SVT's & 8 ohms???

[rodl2005]

I've got an ol' '92 SVTII & love it!!! But I've been reading a few posts lately stating that it IS OK to run an SVT at 8 ohms.

The SVT-CL, VR & my II all have just 4 ohm & 2 ohm ratings listed.

What's the go?? I'd LOVE to be able to run my SVTII into my 8 ohm 1x15" sometimes!!

[JimmyM]

it's a controversial topic in tube amp world. some say that it will stress tubes and output transformer, some say as long as your head sees some sort of load it's fine. i wouldn't do it to my own svt but i have run rental svt's on gigs at 8 ohms and they were perfectly fine. but i've been told that it was because i didn't run them hard enough to cause damage.

[Tim1]

I ran my SVT2nonpro into 8 ohms for years, based on advice I received from Ampeg at the time that it would be perfectly fine. Ultimately the amp became unreliable and I eventually sold it - I have often wondered whether this was related to the speaker load or some other fault.

[anonymous02282011]

Originally Posted by Tim1

I ran my SVT2nonpro into 8 ohms for years, based on advice I received from Ampeg at the time that it would be perfectly fine. Ultimately the amp became unreliable and I eventually sold it - I have often wondered whether this was related to the speaker load or some other fault.

What do you mean by unreliable?

[RickenBoogie]

It's not recommended, but you'd most likely not kill the amp,....right away.

[lem8r]

There is no harm at all to any component. You'll loose some low end and output power.
If you connect 8 Ohm cab to 4 Ohm tap expect about 180-150 W output power.

[kosmicwizard]

Originally Posted by lem8r

There is no harm at all to any component. You'll loose some low end and output power.
If you connect 8 Ohm cab to 4 Ohm tap expect about 180-150 W output power.

Now why would this be? Since it is an all tube amp it doesn't send a different amount of watts at different Ohm ratings ... It delivers 300 watts at 4 or 2 Ohms. I won't run my Mesa/Boogie BASS 400 full out into a 2 Ohm load (it is designed to run on either 8 or 4 Ohms) and I also wouldn't run my Ampeg SVT II at 8 Ohms because it is designed to run on either a 4 or a 2 Ohm load.

To the OP, why not just get a 4 Ohm 1x15 or 2x10? Less cone surface equals less volume, but then again don't crank a SVT into an enclosure that that can't handle 800 watts. Cheers.

[AwkwardLoudness]

An SVT will be fine through an 8Ω cab. There's only a problem if the cabs impedance is less than the amp's selected impedance. The power output will not be optimal with an 8Ω cab since there is not an 8Ω tap.

Originally Posted by kosmicwizard

Now why would this be? Since it is an all tube amp it doesn't send a different amount of watts at different Ohm ratings ... It delivers 300 watts at 4 or 2 Ohms.

Tube amps push about the same amount of power/wattage through each impedance that it has a tap for...as long as the cab(s) impedance matches the tap impedance. If the cab has higher impedance than the tap, then the output power is lowered.

If the cab(s) impedance is less than the amp's selected impedance, then that can be bad news quickly because it causes flyback current that can cook the output transformer.

[Jerrold Tiers]

Originally Posted by kosmicwizard

Now why would this be? Since it is an all tube amp it doesn't send a different amount of watts at different Ohm ratings ... It delivers 300 watts at 4 or 2 Ohms. I won't run my Mesa/Boogie BASS 400 full out into a 2 Ohm load (it is designed to run on either 8 or 4 Ohms) and I also wouldn't run my Ampeg SVT II at 8 Ohms because it is designed to run on either a 4 or a 2 Ohm load.

Well, a tube amp DOES give different wattage to different impedances....... on any given tap.

The tubes have a certain voltage and current level at max power, and they can't magically produce more voltage than is available (except in "spike" voltages).

The job of the transformer is to "match" that to different load impedances that require different voltage and current levels. That's why there is a tap for 2 ohms and a different one for 4. If it had an 8 ohm tap* then it would provide full power at 8 ohms.

So all you get is the 4 ohm tap, which provides the voltage and current needed for a 4 ohm load. But the voltage is too low for 8 ohms, so even though more current than needed is available, you end up with less power. probably around 2.3 of max, for various circuit and tube-related reasons.

