Never, ever put a gas tube or a thyristor based device across a power feed. Once triggered it will conduct until power is cut but only if current is limited to the device power capabilities. If current isn't limited then it is kaboom to something. Hopefully a fuse...
The gas discharge tube (GDT) arrester needs to work faster than the resonance frequency of the OT. Looks like the GDT types work in a uSec., although I don't see that spec'd on the datasheets. And I don't see any spec for time to extinguish the arc either. How about putting a similar V rated MOV and GDT in series? That would reduce the capacitance, and turn off fast via the MOV.
https://en.wikipedia.org/wiki/Surge_protector
https://en.wikipedia.org/wiki/Surge_protector
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Hey Smoking-amp, I am not an engineer and have no clue what a MOV or a gdt (gas discharge tube, ok neon) anyways... more on the GDT.
The gdt will shoot the arc from the tap of the output transformer HV ( sec --> gdt // tube Rk)to ground.
Hopefully something has the time to blow up before things become serious.
The gdt would arc before the transformer does and so protect it from a raising surge.
It will protect both tube failure (reducing the violence of it) and unloaded high level outputs from doing any serious damage to the output transformer.
I serviced a couple of voltage generators which had also a tube relay which sense ground currents and interrupt the main. Most wires inside the instrumentation have a ground wire taped together so that if the insulation fails it will first go through both wires into the ground and stops the machine.
gdt are pretty tough things and almost never need to be changed.
The gdt will shoot the arc from the tap of the output transformer HV ( sec --> gdt // tube Rk)to ground.
Hopefully something has the time to blow up before things become serious.
The gdt would arc before the transformer does and so protect it from a raising surge.
It will protect both tube failure (reducing the violence of it) and unloaded high level outputs from doing any serious damage to the output transformer.
I serviced a couple of voltage generators which had also a tube relay which sense ground currents and interrupt the main. Most wires inside the instrumentation have a ground wire taped together so that if the insulation fails it will first go through both wires into the ground and stops the machine.
gdt are pretty tough things and almost never need to be changed.
Not sure what your nominal idle cathode current is, or your peak signal current, but some margin is a good idea for tube imbalance, so a rounded up 100mA fast blow fuse may be easier to find. An important aspect is that a 100mA fuse is likely to be significantly less than a HT power supply fuse rating (if you have one).
The GDT and MOV both have no issue with speed - they are made for lightning protection, so most of the datasheet tests that indicate timing relate to the standardised lightning test pulses they apply to devices.
A disadvantage of the GDT is it has to be stopped from conducting by a circuit action, whereas the MOV will only influence a circuit when there is sufficient voltage for it to conduct (and in a very soft clipping manner).
If a small disk size MOV is used, along with usually 2 MOVs in series to get a suitable DCV rating, then capacitance is not an issue in practise - even for Williamson grade OTs - as the extra shunt capacitance across a half primary winding is relatively quite minor, and the resonant frequency is related to the square root of the total capacitance.
The turn off time of the GDT still bothers me.
If the GDT doesn't turn off in time (complete de-ionization), it may continue to short out the amplifier after a single spike. (So in range audio V could keep it arcing. OK if the over-voltage problem persists for blowing a fuse, but it's not allowing for transient spikes.)
The related problem with the GDT is that its conduction voltage drops so low that it will not absorb the energy stored in the OT magnetic field for some length of time. Mostly winding resistance for absorption. (so again, conduction overlapping into the next audio cycle)
Since one may need two MOVs in series anyway, to get the voltage rating needed, the series combo of MOV and GDT might be helpful. The MOV providing a voltage drop to absorb the energy, and a fast turn off if necessary. And the series GDT providing low capacitance across the OT. The combo would still provide continuous absorption if the over-voltage condition persists, to blow a fuse.
The higher capacitance of the MOV will likely mean that fast spikes will mostly appear across the GDT initially. So the GDT should be rated for the full clamp voltage. Once the GDT arcs over however, the MOV will have most of the V across it, so it will be clamping to the MOV V level (+ about 50V across the GDT) until the energy is absorbed.
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If the GDT doesn't turn off in time (complete de-ionization), it may continue to short out the amplifier after a single spike. (So in range audio V could keep it arcing. OK if the over-voltage problem persists for blowing a fuse, but it's not allowing for transient spikes.)
