible that the jacks contain some switching themselves to put the internal/external loads in series.

Someone with more knowledge of the amp itself and its jack setup will have to help out on that...sorry!
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Dave Meis


From:
Olympic Peninsula, Washington, USA
Post  Posted 3 Aug 2017 12:11 pm    
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Thanks Randy! I appreciate your time! d
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Randy Schneider


From:
SW New Mexico, USA
Post  Posted 3 Aug 2017 12:26 pm    
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If the labeling does indeed say "4 ohm min." on each jack, and not "4 ohms total," then I would guess that the two jacks connect in series when the ext cab is plugged in (two 4-ohm loads in series would be 8 ohms total).

But I really am guessing - I'll be honest about that!
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Dave Meis


From:
Olympic Peninsula, Washington, USA
Post  Posted 3 Aug 2017 12:32 pm    
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There is a mechanical switch (between the 2 jacks) that opens when you plug anything into the 'ext' hole, so I'm pretty sure you're right about that. Thanks! d
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Mark Draycott


From:
Princeton, Ontario, Canada
Post  Posted 4 Aug 2017 2:15 pm     Speaker Load
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I have a very cool spreadsheet that I got from Duncan Amp pages a while back. It will calculate all your loads for series or parallel and the wattage each speaker sees. I can send to anyone that is interested. I have found it very useful over the years.
Just send me a PM and I can email it.
_________________
1976 Sho-Bud Pro II, 1976 Sho-Bud LDG, BF Fender Deluxe, Evans FET 500 LV, BF Princeton Reverb, '68 Vibrolux Reverb
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Bill A. Moore


From:
Silver City, New Mexico, USA
Post  Posted 4 Aug 2017 3:31 pm    
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I don't believe any of the Twins have a multi-tap OT, like the Hot Rod, and Deville amps have. The '94 Twin, has a 4-8-16 ohm switchable OT, but is not the "Twin" than many of us use. I installed a '94 Twin OT in my Twin clone.
The Twin OT has a 4 ohm output to the speaker main, and the "aux" jack is in parallel to it. A 4 ohm load added to the "aux" would let the output see 2 ohms, (more or less).
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George Piburn


From:
The Land of Enchantment New Mexico
Post  Posted 4 Aug 2017 3:59 pm     solution
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What you are trying to get to is a load at the amp this not too low so as not to tax the amp out.

Going below 4 total at the amp is not recommended.

1 solution is to re wire the 2 twin speakers in series, = 16 ohms , then parallel that load with the other 2 loads = 16 divides by 3 = 5.something. This will satisfy the amp.

What happens is this --- the series will give each of the twin speakers 1/2 of the wattage, = not as loud but will work all night long no problemos.

Basic speaker wattage formula is parallel = same wattage to each parallel speaker.
Series = 1/2 the wattage per 8 ohm speaker.

__________________________________________________

To do as I've suggested , take a 9 volt battery and touch the speaker terminals , ( not hooked to the amp, ) if the cone moves forward , which ever terminal is touching the + on the battery will be hot so to speak.

Reverse the battery and you should see the cones move backwards. This will determine definitively which terminal is hot.

Now go from amp hot out to terminal hot on speaker 1 , from speaker 1 cold to speaker 2 hot - from speaker 2 cold back to the amp out .

Test this again to make sure both cones move in the same direction with the 9 volt battery.

Next use the extension speaker out to go to the other little speaker in parallel, to make for certain test those to make sure they go forward , etc , so that all of the speakers go forward on the battery test.

If any of the speakers go different directions on the battery test , flop the terminals until you get all going forward on the + to hot battery test.

This called "In Phase"

______________________________________________

Worst case you may need to turn up a little , and if you have a master volume , turn up there before turning up the input area to keep it as clean as possible.

Hope this helps you.

GeorgeBoards - Certified JBL speaker tech factory trained. -- aka -- I am supposed to know what I'm talking about.
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Dave Meis


From:
Olympic Peninsula, Washington, USA
Post  Posted 4 Aug 2017 9:13 pm    
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Thanks guys! Smile
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