# Output Transformers...time needed to blow one?



## greco (Jul 15, 2007)

We all fear the possibility of blowing an output transformer (OT) due to it not having a load attached to it. 

I don't remember anyone mentioning anything rergarding the window of time that is available (or not available) before the transformer blows.

My questions are:

1) How long does it take (without the amp being played...i.e., "idling") to cause irreversible damage to the OT? Are we talking less than 5 minutes, 5-10 minutes ?

2) Same question as above, only now you are jamming with a bunch of guys, had a few brews (for medicinal/nutritional purposes) and you don't quite hear yourself in the mix anymore.

3) Doe a larger OT take longer to blow?

These are academic questions. All of my OT's are fine, so don't be worried/concerned. 

Thanks

Cheers

Dave


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## hollowbody (Jan 15, 2008)

I can't help you with #2, but as far as #1 goes, I have a silly story to tell.

I was selling a cab and someone came by to try it out, so I let him play it through my JTM45 for a bit and then he wanted to play it through his head as well, so I could have sworn I flipped my head into standby (stupid beer), but after the dude left 20-30 mins later, I realized that my head was ON with no load. I felt like such an idiot. I thought for sure the OT was gone, but after I shut it down and fired it back up, it was fine.

So, I don't know if it applies to all OTs, but mine sat with no load for quite some time without and damage that I can hear. It still works fine to this day and this event was a few months ago now.


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## greco (Jul 15, 2007)

Thanks Saro. 

What a relief for you to discover that the OT was OK after 20-30 minutes of idling with no load!!

This is the kind of information that I was hoping for...and I hope it will be informative/helpful to others.

I made an 8 ohm dummy load after I had left my amp head idling without a speaker connected. 
The dummy load didn't get all that warm when the amp is just idling (for about 5 mins). However, play through the amp and into the dummy load and it is cooking within a very short time. (the amp is 40 watts and the load is about 80 watts).

Every time I pull the speaker connection out of the back of the amp (for storage/transport), I plug in the dummy load. Just in case it gets turned on without a speaker attached.

Cheers

Dave


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## bolero (Oct 11, 2006)

that's a good idea, a friend of mine had a '60's Ampeg OT blown because he let some friends use it, they obviously put a signal into the head with no speaker/load attached


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## parkhead (Aug 14, 2009)

I think an amp can idle forever without a load and NOT blow the OT 

the real problem is FLYback 

where the OT charges up with a BIG signal ... but the signal has no where to go 
to dissipate the signal has to arc across the windings

once carbonized those windings are perma- shorted 

otherwise the limiting factor is the size of the primary wire 
under load at some point any wire will act like a fuse and POP due to excessive current 

FWIW 
I've ran all kinds of amps without loads and with wrong impedance matches 
and never blown an OT 

However I know people who have had bad luck and cooked a few OTs 

I think the real killer is when the brown pop's are combinded with agressive ignorance 
as in 
"I Can't hear my amp (speaker is unplugged) so I'll crank it to 10 and bash on my geetar till I hear sumpin " 

ON the other hand I've seen some modern amps blow OT's with a proper speaker load an one bad TUBE 

p


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## Wild Bill (May 3, 2006)

parkhead said:


> I think an amp can idle forever without a load and NOT blow the OT
> 
> the real problem is FLYback
> 
> ...



Quite right, Mr. P! An amp would likely idle for years and never hurt the OT! As you said, it's all about "flyback", which is the condition where a large signal is swinging through the OT but there is no load on the secondary. This means that in effect the impedance is "infinity" on both the primary and secondary. In practice of course the primary load is not infinite, due to wire resistances and eddy losses in the iron but still, it is sky high compared to the proper figure. This means that the voltage swing from the signal might rise to several thousand volts, high enough to break down the insulation on the internal wires. As you said, once that insulation is punctured it can't heal itself. The short between windings is permanent.

I've had amps in where they blew the trannie by playing with no speaker load. Usually this is done by somebody with $1000 of rack mount gear and a pirated copy of ProTools who calls himself a studio tech. They like to tap off the preamp to feed their board but have no idea that you still need a speaker load on the power tubes. So they let their clients wail on an amp and of course the OT blows. Last one was a guy in Hamilton who runs a rehearsal hall. Beautiful old JCM800 blown up! Guys like him should only be allowed to run ValveStates and such.

