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

[Jeremy Allen]

Originally Posted by kranahan

use 112 and a 212 = a simple solution

You mean an 8 ohm 112 and an 8 ohm 212? But then the amplifier output would be split equally between two unequal cabinets.

[coreyfyfe]

Originally Posted by 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).

See. Trust this guy.

[dmusic148]

"Tube Amplifiers are a different animal, and many have switches to match the impedance of the cabinets to be connected to them. A given tube amp might be able to send a 400 watt signal to an 8 ohm, 4 ohm, or 2 ohm load, but the switch on the back must be set to the load being used). Generally it’s safe to be up to 100% off on the load hooked up to a tube amplifier from where it’s selected. For a tube amp set for a 4 ohm load that would mean that you could run it into anywhere from an 8 ohm load to a 2 ohm load and still be reasonably safe. The important difference between tube amps and solid state ones is that it’s dangerous to run tube amps above their impedance rating!! Running a tube amp set at 2 ohms into an 8 ohm load can dangerously raise the plate voltage of the amplifier and ruin it. Also, while most solid state amps will do ok with no speakers plugged into them (infinity ohms), tube amps need to have a load connected to them, or they will generally self destruct."

[rodneyat]

Originally Posted by kranahan

use 112 and a 212 = a simple solution

This is not going to sound like 3 112 cabs. 2 112 cabs sound a lot tighter then a single 212 cab.

[kranahan]

isn't it better than having an impedance mismatch?

[Grooveman1961]

So, I'm confused. A v4 has 2 output jacks (at least mine does), so how are you planig on hooking these up? Y cord two together into one output, then one into the other?

that would get you 2.67, so the 2 ohm tap is the one.

2 of the cabs should be louder than the one, but hey, what are you gonna do?

[Jeremy Allen]

I did see the stuff in the FAQ, but I know that not all tube amps are the same and that there are people on here who know the specific behavior of the amplifier I'm talking about.

Originally Posted by Grooveman1961

So, I'm confused. A v4 has 2 output jacks (at least mine does), so how are you planig on hooking these up? Y cord two together into one output, then one into the other?

that would get you 2.67, so the 2 ohm tap is the one.

2 of the cabs should be louder than the one, but hey, what are you gonna do?

I've got newfangled cabinets (two GS112s and a DB112) which each have two speaker jacks on back (well, four, two Neutrik and two 1/4"). The jacks are wired such that you can daisy-chain speaker cabs from one to the next. So you just plug the V4's amplifier output into GS112 #1, go out of that cab and into GS112 #2, and then I suppose either out of that cab into the third cab or you could use the second speaker output on the V4 for the third cab.

I thought our general idea here is that the 2.67 ohm load should actually call for the 4 ohm tap, not the 2 ohm tap...better to send the signal into a too-low impedance than into a too high one. At least, that's what I'm taking away from this discussion.

[johnk_10]

i'm very familiar with V4's and i still think that it'll be the happiest at 4 ohms with your three cabs, but running it at 2 wouldn't hurt it either.

[Grooveman1961]

Originally Posted by Jeremy Allen

I did see the stuff in the FAQ, but I know that not all tube amps are the same and that there are people on here who know the specific behavior of the amplifier I'm talking about.




I've got newfangled cabinets (two GS112s and a DB112) which each have two speaker jacks on back (well, four, two Neutrik and two 1/4"). The jacks are wired such that you can daisy-chain speaker cabs from one to the next. So you just plug the V4's amplifier output into GS112 #1, go out of that cab and into GS112 #2, and then I suppose either out of that cab into the third cab or you could use the second speaker output on the V4 for the third cab.

I thought our general idea here is that the 2.67 ohm load should actually call for the 4 ohm tap, not the 2 ohm tap...better to send the signal into a too-low impedance than into a too high one. At least, that's what I'm taking away from this discussion.

Well Duh. of course, and the reciprocal of 3(.125) is indeed 2.66. That's why I get the micro bucks!

However, I don't see how the four ohm recomendation is the best. I beleive 2 is. It's all about the turns ratio of the OT isn't it?

Won't the connection of a 2.67 ohm load on the secondary that was expecting a 4 ohm load, lower the reflected impedance to the primary, causing a potential increase in current flow for a given Plate voltage? Doesn't this increased current flow potentially overload the primary winding on the OT?

