Tube amp: 2 or 4 ohm setting for a 2.66 ohm load?

[Jeremy Allen]

This question has been asked on here a few times, but not in quite the same way. I would like to drive three 8ohm cabs (Aguilar 112's) with my old Ampeg V4, and I'm not sure whether it is more appropriate to set the amp for a 2ohm load or for a 4ohm load. Or, is this not a big deal (i.e., either one)? Or, again, is it actually a big deal, and I should do neither one and just use two of the cabs (or buy another one and flick the 2ohm switch for a 4x12 stack )?

Total newbie to all-tube amps. Just got the V4 off evilBay after playing through one at my guitarist's house and having a religious experience in so doing, and I also have an old Sunn Sonaro which sounds awesome but needs some serious TLC.

[Laredo]

................You go below 4 Ohms with any combination of cabinets, you will need to use the 2 Ohm setting, or risk damaging your Amp Head!

[DukeLeJeune]

I would use the two ohm setting. I don't know what your amp's protection circuitry is like, but there's less chance it will be needed if the impedance of the load is on the high side instead of the low side.

Also, you'll have a closer match to the actual impedance using the two ohm setting... moreso since virtually all cabs have impedance dips below their rated impedance, so the minimum net impedance is probably very close to two ohms. The minimum impedance is where you're most likely to have problems, so you don't want a big mismatch there.

[B-string]

2 ohms is much closer to the combined than 4 ohms. The old V4's only protection was the line fuse.

[bobyoung53]

Originally Posted by Jeremy Allen

This question has been asked on here a few times, but not in quite the same way. I would like to drive three 8ohm cabs (Aguilar 112's) with my old Ampeg V4, and I'm not sure whether it is more appropriate to set the amp for a 2ohm load or for a 4ohm load. Or, is this not a big deal (i.e., either one)? Or, again, is it actually a big deal, and I should do neither one and just use two of the cabs (or buy another one and flick the 2ohm switch for a 4x12 stack )?

Total newbie to all-tube amps. Just got the V4 off evilBay after playing through one at my guitarist's house and having a religious experience in so doing, and I also have an old Sunn Sonaro which sounds awesome but needs some serious TLC.

2 ohm, won't hurt it at all. I would try two cabs @ 4 ohms and compare it to three at 2 ohms, one more for a total of at 2 ohms would be ideal.

[Tim1]

I always understood that with tube amps the approach is opposite to that with transistors - it is safer to go under the set rating than over it. That is, with a transistor amp that has a minimum load of 4 ohms you would not want to use a 2 ohm speaker, but if it was a tube amp you would be safer with a 2 ohm speaker than say an 8 ohm speaker if the amp was set to 4 ohms. Therefore I would differ from the above advice and set the amp at 4 ohms. Read some of the old Psychobassguy posts for justification.

[Clark Dark]

2.66 (or 2.67 when you have 8ohm + 4ohms) is more resistance than 2 ohms. Transistor amps are not very forgiving when you drop that kind of load on them. Tube types are a little more forgiving but there will be overheating issues. A friend was using an Epi 8ohm cab with a Swr 4ohm cab with a Gallien Krueger MB2-500 Bass Amp and couldn't quite figure out why the amp would shut down while he was playing it. After I told him to go with either of the two cabs alone all was fine.

[Jeremy Allen]

Thanks, all. Interesting. I am indeed familiar with how this works as regards solid state amplifiers and minimum loads, and I'm trying to avoid drawing a conclusion about tubes based on that because I know it's quite a different thing.

I've definitely heard of people running their tube amps into less resistance than they have them set for, but not more; and I'm wary of making assumptions about the nature of output transformers. I imagine that if it were OK to run a tube amp into any higher resistance than a particular minimum load, as is the case with transistor amps, then manufacturers would have just put a 2 ohm tap on the output transformer and let users do what they pleased. (The downside to that being that the amount of power output would be diminished at higher loads; I used to always assume that this was why tube amps had the adjustable impedance setting, to allow you to use the full power rating of the amplifier into any load. It seems like there is more to it than that.)

[alexclaber]

I'd try both settings but I'd err towards the 4 ohm rather than the 2 ohm setting as tube amps are happiest driving a lower impedance than expected (which is why you can't run them without a speaker load as that is perceived as driving an infinite high impedance). I believe it's the peaks at resonance that cause problems with tube amps, the opposite of the dips between resonance that cause s/s amps problems.

