4ohm amp with 8 ohm cab?

Bloc Riff Nut · #1 03-23-2011, 10:29 AM

Can I connect my Trace Elliot V6 head (40hm 300w) to my Markbass CMD210P (8ohm 400w). I will disconnect the markbass's amp.

If I understand correctly the V6 will work fine it will just give out less power due to the higher impedance, *ohm instead of 4 ohm.

Have I got it right or am I about to blow something up?

Yours,
Phil.

billfitzmaurice · #2 03-23-2011, 10:33 AM

What does your manual say? SS and tube are different, so you must be sure, and your manual will say.

Bloc Riff Nut · #3 03-23-2011, 10:45 AM

good point, I didn't think about the difference between SS and Tube.

BigOldHarry · #4 03-23-2011, 10:51 AM

We really need a sticky on this. Really.

The number on the amp - "4 ohm minimum load" mean that the amp needs *at most" a 4 ohm load to operate safely. This is where folks get confused: An 8 ohm cab is *less* of a load than a 4 ohm cab. Add two 8ohm cabs and you get a 4 ohms of load. I don't have the time to re-tell what has been told many many times before about Ohm's law, etc - just trust me: Most amps can handled a 4 ohm load at most; a single 8 ohm cab is fine, two 8 ohm cabs is fine. Two 4ohm cabs, however, makes a 2 ohm load. That would NOT be fine unless you amp is rated to handle a 2 ohm load.

Does that help?

Selta · #5 03-23-2011, 10:57 AM

Quote:

Originally Posted by BigOldHarry

We really need a sticky on this. Really.

The number on the amp - "4 ohm minimum load" mean that the amp needs *at most" a 4 ohm load to operate safely. This is where folks get confused: An 8 ohm cab is *less* of a load than a 4 ohm cab. Add two 8ohm cabs and you get a 4 ohms of load. I don't have the time to re-tell what has been told many many times before about Ohm's law, etc - just trust me: Most amps can handled a 4 ohm load at most; a single 8 ohm cab is fine, two 8 ohm cabs is fine. Two 4ohm cabs, however, makes a 2 ohm load. That would NOT be fine unless you amp is rated to handle a 2 ohm load.

Does that help?

I'm 95% sure the V6 is a tube amp, and hence, a totally different ball game. And there IS a sticky on it, but basically no one reads it.
AMPS FAQ!! Info on OHMS, Allsize RIGS-OverUnderPowerCabs DIY TechTalk-Links

tdub0199 · #6 03-23-2011, 10:57 AM

^ This....

BassmanPaul · #7 03-23-2011, 10:58 AM

The V6 is a tube amp with taps for 2Ω and 4Ω. I would advise against using this amp with an 8Ω load. It's always best to match the load with the tap.

Downunderwonder · #8 03-23-2011, 11:14 AM

Trust Bill, not Harry.

And there is way too much info in that sticky, yet I see no thread linked on under loading a valve amp.

dmusic148 · #9 03-23-2011, 11:22 AM

Quote:

Originally Posted by Ray Salamon

I'm 95% sure the V6 is a tube amp, and hence, a totally different ball game. And there IS a sticky on it, but basically no one reads it.
AMPS FAQ!! Info on OHMS, Allsize RIGS-OverUnderPowerCabs DIY TechTalk-Links

And in it you will find the following:

"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."

Selta · #10 03-23-2011, 11:38 AM

Quote:

Originally Posted by dmusic148

And in it you will find the following:

"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."

I never bought into that myself. There's too many things that can go wrong with a mismatch impedance that I'd rather not risk my tube amps to it. It's just as easy to buy the correct gear in the first place, and you don't risk ruining an OT, power tubes, etc. in the process.

T-Bird · #11 03-23-2011, 11:54 AM

Hi.

Quote:

Originally Posted by dmusic148

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."

Reasonably safe = busy techs.

Fortunately/unfortunately, take Your pick.

Regards
Sam

okcrum · #12 03-23-2011, 01:53 PM

Meh. Most folks who advise against it (Paul is being safe in his recommendation, though) haven't ever run a mismatch for any length of time. What I do know is that it hasn't been a problem running 8 ohms from a 4 ohm tap on an SVT for me. Nor was it a problem on the Traynor head I used before, or the Bogen MO-100A power amp I used before that.

A sufficiently well made amp won't have a problem, other than getting less power and having a slightly different frequency response. An amp with marginal output transformer or which runs its tubes at or near their limits for plate voltage or power dissipation can get into trouble at high drive levels. Your best data would come from the experience of other Trace V6 owners.

GrowlerBox · #13 03-23-2011, 02:08 PM

Quote:

Originally Posted by BigOldHarry

We really need a sticky on this. Really.

The number on the amp - "4 ohm minimum load" mean that the amp needs *at most" a 4 ohm load to operate safely. This is where folks get confused: An 8 ohm cab is *less* of a load than a 4 ohm cab. Add two 8ohm cabs and you get a 4 ohms of load. I don't have the time to re-tell what has been told many many times before about Ohm's law, etc - just trust me: Most amps can handled a 4 ohm load at most; a single 8 ohm cab is fine, two 8 ohm cabs is fine. Two 4ohm cabs, however, makes a 2 ohm load. That would NOT be fine unless you amp is rated to handle a 2 ohm load.

Does that help?

This is slightly confusing, although the sense of it is basically correct: in this context, resistance, impedance and load are more-or-less interchangeable, so increasing the impedance (from 4 to 8 ohms, say) is indeed increasing the load on the source (power output). Reducing the load (from 8 to 4 ohms, say), will increase the current passing through the transformer (or its equivalent), which may be sufficient to damage it, if there is not enough load on the power source to reduce the current.

NB: this is all reasonably straightforward in the case of SS amps; for tube power sections, it's more important to match the impedances within the limits specified by the manufacturer, as noted by most of the above.

At least, this is as far as I understand within my (increasingly apparent) limitations.

greenboy · #14 03-23-2011, 02:12 PM

We need a stickie on reading the stickies.

billfitzmaurice · #15 03-23-2011, 02:58 PM

Quote:

Originally Posted by greenboy

We need a stickie on reading the stickies.

Waste of bandwidth, nobody would read it.

BigOldHarry · #16 03-23-2011, 03:04 PM

Quote:

Originally Posted by Downunderwonder

Trust Bill, not Harry.

And there is way too much info in that sticky, yet I see no thread linked on under loading a valve amp.

Yes, you are right. Tube amps are *magical* and don't obey Ohm's law.

My mistake...

Selta · #17 03-23-2011, 03:08 PM

Quote:

Originally Posted by BigOldHarry

Yes, you are right. Tube amps are *magical* and don't obey Ohm's law.

My mistake...

No one said they don't. But what you stated is factually incorrect for tube amps. If that amp was "OK" from factory with an 8 ohm cab, it would have a toggle for the 8 ohm tap on the O.T. It doesn't. Still Ohm's Law, however, it's application to tube amps is much different than SS.

lem8r · #18 03-23-2011, 04:23 PM

The only true way is to draw a load line for that particular output section when 8 ohm load connected to 4 ohm tap and see is it safe or not.

Selta · #19 03-23-2011, 04:30 PM

Quote:

Originally Posted by lem8r

The only true way is to draw a load line for that particular output section when 8 ohm load connected to 4 ohm tap and see is it safe or not.

Hope you have a good tech around when you try this

aprod · #20 03-23-2011, 04:36 PM

Please listen to Harry. He is telling you the truth. Solid State or tube no difference in regards to ohm's law. If you put a 2 ohm load on a amp rated at 4 ohms you will fry the amp.