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Discussion Starter #1
I have a 1969 silverface Fender Vibrolux which has two 10", 8 ohm speakers wired in parallel, which is 4 ohms total.

This amp has a jack for an external speaker cab. Would it be safe plugging in an extension cab which has one 12" speaker that's 8 ohms?

This extra speaker will drop the speaker load from 4 ohms total to 2.66 ohms total. Will the amp handle this ok?
 

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I have a 1969 silverface Fender Vibrolux which has two 10", 8 ohm speakers wired in parallel, which is 4 ohms total.

This amp has a jack for an external speaker cab. Would it be safe plugging in an extension cab which has one 12" speaker that's 8 ohms?

This extra speaker will drop the speaker load from 4 ohms total to 2.66 ohms total. Will the amp handle this ok?

No, it won't! Fenders wire the extension jack to put the new cab in SERIES with the built-in speakers! They did this to prevent getting too low a total load. Tube amps would rather a mismatch be a bit high rather than a lot low.

This is also why you always plug the first speakers used into the "speaker" jack and not into the "ext." one. The ext. jack is a shorting jack. The main speaker jack goes to ground through the ext. jack, which is shorted if there's nothing plugged in. Running only into the ext. jack doesn't work 'cuz there's actually no speaker connection at all.

So you'll be looking at 12 ohms. What's more. the lower ohm load hogs the power so the internal speakers will be louder than the extension cab.

If you want to run full power into the extension cab unplug the internal speakers and plug the cab into the SPEAKER jack. Don't worry about the mismatch from 4 to 8 ohms. The tubes will laugh at it like Arnold armwrestling Danny DeVito.

Don't worry! Be Happy!:food-smiley-004:
 

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Discussion Starter #3 (Edited)
Hey Wild Bill,

I know that there are amps out there that have an ext.SPKR jack wired in series with the combo's speakers but the ext.SPKR jack on many Fender amps, including mine, look to be wired in parallel with the combo's speakers.

Here's a link with schematic:
http://www.ampwares.com/ffg/schem/vibrolux_reverb_aa270_schem.gif

If I'm not correct on this, please set me straight. If this ext.SPKR jack is indeed wired in parallel as I suspect, then all three speakers should be gettting the same amout of power, right? And the amp should be able to handle this total impedace load of 2.66 ohm?

Thanks again!
 

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I'm curious about this too...I have a SF twin & SF vibrolux & want to use the internal speakers + external cab

thx Wild Bill!! :food-smiley-004:

ps what about mashall's? I have a '66 plexi, just picked up a second cab, both cabs are 8 ohms & I want to run the full stack :banana: what impedence do I need to set it at?
 

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I vote parallel, too. From Weber, "A Desktop Reference of Hip Vintage Guitar Amps," page 211:

"The 'ext. speaker' jack is simply a two terminal jack that is in parallel with the 'speaker' jack, but it has no shorting terminal hookup... When you use the 'ext. speaker' jack while using the 'speaker' jack, the two are in parallel. To get the power divided equally... you should use the same impedance speaker in the 'ext. speaker' jack as you used in the 'speaker' jack... Fender amps are very tolerant of mismatched output impedances as long as they are not more than 100% off."

To my mind, Humbucker's situation (running an ext speaker) requires either a 4-ohm ext cab or a disconnect of one of his two 10's (to get 8 ohms) and then add the 8 ohm ext cab.
 

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Discussion Starter #6
Well, if you look at the schematic that I have there, you'll see that when you plug in an 8 ohm speaker into the ext.SPKR jack, it's simply connected in parallel with the two existing 8 ohm internal speakers which are already wired in parallel.

So, this ends up being three 8 ohm speakers wired in parallel for a total impedance of 2.66 ohms if I'm correct.
 

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On old fender amps the main speaker jack is the shorting jack and the ext jack is in parallel with the main. That is why if you unplug the main speaker and only plug into the ext jack you won't get any sound.

I own six old Fenders and I run extension cabs with mine all the time. My Vibrolux Reverb is set for 4 ohms and I've run it with a 4x12 rated at 8 ohms for years. I have also done the same with my bassman amps and my concert.
 

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Discussion Starter #8
bolero,

If you're plugging in two 8 ohm cabs into your Plexi the impedance tap should be set at 4 ohms. Somebody correct me if I'm wrong, we'd hate to see anything bad happen to an old Plexi!
:food-smiley-015:
 

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Discussion Starter #9
Right on Ripper!