Alex

[johnk_10]

if you must use three cabs, i would run it at 4 ohms. i had this discussion with Leo Fender 30 years ago, and he said that it wouldn't hurt any of his amps to run at half of their normal rated impedance.


of course, on solid state amps, its a different story.

[agreatheight]

[quote=Jeremy Allen;8186196]...or buy another one and flick the 2ohm switch for a 4x12 stack )?

I am a better safe than sorry guy, so I'd buy the extra cab and be flexible - 2 cabs @ 4 or 4 cabs @ 2!

[edbass]

Originally Posted by johnk_10

if you must use both cabs, i would run it at 4 ohms. i had this discussion with Leo Fender 30 years ago, and he said that it wouldn't hurt any of his amps to run at half of their normal rated impedance.


of course, on solid state amps, its a different story.

It seems like most posters are thinking transistor. Regretfully, I never had the opportunity to have a discussion with Mr. Fender, but I know the outputs on his blackface amps I've been up close and personal with default to a dead short with no speakers plugged in, as a protection device. That would be very near zero ohm load on the transformer.
I also bought a two cabinet Sunn 2000S touring rig that had been gigged showing 2 ohms to the 4 ohm tap for decades with no ill effects. The tubes were fresh when I got it, but the other components are factory original and after installing new filter caps it scopes as strong as when new.

[Jeremy Allen]

Originally Posted by johnk_10

if you must use three cabs, i would run it at 4 ohms.

Oh I must, I must.

No, not really, of course; just thought that since I happen to have three I'd give it a try. I may well sell the DB112NT cab and get a 4 ohm something or other to run with the two other 112 cabs when I want to get silly. But on the other hand, the DB112NT is the right cab to have with that old Sunn Sonaro, so it may stay.

Thanks for posting, John! I was hoping you would.

[illidian]

You want the 4 ohm setting.

[coreyfyfe]

Originally Posted by edbass

That would be very near zero ohm load on the transformer.

I believe that would be an infinite load, technically.

In this situation I always site three sources for using 2 instead of 4. Or in my case it was 4 instead of 8 for 5.3.

First the amp techs at ampeg told me so

Second ive read that the fender super six reverb was the same OT as the twin and supers which was 4 ohm driving a 5.3 ohm speaker load.

And third was all the fender tube amps I've had said 4 ohm load minimum.

I caveat all of this by saying I'm not an amp tech, and all of this is based on other peoples opinions. I always trust JohnK's advice and he says different so who knows.

[johnk_10]

those are some pretty good points, however................

actually a super reverb has a 2 ohm output transformer ( four 8 ohm 10's wired all in parallel), and if you plugged in another 4x10 cab into the ext. speaker jack, which is wired in parallel to the main output jack, you'd be running its 2 ohm transformer tap into a 1 ohm load.

likewise, on the Fender showman, it has an 8 ohm output transformer (#125A29A), and if you add another 8 ohm cab, you're running an 8 ohm transformer tap into a 4 ohm load.
its the same situation for a Deluxe Reverb, and vibrosonic.

bassmans, bandmasters, twin reverbs, vibrolux reverbs & dual showmans all have 4 ohm output transformers, and when you add an additional cab, you're running a 4 ohm tap into a two ohm load.

the super six reverb is the only exception where its running a 4 ohm output transformer into a 5.3 ohm load, but the schematic also says that you can add either a 4 or 8 ohm extension cabinet which would be running it at either 3.2 ohms (with an 8 ohm ext cab) or 2.29 (with a 4 ohm ext cab).

[johnk_10]

BTW, Leo also told me the reason that they used a shorting jack on the main output of all of his fender amps is because its better for the output tranny to see a 0 ohm output (shorted), than it would be to see a open load (infinity ohms), as zero ohms is closer to a normal speaker load (i.e. 2, 4 or 8 ohms) than infinity.

[projectMalamute]

Originally Posted by coreyfyfe

I believe that would be an infinite load, technically.

A dead short is 0 ohms.

If you want to think of that as drawing more current and thus 'overloading' the circuit you are arguing semantics.

In concrete terms the impedance does not go to infinity, it goes to zero.

I'd use the 4 ohm tap. If the amp isn't junk neither should blow anything up.

[Jeremy Allen]

As Ralphie Wiggum said on that one "Simpsons" in which he's playing "hangman" on a Coleco computer, "I'm LEARN-ding!"

[kranahan]

use 112 and a 212 = a simple solution