4 ohm parallel amp in to 8 ohm cabinet

[iamtetsuo]

Hi guys...

I recently got a great deal on a second hand Randall RH300 G3 and RS412 XJM cab... Both in immaculate condition, that's only had 1 owner, never left the bedroom, never gigged, never turned up past 1etc. Up to now I've always used 1x12 combi amps but I couldn't pass up the deal, although I'm new to this head and cab thing.

Anyway, I set up the amp and cab, got a great tone and was really happy. I took the rig to rehearsal recently and realised that the cab only has an 8 ohm input with no option for a 4 ohm input... so I've been running a 4 ohm parallel amp output in to an 8 ohm cab input all this time.

Will this cause any major/immediate damage to my amp? The back of the amp states 300 watts @ 4 ohms and 200 watts @ 8 ohms... This implies that the player has the option of (ideally) 4 ohms or (not ideal) 8 ohms. However, If putting the amp to a 8 ohm cab was dangerous, you'd think they wouldn't suggest it on the back of the item.

Returning the rig is not an option so I just need to know if this is a major issue or just a non-ideal setup. I'm getting a great tone and it's damn loud, even if it's not running at full capacity.

Can anyone shed some light on the issue?

Ps. I'm aware that it's not the best amp and cab combo Randall (or any other manufacturer) has to offer, but I'm not the greatest guitar player either. Like I said, I'm new to all this and it was a great deal.

Cheers

 

[GoldenVulture]

Re: 4 ohm parallel amp in to 8 ohm cabinet

The back of the amp states 300 watts @ 4 ohms and 200 watts @ 8 ohms... This implies that the player has the option of (ideally) 4 ohms or (not ideal) 8 ohms.

It's a solid state amp. They're telling you it is quite okay to run an Eight Ohm cab. It just chops the output wattage back. That's the way solid state output works.

 

[iamtetsuo]

Re: 4 ohm parallel amp in to 8 ohm cabinet

It's a solid state amp. They're telling you it is quite okay to run an Eight Ohm cab. It just chops the output wattage back. That's the way solid state output works.

Great! Thanks for letting me know.

 

[devastone]

Re: 4 ohm parallel amp in to 8 ohm cabinet

You could run a 16 ohm cab on it, it would probably cut the output down to around 100W, but it wouldn't hurt it nor have much effect on the sound.

 

[Demanic]

Re: 4 ohm parallel amp in to 8 ohm cabinet

You could run a 16 ohm cab on it, it would probably cut the output down to around 100W, but it wouldn't hurt it nor have much effect on the sound.

That's what I'm doing with my Randall RH200 and it sounds fine. Though may get an 8 ohm cab for it at some point.

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[Sirion]

Re: 4 ohm parallel amp in to 8 ohm cabinet

Yes, this is one of the great advantages of solid state: you can use speaker impedance to scale your rig. With a 2x12 (or 4x12) that can run at either 4 ohm or 16 ohm, the rig suddenly works both for smaller and larger gigs.

 

[Securb]

Re: 4 ohm parallel amp in to 8 ohm cabinet

The rule of thumb to remember when matching cabs and amps for ohms is. Think of the cab as a glass and the amp as a picture of water. You can pour 4 ounces (ohms) into a 8 ounce (ohm) glass. If you try to pour 16 ounces (ohms) into an 8 ounce (ohm) glass disaster will happen.

 

[Sirion]

Re: 4 ohm parallel amp in to 8 ohm cabinet

The rule of thumb to remember when matching cabs and amps for ohms is. Think of the cab as a glass and the amp as a picture of water. You can pour 4 ounces (ohms) into a 8 ounce (ohm) glass. If you try to pour 16 ounces (ohms) into an 8 ounce (ohm) glass disaster will happen.

With solid states, this sounds like a recipe for a very expensive repair.

 

[Demanic]

Re: 4 ohm parallel amp in to 8 ohm cabinet

If you run a 4 ohm SS into an 8 ohm cab, all you should see is a drop in volume from what you would get into a 4 ohm cab. If you do the opposite and run it into a 2 ohm cab, then you're asking for trouble.
From what I know, tube amps run the opposite to SS.

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[Sirion]

Re: 4 ohm parallel amp in to 8 ohm cabinet

That is my understanding too. Switch the roles, and the metaphor works well for solid state – better, possibly, than it does for tube amps, since solid state power amps colour the sound far less.