* We really wanted to add an 8 ohm tap to the various SVT varieties, but the transformer design didn't allow it. To use the original design we could not fit in the extra winding, and rather than change the transformer design, we opted to leave off 8 ohms.....

If we changed the transformer design, we'd have never heard the end of it about our "inferior substitute"...... and the units would likely not sound like SVTs.

[anonymous02282011]

^^^^ straight from the horses mouth

thanks jerrold

[JimmyM]

Originally Posted by Jerrold Tiers

* We really wanted to add an 8 ohm tap to the various SVT varieties, but the transformer design didn't allow it. To use the original design we could not fit in the extra winding, and rather than change the transformer design, we opted to leave off 8 ohms.....

If we changed the transformer design, we'd have never heard the end of it about our "inferior substitute"...... and the units would likely not sound like SVTs.

wise wise wise decision. the one thing about the svt's that makes them rock is that no matter what happens to them, they always sound like svt's. even if you made it sound better with a redesigned transformer with the 8 ohm tap, people would have complained.

[rodl2005]

Originally Posted by Jerrold Tiers

Well, a tube amp DOES give different wattage to different impedances....... on any given tap.

The tubes have a certain voltage and current level at max power, and they can't magically produce more voltage than is available (except in "spike" voltages).

The job of the transformer is to "match" that to different load impedances that require different voltage and current levels. That's why there is a tap for 2 ohms and a different one for 4. If it had an 8 ohm tap* then it would provide full power at 8 ohms.

So all you get is the 4 ohm tap, which provides the voltage and current needed for a 4 ohm load. But the voltage is too low for 8 ohms, so even though more current than needed is available, you end up with less power. probably around 2.3 of max, for various circuit and tube-related reasons.

* We really wanted to add an 8 ohm tap to the various SVT varieties, but the transformer design didn't allow it. To use the original design we could not fit in the extra winding, and rather than change the transformer design, we opted to leave off 8 ohms.....

If we changed the transformer design, we'd have never heard the end of it about our "inferior substitute"...... and the units would likely not sound like SVTs.

Wise Decision INDEED!!! & THANKYOU!!!!

But I'd DEFINATELY NOT be 'pushing' the SVT into the 8 ohm 15" cab. Quite the opposite. Even if the SVTII is only putting out 2/3 of it's max power. I cannot imagine the master volume control(the ONLY volume control ) wouldn't go past 9 oclock- 10 at most.

So are y'all saying I might be sacrificing some of my BELOVED SVTII's longevity by doing thus??
Or would it be 'perfectly FINE to run it, say out of the 4 ohm out socket - into the 8 ohm 1x15" speaker cab?
I DEFINATELY DO NOT wanna shorten my SVTII's LONG life span. Even tho she only does about ONE gig/month at the moment, I still get my tube tech to give her a 'once-over' about every 18-24 months.

THANYOU EVERYONE for the replies so far. I'm greatly appreciating them all!

[JimmyM]

rod, all i can tell you is that the only time i ever ran my svt at 8 ohms was for a brief ab with my vintage 810 and my new svt 210av. i don't care even if it ultimately does no harm...i would never run my svt with an 8 ohm load, except of course for that 10 minute ab.

[kosmicwizard]

I love this place! I learn so much and one day I just might know what I am talking about, or understand why things are the way they are. Cheers.

[Jerrold Tiers]

No, it is not "instant amp death", most likely.......

There are some bad things that can happen, particularly given the not so wonderful tubes that are often sold these days........ So it is "not recommended"........ Like lots of other things that people do every day.

[anonymous02282011]

I know I've caught a bit of slack for saying this before but here's my story ..

I run a 69/70 SVT into a slightly modified TL606 cab with an EVX-155.

