A 4 ohm head with an 8 ohm cabinet

357mag

New member
I was considering picking up a Marshall Valvestate 150 watt guitar head, and using it with my 8 ohm Marshall cabinet. The minimum ohm rating of the head is 4 ohms. I know it's safe to do so, since one fellow at Marshall already told me that's okay, plus in the manual they discuss which jack to use if you are going to be playing an 8 ohm cabinet.

Thing is...what about wattage loss and volume loss? If I use a 4 ohm head on top of an 8 ohm cabinet, is the amp effectively only putting out 75 watts then? And then I will lose some punch and volume?

Is there a reason hybrid amps and all solid-state amps don't have impedance switches like tube amps do?
 
A 4 ohm output head to a 8 ohm speaker cab should pose no problems, might even improve your sound. You should notice only a slight volume drop and should notice better clean tones. The volume drop may be so slight that you wont notice it.
 
A 4 ohm output head THAT IS SOLID STATE to a 8 ohm speaker cab should pose no problems, might even improve your sound. You should notice only a slight volume drop and should notice better clean tones. The volume drop may be so slight that you wont notice it.

fixed.

while most tube amps can also tolerate impedance mismatches, there are a few that cannot, and said mismatch may cause OT to release magic smoke.

a
 
And then I will lose some punch and volume?
......
Is there a reason hybrid amps and all solid-state amps don't have impedance switches like tube amps do?

yes. you will lose volume, as a higher impedance load wont be drawing as much current. in this example, this reduced current should result in lowered volume (speaker dependent). OTOH, speaker efficiency can (a) vary wildly, and (b) can contribute to SPL more than sheer amp power.

for example, (and it isnt a great one, but it proves the principle) i happen to know someone who went to a "real" sub in his car. he had been using an efficient peavey black widow that he had "inherited" (which i was able to snag from his "upgrade"). the kicker sub he installed had sensitivity of like 87dB, while the BW is more like 98dB. guess what? the "old-assed" peavey was much louder, and truly, played almost as low as the sub in his box. i am still using that speaker today.

also, since as impedance increases, current is reduced, and therefore will actually cause your solid-state amp to run cooler. SS stuff only struggles when you decrease impedance below the minimum. actually, lots of amps can handle that for a while---- but the increased current demand puts more stress on the output section, and eventually will cause it to fail (sometimes sooner rather than later). on that note, i used an old big-knobbed peavey CS-800 at 2 ohms stereo fro literally years with not a single hiccup (rated @ 4 ohms min).


a
 
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since one fellow at Marshall already told me that's okay, plus in the manual they discuss which jack to use if you are going to be playing an 8 ohm cabinet.

The volume will not necessarily be reduced significantly. If the head has separate jacks for 8 and 4 ohms and uses an output transformer at the end of the power amp stage, then the 8 ohm output could be of a separate tap on the transformer's secondary side that outputs a higher voltage to compensate for the higher impedance. Such multi-tap arrangments were common on older tube amps -- I don't know about today's stuff.

Tom
 
If it's a tube amp, plug the cabinet into the 8 ohm output. That is the way it was designed to work. The only way you would notice a drop in volume is if you generally run the thing flat out into a 4 ohm cabinet. The difference would be (at most) 3db spl.

If it's a solid state amp, you have no worries. 4 ohms in the minimum load, meaning that is the limit, don't go below that. Anything above will work just fine.
 
The only way you would notice a drop in volume is if you generally run the thing flat out into a 4 ohm cabinet.

wait, what?

the only scenario where this would be true is running a 4 ohm cab off of the 8 ohm tap of a tube amp. with a solid-state amp, the 4 ohm load would draw approximately 30% more current, and all other things equal, will be significantly louder than an 8 ohm load. 3dB is a lot; all things considered.

his valvestate amp is definitely solid-state.

a
 
If you are still concerned, open up the speaker cab and reconfigure the wiring (series/parallel) to make it a 4 ohm cab. Perceived volume is often more a matter of speaker efficiency rather than the power handling rating. If your amp has multiple speaker outs, use the one which matches your speaker cab impedance.

Another alternative would be to plug another (8 ohm) speaker cab into the parallel speaker jack on the speaker cab, thereby making it a 4 ohm system. If you don't need the extra cab for your up front sound, use it for a monitor so the drummer or bass player can hear you better.
 
If you are still concerned, open up the speaker cab and reconfigure the wiring (series/parallel) to make it a 4 ohm cab.

cant.

no way to wire an 8 ohm 4x12 into 4 ohms (at least not without something unorthodox like a big 8 ohm resistor). 2, 8 , or 32.

a
 
I do not play through a 4 x 12. My 1936 is a 2 x 12. I think 4 x 12 cabinets are overkill for average bars.
 
I do not play through a 4 x 12. My 1936 is a 2 x 12. I think 4 x 12 cabinets are overkill for average bars.
An 8 ohm 2x12 cabinet can not be wired for 4 ohms either. If it's two 4 ohm speakers, the only other alternative is 2 ohms. If it's two 16 ohm speakers, the other alternative is 32 ohms.
 