The related problem with the GDT is that its conduction voltage drops so low that it will not absorb the energy stored in the OT magnetic field for some length of time. Mostly winding resistance for absorption. (so again, conduction overlapping into the next audio cycle)
Since one may need two MOVs in series anyway, to get the voltage rating needed, the series combo of MOV and GDT might be helpful. The MOV providing a voltage drop to absorb the energy, and a fast turn off if necessary. And the series GDT providing low capacitance across the OT. The combo would still provide continuous absorption if the over-voltage condition persists, to blow a fuse.
The higher capacitance of the MOV will likely mean that fast spikes will mostly appear across the GDT initially. So the GDT should be rated for the full clamp voltage. Once the GDT arcs over however, the MOV will have most of the V across it, so it will be clamping to the MOV V level (+ about 50V across the GDT) until the energy is absorbed.
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nope no change. its too benign when you connect the speaker.
that is the beauty of it.
2000 VAC?
Ok.... I have some very budget Japanese OPT's here that I would not trust anywhere near 1000VDC.
Hi,
I have the following suggestion. What about install a fuse inline with the high voltage wire or from the power supply that supply the high voltage to the tubes. Since the MOV will be connected from the tube plate to ground and if the MOV it is energized by a high voltage it will discharge it to the ground at the same time blowing the fuse. This sequence of event will consequently removed the high voltage from the tubes.
I have the following suggestion. What about install a fuse inline with the high voltage wire or from the power supply that supply the high voltage to the tubes. Since the MOV will be connected from the tube plate to ground and if the MOV it is energized by a high voltage it will discharge it to the ground at the same time blowing the fuse. This sequence of event will consequently removed the high voltage from the tubes.
yup. I can see that now Tim. Thanks for clearing this up for me (and others too I hope).
btw - the fuses I use in the cathodes of my 2a3's are slow-blow ones from littlefuse. I don't run more than 50mA on my 2a3's but a 100mA fuse would probably work well enough too.
I have the 2a3's directly coupled, so if for some reason the input tube circuit fails, is pulled or whatever..., causing the grid to rise, my 2a3's will not be immediately destroyed. Of course, they are inexpensive china 2a3's, but it would be a pity for them to die such a death.
My thinking was that a MOV would then would a nice combination. If the B+ then rises high enough, the MOV will conduct, causing a short which would blow the mains fuse. I like the idea of the mains fuse blowing as a consequence.
The resistor across the speaker terminals is another good fail safe against the speakers suddenly becoming disconnected. It is a measure that I hope to test soon.
Ian
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I have done this previously as well. Look for sand filled slow blow, and if you can, get longer ones, especially if your B+ could go very high.
Its important to consider where best to place them.
2000 VAC?
Ok.... I have some very budget Japanese OPT's here that I would not trust anywhere near 1000VDC.
Yes. keep in mind that in 'theory' the voltage is spread from winding to winding in a transformer so it is not that high potential for the windings.
If you see what a 400 V anode triode can do you would be amazed!
I just have an amplifier which had a bad n.o.s. (so far that is the hypothesis). and it blew across 4 cm sparking at least 5000 V , the speakers are fine.
I will see if the output transformer is fine later.
I have a lot of experience with experiments with up to 10 000 V ac and dc from generators and protections are never too much. Tubes can strike too 0.0
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Ian, MOVs have a wide tolerance on when they start to conduct, and a soft conduction characteristic. As such, a MOV shouldn't be thought of in the same manner as a zener diode. The DC turn-on voltage spec for a typical MOV has about 20-25% tolerance for 1mA conduction. It is likely to need that turn-on voltage to increase by another 30% to force about 1A conduction (as an example). If a fuse is placed in series with the MOV, then the fuse will need a lot of over-current to blow in a short time. I'm not sure all that would blend together in a suitable manner ?
Yes, I was reading up on this wide tolerance for MOV a few times so I was surprised you were suggesting MOV in the first place.
Do you use the MOV to simply limit B+ were it to rise? you don't expect it to actually go to fully conduct to cause a shutdown and prevent disaster? Now perhaps I am quite confused....
Maybe not in this thread, but I remember this as an option as well, so you must have suggested them as well. I think its also an interesting option - especially since I have a lot of them...
How do they fare when B+ rises beyond tolerance?