It's extremely rare but you can also lose an OT or power transfromer just from some slight internal flaw eventually showing up as an internal short. In these cases even a fuse can't save it. However, as I said it is very rare.

Judging by the posts we keep getting on this subject some guys have been given the impression that OTs are very fragile and forgetting to plug in the speakers means instant destruction! I wish I knew who keeps spreading this guff. Transformers are probably the toughest parts in your amp! They are not much more than copper wire coiled around some steel. Hardly fragile!

WB


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## greco (Jul 15, 2007)

Wild Bill said:


> *Judging by the posts we keep getting on this subject some guys have been given the impression that OTs are very fragile and forgetting to plug in the speakers means instant destruction! *I wish I knew who keeps spreading this guff. Transformers are probably the toughest parts in your amp! They are not much more than copper wire coiled around some steel. Hardly fragile! WB


Wild Bill & Parkhead...Thanks for the responses and valuable information. 

Wild Bill's quote above is exactly why I posted this thread. I just wanted to get some factual information re: the time involved before damage to the OT could be expected/feared.

Cheers

Dave


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## Fader (Mar 10, 2009)

It's great to get the straight dope and no guff.


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## parkhead (Aug 14, 2009)

One day I was driving along and saw a chevy cavalier at the side of the road with a small fire 
under the hood ...

we slowed down to watch as the owner and passenger ran accross a farmers field away from the car screaming ... 

they were running for their lives ... since their experience with car fires was the hollywood version 
where the car goes up like an A-BOMB after about 4 seconds 

in the real world they don't blow up they just burn for a while and your insurance buys you a new car


Bottom line People love a good explosion story...
but most disasters are not very spectacular in the real world 


99% of people who panic thinking they BLEW an OT really just need a new pre amp tube 




p


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## Greenbacker (Mar 29, 2007)

When I first started playing bass, I didn't know any of this. I found an SVT for a good price. I recorded with it for HOURS into a recording interface with no speakers attached. I've realized my mistake since but the amp must have about 100 performances on it since and it sounds/functions great!


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## hollowbody (Jan 15, 2008)

greco said:


> Wild Bill & Parkhead...Thanks for the responses and valuable information.
> 
> Wild Bill's quote above is exactly why I posted this thread. I just wanted to get some factual information re: the time involved before damage to the OT could be expected/feared.
> 
> ...


Yeah, thanks guys for clearing that up. I gotta say, I felt like a total tool when I discovered my amp idling with no load. I thought I had fried it and there was no one to blame but me. 

I'm still not happy about it, because I should have been more careful, but at least I know that it's not the end of the world for an amp to idle with no load.


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## greco (Jul 15, 2007)

Given this thread now appears to be winding down, I will offer to make a dummy load for anyone interested. Please PM me.
They are not very expensive. 

I'm not sure (especially now...LOL) that a dummy load is all that "essential"...but it certainly makes me feel more "relaxed" .

Cheers

Dave


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## nonreverb (Sep 19, 2006)

Interesting discussion...some of the worst failures I've seen are with tube power transformers. I've had repairs come into the shop with amps that have been left on for weeks. Tube power transformers are what I believe called medium duty. That means they are spec'ed to be on for a given amount of time but not for extended durations. Anyway, tube transformers generate a considerable amount of heat. Not so much due to the b+ but rather due to the filament current present. After several days, this can cause heat related failure even if there's no signal applied. The majority of the failures of this type that I've seen have been in old transformers which is probably due to age and materials used at the time. The result is a gooey mess around a dead transformer. All to say, not a good idea to leave your amp on for days...or weeks


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## bolero (Oct 11, 2006)

wow, never heard of that before, but it makes sense

is it any better if it's on standby?


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## nonreverb (Sep 19, 2006)

If the failure is due to excessive heat from the heater winding then no.
Interesting thing is that these failures take place in spite of the primary fuse. However, 6V @ 20A on the filament is only 120V @ 1A on the primary...you get the picture. Probably why most if not all decent amps now have heater fuses 



bolero said:


> wow, never heard of that before, but it makes sense
> 
> is it any better if it's on standby?


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