It's an 8ohm cab

The guy who got me into the set up is a 30+ year veteran in sound engineering .. his credentials are respectable to say the least

I've been running the setup for a few years with no issues .. note: I can't even turn the amp up past 10 or 11 oclock in the situation i play in .. it's just too loud

The aforementioned sound engineer currently owns 3 vintage SVTs and has been running them into this modded TL606 cab for 20+ years with no problems

so in my (and his) experience, if you don't push the amp into this particular cab you'll be OK

the thing that I keep in mind when considering amp impedence is that it varies with each frequency .. some cabs/speakers will vary greater than others .. so the nominal impedence may be higher or lower than the actual speaker rating .. IMO/IME etc etc

[Rockmusician]

Originally Posted by kosmicwizard

Now why would this be? Since it is an all tube amp it doesn't send a different amount of watts at different Ohm ratings ... It delivers 300 watts at 4 or 2 Ohms. I won't run my Mesa/Boogie BASS 400 full out into a 2 Ohm load (it is designed to run on either 8 or 4 Ohms) and I also wouldn't run my Ampeg SVT II at 8 Ohms because it is designed to run on either a 4 or a 2 Ohm load.

To the OP, why not just get a 4 Ohm 1x15 or 2x10? Less cone surface equals less volume, but then again don't crank a SVT into an enclosure that that can't handle 800 watts. Cheers.

A tube amp does run at a different impedance based on the ohm rating of the cabinet. However, you will be ok if you run an 8 ohm cab. You will lose some power but that's all. What you can't do is go in the other direction such as running a head that's designed to run at 4 ohms through a 2 ohm load.

[JimmyM]

Originally Posted by Rockmusician

A tube amp does run at a different impedance based on the ohm rating of the cabinet. However, you will be ok if you run an 8 ohm cab. You will lose some power but that's all. What you can't do is go in the other direction such as running a head that's designed to run at 4 ohms through a 2 ohm load.

that isn't what guys like jess oliver say. jess often installs 4 ohm speakers in b-15's designed to run at 8 ohms. most techs i've taked to say going to a lower impedance than the transformer is designed for is easier on your amp than going higher. me, i know little about it, so i let the experts do the talking.

[rodl2005]

Well, Don't think I'll try it. Not for any length of time anyway.

Thanks guys.

[Jerrold Tiers]

Originally Posted by JimmyM

that isn't what guys like jess oliver say. jess often installs 4 ohm speakers in b-15's designed to run at 8 ohms. most techs i've taked to say going to a lower impedance than the transformer is designed for is easier on your amp than going higher.

yeah.... it may be a little harder on the tubes, but overall it may be easier on the general electronics.... voltages are lower, and some otehr factors are better also.

When you go too high on impedance, a couple things can occur.... not that 8 ohms is necessarily too high*....

First, all the voltages are higher, and "spike " voltages from clipping the amp tend to be higher also. if the tubes are going to arc inside**, they are more likely to do that with the higher spike voltages. if something else is ready to break down, again it is more likely to do it with higher spike voltages.
While the nominal voltage is around 660-700V, the normal maximum peak voltage is 1200 to 1400V. Under-loading the transformer may provide spike voltages of 2000 to 2500 volts (or higher).
The spike voltage issue is more a problem if you really push the amp. So if you want overdrive, and you crank it hard, you may get more trouble than if you just put on a wrong-impedance speaker and play "normally".


Second, if, as with the SVT, there is "feedback" used in the amplifier to stabilise it, under-loading the transformer may change that.... phase and amplitude may be wrong, and so the amp might oscillate , either in short bursts at certain times, or continuously. That is usually spectacularly bad for the tubes, which may be aged 20 years in 30 seconds, if the oscillation makes the current go way up and the tube plates turn red...... After that the power tube is usually toast.
You don't have to push the amp to get this problem.

*
One thing is that speaker impedance is really variable.... a nominally 8 ohm speaker might be anything from 6 to 10 or 12 ohms in standard measurement, and vary even more with frequency (JBL was usually low, others were often high). So you may not really KNOW what you are connecting.

**
Extra high voltages can cause the tube to arc if there is some little piece of crud loose in it. In some cases the tube will then start a "light-show" of "white fire" inside, after which it generally is no good.
Manufacturers are SUPPOSED to spark-out their tubes so that doesn't happen, but we got shipments where we had to do that ourselves. And they usually only do it to the normal voltage the tube should withstand, and high spike voltages may cause an issue.