I don't know what ohm the speakers are in my cabinet. But even if they could be wired to produce a 4 ohm cabinet, I would probably leave the cabinet alone, in case I want to revert back to an 8 ohm cabinet to use with my all tube head.

Although I think my Marshall head does have an ohm switch so I could use that head with a 4 ohm cabinet also.
 
come-on guys it's a moot point for the most part... its a solid state head.... as long as it's notlower than 4ohms the head dont care... could be 108ohms...
 
It's a solid state head, it doesn't care about the impedance as long as it is above 4 ohms.

It is not a 4 ohm head it just can't handle anything below 4 ohms.
 
Ampeg VT-60

This is kind of on topic to this...

I don't quite follow how ohms works vs. watt power. My Ampeg VT-60 (60 watt combo tube amp) has 5 different speaker plugs in the back, one for a single 16ohm speaker, two for 2 x 16ohm speakers and then 2 x 8ohm speakers. Since I only have one speaker in the amp I've always just left it as it came, which is being plugged into the single 16ohm plug. I don't know what the specs on the speaker are right now, but would I gain anything in volume by using one of the other speaker jacks? Is there anything I can do to gain a little more volume by setting it up differently?

Another question for you...

What will the Pentode/Tritode switch do to the sound?

and what will the High dampening/Low Dampening switch do? There is a presence rotary dial but it looks like it is only functional with high dampening on via a line graphic drawing on the amp.

Thanks

P.S. my power tubes are mis-matched right now and the replacement tube is unbiased I think, so is there anything I should be careful with changing that could throw it over the edge? So far it plays fine. I'll fix this soon with new tubes.
 
This is kind of on topic to this...

I don't quite follow how ohms works vs. watt power. My Ampeg VT-60 (60 watt combo tube amp) has 5 different speaker plugs in the back, one for a single 16ohm speaker, two for 2 x 16ohm speakers and then 2 x 8ohm speakers. Since I only have one speaker in the amp I've always just left it as it came, which is being plugged into the single 16ohm plug. I don't know what the specs on the speaker are right now, but would I gain anything in volume by using one of the other speaker jacks? Is there anything I can do to gain a little more volume by setting it up differently?
No. With a tube amp it is very important to match the output of the amp to the impedance of the speaker. Your speaker has to be 16 ohms or else you would have blown the output transformer by now.

The easiest thing you can do to get more volume is to get a 4x12 cabinet. I would suggest a 16 ohm cabinet to match the speaker in the combo. You would them plug both of the cabinets into the 8 ohm jacks. (because two 16 ohm cabinets running together is 8 ohms total)


What will the Pentode/Tritode switch do to the sound?
it will change the distortion characteristic.

and what will the High dampening/Low Dampening switch do? There is a presence rotary dial but it looks like it is only functional with high dampening on via a line graphic drawing on the amp.
These things just change the way the amp responds. You will need to experiment to find what is right for you.

P.S. my power tubes are mis-matched right now and the replacement tube is unbiased I think, so is there anything I should be careful with changing that could throw it over the edge? So far it plays fine. I'll fix this soon with new tubes.
Get a set of matched tubes as soon as possible. That will probably take care of your volume problem. Having mis-matched tubes is a no-no. It's OK to do to get you through a show, it shouldn't ruin anything, but the amp will not sound as good as it could. Get matched tubes and get the amp biased...In that order.
 
A lot of my questions are answered here, thanks.

But there's one option that wasn't talked about.

If my SolidState Laney Linebacker 50 reverb (from the 80's) head has 2 speaker outputs rated at 4 Ohm. Still don’t how they are setup exactly, parallel…I’m waiting for a response from Laney.

And my cabinet is a Marshall MG412B, 120W RMS, 4 x Celestion 12" 8Ohm plugged in the serie/parallel setup. wich gives an input impedance of 8 Ohm.

Ok I now it should be fine. But? The thing his, with my new LesPaul baritone, the added low-end and a Big Muff Pi engaged, it makes my Celestions sounds like they work too much so I lowered the bass boost and the extra gain of the head and manage to get something more balance.

But what if ad an extra mono plug on the cabinet and hook-up two Celestion 12" 8 Ohm in parallel to it?
In other words, one parallel pair per plug. wich gives two 4ohm inputs, right?

So then i can plug the two 4 Ohm output of the head in the two 4Ohm input of my cabinet...wich will give me the max I can get from my cabinet.

I read it would clear-up the sound with less low-end…but more definition.
I have plenty of low-end now so it’s not an issue.

what do you guys think?

The more $$$ option, would be to get a hand on a full-size cabinet like a Marshall 1960
or Laney GS412 or the best, a Mesa Rectifier! All these have GT12-75W or Vintage V30(70W) Celestion instead of my 30W

thanks for any help!
 
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