vintage svt question.

[68Goldfish]

Hey dudes. I just recently aquired a 72 svt, I've paired it with a newer 8/10e and I'm loving it......no, I'm realy realy loving it. My question is, there's going to be several gigging situations where it's not going to be worth it to take the 8x10 along. Now I do have a four ten cab, an swr golliath3. the problem is is that it's an 8 ohm cab. Am I correct in thinking that the svt can only do a 4 ohm load? Would anything bad happen to the amp if I used the 8 ohm cab. For some reason I thought you could go up in ohms but not down. Any help would be greatly appreciated.

 

[GabeN]

You can run it at 8ohms but your wattage will be halved. Most SVTs can run between 4 and 8 ohms.

 

[Psycho Bass Guy]

Am I correct in thinking that the svt can only do a 4 ohm load?

The SVT does a 4 ohm load or a 2 ohm load if you use the extension speaker jack.

Would anything bad happen to the amp if I used the 8 ohm cab.

You might blow your amp up, but other than that, no big deal.

For some reason I thought you could go up in ohms but not down.

The SVT, like most tube amps, uses an output transformer, which means the rules for connection are different that what the mouth-breathing counter jockeys at your local GC may tell you. This has been discussed and explained here many, many, many, many, many times and is even included in the FAQ.

You can run it at 8ohms but your wattage will be halved. Most SVTs can run between 4 and 8 ohms.

Your advice is wrong, dangerously so.

 

[alembicguy]

I have seen SVT amps used at 8 ohms with the same bad out come(damage to the amp)

 

[bobyoung53]

You might blow your amp up, but other than that, no big deal.

Haha!

 

[anderbass]

I just recently aquired a 72 svt, I've paired it with a newer 8/10e and I'm loving it......no, I'm realy realy loving it. My question is, there's going to be several gigging situations where it's not going to be worth it to take the 8x10 along.

Those are the exact reasons you see so many people on this forum raving about they're NV610's. Its a 4ohm cab with the same vibe as the fridge, but it's constantly reported to out perform it in every way. You'll also find its much easier to haul/load than your Goliath.

Here's lots more NV610 info:
http://www.google.com/custom?domain...;S:http://www.talkbass.com;LP:1;FORID:1&hl=en

 

[68Goldfish]

You might blow your amp up, but other than that, no big deal.

OK then. I guess that sums that up.

 

[denton57]

I am in the same boat. I was thinking the fliptops 1x15 in 4ohm would be a perfect solution...and it will match!

 

[brothernewt]

I am in the same boat. I was thinking the fliptops 1x15 in 4ohm would be a perfect solution...and it will match!

pretty sure they can make a matching 4 Ohm 4-10 cab too if ya ask 'em...

 

[anonymous02282011]

I've been running my 74 SVT with an 8ohm 15" speaker cab. Needless to say, it's been fine. But now I'm worried. I read through the FAQs, but I'd like some help here: how exactly can i damage the amplifier by running an 8ohm load? and why haven't I damaged it yet?

 

[Psycho Bass Guy]

http://www.talkbass.com/forum/showthread.php?p=5218766#post5218766

http://www.talkbass.com/forum/showthread.php?p=3779979&r#post3779979

 

[anonymous02282011]

Thanks Psycho. Since I haven't damaged the amp, is there any reason to stop running an 8ohm load? is there potential for damage in the future? I've played this rig on countless gigs, many hours at a time and have never had a problem. Is there some tell-tale sign that I may be doing damage?

 

[Psycho Bass Guy]

Thanks Psycho. Since I haven't damaged the amp, is there any reason to stop running an 8ohm load? is there potential for damage in the future?

It's possible that the resonance of the speaker is low enough that with your particular cab, there isn't an appreciable spike in your tubes' plate voltage. A higher load isn't necessarily instant death, BUT conversely, the amp was not designed for it, and a higher impedance load makes the chance of serious damage that much greater.

I've played this rig on countless gigs, many hours at a time and have never had a problem. Is there some tell-tale sign that I may be doing damage?

Short of disassembling the amp and the output transformer, unless it's acting funny, there is no way to tell. However, if you take the amp apart and notice burn marks on your tube sockets and/or OT(which would have to be disassembled itself), you've done damage. Also, since it puts your output tubes above their rated limits, you may have had to replace tubes much sooner than you should under normal conditions.

 

[anonymous02282011]

Very interesting!

Thanks for the edification. I really do appreciate that.

Now just for the peace of mind, I'll have to find a 4ohm 15"cab! Either that or lug the second one to make it a 4ohm load.

 

[ReidK]

I think PBG is overstating the risks of running an SVT into an 8 ohm load. Here's why:

http://www.talkbass.com/forum/showthread.php?p=5218766#post5218766

http://www.talkbass.com/forum/showthread.php?p=3779979&r#post3779979

Those writeups do a good job of explaining why tube amps must never be run with no speaker attached (theoretically infinite ohms) but are relatively safe into a short circuit(theoretically 0 ohms), while transistor amps are just the opposite.

