No speaker load and the output transformer

PowerTube44

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Okay, so I've known for years that one should never power on a tube amp with no speaker load attached. I've also never done it.

But I was in Guitar Center on Saturday to try out the 5150 Iconic head and cab (again). There was power going to the head, but no speaker cable running to the cab. I checked first, so I didn't power on until I found a speaker cable.

Here's my question: If it's going to blow the output transformer, is this usually an instant thing, or can damage be caused over time? What if other people DID walk up and power it on with no load? Could it have caused damage, or does the simple fact that it powers on and works mean that it's fine?
 

Jon Snell

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Damage can occur if the amplifier is being played loud with no speaker connected. This is because the output transformer is an inductor.
If there is no load (no Speaker connected), the power has no where to go and is reflected back onto the valves and seriously high voltages can occur that can damage the valves and indeed exceed the electrical insulation voltages of the actual windings causing short circuits and permannent damage.
Some amplifier designs have clamp diodes that help to protect against this phenomenon.
 

Pete Farrington

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With sufficiently high control settings, even with no guitar plugged in and the shorting contacts on the input jack functioning, the amp could oscillate. Especially so with a super high gain amp. It’ll probably cope for a minute or 3, but eventually a valve screen grid or anode circuit insulation will short.
 

PowerTube44

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Thanks, guys. For some reason, I've always heard people say that it was the output transformer that was in danger.

So going back to the question above, hypothetically, if people had been powering it on with no speaker load, but it works fine now, could there still have been damage done over time?
 

Edgar Frog

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If anyone powered it on and tried to crank it to get any sound out of it or plugged in a guitar trying to play it, I would say definitely yes. People screw with stuff all the time on the floor. It would be impossible for anyone to say what has and hasn't been done with the amp while it sat there plugged in with no speaker load. Someone could have turned it on turned it up and walked away with it still on and powered down some time later when someone noticed it. I don't know much on the technical side but I always took all precautions and play it safe by never powering on a tube amp without a proper cab hooked up. I never buy floor model stuff because I always assume the worst has been done to it. There's a lot of clueless dummies out there and a lot of people that just like to screw things up just for the hell of it.
 

PowerTube44

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If anyone powered it on and tried to crank it to get any sound out of it or plugged in a guitar trying to play it, I would say definitely yes. People screw with stuff all the time on the floor. It would be impossible for anyone to say what has and hasn't been done with the amp while it sat there plugged in with no speaker load. Someone could have turned it on turned it up and walked away with it still on and powered down some time later when someone noticed it. I don't know much on the technical side but I always took all precautions and play it safe by never powering on a tube amp without a proper cab hooked up. I never buy floor model stuff because I always assume the worst has been done to it. There's a lot of clueless dummies out there and a lot of people that just like to screw things up just for the hell of it.
That was my concern. I posted in another thread about my fear that if I order it from Guitar Center, are they just going to re-box the "new" one in my local GC, the one that's been out on the floor for three weeks, to save on shipping?

On the risk that they might do exactly that, I'll order from Sweetwater or Amazon if I get one.
 

Norfolk Martin

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The first question is whether it has a shorting output jack or not . Most manufacturers wire the jacks so there is a short across the output winding of the transformer if there is no plug in the output jacks. Although it's not advisable to do it at high power for a long time, IME most tube amps do not suffer damage form running into a short circuit load . I've see the problem mostly where something is plugged into the speaker out, but the cord is broken, or it isn't plugged into anything at the other end. Then you are trying to drive current into an almost infinite impedance . This will fry things fairly quickly.
 

Pete Farrington

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It’ll probably cope for a minute or 3, but eventually a valve screen grid or anode circuit insulation will short.

For some reason, I've always heard people say that it was the output transformer that was in danger.
The OT primary is a key component in the output valve anode circuit. Sorry I didn’t make that more clear.
hypothetically, if people had been powering it on with no speaker load, but it works fine now, could there still have been damage done over time?
If you can crank it to overdrive the power amp, and it sounds fine, then all is well.
Most manufacturers wire the jacks so there is a short across the output winding of the transformer if there is no plug in the output jacks
Dunno about most; some manufacturers do, some don’t.
Bear in mind that Marshall don’t, and this is a Marshall forum.
It’s not feasible to do it if, rather than a load impedance selector, different impedance taps are wired to different jacks.
 

SwedishWings

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The OT primary is a key component in the output valve anode circuit. Sorry I didn’t make that more clear.

If you can crank it to overdrive the power amp, and it sounds fine, then all is well.

Dunno about most; some manufacturers do, some don’t.
Bear in mind that Marshall don’t, and this is a Marshall forum.
It’s not feasible to do it if, rather than a load impedance selector, different impedance taps are wired to different jacks.

Are there any drawbacks? Been thinking of doing this to my home brew amps, but was not sure it was a good idea.
 

PascalP2

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The 5150 iconic can run without a cab if the "power section switch is off" I know I own one. So with that you can run the amp and record silently. you have to make sure that the switch is off. there's also a XRL out for recording.
 

neikeel

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The earliest Marshall’s did have a resistor on the jacks to ameliorate the risk of no load open secondary.
 

Pete Farrington

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Yes, but weirdly it was never shown on the schematic. So copies, even the RIs, don’t include it.
It seems a very retrograde change that they dispensed with it by the late 60s; if hardwired across the 16ohm output, it could have saved a lot of OTs, eg from a failing impedance selector.
I think such a safety back up load resistor is a better approach than a shorting jack on the speaker output, as the latter is no use if the speaker cable or cab fails intermittent / open circuit.

Some Vox and Traynor schematics show a safety back up load resistor.
 

PelliX

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The OT primary is a key component in the output valve anode circuit. Sorry I didn’t make that more clear.

If you can crank it to overdrive the power amp, and it sounds fine, then all is well.

Dunno about most; some manufacturers do, some don’t.
Bear in mind that Marshall don’t, and this is a Marshall forum.
It’s not feasible to do it if, rather than a load impedance selector, different impedance taps are wired to different jacks.

Not trying to nitpick here, but in theory the OT (and the valves) could have already suffered some stress from the scenario outlined, making them more prone to early failure. As far as I'm aware there really is no practical method to determine the state of the windings/lamination in a transformer unless the electrical properties have already been affected. That said, give it hell during the warranty period - you could always end up with a compromised unit new in box, too - the element of luck is always involved here.

I think such a safety back up load resistor is a better approach than a shorting jack on the speaker output, as the latter is no use if the speaker cable or cab fails intermittent / open circuit.

It's often puzzled me why more manufacturers don't/didn't do this - I doubt it's much more expensive to implement than the switching jack route. :shrug:
 

fitz288

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That was my concern. I posted in another thread about my fear that if I order it from Guitar Center, are they just going to re-box the "new" one in my local GC, the one that's been out on the floor for three weeks, to save on shipping?
Did you ever call GC and ask about NIB?
 


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