is an 8ohm load safe for a fender twin

Randy Carson · Post by **Randy Carson** » 11 Mar 2003 11:43 pm

im in the middle of putting an amp together and recently bought an old dual showman/same as a twin chassis but im not sure what is safe. i know the twin was made for a 4ohm speaker load but that of course was two 12" speakers. i want to build a special cabinet for just one 15" but finding a 4ohm speaker that you want is near impossible. so what about all the twins being converted to a single 15 are they mainly 8ohms and the amp will go either way. a little input would be nice from some guys that have been doing this alot longer than me.
THANK YOU IN ADVANCE
RANDY CARSON

Herb Steiner · Post by **Herb Steiner** » 12 Mar 2003 5:26 am

Randy
Sorry I didn't get back to you by phone but I've been so swamped with busy work from the Dallas Jamboree, and now SXSW, that I've been running around like a headless chicken.

What guys have been doing is cutting a new baffle board and installing either 1) a reconed JBL D-130 in 4 ohms, or 2) a Black Widow, or 3) a 4 ohm Evans (Eminence) speaker.

I talked to Derrell Stephens this weekend in Dallas and he said that an 8 ohm load into a 4 ohm amp is okay, but that you'll get only about 70% of the potential power, which would knock a Showman down to around 65 or 70 watts of power.

I'm not a tech-head, but that's what my tech-head told me, anyway.

Glad you're feeling better, my friend.

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Herb's Steel Guitar Pages
Texas Steel Guitar Association

C Dixon · Post by **C Dixon** » 12 Mar 2003 6:13 am

It is never a problem (excluding loss of power) to "up" the omhage load on an amp. IE, putting an 8 Ohm speaker on an amp rated at 4 ohms is no problem whatsover (except of course there WILL be a loss of power). Maximum transfer of power is only possible when the source and load are perfectly matched.

Putting a load "less" than an amps rated ohmage can run the risk of blowing something. But usually this does not happen. Here is why. ONLY when an amp is run at Peak RMS power is there enough current to blow power components. And since 90% of playing is not a peak power, most can get away with reducing the load ohmage.

One caveat is solid state amps where it is never recommended to go below 2 ohms. Reason is; below this, output stage currents rise rapidly and dangerously high; almost guaranteed to blow one or more output transistors. This assumes again that a player plays at very high volume levels.

Incidently, there is Zero current flowing thru a speaker when there is no sound coming out of it. ONLY when it is putting out sound is there EVER any current flowing thru the voice coil. The opposite is also true. When a speaker is putting out very large levels of sound, very large "AC" currents are flowing through the voice coil. And it is current that blows ANY thing in any electrical devices; NOT the voltage.

carl

James Quackenbush · Post by **James Quackenbush** » 12 Mar 2003 3:09 pm

Randy,
Tube amps in general will have a loss in power if they are say rated at 4 ohms, and an 8 or 16 ohm speaker is used...What you can't do is go DOWN in ohms ...In other words if you have an amp that is rated at 16 ohms, you really shouldn't run 8 or 4 ohm speakers or you stand a chance of frying the amp !!..Jim

ajm · Post by **ajm** » 12 Mar 2003 7:14 pm

From Mr. Dixon (quote): "It is never a problem (excluding loss of power) to "up" the omhage load on an amp."

I assume that you are assuming that the operator will not go much higher than, say, 16 ohms. I don't understand all of the technical details behind it, but I have read that it is NEVER permissible to run a tube amp with no load connected. I know that I'm being picky here, but an open load is as far "up" as you can go.

On the other hand, I have heard that solid state amps don't care if you turn them on with no load connected.

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Artie McEwan

C Dixon · Post by **C Dixon** » 12 Mar 2003 9:04 pm

"I have read that it is NEVER permissible to run a tube amp with no load connected.

Very respectfully, the information you read is incorrect. Here is the way it is easiest to understand:

If there is NO power transferred (source to load), there is NO output current. If there is No output current, there is NO possiblility of damage; since exceeding maximum "design" currents is always what harms electronic components.

This can be proven by the following forumla:

P is equal to voltage times Current.

Put another way if the power is zero (as it would be with NO load) the current is also Zero at any given voltage. Thus, NO harm.

carl

Gino Iorfida · Post by **Gino Iorfida** » 13 Mar 2003 5:38 am

Ok, I've been sitting back here, listening before I chime in.

1) Fender amps, although this isn't recommended, have been proven over years to be able to handle a mismatch either DOUBLE the rated load, or HALF the rated load. However in both cases you WILL lose power. Tube amps unlike solid state don't 'gain' output by 'halving ' the load, due to the use of the output transformer,, the 'wrong' load will reflect the wrong impedance to the power tubes that will now work inefficiently.

