Protections against being run without a speaker

[2L man]

Jack which has shorting switch does not short if plug is still inserted so it does not protect if contact has come bad. Most jack Switches are specifier only for 300mA or 1A so it is not bullet proof.

When changing jacks it is worth checking if it is possible to change a jack which is specified for heavy duty instrument amplifiers. They have a locking mechanism and plug is turned when inserted. They also handle more power.

There is also a jack for 6,3mm plug which has a locking and higher current spec.

Highest danger for "Flyback Voltage" is if loudspeaker load is lost when playing loud. Even if sound is lost very short just hearable time it can be long enough to cause tube amp problem.

 

[W.L.Weller]

The only saving grace of my Valve Jr abuse is that I did it back when ClassicTone was still making transformers, so I got to learn first-hand (first-ear?) what a difference a trafo makes.

 

[printer2]

Transformers are like a transmission in a car as previously said. You have your foot on the gas and you are going down the road at 30 mph with the engine running at 2,000 rpm. It is winter and you hit a section of road with ice on it. Before your foot has time to react the lack of a load has your tires spin and the rpm jumps up to 4,000 rpm. You back off of the gas and all is good. Now say you are going in a hurry and the rpm is 4,000. Then you hit the ice and the rpm jumps to 6,000. Sadly the engine has a 5,500 redline and parts start flying.

Now say you have an output transformer. You have the amp running at ten watts which is the same as the Volt-Amp being delivered into a frequency where the amp sees 10 ohms at the speaker. So the amp is delivering 10 VA. The transformer converts the VA in the primary into magnetic flux in the core and back into electrical energy in the secondary. When the secondary does not absorb the magnetic flux when it is open the flux has to go somewhere. We have 10VA of energy and say the load drops down by a factor of 10 (resistance goes up by 10X, or actually the impedance). The current goes down 10X and with the conservation of energy the voltage has to rise 10X to keep the VA constant. So we end up with 100V rather than 10V.

The power gets reabsorbed by the output tube (hopefully). But then we crank the amp and put in 50VA. The power then gets transformed and we get 50V rather than 10V. The secondary should be fine as the insulation on the winding should be rated higher than that. But what is happening on the primary side? Say we have a transformer with an 8k primary and 8R secondary. Dividing the 8k by 8 and then taking the square root gives a ratio of 31X. The 500V is multiplied by 31 so the primary now has 16kV across it. The insulation of the primary may not be thick enough to prevent the voltage to arc across the windings or to the core. The voltage will not get this high as the plate resistance will absorb the energy. The amp may still be fine, but what if the amp is run into clipping when this happens? There already is a flyback voltage when we clip, with no load from the tube (plate has a very high resistance) the flyback voltage has nowhere to go but up.

Say the voltage is high enough to punch through the magnetic wire's insulation. There is now current flowing with little resistance and the area gets hot. If it is only for a brief time the arc can stop and the insulation solidify again. So the amp runs like normal but where the arc was the insulation is weakened and there might even be carbon traces. If the transformer is abused again this spot will likely see more damage but require less than the 50VA of the original injury. Some adjacent layers of the winding may have a short between them and the layer bypassed, with the amp sounding 'tired' as compared to normal.

More or less. Amps I have concerns of I put a 50R resistor across the terminal. High enough to help reduce the damage but low enough not to impede the power much.

 

[schmee]

Prevention: Well, replace those dirty old jacks!

 

[Peegoo]

dirty old jacks

 

[ZackyDog]

This is presented in a way even a nug like me can unnerstand

Nug? Surely you jest.

Nug

 

[Peegoo]

Nug? Surely you jest.

You can trust me.

I'm not a drummer.

 

[RLangham98]

@Peegoo

Thanks for the link! At some point in this conversation I found the formula for reflected load, but it's always good to have Uncle Doug's straightforward explanations and practical examples.

It may come as no surprise that I'm working through his videos on electrical concepts in order--I thought I understood Ohm's law, resistance, capacitance and inductance before, but a lot of the practical implications of these concepts escaped me--for instance, that plate load is what "turns" plate current into the voltage to drive the next amp stage or the OT. Uncle Doug and Rob Rob have taught me a lot in a very short time

But as you can see, I hadn't gotten to transformers yet!

So correct me if I'm wrong, the thing that cooks the OT when in open secondary condition, is that the output tubes are seeing an extremely small load and so instead of a high voltage, a high current begins to flow through the primary, rapidly overheating it?

 

[bbetancourt]

@RLangham98
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.

Wrong - at least for an open circuit. With no load connected there will be no power to heat up the windings (at first).
But without a load there is no current in the coil to produce a reactive magnetic field and the output voltage will rise above the dielectric strength of the windings. This sparkovers will burn through the wire-enamel and creating a interwinding fault which will then finally create a short that burns up the winding.

 

[enorbet2]

Loose or dirty contacts? Simple answer - Maintain your gear. No flame intended. I've been a tech as well as a player for decades and as a tech I've witnessed global Pros at big shows who either don't know how or just didn't bother to "keep their tools sharp", but it is an important habit to create. Basic examination, cleaning and soldering skills aren't Rocket Surgery.

 

[RLangham98]

Loose or dirty contacts? Simple answer - Maintain your gear. No flame intended. I've been a tech as well as a player for decades and as a tech I've witnessed global Pros at big shows who either don't know how or just didn't bother to "keep their tools sharp", but it is an important habit to create. Basic examination, cleaning and soldering skills aren't Rocket Surgery.

It was a cabinet I was trying at a store using my own amp, and a cabinet that I had just bought. I don’t let this go on very long with my own property.

But you know the thing is… it’ll happen again one way or another, so it’s good to know what the consequences are. If not dirty contacts then one day I may just forget when I’m setting up, or a speaker may die on me, or something else could cause an open circuit.

 

[MonkeyGym]

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.

This is a good analogy. To extend analogy, if you don’t try very hard or very long, your arm will be fine. If you keep trying to throw the ping pong ball or the bowling ball as hard as you can to the point of failure, you’re going to pull a muscle or injure your shoulder or otherwise hurt yourself.

 

[Stratocast]

y

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.

you can lessen the occurrence of oxidation if you quit playing outside..