Twin Reverb Extension Cab?

Brian LeBlanc · Post by **Brian LeBlanc** » 11 Jul 2006 1:01 pm

- Playing 70' ShoBud out of a 66 Twin (BlackFace)
- ie, 2 12's
- Can I /should I use a 15" extension
- been reading posts and totally confused!

Ken Fox · Post by **Ken Fox** » 11 Jul 2006 4:00 pm

The Twin Reverb is running into a 4 ohm load now. A lot of folks will say yes you can do it. I would not say it is a great idea. I know that adding an 8 ohm extension speaker would result in about a 2.67 ohm total load. If you were to use a 4 ohm external speaker the result would be a 2 ohm total load. It is a might rough on the power tubes due to reflected voltage (flyback) caused by the impedance mismatch (remeber the Twin has a 4 ohm output). Is there a way to do it? Yes there are several ways. One way is to install 16 ohm speakers in the Twin, resulting in an 8 ohm internal load. Adding the 8 ohm extension would result in a 4 ohm total load.
Certainly not an economical solution!

Another solution is a newer output transformer from Fender with multiple output taps (16/8/4 ohm taps). By changing that and using a switching jack for the aux jack you can get a 4 ohm transformer tap when just the internal speakers are in use or an 8 ohm tap whem running a 4 ohm extension. The wiring scheme would place the internal and external loads in series, a resulting 8 ohm total load. This can be seen on the late 70's and early 80's Twin Reverb amp (135 watt model).

I use a similar transformer for my 1959 Bassman clone. I have 8/4/2 ohm taps on it. My internal load is two P12N speakers in parallel, a 4 ohm load. My exteranl 2-12 cabinet is another 4 ohm load. When it is plugged into the aux jack I get a 2 ohm tap.[This message was edited by Ken Fox on 11 July 2006 at 05:02 PM.]

John Bechtel · Post by **John Bechtel** » 11 Jul 2006 9:11 pm

Ken; The owner's manual to my ’65 Re-Issue Fender Twin~Reverb Custom™ 15” 4-ohm Eminence Speaker says in regards to the External Speaker Jack, that to use the External-Jack you can use (1) 8-ohm speaker so long as you have another 8-ohm speaker plugged into the Main=Jack. That would amount to (2) 8-ohm speakers in parallel, if I understand what I'm reading! Does this concurr with your opinion and do you agree with this combination? And also, would you advise doing it this way? I guess it would be a match for the Amp.! (???)

------------------
“Big John”
a.k.a. {Keoni Nui}
Current Equipment

Post by **Dave Mudgett** » 11 Jul 2006 9:46 pm

I agree with Ken - I don't like running Twin Reverbs at 2.67 ohms. I also like the idea of temporarily replacing the original output transformer - you can pack away the original one. Just make sure you get a retrofit replacement that fits in the existing screw holes - for heaven's sake, don't drill any holes in the chassis of your '66 Twin Reverb - you will regret it! Now your original output transformer is safe - I have fried several old Fender output transformers in the last 2 decades, even run at the correct impedances. Ahh, we shouldn't have been playing so loud, but you know how it goes.

But here's another possible solution - I think this is in line with what Big John is suggesting. Disconnect one of the 12" speakers in the Twin, then add an 8 ohm 15" extension speaker. Now, the total load = 4 ohms. I've done this, it works fine if you just have to have a 15" speaker. Just be sure to tape up the leads to the disconnected speaker so they don't short out.

Rick Johnson · Post by **Rick Johnson** » 12 Jul 2006 3:29 am

Brian
Ken is right about the mismatched ohms.
I would say that problem has been the
downfall to many good amps.

I would suggest a new 15" baffle
for the Twin Rev and a new 1/15
extension cab, both with 8 ohm
speakers, then your amp will see
a 4 ohm load, problem solved.

Rick
www.rickjohnsoncabs.com

Ken Fox · Post by **Ken Fox** » 12 Jul 2006 4:54 am

I can't comment on the Twin Custom 15, as I can not find a schematic for it. I looked on the fender website, no luck there. I would never assume that the jacks are in parallel with a Fender amp! In the ultralinear amps (lates 70's) they placed speaker in series in some amps and parallel in others by use of a switching jack at the aux speaker out. They are now using a jack on the Hot Rod amps to swap output transformer taps! A real upgrade from the Blues Deluxe and Blues Deville amps that preceeded the Hot Rod Series.

Excellent suggestion by Dave above too! [This message was edited by Ken Fox on 12 July 2006 at 05:56 AM.][This message was edited by Ken Fox on 12 July 2006 at 05:59 AM.][This message was edited by Ken Fox on 12 July 2006 at 06:00 AM.]

Donny Hinson · Post by **Donny Hinson** » 12 Jul 2006 7:36 am

Speaking from personal experience, I've run Fender 4-ohm amps as low as 2 ohms (for long periods of time), and have not had a problem. Keep in mind that speaker impedance is dynamic, not fixed. Saying a certain speaker is "8-ohms" is like saying "It's a long walk from Nashville to Hendersonville". It gives you a rough idea, but there's really nothing "exact" about it. In fact, most 8-ohm speakers have an impedance curve (if you can call it that) that varies from 6 or 7 ohms to around 30, depending on the frequency they're reproducing. Fender obviously did studies on this sort of thing before they put an extension speaker jack in their amps. Unless you're playing at "stadium volume", I don't think there's anything to worry about adding an 8-ohm speaker.

