Protections against being run without a speaker

[RLangham98]

I know this is a stupid question but here goes..

On several occasions I've dealt with old cabs where the jack is loose or oxidized and for a moment had a tube amp running and even began to play into it... while the speaker wasn't in circuit. In most older amps I've seen schematics for, like 5f1, 5f1A, 5E3, et cetera, there's either a simple jack, which would show the OT an open circuit, or at best a shorting jack that would show the OT a dead short across the secondary. In either event, if the speaker cable is plugged into the amp but not plugged into the speaker, it should be seeing an open circuit.

Now obviously, both of those are a very bad scenario that can quickly fry the amp. If I had to guess, the open circuit would fry the OT and the dead short might fry the output tubes?

However, on both my Champion 600 and my 6505 MH, I've now accidentally run them momentarily, with input, with a speaker cable plugged in but not making good contact with the cab jack, and they're both fine. Now, to be sure, I realized immediately what was happening and scrambled to turn the power off, but I think the 6505 may have been in this situation twice now, and has no problems. At least, no problems it didn't have when I bought it.

So I'm just looking for clarity. Is there a safety mechanism on modern amps, even ones as cheap and simple as the Champ 600 RI? Did I just get lucky? Or does it take time for this to do damage? If so is it because these are both low power amps? Seems like a safety mechanism would have to be a current limiter inline with the output, since a switching or shorting jack would only work if there was no cable plugged in. And I think a current limiter would cause problems of its own.

And yes, I've looked for schematics of the 6505 MH, can only find the original 100W combo. Haven't looked at the Champ 600 schematics.

 

[Peegoo]

@RLangham98

Based on my own experience... To fry the OT, it needs to (1) see an open circuit on the secondary for more than several seconds, and (2) the volume control on the amp needs to be cranked up a bit.

What kills the OT is heat buildup in the windings. The thermal mass of the laminated steel core acts as a heat sink to prevent it popping like a fuse.

 

[tubedude]

You've been lucky. The best protection for open speaker circuit is a higher value resistor across the output jack. You do lose a small amount of power though. I've used 27 Ohm 16W resistors before.

 

[elpico]

The champ 600 RI schematic shows the standard shorting jack. I'm not familiar with the other amps.

The only other protection commonly seen is a load resistor in parallel with the jack.

I think what you're discovering is just that amps shouldn't really die from a brief run in with no load operation.

 

[RLangham98]

You've been lucky. The best protection for open speaker circuit is a higher value resistor across the output jack. You do lose a small amount of power though. I've used 27 Ohm 16W resistors before. View attachment 1391941

Oh you know that makes sense. I'm sure you would want a 40W+ rating for the 20W amp, though, right?

The champ 600 RI schematic shows the standard shorting jack. I'm not familiar with the other amps.

The only other protection commonly seen is a load resistor in parallel with the jack.

I think what you're discovering is just that amps shouldn't really die from a brief run in with no load operation.

Yeah I feel like most of the stories I've heard where this happened have either been high-watt amps or amps that were cheaply built in their own time... my buddy has a Silvertone 1483 that he says will go pretty quickly if run with no load. But that's 40W and... well, to be blunt it's a Silvertone.

 

[Ricky D.]

Oh you know that makes sense. I'm sure you would want a 40W+ rating for the 20W amp, though, right?

Yeah I feel like most of the stories I've heard where this happened have either been high-watt amps or amps that were cheaply built in their own time... my buddy has a Silvertone 1483 that he says will go pretty quickly if run with no load. But that's 40W and... well, to be blunt it's a Silvertone.

Those were built for Sears by Danelectro.

 

[RLangham98]

Those were built for Sears by Danelectro.

I was about to edit my post in fact, it's not Silvertone 1483 it's some Harmony head.

EDIT: It's a 535.

 

[elpico]

Oh you know that makes sense. I'm sure you would want a 40W+ rating for the 20W amp, though, right?

Usually the resistor is chosen to be a very poor match. The amplifier is unable to develop much power into such a mismatched load so you don't need a powerful resistor with this approach. The 27ohm value mentioned earlier is low enough that you'd need some power handling, but not more than the amp's output rating like that.

 

[tubedude]

Oh you know that makes sense. I'm sure you would want a 40W+ rating for the 20W amp, though, right?

Yeah I feel like most of the stories I've heard where this happened have either been high-watt amps or amps that were cheaply built in their own time... my buddy has a Silvertone 1483 that he says will go pretty quickly if run with no load. But that's 40W and... well, to be blunt it's a Silvertone.

Well at 27 Ohms you're not developing full power, and if there's no sound, you'll figure out there's a problem, and stop playing to troubleshoot. The likely short exposure period and less than 100% duty cycle coupled with the 18 Continuous Watt rating it will be fine. After all it's protection, not a dummy load.

 

[RLangham98]

Usually the resistor is chosen to be a very poor match. The amplifier is unable to develop much power into such a mismatched load so you don't need a powerful resistor with this approach. The 27ohm value mentioned earlier is low enough that you'd need some power handling though.

Well at 27 Ohms you're not developing full power, and if there's no sound, you'll figure out there's a problem, and stop playing to troubleshoot. The likely short exposure period and less than 100% duty cycle coupled with the 18 Continuous Watt rating it will be fine. After all it's protection, not a dummy load.

