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Dumb SVT3-Pro question about ohms

E

EastDakota

Mar 9, 2024
196
444
#1
I love the sound of my older SVTIII-Pro for bass but I also love it for guitar (into my cab sim at least). My bass cab with a Bag End E-15 speaker sounds exactly how I want for bass and does decently well enough for guitar, but I've also been on the hunt for a guitar speaker to run from it that's close to my favorite guitar IRs, and that I could run as an open back cab. There's some good high-wattage guitar speaker options I'm looking at, but I'd also had the thought that using a 16ohm cab would expand the suite of possibilities with the reduced power at that impedance (to avoid speaker blow-out warnings, this would be guitar into a guitar speaker, and I'd run a HPF). I searched around to see if this was a viable option, and there's some conflicting responses for other solid-state power amps online, but I can't find an answer for this model. I found a thread elsewhere with an SVT4-Pro, and the justification as to why not was to avoid breaking the warranty. I reached out to Ampeg to ask if it was safe to run at 16ohms and they said:

"This amp is only rated for use with 4- and 8-ohm loads. Best use practice would be to stick with those impedances."

I like my amp in it's current, awesome, working state, so I'm going to listen to Ampeg, but I'm curious as to why a 16ohm load could possibly be harmful for an amp (or for the speaker?) designed for 4 and 8ohm loads. I'd been under the impression that higher impedance speakers were generally safe for solid-state amps. Or, is it just that the customer service rep followed the manual for their answer (which states 4 or 8 ohm load), and 16ohm bass cabs are such a rarity that they simply don't ever get mentioned?
 
#2
There is no technical reason why the SVT-3Pro can't be used with 16 ohms. It's just not common to rate at 16 ohms because that's a rare load in the bass world.

Somebody must have confused this amp with one of the tube output section power amps which is NOT rated for 16 ohms.
 
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#4
Wolford said:
@agedhorse Is it safe to use a tube SVT CL-300 with a series cable into 2 8-ohm cabs? 16 ohm load. I imagine it couldn't hurt the cabs, but might overwork the head? Thanks!
Click to expand...
Not a good match for the head, and unless the cabinets are identical, the results might not be all that great even if you could.

In general, parallel connections are better.
 
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#5
Wolford said:
@agedhorse Is it safe to use a tube SVT CL-300 with a series cable into 2 8-ohm cabs? 16 ohm load. I imagine it couldn't hurt the cabs, but might overwork the head? Thanks!
Click to expand...

Why would you even consider this? The designer of the original SVT said the amp is safe at 8 ohms on the 4 ohm output. Let's call this a 2 to 1 mismatch. The consequence of a 2 to 1 mismatch is the amp will make less power and the tubes will be stressed harder. A 4 to 1 mismatch is probably fairly risky for a most tube amps. The resulting repair could be very expensive.

SVTs are rated for 2 ohms or 4 ohms. Two 8 ohm cabs in parallel is 4 ohms...exactly what the amp wants. Since you have two 8 ohm cabs, give the amp what it wants.

Not sure if you understand this. The SVT 3-Pro has a solid state output section. You need to be concerned with the minimum impedance the amp can handle which is 4 ohms. It's safe if the impedance is higher than expected. Solid state is generally safe with an open circuit, but cannot handle a short circuit.

The SVT CL has a tube output section. With tube output sections you are concerned with matching impedance. Tube amps are safer with a short circuit than an open circuit. In fact, some tube amps short the output jacks unless a speaker is plugged in. I am not recommending that you show short the output of a tube amp. But definitely do not operate a tube amp without a speaker or dummy load attached.
 
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