Im[edance question: 4 ohm load on an amp set to 8ohms.

Lenny · #1 03-03-2011, 11:05 PM

Set up my rig at the new rehearsal spot tonight. Flipped the head on to standby to let it warm up for a bit. About ten minutes later I had a look at the back of the head and noticed it was set for 8 ohms. I had two 8 ohm cabs plugged in.

So I know I had a 4 ohm load on the amp, and I noticed before it came off standby and I played tonight. I'm not sure if it was set to 4 or 8 the last time we jammed though. I'm usually pretty cautious when it comes to making sure it's set to 4 ohms.

Anyway, what would this do to the amp? Anything if it was only on standby?

Thanks in advance, Lenny

T-Bird · #2 03-03-2011, 11:30 PM

Hi.

If it was only on standby, nothing bad can happen.

Regards
Sam

Lenny · #3 03-03-2011, 11:42 PM

Thanks for the reply.

What could potentially happen if I'm wrong about the switch being at 4 ohms the last time I used the head? It was used for a couple of hours at half volume or so.

Lenny · #4 03-04-2011, 06:22 AM

Does the amp explode into a million pieces? Or does it just stop functioning? Would it still work, but at a reduced output? Would it still work, but change the tonal charecteristics?

I read the FAQ sticky on impedance and did a search, but I can't find the answers to these specific questions.

Thanks, Lenny.

username1 · #5 03-04-2011, 06:33 AM

It wont explode on you but it does put an extra strain on the transformers and tubes.

billfitzmaurice · #6 03-04-2011, 06:39 AM

Quote:

Originally Posted by Lenny

What could potentially happen if I'm wrong about the switch being at 4 ohms the last time I used the head?

Nothing. I assume this is a tube amp. Tube amps are perfectly happy with loads half that of the tap rating; what they don't care for is higher impedance loads. With SS the opposite is true.

Lenny · #7 03-04-2011, 06:41 AM

No, its the amp in my sig. Terror Bass 500. What effect would straining the transformer and tubes have? Sounds to me like the sound quality could be changed/degraded by straining the tubes?

billfitzmaurice · #8 03-04-2011, 07:07 AM

Quote:

Originally Posted by Lenny

No, its the amp in my sig. Terror Bass 500. What effect would straining the transformer and tubes have? Sounds to me like the sound quality could be changed/degraded by straining the tubes?

I don't know what that amp is, tube or SS. I assume you do.
With a tube amp if you run with a higher impedance load on a regular basis eventually you might damage the output transformer. With a lower impedance load, no worries. Within reason, of course, I wouldn't run 2 ohms on an 8 ohm tap.
With SS it's the other way around. Run too low a load and the output stage will overheat, and the protection circuits will shut the amp down. There's no such thing as too high a load with SS, the amp just won't drive the speakers to full output.

Lenny · #9 03-04-2011, 07:16 AM

Sorry, its a hybrid, tube pre and SS power amp. 500 Watts at either 4 or 8 ohms. There's a little switch on the back that let's you select the load.

billfitzmaurice · #10 03-04-2011, 07:21 AM

Quote:

Originally Posted by Lenny

Sorry, its a hybrid, tube pre and SS power amp. 500 Watts at either 4 or 8 ohms. There's a little switch on the back that let's you select the load.

Quote:

With SS it's the other way around. Run too low a load and the output stage will overheat, and the protection circuits will shut the amp down.

That.

Lenny · #11 03-04-2011, 07:31 AM

Thanks! Its never shut down on me, so maybe the switch wasn't on 8 ohms last time I used it. The reason i'm asking all the questions is because my sound doesn't seem to be as full and rich as it used to be. Only noticed this the last two practices, so I thought maybe the improper load (if there even was one) might be the cause. This rig blew me away when I first got it. The last couple of rehearsals it just seemed to lack warmth and output. Put new string on just to eliminate that possibility, but same thing.

Could the tubes have something to do with that? Pretty general questions I know, but I seem to be in some kind of tone hell at the moment. The amp is only four months old and its never been pushed really hard.

Thanks, Lenny.

Gar Bear · #12 03-04-2011, 08:45 AM

I suggest cleaning every input and output jack with a spray cleaner/lube. You can use the one available from Radio Shack even. Don't over do it. After you spray, excercise the jack with the appropriate plug. I could be some crud or oxidation has built up and is effecting your tone adversely.

Treat your bass too and all cable ends. Hope this helps.

AwkwardLoudness · #13 03-04-2011, 09:16 AM

You can damage the output transistors if you play long enough through cabs with total impedance less than the selected (or fixed) minimum impedance of the amp. If cab(s) impedance is higher than the amps minimum impedance, then the amp's output power is lowered but does not cause harm.

Lenny · #14 03-04-2011, 09:34 AM

So two hours of playing with the wrong impedance isn't likely to have caused any lasting damage? I think i'm barking up the wrong tree here. I'm pretty sure the second last time I jammed the amp was set to four ohms (where it should be). Like I said before, last night it was set to 8 ohms, but I noticed while it was still warming up on standby.

I suppose a good cleaning of all the cables and pots wouldn't hurt for sure.

lbwdog · #15 03-04-2011, 09:55 AM

What's the point of an impedance switch on a solid state amp anyway?

billfitzmaurice · #16 03-04-2011, 10:33 AM

Quote:

Originally Posted by lbwdog

What's the point of an impedance switch on a solid state amp anyway?

If it was one I designed it would have two power amps series bridged for maximum voltage into 8 ohms or parallel bridged for maximum current into 4 ohms. I can't speak for Orange.

Quote:

You can damage the output transistors if you play long enough through cabs with total impedance less than the selected (or fixed) minimum impedance of the amp.

Possible, but extremely unlikely. Well designed amps have over current protection circuitry that will shut the amp down with plenty of margin to prevent that.

MikeyMelons · #17 03-04-2011, 02:02 PM

I'm fairly sure that amp can supply 500w at 4 or 8 ohms hence the impedance switch.

Lenny · #18 03-04-2011, 02:05 PM

It can for sure. Two 8 ohm cabs = 4 ohm load, but the amp was set to 8.

okcrum · #19 03-04-2011, 02:51 PM

Quote:

Originally Posted by Lenny

Sorry, its a hybrid, tube pre and SS power amp. 500 Watts at either 4 or 8 ohms. There's a little switch on the back that let's you select the load.

If it's the amp I'm thinking of, that switch just sets the limiter threshold for the power amp. If you didn't clip the amp, it made no difference.

In other experience, within 2:1 either direction of matched on a tube amp won't hurt a decently designed amp.