Ohm question

[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.  

[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.

[Hornisse]

Thanks for all the input guys!  It is a Peavey Tour 410 cabinet.  Not sure it can be rewired to 4 ohm.  Will probably get another 8 ohm cab or try to find a good 4 ohm 410 cab. 

[hollowbody]

You should be able to rewire the cabinet to 2 ohms, but you wouldn't be able to use it with a lot of other amps.  I have a diagram on the 'puter somewhere that details how to rewire and add a switch to go between 2 and 8 ohms.  I'll dig it up for ya.

EDIT: You could only rewire the cab for this IF the each speaker is 8 ohms.  You'll have to pull one and measure it.

[Hornisse]

That would be great!  I really like the cabinet and have used it a lot with other heads.  This is the first amp head I've used with it that weighs more than the cabinet!

[hollowbody]

This is the diagram I saved when I was going to modify my Acoustic 408 cab.  I can't bear to make any mods to it so it will stay 2 ohms.

Note that this will only work if your speakers are individually rated at 8 ohms.  You should be able to contact Peavey with the model number and I'm sure they could tell you.

[Chris P.]

A lot of times the amount of Ohms is on the back of the speaker.

[Hornisse]

Thanks for the cool diagram.  I may wire it without the switch.  I'm going to pull a speaker tonight and see what it measures. (if it is not written on back of the speaker)  I'm interested to see what the Peavey Neo speakers look like.