But that's not the situation in this case. Running an amp into an 8 ohm cab is very, very different from running one with no speaker attached, for the simple reason that 8 ohms is much closer to 4 ohms than it is to infinity (or millions of ohms, which is the situation in reality). Assuming good tubes and a good output transformer, an SVT is not going to die from running into an 8 ohm cab. If it does, either the amp was was dying anyway (probably from having been run previously without a load attached), or the speaker simply blew and failed open, which can happen with any speaker regardless of its impedance rating.

As PBG correctly described, higher impedance on the secondary or output side of the transformer causes higher voltages to develop on the primary or tube side. This is the mechanism that kills transformers when there's no speaker attached (which equals very high impedance); the voltage on the primary can get so high that it destroys the output tubes, punches through the wiring insulation in the transformer, or both. In the case of an open output (no speaker), the voltage spike can get up into the range of thousands of volts, and few audio tubes or transformers can withstand that. But using 8 ohms instead of 4 does not produce that kind of voltage rise, and only defective or marginally-designed equipment (and the SVT does NOT fall into that category) will die from an output impedance mismatch of 2:1.

I know PBG has a good rep here, and you may not believe me, and that's OK. You do have another option, and in fact it's the best one. It's pretty easy to rewire a 4-driver 8-ohm cab so that it beomes a 2-ohm cab which the SVT can drive. If you're not up to that yourself, any good electronics tech can do it, and they can even add a switch so you can run the cab either way. Issues of amp stability aside, you'll get the best sound and best power from doing that.

EDIT: If the SVT has an extension speaker jack rather than a switch (I know the oldest ones did), and you don't want to modify the SVT, you can still rewire the cab, but use two input jacks instead of one. The cab effectively becomes two separate 4-ohm cabs in one box. Then run a cable from each output on the amp to an input on the cab (two cables, in other words). You can still have a switch on the cab to allow 8-ohm operation as well.


Hope this helps.

Reid

 

[anonymous02282011]

What's not to believe? you are both stating the same technical information. It sounds like Psycho is perhaps a bit more cautious.

From my experience the SVT through my 8ohm EV-Thiele cab sounds great and I haven't noticed any damage to the amp over hundreds of hours of use. So i think i'm in the clear. None the less, I learned something about these old amps, which to me is priceless.

I don't run a 4 driver cab -- just single 15s. Again, one is stupid loud for most occasions, but the second certainly adds depth and presence even at low volumes. Now, practically speaking, schlepping 2 15s to a gig is a work out and volume wise it's overkill. Most importantly, when my bandmates see those two 15s, no amount of tech talk is going to deter them from poking fun at me. So I think i'm just going to keep running the amp into an 8ohm load.

If anything blows up, this will be the first place I go for help!

 

[Kindness]

As PBG correctly described, higher impedance on the secondary or output side of the transformer causes higher voltages to develop on the primary or tube side.

And since in the SVT in particular the voltages are already at extremely high levels, not many tubes are going to be forgiving with increasing the voltages - especially the levels seen when driving an SVT out of its linear range, which is more likely when using the typically less sensitive 8 ohm cabinet (in other words, a user is more likely to push their SVT harder into an 8 ohm 1x15 than into a 4 ohm 8x10).

 

[ReidK]

I don't run a 4 driver cab -- just single 15s.

Actually, that was for the OP, 68goldfish, who said:

Now I do have a four ten cab, an swr golliath3. the problem is is that it's an 8 ohm cab

Reid

 

[anonymous02282011]

ha. seems i was getting "self-absorbed"!

 

[Psycho Bass Guy]

I think PBG is overstating the risks of running an SVT into an 8 ohm load.

You don't have to take my word for it:

http://www.talkbass.com/forum/showpost.php?p=5194413&postcount=32

...and there are more by Jerrold that go further into detail. This isn't a 'Psycho being over-cautious' issue; this is a 'that's the reason output transfromers have ratings' issue.

Running an amp into an 8 ohm cab is very, very different from running one with no speaker attached, for the simple reason that 8 ohms is much closer to 4 ohms than it is to infinity (or millions of ohms, which is the situation in reality).

The output transformer is rated to accomodate the range of impedance presented by a four or two ohm load. If eight ohms, which is different from four ohms by the same percentage as two to four, is no big deal, WHY did Ampeg bother to specify two separate taps? Increasing the output impedance 100% above rating shifts the range of speaker impedance response into a much smaller "window" of safe operation.

Assuming good tubes and a good output transformer, an SVT is not going to die from running into an 8 ohm cab.

Since high impedance can also over-arc the output transformer you've just assumed that part of the damage from too high a load doesn't exist. On top of that, all it takes is one resosant spike do do serious damage. You may have been polite about it and attempted to qualfiy the likelihood of damage to a minimum, but your supposition has no basis in fact.