The speaker jack for the internal speakers is a shorting jack, I believe this is there to try to protect the output transformer in the event of having no speaker load plugged in. The ext.SPKR jack is simply wired in parallel with the main speakers in the combo amp.

I was just concerned that running the amp lower then 4 ohms(2.66 ohms) might make the amp overheat, if I had to bet, I think that the amp would handle this okay but I'm not 100% sure about this.
 

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With the plexi if your running two cabs, set the impedance to 4 ohms and you will be good.


I've run my fenders like this for years (I got my Vibrolux in 1977) and I've never had a problem.

Check this link out it gives an idea of the mismatches that old fenders will tolerate no problem. http://www.21frets.com/speakers.html

Tube amps are very very tolerant but again I wouldn't recommend running them at full tilt for three hours like that either.
 

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Mea Culpa!

Son of a gun!

I am dead wrong and stand corrected! Pete Traynor would kick my ass!

I've never looked closely at how the shorting jack was wired before. When I do my own amps I just wire an extra jack in parallel and lecture the player! I've been carrying an assumption about Fenders for years, obviously from some other make of amp. Ampeg maybe? Dunno, I went to Schematic Heaven to see an old schematic and it's down! Something to do with contacting the Accounting Dept. Maybe not enough donations.

Anyhow, I'm pleased to see how many folks jumped in to set the fellow straight. Nice to know there's good backup on this forum. I hate being taken as gospel. If I make a mistake who suffers? I'm here to help folks, not to start a Jim Baker church and scoop your money.:smile:

About Marshalls, everyone I've seen is just a parallel ext jack.

Anyhow, putting 8 in parallel with 4 is still not gonna put things low enough to blow something up. You'll still have the problem of the lower 4 ohm load hogging the power.
 

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Did it again!

Sorry, Humbucker! I did it to you again!

Just reread your earlier post and noted that you're quite right. If you have 3 - 8 ohm speakers in parallel they each will get a third of the power. Two thirds will be with the 2 in the amp, leaving 1/3 in the extension cab. So the power is shared equally but you have double coming out of the amp itself.

This time, everybody send a beer to Ripper and not me!

:food-smiley-004:
 

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Discussion Starter #13
Thanks to all that took the time to put in their two cents!

I figured that it would work okay but it's nice to have a few extra opinions thrown in too.
:food-smiley-004:
 

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Hey Bill,
I think you are right on the old Ampeg's being wired that way. I've come across some old garnets that were done that way too.

Hey Humbucker, is that old Vibrolux of yours still using the original speakers? I changed mine out awhile back for some older eminence and boy did that make a huge difference. Mine is a 69 as well. It made the old amp sound fuller and louder as well.
 

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Discussion Starter #15
Ripper,

Yes, the two 10" speakers in my Vibrolux are the original, blue label Oxford speakers. They're not the best speakers out there but they're not bad either I guess.

I'd like to hear what this amp would sound like with some better speakers though. Maybe some Weber speakers or something else.
 

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With the plexi if your running two cabs, set the impedance to 4 ohms and you will be good.

nice, thx!

it's got a 4/8/100v tap on it, so I'm glad you didn't say 16 ohms!! heh
 

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I think it would be worthwhile to read my 'cut & paste' from a thread in TGP. I left a link to the original thread below. Read the whole thing. It's interesting to see how SS and tube amps differ in this regard.


Originally Posted by TENAX

the speaker resistance the amp sees at the load (speaker end) should always be more than the amps output ohm rating..so for example, 8 or 16 if you have a 4, 8 , or 16 ohm output options on your amp..you could then use for example:

4 or 8 ohm amp outputs with 8 or 16 ohm speaker resistance...or...

4 ohm output with a 4, 8 or 16 ohm speaker load resistance.

i have a valve jr that can do either 4, 8 or 16 ohm output. i have a 2 x 12 with 2 16 ohm speakers in parallel so it's an 8 ohm speaker load. so i would use either the 4 or 8 ohm outputs on the amp..but not the 16..

speaker load always to be matched or higher than the output on the amp or it could cause too much current to flow in your amp and fry your transformer.

easiest way i find to look at it is why is it said to always make sure you have a speaker hooked up..or a "dummy load" when you turn on an amplifier?" because otherwise, there would be no resistance in the circuit so say a transformer designed for 4 ohm resistance is presented with zero load with no speaker connected..too much current flowing..flames too follow quite possibly

Here's the response....

Sorry, this is absolutely wrong (again - every time this question comes up someone posts this)... especially the bits I highlighted.