George McLellan · Post by **George McLellan** » 12 Jul 2006 8:13 am

I have a 65 Twin Reverb RI and a 63' Fender Pro. Both have extention speaker jacs. This may not be good for the amp, but I've used both a 2x12" or a 1x15" (8 ohm) extention speaker cab with either amp at probably no more than 3 or 4 on the volume control. I haven't noticed any problems so far. I never run an amp over that volume. If I need more, then I have the amp miced. There is a big differance in what the amp will sound like, ie: "bigger" or more full.

The best overall tone I ever heard from my "Willy" was with my NV112 into the 63' Fender Pro w/15" extention speaker as a slave. At a indoor concert, my volume on the NV was Pre-Gain2½ and Master 3. The Pro was set to 3½. (jpg pictures of the amp set up available if anyone wants to see what I mean)

Geo

Post by **Dave Mudgett** » 12 Jul 2006 8:16 am

I think it's fine to run a slightly mismatched speaker load if the amp isn't being pushed hard - I do think that Fender somewhat overdesigned their output transformers a bit, unlike something like a Marshall, where impedance matching is more critical. But the idea of adding a big 15" speaker seems - to me - to be pushing in the direction of wanting a more solid bottom end and overall louder sound. To me, this implies the amp is likely to be pushed harder - I assume we're talking about gig context. Overall, I think the mindset these days is to push these amps quite a bit harder than people had in mind back in the 60s. As that mindset became more obvious in the later 70s and 80s, Fender started using tapped output transformers to permit a closer match. It's with this in mind that I am reluctant to suggest running a mismatched load.

Of course, the other reason to match the load is that one gets maximum power transfer when output stage and load impedances are matched as closely as possible. Of course, complex impedance is frequency-dependent, but the speaker coil resistance sets the baseline for that, especially at low frequencies - the ones that the big 15" speaker will emphasize. All IMO.

Brian LeBlanc · Post by **Brian LeBlanc** » 12 Jul 2006 9:31 am

TOO MUCH INFORMATION
(HA!)
Sounds like:
1) NO - weld a broken philips head into jack
2) replace 2 12's with 1 15"
3) cut out 1 12 and run extension 15"

... routinely run volume 4-7 range!

John Bechtel · Post by **John Bechtel** » 12 Jul 2006 11:06 am

I have the schematic for my ’65 Re-Issue Twin~Reverb Custom™ 15 and the Speaker Jacks are wired Paralell. Therefore, if I replace the Original 15” 4-ohm with an 8-ohm 15” and add an 8-ohm 15” Extension~Cabinet, I'm good to go, but; basically (best) only when using both speakers! {Yes, NO, Maybe?}

------------------
“Big John”
a.k.a. {Keoni Nui}
Current Equipment

Woody Woodell · Post by **Woody Woodell** » 12 Jul 2006 1:39 pm

I'm inclined to agree with Donny Hinson. I have always thought that, since the Twin came with two 8 ohm speakers (4 ohms) and an external speaker jack that the transformer was designed to operate into 2 ohms, if need be, since most people would use the same load for the external as the internal. It certainly would not hurt to use an 8 ohm speaker and as Ken said, in that case you would have 2.67 ohms but you probably would not get the same volume from the external as the internal.
I just reconed 2 JBL D130Fs to 4 ohms each for Lloyd Green. He uses a Twin with 3 cabinets, one with the amp in it, one with a 4 ohm D130F that has an external speaker jack and he can come out of that cabinet to the other 4 ohm cabinet. His external jack on his amp has been converted to a LINE OUT jack. He loves the sound but became concerned that it would damage his amp using 2 ohms after he read some of the discussion on the Forum. He is doing an album in the next few days using the two 4 ohm speakers. I believe the artist and the producer might have had Lloyd in mind in particular for the songs they are doing and there will be a lot of focus on the steel. It will certainly be interesting to hear the effect that the two speakers have.
Boy! If that amp blows up in the middle of a session I know someone who's going to have a little explaining to do.
Woody Woodell

Woody Woodell · Post by **Woody Woodell** » 12 Jul 2006 1:44 pm

By the way, the guy that made Lloyd's cabinets did a fantastic job. The workmanship is marvelous. I forget his name but he lives somewhere in Kentucky.
Woody Woodell

Post by **Dave Mudgett** » 12 Jul 2006 6:32 pm

As I said in my last post, I don't think it's a big issue to run a bit mismatched if the amp isn't pushed hard - and very importantly, the output transformer is in good shape in the first place. Playing guitar live, I tend to run old Fender amps on the edge, think Roy Buchanan. I want my impedances matched up as closely as possible in that case, and even then, the transformers are often hot at the end of the gig.

