Turning a head on without a cab

mr_goodbomb · #1 07-16-2010, 01:39 AM

Stupid.

I just got a new SVT Classic. Went to practice, this was the second time I've used it. Turned it on, turned standby off, no sound. Hmm. Turned it off. Checked to be sure everything was plugged in. Turned it back on. No sound. Hmm. Check again. Turns out, the guitarist unplugged it from the cab so he could better reach the mini fridge. Ugh.

I was worried it had caused the amp harm. Practiced, it was fine, whatever. Played live tonight. Sounded fine. Half hour set, 3/4 through the last song, the bass entirely cuts up. We had to tear down and let the next band on, but I was worried. Didn't get to try it, it was so late when we got back the other guys unloaded and went home without me. So, I won't get to check for a few days.

Worst case scenario, if there was damage done when the drunk needed his beer, would it have worked for hours afterwards, and the just suddenly not?

blendermassacre · #2 07-16-2010, 01:40 AM

running a tube head without a load is a bad, bad plan.

spaz21387 · #3 07-16-2010, 01:57 AM

Quote:

Originally Posted by blendermassacre

running a tube head without a load is a bad, bad plan.

beat me to its very bad for tube heads. solidstate heads can handle it but its still not good to run a head on its own... looks like your guitarist just bought an svt cl then.

JimmyM · #4 07-16-2010, 02:20 AM

while it IS bad, there's an outside chance that it wasn't a transformer failure. but you can't blame the guitarist...he needed a beer. and you should check everything every single time you plug it in as a matter of course.

however, yes, it could be a tranny failure. if you melted the inside of it, it could have taken some extra time for it to fail completely. only one way to find out at this point...tech time.

johnk_10 · #5 07-16-2010, 03:09 AM

if you only turned it on it briefly and didn't crank it up loud, IMO, it's pretty unlikely that you damaged an output tranny. but nevertheless, when you get a chance to try it again, if it doesn't come on, or the bias lights aren't lit as per spec, it's time for a tech to have look at it.

Sartori · #6 07-16-2010, 03:46 AM

One thing is clear. This is cause for beating your guitarist to within an inch of his life. Or at least threatening to do so if he ever does something that stupid to your gear again.

Smokin' Toaster · #7 07-16-2010, 07:43 AM

At least smash his guitar against his head as a reminder not to do it again.

blendermassacre · #8 07-16-2010, 07:52 AM

considering the OP made a post right after this one, saying that his bass cab was 8ohms, I'm pretty sure he double whammy cooked the amp...

Grooveman1961 · #9 07-16-2010, 08:27 AM

turning it on with out a load plugged in will not harm it. Whatever caused the problem, its not that.

plugging an 8 ohm cab in won't do anything negative either.

StuartV · #10 07-16-2010, 08:31 AM

There are several posts on here explaining clearly that (and how) running a tube head (or at least an SVT tube head) with an 8 ohm load (when the head is only designed for 2 or 4 ohm loads) CAN damage it.

Big_Stu · #11 07-16-2010, 08:35 AM

Quote:

Originally Posted by Grooveman1961

turning it on with out a load plugged in will not harm it. Whatever caused the problem, its not that.

Having it switched on without a cab would do nothing, true. Putting a load on the amp by sending a signal to by trying to play through it, yeah that will, big time. I take it that's what the OP did when he says "no sound". He did it twice.

As far as whether anything is cooked even after lasting 90% of a gig, it's possible too. Think of it as twisting your ankle a bit while out walking, thinking "Mmm, it feels OK" & then going out & playing a game of football then wondering why you have to take yourself off the pitch early.

As said before; it's definitely tech time.

BassmanPaul · #12 07-16-2010, 08:38 AM

Quote:

Originally Posted by Grooveman1961

turning it on with out a load plugged in will not harm it. Whatever caused the problem, its not that.

plugging an 8 ohm cab in won't do anything negative either.

Sorry Grooveman but you are out to lunch on both these statements.

Paul

christw · #13 07-16-2010, 08:39 AM

Quote:

Originally Posted by StuartV

There are several posts on here explaining clearly that (and how) running a tube head (or at least an SVT tube head) with an 8 ohm load (when the head is only designed for 2 or 4 ohm loads) CAN damage it.

+1

Incorrect impedance is not good.