It is is true for solid-state amps, as is the reason - causing too much current to flow - but it is NOT true with a tube amp, as the mechanism for damage is entirely different. Excess current is not the problem, as it's limited by the loss of efficiency of the OT as the mismatch becomes more extreme. Flyback voltage causing insulation breakdown is the risk, and occurs when the load is too high impedance, not too low.

You're also confusing 'no load' with 'no resistance' in the last paragraph.

'No load' means infinitely high impedance - an open circuit - and is dangerous for tube amps (although almost never for solid-state ones). 'Zero impedance' - a short circuit - is in fact usually safe for a tube amp, at least for a short time (although it's very hard on the tubes), but will fry a solid-state amp.

Generally, tube amps are safe with between half and double the correct matching impedance. Lower is more risk to the tubes, higher is more risk to the transformer and other parts, and it is NOT a good idea to run a 4-ohm tube amp into a 16-ohm load. If you must mismatch, it's safer to go lower rather than higher, with most tube amps.

Here's the link to the thread at TGP:

http://www.thegearpage.net/board/showthread.php?t=274034
 

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Ripper,

Yes, the two 10" speakers in my Vibrolux are the original, blue label Oxford speakers. They're not the best speakers out there but they're not bad either I guess.

I'd like to hear what this amp would sound like with some better speakers though. Maybe some Weber speakers or something else.
I changed the speakers in mine and I finished the blackfacing off (you'll find the circuit in yours is already about 3/4 a blackface circuit), and wow what an amp.
 

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I think it would be worthwhile to read my 'cut & paste' from a thread in TGP. I left a link to the original thread below. Read the whole thing. It's interesting to see how SS and tube amps differ in this regard.


Originally Posted by TENAX

the speaker resistance the amp sees at the load (speaker end) should always be more than the amps output ohm rating..so for example, 8 or 16 if you have a 4, 8 , or 16 ohm output options on your amp..you could then use for example:

4 or 8 ohm amp outputs with 8 or 16 ohm speaker resistance...or...

4 ohm output with a 4, 8 or 16 ohm speaker load resistance.

i have a valve jr that can do either 4, 8 or 16 ohm output. i have a 2 x 12 with 2 16 ohm speakers in parallel so it's an 8 ohm speaker load. so i would use either the 4 or 8 ohm outputs on the amp..but not the 16..

speaker load always to be matched or higher than the output on the amp or it could cause too much current to flow in your amp and fry your transformer.

easiest way i find to look at it is why is it said to always make sure you have a speaker hooked up..or a "dummy load" when you turn on an amplifier?" because otherwise, there would be no resistance in the circuit so say a transformer designed for 4 ohm resistance is presented with zero load with no speaker connected..too much current flowing..flames too follow quite possibly

Here's the response....

Sorry, this is absolutely wrong (again - every time this question comes up someone posts this)... especially the bits I highlighted.

It is is true for solid-state amps, as is the reason - causing too much current to flow - but it is NOT true with a tube amp, as the mechanism for damage is entirely different. Excess current is not the problem, as it's limited by the loss of efficiency of the OT as the mismatch becomes more extreme. Flyback voltage causing insulation breakdown is the risk, and occurs when the load is too high impedance, not too low.

You're also confusing 'no load' with 'no resistance' in the last paragraph.

'No load' means infinitely high impedance - an open circuit - and is dangerous for tube amps (although almost never for solid-state ones). 'Zero impedance' - a short circuit - is in fact usually safe for a tube amp, at least for a short time (although it's very hard on the tubes), but will fry a solid-state amp.

Generally, tube amps are safe with between half and double the correct matching impedance. Lower is more risk to the tubes, higher is more risk to the transformer and other parts, and it is NOT a good idea to run a 4-ohm tube amp into a 16-ohm load. If you must mismatch, it's safer to go lower rather than higher, with most tube amps.

Here's the link to the thread at TGP:

http://www.thegearpage.net/board/showthread.php?t=274034
This is absolutely positively the whole unvarnished truth!

How wrong assumptions get started so easily is beyond me. Maybe from folks trying to get you to blow your amp up so they can sell you another one!:mad:

The difference between a short and an open load and how the effect is different between tube and solid state amps has been stated time and time again and yet the wrong ideas still persist.

It's like a quote from Carl Sagan, the famed cosmologist from that Nova show on PBS:

"Tell a man there are billions and uncounted billions of stars in the universe and he will believe you without question. Yet when he sees a sign saying 'Wet Paint' he has to touch it!"

:food-smiley-004:
 
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