If I'm playing steel live, sometimes it's more laid back, and sometimes not. I do remember playing steel through my old Deluxe Reverb with both the regular 12" speaker and an extension cab with an orange JBL D-130F - total load 4 ohms, when 8 was nominally expected. It was a very laid back, quiet gig, the Deluxe was not straining at all, and everything sounded great - that is until the bass player tapped me on the shoulder to look around - there was smoke coming out of the low-input jack. Sure enough, the output transformer was frying before our eyes and the amp cut out a moment later. I expect this was the result of a lot of years of being pushed - that was my main guitar gigging amp for a long time. But this was apparently the straw that broke the camel's back. I was able to eventually find another old output transformer (thanks Mark Ross), but it was a royal pain.

I guess my point is that I'm reluctant to just give carte blanche advice to ignore impedance matching, especially if I don't know the person or how the amp will be used. Under some circumstances, it may be OK - I've done it myself and know others that do it routinely. But I also think there is the potential to do damage, especially if the output transformer already has issues.

James Morehead · Post by **James Morehead** » 12 Jul 2006 7:11 pm

Could it be Rick Johnson built Lloyd's cabs??

Rick Johnson · Post by **Rick Johnson** » 13 Jul 2006 8:54 am

I'm guilty.

Rick
www.rickjohnsoncabs.com

David Doggett · Post by **David Doggett** » 13 Jul 2006 11:05 am

Because of the confusion that Ken Fox mentioned about how the external speaker jacks are wired on old Fender amps, I don't use the external jack at all, but run external extension speakers from the internal jack, so I can wire in parallel or series as needs be to keep the impedance exactly matched to the amp. I have a Dual Showman reverb head and a Super Twin head. Both want 4 ohms at the internal speaker jack. I have an external cab with single Eminence 4 ohm 15" speaker. I also have two 8 ohm JBL 15s. Using a Y connector, I run them in parallel to the internal jack for a 4 ohm load.

The '70s Vibrasonic, unlike the other big Fender amps, was made for a single 8 ohm speaker on the internal speaker jack. I have no idea what it wants on the external speaker jack. But you could run two 4 ohm speakers in series on the internal jack and give it the 8 ohm load it was built for.

It's easier to deal with this with amp in a head cabinet, because then you can choose a single speaker to match, or two separate speakers to match. But if you have a single internal 4 ohm speaker (or two internal 8 ohm speakers in a 4 ohm parallel circuit) in a 4 ohm combo, you can't keep the internal speaker and add an external speaker in series or parallel and get exactly 4 ohms. I guess that's why Fender kept trying different wiring arrangements to do the impossible. This is just one more argument for amp heads versus combos. Most combos are really designed to run properly with just the internal speaker. Amp heads a designed to be more flexible, and many of them have switchs for different ohm loads.

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Student of the Steel: Zum uni, Fender tube amps, squareneck and roundneck resos, tenor sax, keyboards

[This message was edited by David Doggett on 13 July 2006 at 12:15 PM.]

Post by **Dave Mudgett** » 13 Jul 2006 8:06 pm

I think the question was about a 66 blackface Twin Reverb. I probably haven't seen every possible configuration, but the extension speaker output jack on all the classic old Fenders I've seen, non-master silverface and earlier, have been wired in parallel with the main speaker jack. With that wiring, there's no problem disconnecting the internal speaker and just hooking up the extension speaker to that jack, or running them together.

Further, I believe tapped output transformers for impedance switching came along late in the silverface era. I don't recall ever seeing an original early silverface, blackface, and earlier Fender amp - head or combo - with a tapped output transformer. I've seen both later heads and combos with impedance selection capability. Of course, I like heads to distribute the weight load, but an early silverface Dual Showman Reverb chassis is identical to an early silverface Twin Reverb chassis - I've had several of each, they're were all the same except for the box and speakers. Of course, there was no blackface or earlier Dual Showman Reverb.

David Doggett · Post by **David Doggett** » 13 Jul 2006 9:28 pm

Yes, Dave, I know a Dual Showman is identical to a Twin. But my point is that the Dual is unencumbered with an internal speaker. Both amps want to see 4 ohms speaker load. On the Twin you already have two 8 ohm 12s in parallel for 4 ohms. If you add another 8 ohm speaker as an extension, it will be 12 ohms in series, and 2.7 ohms in parallel. If you add a 4 ohm speaker it will be 8 ohms in series, and 2 ohms in parallel. There is no way to use the internal speakers and an external one and get exactly 4 ohms. But with a Dual, it is simple to get 4 ohms: one 4 ohm speaker; or two 8 ohm speakers in parallel.

This is the situation because vintage speakers were either 4 ohm or 8 ohm, mostly the latter. So starting from scratch with the Dual you can easily arrange for a 4 ohm load with either one or two speakers. But starting with the internal speakers of the Twin, you cannot add 4 ohms or 8 ohms in series or parallel and get 4 ohms. It's just the way the arithmetic works out. If you want to run external speakers, skip the combo and get something designed for external speakers, an amp head. [This message was edited by David Doggett on 13 July 2006 at 10:38 PM.]

Rick Summers · Post by **Rick Summers** » 14 Jul 2006 6:52 am

Here ya go....

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Rick Summers -
RAWOODS Custom Cabinets
www.rawoods.com