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Fender Twin Reverb Extension Cab
http://forums.fender.com/viewtopic.php?f=13&t=39863
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Author:  Earth [ Sat Jun 05, 2010 11:27 am ]
Post subject:  Fender Twin Reverb Extension Cab

Hey guys, this is my first post!

I was wondering about putting an extension cab with my fender twin reverb for more power and volume, yet I know nothing about ohms and all of that. I am asking this because I had an old bass cab hooked up to it and I had trouble with the tubes.

I am looking into buying a fender 4x12 so I guess its time to learn about if it is compatible.

Any and all answers are much appreciated, thanks guys.

Author:  Retroverbial [ Sat Jun 05, 2010 12:10 pm ]
Post subject: 

The Twin Reverb re-issue is optimized to deliver 85 watts into a 4Ω load. Decreasing the total speaker impedance by adding an extension cab will not increase the power and may well cause damage to the output tranny under prolonged conditions. There may be a perceived increase in volume but it's a fool's errand -- better to mic your amp through a PA to increase the sound coverage.

HTH

Arjay

Author:  Earth [ Sun Jun 06, 2010 8:24 am ]
Post subject: 

Hey thanks for the info, sure saved me alot of money.

I have been using a cab with my twin reverb for probably like 6 months now, my tubes sorta act funny.

How do I ensure there hasn't been any damage?

Author:  Shockwarrior [ Mon Jun 07, 2010 7:05 am ]
Post subject: 

Earth wrote:
Hey thanks for the info, sure saved me alot of money.
I have been using a cab with my twin reverb for probably like 6 months now, my tubes sorta act funny.
How do I ensure there hasn't been any damage?


When you replace the tubes if it still sounds funny on the stock speakers you got a problem.
That is about the only non technical thing you can try. The rest involves scoping the different amp stages with a test signal with a load placed on the output.

Author:  Big The Cat [ Wed Jun 09, 2010 5:45 am ]
Post subject: 

Wow, how can you need more volume from a twin! twins are deadly loud!

However by adding a cab your just doubling the work load by 2 so all the difference thats going to make is more wear on your output tubes.

Thats what all the pro's here said anyways. And im sure if it was dangerous or bad for your amp fender wouldnt have added an extension cab out.

Author:  Retroverbial [ Wed Jun 09, 2010 7:34 am ]
Post subject: 

Big The Cat wrote:
And im sure if it was dangerous or bad for your amp fender wouldnt have added an extension cab out.


The problem lies in the fact that all of the '65 re-issue amps -- like the originals that preceded them -- are equipped with an extension speaker jack but not much guidance is provided on the amp itself regarding the wisdom of their use. To its credit, FMIC explains this feature in the operator's manuals supplied with these amps. However, it's been my observation over some 40+ years of owning, playing, servicing, and repairing them that most musicians fail to read these manuals, or do so in only the most cavalier fashion. Likewise, an unfortunate number of these manuals go out with the trash when the musician unpacks his latest purchase in an impatient quest to "get it on".

The original poster did the right thing by asking a simple concise question in order to avoid a costly calamity. There are many "stupid" answers but the only "stupid" question is that which is never asked.

Arjay

Author:  Big The Cat [ Wed Jun 09, 2010 8:47 am ]
Post subject: 

was just saying that it doubles the load using a 4ohm cab.....

Author:  Retroverbial [ Wed Jun 09, 2010 9:11 am ]
Post subject: 

Big The Cat wrote:
was just saying that it doubles the load using a 4ohm cab.....


No, it does NOT.

The jacks are wired in parallel. Adding a 4Ω load to the existing two 12's halves the total impedance load to .

Arjay

Author:  Big The Cat [ Wed Jun 09, 2010 10:29 am ]
Post subject: 

Ok 2ohms it is...

Author:  Retroverbial [ Wed Jun 09, 2010 10:43 am ]
Post subject: 

Correct.

And inasmuch as the dynamic "impedance" of a loudspeaker (or loudspeakers) is actually a variable inductive resistance, when the input signal frequency decreases, so goes the resistance. Thus, under high power conditions at low frequencies, the resistive load may actually approach that of a dead short across the secondary windings of an output transformer. When that moment occurs, the amp catastrophically fails the proverbial "smoke test" and it's time to spend BIG $$$ on repairs.

Arjay

Author:  filerj [ Thu Jun 10, 2010 3:44 pm ]
Post subject: 

Retroverbial wrote:
Correct.

And inasmuch as the dynamic "impedance" of a loudspeaker (or loudspeakers) is actually a variable inductive resistance, when the input signal frequency decreases, so goes the resistance. Thus, under high power conditions at low frequencies, the resistive load may actually approach that of a dead short across the secondary windings of an output transformer. When that moment occurs, the amp catastrophically fails the proverbial "smoke test" and it's time to spend BIG $$$ on repairs.

Arjay

Arjay, if I understand you correctly and you want to use the extension jack you should should disconnect the amps speakers and plug two 8 ohm cabinets into the main and extension jacks. Pardon my ignorance if I'm wrong. I was thinking a building a 2X12 extension cab for my twin and was going to wire 2 8 ohm speakers in parallel just like the speakers in the amp. If you want to use the amps speakers and an extension cab, what should the resistance on the extension cab be? :?

Thanks,
filerj :D

Author:  Retroverbial [ Thu Jun 10, 2010 7:35 pm ]
Post subject: 

There is a bit of "fudge factor" that you can work to your favor. Most of the re-issues have trannies that are built robustly enough to permit some impedance mismatch without risk of damage.

In your specific case (adding a 2 x 12 extension cab to the extant 2 x 12 load) I'd recommend installing 16Ω speakers wired in parallel into the extension cab, for an 8Ω total. Thus, with the Twin's normal complement of drivers and the added 8Ω enclosure, the total load your amp will see is 2.66Ω which IMO should be safe. I have used this configuration myself for a number of years with absolutely no issues whatsoever. In my case, the added cab was a Dual Showman equipped with a pair of 15-inch 16Ω JBL D130Fs......

Image

Be forewarned though -- a rig like this puts the capital "L" in the word Loud.

:mrgreen:

Arjay

Author:  filerj [ Fri Jun 11, 2010 4:11 am ]
Post subject: 

Arjay, Thanks for the info, my amp is 79 ultralinear not a reissue so will the same info hold true?
Thanks,
filerj

Author:  Retroverbial [ Fri Jun 11, 2010 9:30 am ]
Post subject: 

The ultra-linear Twin Reverbs use a radically different output tranny to permit the use of a variety of impedance loads.

To understand the operating parameters, view the schematic here......

http://www.schematicheaven.com/fenderam ... _schem.pdf

Thus, it's completely safe to run this amp with its internal speakers in conjunction with a 4Ω extension cab.

Arjay

Author:  filerj [ Fri Jun 11, 2010 12:10 pm ]
Post subject: 

Arjay, Thanks a lot for the info, now I know I can go ahead with the speakers I wanted to use set up the way I originally planned. Hope you don't mind me picking your mind for the info.
Thanks again,
filerj :D

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