OTOH, connecting a slightly higher impedance across the 2 ohm tap will raise the plate impedance, lowering current flow somewhat?

Now its true that speaker impedance ratings are nominal, and as such vary across the freq response of the driver, so the tranny design must be able to deal with extra current and voltage conditions with in a range.

But I still agree with B string, that the 2 ohm tap is more appropriate.

[B-string]

I don't think harm will come either way, my only reason for suggesting the 2ohm setting is closer matching of the load to the output tube plates. That is the whole reason for multiple taps on the secondary of the output transformer. It is a very fluid load as impedance changes with frequency. We used to use a 100watt bogen tube amp with a signal gen. set to 50Hz to test turntables. I hooked the same amp (had a 115volt tap) to the master clock at the factory with the gen set to 240Hz. Time flew by (and I almost got fired), they locked to room to the master clock after that.

Looks like I was just a little late in posting, but Grooveman 1961 got where I was going with my suggestion.

[Grooveman1961]

Wow

Simultainious posting....the nirvana of forum participants

[B-string]

Originally Posted by Grooveman1961

Wow

Simultainious posting....the nirvana of forum participants

Here in the twilight zone

[Jeremy Allen]

I wish I had any idea what you cats were talking about. But you've got to understand...I'm actually a double bassist. We mostly talk about sheep intestines and tree sap. (And whether or not we get fed on the gig.)
Very interesting though!

[Grooveman1961]

Dude, what we're saying is that the BEST choice, based on scientific gobldygook of tubes is...set it on 2 ohms.

And leave the standby switch on between sets.

You're gonna play a double bass throught hat rig? SaaaaWeeeet. Everet H. will be happy.

[B-string]

Well keep in mind that a speaker cab with anything but a single PURE note going into it (instruments are anything but a single pure tone) will never present a constant load to the amp. Just because the speaker says 8ohm, it does not load the amp with a fixed 8 ohm load. Most times it will be anything but 8ohms. What the attempt is, is to not overload the amp with a load that can go too low for the amp driving it (most important with solid state, tube amps are more forgiving until you run them with too light of a load ie high impedance numbers). 2ohms heavy load 1,000ohms light load.

[illidian]

I think the most correct thing to do here is plug in your washer and dryer. Then use the two ohm setting.

[tifftunes]

Originally Posted by 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.

Originally Posted by 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.)

Originally Posted by 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.

+1 for 4ohms. You can't run a tube amp without a load, and you can't over load it either. Its better to run the amp switched to 4ohms. At the 4ohm setting, you can drive 2, 4 or 8 ohm loads.

[Jeremy Allen]

Originally Posted by Grooveman1961

You're gonna play a double bass through that rig? SaaaaWeeeet. Everet H. will be happy.

Yeah, every once in awhile. Mostly it will be for when I take a walk on the other side of things; I have an old P-bass and play in a rock band too, and I'm also interested to see what the V4 will be like in all of the settings where I typically use a high-powered SS amp (fusion band, afro-cuban group). And I have this Stambaugh six string bass which, when played with the neck pickup only and in single-coil mode, sounds like Grant Green's or Kenny Burrell's guitar, and especially when I play it through a tube amp.

Basically I'm tired of "faking" the tube amp sound with pedals and compressors and a single dinky 12AX7 in a pre slot, and when I played through my friend's V4 at a preposterously loud rehearsal I attained enlightenment (or at least profound satisfaction).

[anderbass]

Have your amp tech bench test max clean output of your V4 into a 2.67 dummy load, preforming 2 separate tests using both the 2 and 4 ohm tap switch settings. The test results should prove what you wanna be using.

[Jeremy Allen]

Just got the amp and all the cabs in the same room at the same time for the first time.

OK fellas, this was a very interesting discussion and I certainly learned a lot. But the truth is, now I see that there is no way on EARTH that I will ever use more than two of those 112 cabs with the V4. With the EQ flat and a P-bass, I got the volume to about 9:30 and at that point I was deafened and sh!t was falling off of shelves upstairs.

I know it's different live, and when I've played a V4 through a 2x15 cab I was able to have it up about halfway in an insanely loud situation. Big stages, outdoor gigs, it's different. But this discussion just became hypothetical for me.

Man, I love the V4.