How long was it run without a cab?

blendermassacre · #14 07-16-2010, 08:43 AM

Quote:

Originally Posted by christw

+1

Incorrect impedance is not good.

How long was it run without a cab?

not very long, but I'm pretty sure he played a whole show with an 8 ohm cab...

christw · #15 07-16-2010, 08:47 AM

I understood that he used/uses an 8 ohm cab with it regularly? I know tube amps are resilient beasts, but still... I killed a tube guitar amp with incorrect impedance in my noob days.

Foz · #16 07-16-2010, 08:47 AM

Its all of a piece... toob heads have an output transformer that is intended to be matched with the load [the cab] when properly set up. Usually the OT has more than one tap so it can be used with more than one level of load. This is why toob heads usually have more than one speaker jack or some sort of switch [example:. a 400+ has six speaker jacks - a pair of jacks each for a 2 ohm, a 4 ohm or an 8 ohm total load]. When you have the wrong load [use the wrong jack or or run the head with no cab attached] the current through your power section runs wild because it has an imbalanced load [or no load] to work against.

Doing this - and keeping it up - will burn the OT, much like driving with your parking brake on - and that is an expensive failure. That said, turning the amp on without a cab attached and asking yourself "why the hell doesn't this thing make noise?" for a minute or two isn't usually a problem. OTOH running the head with an 8 ohm cab into a 4 ohm jack will make for a hard life - and sooner or later [probably sooner] that'll kill it.

Therefore I reckon there is no need to beat your guitard to death. Instead simply punch him in the face [once], and pour his beer out into the back of his amp and ask him how he likes that bowl of bitch slap soup.

jeff62 · #17 07-16-2010, 08:49 AM

Funny, Mesa Boogie support has directly told me that unmatched load, as long as it is higher than the amp, may shorten tube life but will otherwise cause no damage.

I ran a Boogie guitar amp for *years* with a 16-ohm load from the 8 ohm output and never had a problem. When I finally did need to replace a tube, it was just a microphonic pre-amp tube. Just sayin...

It wasn't until I started reading this stuff that I got paranoid.

blendermassacre · #18 07-16-2010, 08:53 AM

Quote:

Originally Posted by jeff62

Funny, Mesa Boogie support has directly told me that unmatched load, as long as it is higher than the amp, may shorten tube life but will otherwise cause no damage.

I ran a Boogie guitar amp for *years* with a 16-ohm load from the 8 ohm output and never had a problem. When I finally did need to replace a tube, it was just a microphonic pre-amp tube. Just sayin...

It wasn't until I started reading this stuff that I got paranoid.

there are some huge differences between bass and guitar tube amps. Mainly the fact that the output transformer is 2 to 3 times the size.

John D · #19 07-16-2010, 10:03 AM

Quote:

Originally Posted by jeff62

Funny, Mesa Boogie support has directly told me that unmatched load, as long as it is higher than the amp, may shorten tube life but will otherwise cause no damage.

I ran a Boogie guitar amp for *years* with a 16-ohm load from the 8 ohm output and never had a problem. When I finally did need to replace a tube, it was just a microphonic pre-amp tube. Just sayin...

It wasn't until I started reading this stuff that I got paranoid.

FWIT I ran a Fender Showman (Not a Dual Showman) rated for an 8-ohm load into a 4-ohm cabinet (two 8-ohm D140F's wired parallel) for several years without any noticeable damage.
OTOH, I forgot to plug in the speaker cable one night. I switched on the power and standby switches and walked away to set up some additional equipment. When I looked back at the amp, there was an orange glow on the wall behind the amp. All four 6L6's were glowing cherry red. Needless to say, I was unable to use that amp that night. When I opened it up the next day, I found all four screen grid resisters burned open. Replaced the resistors and never had another problem.
Lesson learned: never take the amp off standby until you are ready to play and have checked that everything is connected correctly.

BassmanPaul · #20 07-16-2010, 11:04 AM

Quote:

Originally Posted by jeff62

Funny, Mesa Boogie support has directly told me that unmatched load, as long as it is higher than the amp, may shorten tube life but will otherwise cause no damage.

It IS actually the other way around. It's better for a tube amp for the load to be lower than specified if you cannot match exactly. Best is to match impedances exactly.

Paul