Vintage SVT ohm question

gearHed289 · March 05, 2012, 10:35:01 AM

Can I run a 4 ohm cab out the standard speaker jack, and a 8 ohm cab out the extension jack on an old SVT head?

stiles72 · March 05, 2012, 02:58:59 PM

SVT's are designed to run at either a 4 ohm or 2 ohm load. Running a 4 ohm cab and an 8 ohm cab will give you a mismatched impedance. It might work just fine - but I wouldn't chance it.

rahock · March 06, 2012, 05:13:57 AM

It may sound fine, but I would be concerned about long term damage to the head. I'm sure PBG will chime in with much more solid advice.
Years ago I had a buddy with an old Vox T60(I think) with a 15 and and a 10 in the cab. He mismatched the Ohms on the two speakers,used the wrong wire and did everything wrong that you can think of. I advised him not to do it, but it turned out to be one of the best sounding bass amps I ever heard. He did have some head problems down the road but I don't really know if it had anything to do with the mismatch

.
Rick

jumbodbassman · March 06, 2012, 02:49:52 PM

IIRC the 4 and the 8 i believe would run at 3 ohms so should be ok with the 2 ohm setting. they will be mismatched however.

Psycho Bass Guy · March 07, 2012, 02:48:23 AM

You'll be putting a 2.66 ohm load across the 2 ohm tap, not a good idea. If your amp is in good health and has good output tubes, you might get away with it, but I wouldn't do it.

jumbodbassman · March 07, 2012, 08:39:33 AM

I am somewhat confused. If the amp has a 2 ohm out isn't that th lowest you can go so a 2.67 should be ok or is that just a SS thing???

gearHed289 · March 07, 2012, 09:56:04 AM

I'm going to take PBG's word for it. I thought someone had told me that years ago, just wanted a second opinion. Not something I NEED to do. Just thought it might be fun at some big fests over the summer. Guess I'll bring along the SVT III for the extra cab.

Doesn't hurt to have a backup for a 38 yo old amp anyway...

FrankieTbird · March 07, 2012, 12:43:43 PM

Quote from: gearHed289 on March 07, 2012, 09:56:04 AM
... Just thought it might be fun at some big fests over the summer. ...

Just run a second 8-Ohm cab in parallel with your other one, set the amp to the 2-Ohm setting, and run all three cabs. Then you'll be all set!

Dave W · March 07, 2012, 04:28:40 PM

Quote from: jumbodbassman on March 07, 2012, 08:39:33 AM
I am somewhat confused. If the amp has a 2 ohm out isn't that th lowest you can go so a 2.67 should be ok or is that just a SS thing???

It's an SS thing.

Psycho Bass Guy · March 07, 2012, 07:04:06 PM

Quote from: jumbodbassman on March 07, 2012, 08:39:33 AM
I am somewhat confused. If the amp has a 2 ohm out isn't that th lowest you can go so a 2.67 should be ok or is that just a SS thing???

With tube amps, higher than rated impedances are more dangerous than lower ones. The higher secondary impedance is reflected back into the output tubes by the output transformer, which causes their plate voltage to rise proportionally to the difference. Lower than rated impedances can age tubes prematurely, but in most cases would blow the mains fuse before any serious damage would occur. Higher imepdances just blow up the amp with no warning. The difference with 2.66 ohms and 2 ohms is not so huge that it means instant death, but an SVT is already skating the limits with lots of tubes.

Quote from: FrankieTbird on March 07, 2012, 12:43:43 PMJust run a second 8-Ohm cab in parallel with your other one, set the amp to the 2-Ohm setting, and run all three cabs. Then you'll be all set!

This would work fine, and BTW on vintage SVT's, any time you use the "extension speaker" jack, you're tripping an internal switch that connects the 2 ohm tap of the output transformer whether you use both jacks or not. Unlike modern SVT's, there is not an external selector switch.

gearHed289 · March 08, 2012, 09:24:49 AM

Great info. Thanks!

jumbodbassman · March 08, 2012, 09:32:27 AM

Ditto.

Old tube amps!!!!!!!!

jumbodbassman · March 21, 2012, 08:46:44 AM

BTW what happens regarding the newer inconsistent driver sout there. old days a 8 ohm driver was 8 ohms. now could be 7.2 to 8.4

Psycho Bass Guy · March 21, 2012, 11:56:20 PM

Impedance is not constant; never has been. Resistance is constant. True impedance is resistance and reactance, factoring in the electrical and mechanical forces acting upon and generated by the driver. It varies with frequency, sometimes greatly, and the mechanical and electrical resonances of a driver and cabinet. What the rating gives is a "range" that so long as the driver's tuning parameters are obeyed, will not exhibit potentially damaging resonant impedance spikes. Typically for bass drivers, as frequency decreases, impedance increases. This occurs because low frequencies require a much higher amount of current to sustain them and the physical excursion of the driver increases. This is the reasons lows are so hard to reproduce. In a sealed cabinet, the impedance to frequency ratio is an almost perfect 45 degree angle, but when you add things like ports and vents, things can get REALLY wacky.