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Question re: 4 ohm cabinet

#5
Virtually any head will run fine at 4 Ohms. There will be very few exceptions for used heads. I cannot think of a single, currently available new head that would not be happy at 4 Ohms.
 
#6
T-Bird said:
IME slim on paper, but in reality 50/50 if You're talking about used SS heads.

While the manufacturers claim that their power aplifiers are 4Ohm stable, running them with anything less than 8Ohm is a gamble.
The older SWR's are a perfect example of this.
Click to expand...
This is the first time I've ever seen anybody say that, and it doesn't match anything I have ever experienced or heard of.

Can you provide some sort of link or source or anything that supports your statement?
 
#8
T-Bird said:
Hi.

IME slim on paper, but in reality 50/50 if You're talking about used SS heads.

While the manufacturers claim that their power aplifiers are 4Ohm stable, running them with anything less than 8Ohm is a gamble.
The older SWR's are a perfect example of this.
IME anyway.

Regards
Sam
Click to expand...

Careful. This is not really correct. A monoblock amp that is designed to run at 4ohms is what they are designed to do, and you will get the optimum output. There would be absolutely NO issue in reliability, etc. Zero issue.

However, your example is valid, if very rare. Like the old SWR's, which were 'dual mono designs', when you bridge two power amps and run then into a single 4ohm cab, that results in each power amp actually seeing a 2ohm load. And, you are correct, running two powerful power amps that are crammed into a relatively small space at 2ohms can cause a LOT of heat. That is a rare, and VERY different situation than running a typical mono block amp at its designed optimal performance at 4ohms.

Of course, there are amps that limit their power output (i.e., don't increase from 4ohm to 2ohms), like the big Genz heads, the Mesa M6/9, and that results in no additional heat/issues.

To the OP, there are some amps that are dual mono design that either are not 'safe' at 4ohms (like the original WT800 before the heat control upgrad, the original Puma1000, etc.). This is very rare. Virtually every other solid state amp can run safely, and actually optimally (i.e., getting your full bang for your buck) at 4ohms.

One more point though. Getting a single 4ohm cab is a good idea IF, and only if:

a) the cab can actually use the extra power from your particular head running at 4ohms. A 410 or 212 with a moderate powered ss head... wonderful (you will notice a bit more volume, and less compression/limiting from the power section as you crank it). A 112, of a very high powered head, you would never notice.

b) you know you will only use one, and only one cab (assuming you have a 4ohm minimum head).
 
#9
Hi.

bongomania said:
This is the first time I've ever seen anybody say that, and it doesn't match anything I have ever experienced or heard of.

Can you provide some sort of link or source or anything that supports your statement?
Click to expand...

Really?

You've never seen a thread about ST220 or SM400 letting up the magic smoke when the unsuspecting and led-by-the-nose-ring owner or user of said amps has gone from 8Ohm tho 4Ohm in the search of the last squeezeable watt out of their "shy on power" amps?
In vain no less.

Or about the numerous SS Ampeg threads about the same subject?

Or how about the combo amp threads where the manufacturer approved 8Ohm external speaker has led to a sudden death for the amp?

I don't have the said threads listed anywhere, they don't concern me the slightest since I either choose twice the minimum recommended impedance, or just run tube amps, but perhaps Stumbo has.
He seems to have an extensive library of the TB threads sorted by the topic.
Something I'm very envious about actually.

Granted, in both aforementioned cases the problem has (most likely) been the owner either neglecting the periodical bias adjustment and/or neglected the necessary cooling air circulation when rack-mounted, but the end result is the same: The amp works nearly indefinitely to 8Ohm nominal load, but overheats or quits or breaks down on 4Ohm load.

Floyd Eye said:
Maybe he meant 2 ohm. Even that isn't really true anymore.
Click to expand...

Nope, I did mean 8Ohm.

KJung said:
Careful. This is not really correct.
Click to expand...

Correct or not, with only about two decades of experience, it's been my observation. You may have considerably more experience, and I do respect that.
Experience with my own gear and the occasional repairs I'm coarced to perfom for others.