Okay, I see this is one of those things where I still have a lot to learn.

I assumed that, since the primary windings are acting as the load on the output tubes, they would happily chug along as long as there was some secondary current, and the secondary and resistor would just soak up the power as best they could.

But I still don't have a great grasp of how transformers behave. I guess it's easy enough to intuit that the inductance of the primary would vary with the load on the secondary, but I don't know any formula for calculating this.

 

[elpico]

A simplified analogy:

You've got a ping pong ball, a baseball, and a bowling ball in front of you. Which one can you throw the farthest?

The ping pong ball is too light to put any really heft into it and the bowling ball is too heavy.

Unsurprisingly the baseball is just about the perfect weight for human arms to throw it across a whole field, we designed that way on purpose to be an excellent *match* for our arm.

Tube amplifiers also work best into an ideal match. When they get a load near the ideal value they develop good power. Faced with a much heavier or much lighter load though, they can't muster up as much. Like throwing a ball that's too light or too heavy.

The reality is more complicated than this, but maybe that gives some kind of feel for it.

 

[RLangham98]

A simplified analogy:

You've got a ping pong ball, a baseball, and a bowling ball in front of you. Which one can you throw the farthest?

The ping pong ball is too light to put any really heft into it and the bowling ball is too heavy.

Unsurprisingly the baseball is just about the perfect weight for human arms to throw it across a whole field, we designed that way on purpose to be an excellent *match* for our arm.

Tube amplifiers also work best into an ideal match. When they get a load near the ideal value they develop good power. Faced with a much heavier or much lighter load though, they can't muster up as much. Like throwing a ball that's too light or too heavy.

No I mean I understand impedance matching at least tentatively, but it seems like transformers are, in the abstract, capable of changing the output impedance of a circuit, and of isolating parts of a circuit from each other, so in the absence of any rule of thumb on how a transformer behaves, I guess I imagined an amp could still develop power under that condition.

 

[elpico]

The transformer is like a lever or gear box in that analogy. It scales up whatever load you put on the output side by a fixed ratio to make it look like a lighter load on the input side. In the ballpark of 1000:1.

If you put an unusually light load on the output side, the transformer dutifully scales that up to an unusually light load for the tubes. And then we're back to - the tubes can't make much power into an unusually light impedance.

 

[RLangham98]

And then we're back to - the tubes can't make much power into an unusually light impedance.

Okay, the transformer thing I get now, although I want to do further reading on how resistive vs reactive impedances are transferred from the secondary to the primary, whether that's different or not.

But when you say an unusually light impedance... I understand that that makes sense from the perspective of the ball analogy but it doesn't seem to make sense with how I know tube amps actually behave. If you underload them they run hot.

EDIT: Actually, in the ball analogy, power is how hard you swing your arm, and the small mass of the ping pong ball isn't preventing you from doing mechanical work, it's just not receiving very much of it, so that mechanical work will be dissipated in other ways, such as the stress on your arm as it rapidly decelerates, without having transferred much energy to the ball. And that does kind of explain why an underloaded amp would run away.

 

[Engine Swap]

 

[W.L.Weller]

@Peegoo mentioned an important part of the equation, where the volume/gain is set on the amp. Were you playing and not hearing anything with the volume on 2? 5? Wide open?

I cooked the OT in my Epiphone Valve Jr. by hitting the strings with increasing vigor, while the amp was dimed, wondering why I couldn't hear it until I smelled something and realized the speaker cable wasn't connected. Took about 2 minutes.

 

[RLangham98]

Had no idea. Totally new information.

 

[elpico]

But when you say an unusually light impedance... I understand that that makes sense from the perspective of the ball analogy but it doesn't seem to make sense with how I know tube amps actually behave. If you underload them they run hot.

Yeah that analogy definitely only makes sense in a very surface level way. Visualizing what will happen inside a pentode tube as you vary it's transformer coupled load requires a specific mental model of those devices and how they interact.

The simplest description I can think is that the tube and load have to split a resource between them, and that can lead to outcomes where less power developed in the load means more power (heat) left in the tube, but that's over simplified too.

 

[Peegoo]

I want to do further reading on how resistive vs reactive impedances are transferred from the secondary to the primary, whether that's different or not.

Looky here. You'll have it all in your bag in about 25 minutes.

This is presented in a way even a nug like me can unnerstand There are no stupid questions when it comes to electricity.

 

[elpico]

That probably needs a warning that he's describing what would happen if a transformer was driven by an ideal voltage source, not a tube amp. Switching from a 10ohm load to a 5ohm load does not double the current through your speaker or output transformer in a tube amplifier like he's calculated there.

This is unfortunately more complicated than can be explained in a short time. What happens when you vary the load depends a lot on the design of the amplifier so there's no pat answer you can give.

In the champ 600 mentioned earlier the 6V6 tube runs the hottest when you're not playing anything at all. When you do start playing, the plate actually gets *cooler* not hotter. The louder you play the cooler it gets, no matter what you're doing with the load.

The screen on the other hand gets hotter the louder you play and mismatching the load does affect this, with a lighter than normal load causing the most extra heating of the screen.

What happens in a Class AB amp is different from that, and probably different from another AB amp, so again, there's no simple answer you can give that would hold true for all amps. As nice as that would be.