I need to replace the toasted speaker output jack on my Crate BV-60H Blue Voodoo guitar amp. It seems odd that it would short out the output transformer when nothing is plugged in. Probably why the contacts are toasted. (From the previous owner.) I would think that accidently powering the amp up with no-load, (open circuit), for a moment or two, would be better than shorting the output transformer.
Then again, I don't want to 2nd-guess the design engineers. Before I order the replacement part, does this seem like a good idea? Any harm if I install a non-shorting output jack?
Thanks all, for any info.
Artie
Then again, I don't want to 2nd-guess the design engineers. Before I order the replacement part, does this seem like a good idea? Any harm if I install a non-shorting output jack?
Thanks all, for any info.
Artie
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If you send a signal through a guitar amp, the odds are it will be fairly noisy.
If there is no load across the speaker jack, the output transformer will send a back EMF into the amplifier and most likely destroy itself.
That is why there is a shorting jack, to guarantee this won't happen.
The jack is probably just worn or needs cleaning.
If there is no load across the speaker jack, the output transformer will send a back EMF into the amplifier and most likely destroy itself.
That is why there is a shorting jack, to guarantee this won't happen.
The jack is probably just worn or needs cleaning.
This appears to be a tube amp. Running a tube amp without an output load can lead to bad things happening to the OT (aka letting the smoke out). It's actually much safer to run with a shorted output than an open one.
Every old Fender, Marshall, Vox and Hiwatt has a shorting jack speaker output.
This is sufficient justification for this solution in my eyes... 😉
T
This is sufficient justification for this solution in my eyes... 😉
T
Thanks guys. To be clear, I'd never run the amp with no load. But a shorting jack it is. Will place my order now.
Thanks for the info and speedy response.
Thanks for the info and speedy response.
Some varistors across output connectors with a run voltage over maximum during normal operation can protect the amplifier in case the cable is broken.
My preference would be to have that jack connect a resistor across the amp's output & ground, rather than a dead short to ground. Maybe you could implement that in your amplifier.
Shorting a tube amp is generally safe.
The power is limited by the tubes.
The opposite open output is dangerous as very high voltages might occur, this may cause flash-over in transformer
tube sockets or somewhere else.
Shorting a transistor amp is generally to be avoided and might destroy output transistors in milliseconds
The power is limited by the tubes.
The opposite open output is dangerous as very high voltages might occur, this may cause flash-over in transformer
tube sockets or somewhere else.
Shorting a transistor amp is generally to be avoided and might destroy output transistors in milliseconds
Famous last words 😄
You won't ... on purpose.
But a guitar amps are portable and moved often, from rehearsal room and back, all the time.
Many occasions to forget plugging cable in the midst of chaos.
You took the words right out of my brain. I'm considering doing exactly that. This would allow me to use the "Line Out" jack silently. I'm in the process of removing the circuit board, (which is a PITA), to examine the traces and see if that's reasonably feasible, without butchering the circuit board.
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The only time this beast will be moved, is if I put it on Craigslist, and someone comes and gets it. (But point taken.) 😉
Use a resistor of a little higher resistance, such as maybe 100 ohms or so. You don't want to put a load that draws significant current from the amp's output; just draw enough current to keep the unterminated output from flapping in the breeze. Choose a wattage rating such that if the amp is putting out full power for some reason, the resistor won't be damaged.
I was thinking that too. My only concern was . . . tubes amps are known for wanting the load that they are designed for. Thus, the 8/16 ohm impedance switch. I'm not sure what the affect would be on the output tranny to have, say, 8 ohms selected, and a 100 ohm load. I'm guessing that the low current draw would mitigate that concern.
When the tube amp is required to deliver its full output power, then the proper 8-ohm load would be needed for best protective results. If the amp is idling or delivering small power, then the larger resistor would work fine, since the resistor's only purpose is to maintain amplifier stability. For a solid-state amp, no resistor is needed at all on its output, since that kind of an amp is a very low impedance hard voltage source that changes little, loaded or unloaded.
Shorting jack is a must IMHO and protects a tube amp output quite well... if you keep the shorting time.. well, short. The tubes can take the excessive current for some seconds and then ideally the HT fuse should trip. We better not rely on that, though.
The only halfway bullet-proof option is a pre-load, preferably a separate one for each tap of the transformer. Something like 7..10x the nominal load impedance per tap.
We might not catch all kinds of abuse scenario but IMHO owning/operating a tube amp means that known best practices should be followed, like for a car or any other serious toy.
If you don't get any output with very low (master) volume setting then never be tempted to crank up the volume. Check and debug the situation, carefully.
The only halfway bullet-proof option is a pre-load, preferably a separate one for each tap of the transformer. Something like 7..10x the nominal load impedance per tap.
We might not catch all kinds of abuse scenario but IMHO owning/operating a tube amp means that known best practices should be followed, like for a car or any other serious toy.
If you don't get any output with very low (master) volume setting then never be tempted to crank up the volume. Check and debug the situation, carefully.
If you add the resistor, though, then the amp is still protected and no fuse blows. If the amp is a no negative feedback design, then the shorted output will not be as bad for the amp. With feedback, the amp will bust its, um, tubes trying to force the commanded voltage across the dead short at all costs. I would not want to short the output on a tube amp, especially high-powered ones, that uses negative feedback for that reason.
I read it, and appreciate the suggestion.
But here's the problem: (If I understand a varistor correctly.) A varistor presents itself to a circuit as a high impedance load. In other words, it's virtually invisible under normal operating conditions. But as the voltage, and/or current increases, it's "load" increases. (Resistance lowers.) This is great for a power supply with decent current limiting circuitry. As the load increases, the power supply ramps down.
A basic tube guitar amp output stage doesn't have that current limiting circuitry. So as you approach overload, the varistor "morphs" to become the virtual short you'd have to begin with. (With the simple shorting jack.)
Again, if I understand a varistor correctly.
But here's the problem: (If I understand a varistor correctly.) A varistor presents itself to a circuit as a high impedance load. In other words, it's virtually invisible under normal operating conditions. But as the voltage, and/or current increases, it's "load" increases. (Resistance lowers.) This is great for a power supply with decent current limiting circuitry. As the load increases, the power supply ramps down.
A basic tube guitar amp output stage doesn't have that current limiting circuitry. So as you approach overload, the varistor "morphs" to become the virtual short you'd have to begin with. (With the simple shorting jack.)
Again, if I understand a varistor correctly.
Peak voltages into a 100 ohm load instead of 8 will go higher due to output trafo inductance, but probably not dangerously high. Not like taking the load off entirely.
But some of the current that was intended to go into the plates will go to the screens, running them hot. When the voltage peaks up on one side it goes way down on the other. The screen then becomes the anode.
This of course being tube amplifiers. 100 ohm load on a solid state amp is perfectly safe unless the designer is just stupid (been known to happen).
But some of the current that was intended to go into the plates will go to the screens, running them hot. When the voltage peaks up on one side it goes way down on the other. The screen then becomes the anode.
This of course being tube amplifiers. 100 ohm load on a solid state amp is perfectly safe unless the designer is just stupid (been known to happen).