KJung said:
A monoblock amp that is designed to run at 4ohms is what they are designed to do, and you will get the optimum output. There would be absolutely NO issue in reliability, etc. Zero issue.
Click to expand...

Absolutely true.

However, what the marketing department claims, doesn't always equal what the designers were able to acchieve within the parameters set by the management.


KJung said:
However, your example is valid, if very rare. Like the old SWR's, which were 'dual mono designs', when you bridge two power amps and run then into a single 4ohm cab, that results in each power amp actually seeing a 2ohm load. And, you are correct, running two powerful power amps that are crammed into a relatively small space at 2ohms can cause a LOT of heat. That is a rare, and VERY different situation than running a typical mono block amp at its designed optimal performance at 4ohms.
Click to expand...

I wouldn't call it rare, except for the fact of course that most of the amp users don't ever try to run the lowest possible load. Not twice anyway ;).

I've rarely came across a PA amp in my bench tests that would've had a PSU be sturdy enough to equal the performance of 8Ohm when ran to a 4Ohm load either.
That obviously doesn't mean they're not there, or that I think they're not there, but just not in my comfortable price range ;).

AFAIK SWR never claimed that their vintage heads could be bridged to 4Ohm, so that would be just going against the manufacturers recommendations.

KJung said:
Of course, there are amps that limit their power output (i.e., don't increase from 4ohm to 2ohms), like the big Genz heads, the Mesa M6/9, and that results in no additional heat/issues.
Click to expand...

Unfortunately IME that level of dedication to the details includes, what, 5-10% of the traditional PS MI amps in the market. From my experience anyway.

If You look at my first reply more closely, I claimed that IMHO/IME only about 50% of the used SS heads may become unstable at 4Ohms even though the manufacturer claims otherwise.
That still leaves 50% that will happily trot along with 4Ohm.
A glass half empty or half full and all :).

Regards
Sam
 
#10
The only two SS heads that I have ever seen/used/had that required an 8 ohm minimum load were the old GK 200RB and the GK BL 250 out of the Backline series ...

... Combo's there are several, but the OP was regarding heads ...
 
#12
Believe it or not, the much beloved Ampeg B15N head needs an 8 ohm load. Alot of tube amps can go lower, but not higher than they are rated, but not this one. It also has an extension speaker outlet that would make you believe that it could go to 4 ohm, but it has an internal switching device that changes the load to 16 ohm, so that each cab is still seeing 8 ohms. Of course, you are probably never going to see a B15N head only as a backline, but the OP did ask if ANY bass head would not go to 4 ohms. :)
 
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#13
T-Bird said:
Really?
Click to expand...
Yes, really. In the however many years I've been on this forum, I never saw the examples you mentioned. Even if I had though, "50%" would be an absurd exaggeration. Thank you for mentioning the few rare examples you know of--this demonstrates it is a possibility, which is something I would not have argued--certainly it is a possibility.
 
#14
T-Bird said:
Hi.

IME slim on paper, but in reality 50/50 if You're talking about used SS heads.

While the manufacturers claim that their power aplifiers are 4Ohm stable, running them with anything less than 8Ohm is a gamble.
The older SWR's are a perfect example of this.
IME anyway.

Regards
Sam
Click to expand...

I found my SWR 350 silverface (2001, iirc) worked very well at 2 ohms. Hot, yes. Fail, no. :D

You are much less likely to find a 2 ohm capable head, than one that can't do 4 ohms. BTW, there is a link to a list (pdf) of 2 ohm capable heads in the ohms faq stickie.
 
#16
A lot (if not all?) late 60's 50watt Guild tube amps and I think some of the Univox ones from the same period only run at 8ohms. They do have an 8ohm extension speaker jack, but it does the same thing mentioned above about the Ampeg B15n.
My Guild Thunderstar Bass amp was like this. I think the Univox 1235 amp was of the same design.
 
#17
I don't know what they're putting in the electricity in Finland. I wouldn't think twice about connecting two 8 ohm bass cabs to a bass amp unless it was something a bit different.

Having said that a cheap Line6 combo once popped its clogs on me for only possible reason that an extension cab stressed it. To me that's Darwinian amp selection at work.
 

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