I ran my amp at 4ohms into an 8phm cab whoops

panama · #1 09-13-2010, 11:36 AM

So this weekend I jammed with a new band and rather than haul my cabs one of the dudes said I could use his Emperor 4x12, which is rated 8ohm. I forgot to switch my VB-2 from 4 to 8 ohms. The cabinet didn't fart or blow or anything, which was a relief, but I did notice my head was pretty hot after about an hour of jamming. No harm no foul? Which piece of equipment would have got the short end of the stick if something went wrong? Did I shorten the life of my tubes?

Spinal Tapper · #2 09-13-2010, 11:38 AM

if it didn't crap out, you'll probably be ok. just remember for the future

perogato02 · #3 09-13-2010, 11:40 AM

I am a newbie on tube grounds, but I think you just shortened the life of your power tubes. Maybe some potentional harm to your trannies?

panama · #4 09-13-2010, 11:41 AM

Yeah I will. I don't have the cash to be replacing someone;s blown speakers!

That said - the Emperor sounded pretty incredible, but I like the tightness of my 10s.

panama · #5 09-13-2010, 11:45 AM

Quote:

Originally Posted by perogato02

I am a newbie on tube grounds, but I think you just shortened the life of your power tubes. Maybe some potentional harm to your trannies?

Well, after we jammed and I had turned off my amp and realized what I had done I promptly switched it to 8 and turned it back on. Everything seemed to be in fine working order. If I have to replace my power tubes sooner as a result then oh well.

perogato02 · #6 09-13-2010, 11:52 AM

Quote:

Originally Posted by panama

Well, after we jammed and I had turned off my amp and realized what I had done I promptly switched it to 8 and turned it back on. Everything seemed to be in fine working order. If I have to replace my power tubes sooner as a result then oh well.

Then no harm done!

panama · #7 09-13-2010, 12:08 PM

I'm new to the tubes too

Grooveman1961 · #8 09-13-2010, 02:56 PM

No harm done

You didn't hurt anything. You ran at lower power and slightly reduced freq response, thats all.

rickdog · #9 09-13-2010, 09:04 PM

The thing about tube amps that causes problems with mismatched impedances is that tubes are current output devices. The signal voltage into a tube (on the grid) controls the current through the tube (plate current); within limits, the plate current on output tubes is going to be what the grid says it should be, no matter what voltage that causes on the plate.

What this means is that if the output impedance is too high, the output voltage will also be too high. (Ohm's Law: voltage equals current times resistance, if the resistance/impedance goes up and the current is constant, the voltage goes up). This can exceed the maximum plate voltage rating of the output tube. If you don't exceed it by too much, or for too long, you won't cause serious damage (tubes are a lot more forgiving this way than transistors).

I would expect that if the amp has some negative feedback from the secondary of the output transformer (which is pretty commonly the case), you wouldn't even have this problem - you'd just lose some output power because the feedback reduces the current to try to hold the voltage constant.

In extreme cases, the voltage can get high enough to break down the insulation in the output transformer windings, causing permanent damage. Or it can arc over from the plate terminal on the tube socket to the chassis. I once repaired a tube amp that had been run with no speaker (open circuit) that had burned up the sockets this way.

rbbrchkn · #10 09-13-2010, 09:32 PM

A properly built tube amp should be able to handle a load 100% above or below the proper selected impedance. (2 or 8 ohms if set to 4 ohms, etc.) Not the best thing to do, especially if you're running it hard, but you can run it like that for an extended period of time and be totally ok.

JimmyM · #11 09-13-2010, 10:42 PM

ya, for that short a time, i doubt you did anything to your amp at all, including your power tubes. if you did shorten their life, you probably didn't shorten them by much at all.

dmusic148 · #12 09-13-2010, 10:46 PM

Quote:

Originally Posted by rbbrchkn

A properly built tube amp should be able to handle a load 100% above or below the proper selected impedance. (2 or 8 ohms if set to 4 ohms, etc.) Not the best thing to do, especially if you're running it hard, but you can run it like that for an extended period of time and be totally ok.

QFT. Look it up here in 'Amps, Ohms,' etc.

Destructobot · #13 09-14-2010, 12:27 AM

You won't hurt your amp running with too much load on it. It'll just sound bad. The upside is that once you realize what's wrong, it'll sound HUGE when you switch the impedance to the right ohmage.

panama · #14 09-14-2010, 08:07 AM

I didn't think it sounded bad though! haha