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Ohm question

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Hornisse:
I have an Ampeg SVT-2 Pro which is rated for 4 or 2 ohm loads.  I have an 8 ohm cabinet.  Is it safe to run the amp into an 8 ohm load?  Thanks for any help!

chromium:
Wow just looked that thing up - that's a monster head!

When I got my Orange several years back, I had found some helpful info on their forum.  I couldn't locate the original post, but I found it quoted and being discussed on some other forum:

http://www.bossarea.com/forum/topic.asp?TOPIC_ID=3960

There's some good info in there, but to paraphrase it:

Solid State amps: Safe with rated load or with any higher impedance speaker load (up to & including an open circuit ie infinite ohms.) The amp will develop less power as the impedance increases.  Do not short circuit (0 ohms) as this spells sudden death for the output transistors, but they'll sit happily all day with no speaker load applied.

Valve amps: Match impedance if possible. If mismatching, it is safer for the amp to mismatch low (e.g. connecting 2-ohm speaker load when 4-ohm is the stated minimum). This will wear the valves but the amp shouldn't suffer. A short circuit (0 ohms) is normally survivable.  It's the circumstance where you try to put a signal into an open circuit (i.e. not connecting any speaker load, creating virtually infinite ohms) that is a real killer.


Based on that, it doesn't sound like it would be advisable to run an 8 ohm load there.

What kind of cab do you use?  Depending on the impedance of the speakers, it might be possible to rewire it to match the impedance requirements of the head.  Either that, or it gives you an excuse to buy a second cab.   ;D

Dave W:
No, it's not safe. Risk of burning out the output transformer.

Highlander:
What's in the cab...? possible to rewire if the values are right...

Dave W:
I'm no amp tech, this is just what was explained to me: that it's possible to run a mismatch up to 2x either way, but far more dangerous to run at too high impedance. It can cause flashback voltages in the output transformer that can kill it.

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