2) tube amps are opposite of solid state. In other words, it's SAFER to double the impedance load on a solid state amp than to halve it. A tube amp would prefer to see the load halved than doubled. If you run a solid solid state amp with no speaker, chances are things will be ok, run a solid state amp into a dead short, and you'll fry the output transistors. Run a tube amp without a load for even a second, you'll take out the power tubes AND in a lot of cases the output transformer (BIG $$$), running a tube amp into a dead short, although not recommended IS safer than nothing (in fact, old fender amps had shorting jacks in the output, so if you forgot to plug in the speaker, the output would be shorted, keeping flyback voltage spikes from eating away the transformer insulation.

In short, yes, you COULD run your twin with an 8ohm speaker, but expect shortened tube life, however I'd strongly recommend either trying to find a 4 ohm speaker, or havign a tech put in an output transformer that has multiple output taps. The $100-$200 you spend NOW, will save you a LOT in the future.

Jody Carver · Post by **Jody Carver** » 13 Mar 2003 7:20 am

Gino
Running 4 16 Ohm speakers through a 4 ohm load as opposed to an 8 ohm load, what is your opinion on this.?...some of my amps have a choice of 4 or 8 ohms.

Gino Iorfida · Post by **Gino Iorfida** » 13 Mar 2003 8:05 am

Jody,
If I'm reading you right, you want to run a twin through (4) 16 ohm speakers? You would be ok, if you rant eh speakers in parallel, since 4 16's in parallel would be 4 ohms, and what the output transformer prefers.

Now if you wanted to run say a 16 ohm load off of a 4 ohm transformer, I would say BAD idea. The power tubes would be so mismatched (think of how the output of a tube amp works like a car-- if you floor the accelerator while you have the clutch pushed in (or car in neutral for those w/automatic), you'll blow your engine-- that's how too high of a load/no load would act. If you ran it into too low of a load, or a dead short, it would be like driving with yoru parking brake on, yea, you'll move, but you'll lose power, and yoru brakes will wear out quicker.

I've seen output transformers that had 'universal' taps, for 2, 4, 8, and 16 ohm loads (among others).

Ricky0ne1 · Post by **Ricky0ne1** » 13 Mar 2003 10:44 am

I recently installed a new output transformer in my Bandmaster Reverb with taps for 2, 4, 8 ohm impedence. Very easy to do, cost was about $70. No mismatches with this here head! Versatality to boot...

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Rick Summers

Randy Carson · Post by **Randy Carson** » 13 Mar 2003 11:02 am

thanks guys i appreciate your help. i was aware of how the load will affect the power of a solid state amp sometimes cutting it in half or almost doubling it depending on if you went up or down with the load but wasnt sure how much it would affect an old dual showman or (twin). since this is a black face amp its only 85 watts so i cant afford to loose any power so i ordered a 4 ohm speaker from evans this morning. i know it would have been alot easier and whole lot cheaper to buy a peavey amp made for steel but being a lead player for the first half of my life i just could never go solid state. so i guess when i get to the point where im giggin they'll just have to mic me or play at a lower volume, which i prefer anyway.
THANKS AGAIN!!
RANDY CARSON

Ken Fox · Post by **Ken Fox** » 13 Mar 2003 11:09 am

With all due respect I must state that it is not safe to run a tube amp with an open secondary. I agree with Gino in what he said above! The primary windings will then become the load and fry quickly (seen this in my own shop, once was enough for me!). That is why Fender puts a shorting jack on the main speaker output, so there is a load on the secondary and the fuse will likely go before the OT burns up.<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Ken Fox on 13 March 2003 at 11:11 AM.]</p></FONT>

Alvin Blaine · Post by **Alvin Blaine** » 13 Mar 2003 4:37 pm

If this is for a blackface "Showman" you should check to see what transformer you have first.
I have seen "Dual Showman" amps that have an 8ohm trans. and I have a '66 "showman" that has a 4ohm transformer.
4 ohm transformer number is: 125A29A
8 ohm transformer number is: 125A30A

Jody Carver · Post by **Jody Carver** » 13 Mar 2003 4:41 pm

Thank You Gino.

Iam running these speakers through a 212 HD 150 MusicMan amp. I have a choice of 4 or 8 ohm selector switches on the backside of the chassis. The Altec speakers are wired in
parallel. I am running 4 12" Altecs through the MusicMan amp, I have an extension cab for the additional 2 speakers.

I dont use my Fender amps. Only once in a while. Thank you.