Ampeg SVT-CL impedance question...

[Bob Lord]

Hi amp gurus, need your wisdom...

The SVT-CL will go down to 2 ohms. There's a switch on the back for you to select whether your load is 2 or 4 ohms. So, no problem plugging it into two 4 ohm cabs in parallel...

But what if I have a 4 ohm and an 8 ohm cab? Would they draw the same power? And would the impedance of those cabs in parallel (2.67ohms) be likely to cause any issues if the amp was switched to the '2 ohm' setting? What about the 4 ohm setting?

Cheers!

[itu]

Impedance itself is not a steady parameter. It changes over the frequency range.

Depending on your cabs' efficiencies, the power is distributed basically according to their impedances. On the other hand the efficiency may level the difference. This can be verified by playing through the cabinets.

Maybe the difference is so big that you'll leave the other cab away. But this needs testing.

If you drive the head to its extremes, the choice may be more crucial, but if you do not need all headroom, play with both and check the temp of the transformer. That probably gives most tangible result to your question. Cooler is better.

[skidder652003]

hmmm Im no expert but as the owner of a SVT-CL I wouldn't risk it, those valves make for strange bedfellows with impedance IMO,  2 x 8 ohm, fine, 2 x 4 ohm also good, but I'd be nervous mixing them up, I had an 8 Ohm cab when I first gor the Ampeg, it was fine but I heard they didn't like it ( true or not, no idea), so I moved sharpish to the 410 Ampeg 4 Ohm and left it at that. No doubt someone who knows what they're actually talking about will be along to offer more sensible advice, great choice with the amp btw, how's your back?

[Bill Fitzmaurice]

With an SS amp the rated impedance is minimum, with valve it's maximum. Using a 2.67 ohm load with a valve amp you'd probably be better off with the 4 ohm tap, but you're not going to have a problem with the 2 ohm tap. Using an 8 ohm load on the 2 ohm tap would be a different story.

[Lozz196]

If I understand that correctly Bill - please bear with me as not too good on this stuff - if a valve amp has 16ohm & 8ohm selector then it’s ok to use a 4ohm cab and then just choose the 8ohm setting?
 

Am asking as our guitarist has a Marshall with the above selector switch but often there are only 4ohm cabs available.

[Bill Fitzmaurice]

Yes. Valve amps have no minimum load. I remember running 4 ohm rated Fenders with 1 ohm loads with never an ill effect. They even handle a dead short. OTOH while you can run an SS amp with no speaker that will damage a valve amp. To protect against that Fenders have switched output jacks that short the output if there's no speaker plugged in.

[thodrik]

12 hours ago, Bill Fitzmaurice said:

With an SS amp the rated impedance is minimum, with valve it's maximum. Using a 2.67 ohm load with a valve amp you'd probably be better off with the 4 ohm tap, but you're not going to have a problem with the 2 ohm tap. Using an 8 ohm load on the 2 ohm tap would be a different story.

This is why I am cautious with my old Trace Elliot V6 with 2 and 4 ohm taps.

I have seen Trace Elliot documentation saying that the V6 and V8 will run fine with an 8 ohm cabinet (which is just as well given that the matching 4x12 cabinets were 8 ohm). Given what little I know about valve amps it just doesn't make sense to me. Given how expensive the V6 is to repair and how little actual information there is on the V type line, I am not taking any chances.

[stewblack]

My amp tech just finished overhauling my SVT-CL and advised me to match the impedence exactly. 4 Ohms or 2 Ohms, switch set to whichever I'm using. 

They're delicate great lumps, best not messed with. 

[Bill Fitzmaurice]

Look at the impedance chart for a speaker and then reconsider that advice. This is a typical 8 ohm sealed speaker:

This is a typical 8 ohm ported speaker:

In neither case is the impedance 8 ohms at more than a few frequencies, so there is no way to exactly match the amp to the speaker. You use the tap that's equal to or higher than the speaker's rating, which is a nominal value, not an exact one.

 

[stewblack]

23 minutes ago, Bill Fitzmaurice said:

Look at the impedance chart for a speaker and then reconsider that advice. This is a typical 8 ohm sealed speaker:

This is a typical 8 ohm ported speaker:

In neither case is the impedance 8 ohms at more than a few frequencies, so there is no way to exactly match the amp to the speaker. You use the tap that's equal to or higher than the speaker's rating, which is a nominal value, not an exact one.

 

I have no argument with you Bill, I don't have the remotest scintilla of the necessary knowledge to debate with you. But I know on my budget I take no chances with an amp that just cost me more than i earn in a month to have serviced. So switch will be at 4 ohms and I will use two 8 ohm cabs.

[Bill Fitzmaurice]

That's fine. The trouble with your tech's advice is what does one do it they have a 2.67 ohm load as in the case of the OP? How does one match that exactly with the tap choices being 2 and 4 ohms? You can't. You may use either, though, and neither will bother the amp in the slightest. Amps aren't designed to operate into a precise impedance load because there's no such thing as a precise impedance load.

[stewblack]

12 minutes ago, Bill Fitzmaurice said:

That's fine. The trouble with your tech's advice is what does one do it they have a 2.67 ohm load as in the case of the OP? How does one match that exactly with the tap choices being 2 and 4 ohms? You can't. You may use either, though, and neither will bother the amp in the slightest. Amps aren't designed to operate into a precise impedance load because there's no such thing as a precise impedance load.

Not a problem for me, but yes a problem if you only have speakers which stack up to 2. 67 ohm. 

[Bob Lord]

On 05/12/2019 at 20:57, skidder652003 said:

hmmm Im no expert but as the owner of a SVT-CL I wouldn't risk it, those valves make for strange bedfellows with impedance IMO,  2 x 8 ohm, fine, 2 x 4 ohm also good, but I'd be nervous mixing them up, I had an 8 Ohm cab when I first gor the Ampeg, it was fine but I heard they didn't like it ( true or not, no idea), so I moved sharpish to the 410 Ampeg 4 Ohm and left it at that. No doubt someone who knows what they're actually talking about will be along to offer more sensible advice, great choice with the amp btw, how's your back?

it's fine so far, but I always get someone else to two the head with me. I try to be careful. Hardest bit is hoisting the head onto the 8x10. It's just a bit too high and a bit too heavy. Wonder why on earth i'm dragging this stuff about when everyone else seems to be using super light cabs and tiny class D amps, but when I start playing I always glad I bothered