 

[Little Pigbacon]

Re: 4 ohm parallel amp in to 8 ohm cabinet

Solid-state amps are usually modeled as non-ideal voltage sources with substantial output impedances. If you look at that output impedance, it's in series with the load impedance of the cab, so it makes a voltage divider for the output section to fall across.

When the cab's impedance is high compared to that of the amp's output impedance, most of the power generated by the amp will be dissipated in the speaker load. This is very safe for the amplifier, but with such a high load impedance, it might not be generating as much power as it could.

When the cab's impedance is low compared to that of the amp's output impedance, more power is actually dissipated in that output impedance than in the speaker load. This is a bad situation for the amp, because it's putting more power into heating up its own output section than it's delivering to the speaker cabinet. This is moving in the direction of running the amp into a "short", which would be the ultimate case of a speaker load with too low an impedance.

The best situation is usually when the speaker load and the amp's output impedance are about the same. That's when you get optimal power delivery to the speaker cabinet. In general, all this is why we like solid-state power amps to have low output impedance and the ability to deliver high currents into low-impedance loads.

 

[Demanic]

Re: 4 ohm parallel amp in to 8 ohm cabinet

Right. But SS amps tend to handle mismatches with a higher load cab. Tube amps don't like higher load mismatches. Too much of one, like 4 ohms into 16 ohms could fry the OT.

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[Little Pigbacon]

Re: 4 ohm parallel amp in to 8 ohm cabinet

Right. But SS amps tend to handle mismatches with a higher load cab. Tube amps don't like higher load mismatches. Too much of one, like 4 ohms into 16 ohms could fry the OT.

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Yes. SS is safe with a higher impedance speaker load. I don't understand power tubes or output transformers well enough to want to mess around; I always make the impedances match.

 

[Demanic]

Re: 4 ohm parallel amp in to 8 ohm cabinet

With tubes you can run a slight (1 or 2) lower ohm mismatch. But if you go higher than what the OT wants to see, it tries to squeeze more power through to complete the loop at the same ohmage. It generally doesn't try for too long.

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[Securb]

Re: 4 ohm parallel amp in to 8 ohm cabinet

With solid states, this sounds like a recipe for a very expensive repair.

I said rule of thumb not law of physics. With that being said anytime you mismatch ohms in heads and cabs with tubes or solid state it is a crap shoot. My bass player has a 16-ohm vintage tube head he has been running into an 8-ohm cab at high volumes for years. He has never run into a problem and it sounds great.

 

[GoldenVulture]

Re: 4 ohm parallel amp in to 8 ohm cabinet

Solid State outputs scale themselves to the load attatched so pretty much any Ohmage 4 Ohms and above will work safely. 2 Ohms and under in most cases will short out the outputs.
As it was formerly stated in the OP

The back of the amp states 300 watts @ 4 ohms and 200 watts @ 8 ohms... the player has the option of 4 ohms or 8 ohms.

The obvious equation here is "as the load [ ohmage of speakers ] increases , Power Output decreases ".

 

[Sirion]

Re: 4 ohm parallel amp in to 8 ohm cabinet

I said rule of thumb not law of physics.

And anybody with a solid state amp (like the OP) is likely to harm their gear if they follow your "rule of thumb". Do the exact opposite of what you wrote, and solid states will be fine. It really is that simple.

With that being said anytime you mismatch ohms in heads and cabs with tubes or solid state it is a crap shoot.

Having higher ohmage cabs is very common with solid states; it is in fact an easy way to scale the size of your rig. Most solid state amps just give a maximum [EDIT: minium] ohmage, usually but not always 4 ohm.

 

[Little Pigbacon]

Re: 4 ohm parallel amp in to 8 ohm cabinet

Minimum.

 

[Sirion]

Re: 4 ohm parallel amp in to 8 ohm cabinet

I made myself guilty of the same mistake I was chastising Securb for there, didn't I?

 

[Securb]

Re: 4 ohm parallel amp in to 8 ohm cabinet

I made myself guilty of the same mistake I was chastising Securb for there, didn't I?

Chastising? I was referring to tube amps I should have made that clear. Yes, I know the OP has a solid state. If I misspeak and you correct me that is fine. That is how we all learn. If you think you are chastising me, you have a lot to learn about me. Let's hope that doesn't happen.