fusing OT center tap?
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Stevem
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fusing OT center tap?
Looking for opinions on fusing the center tap on a OT ?
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: fusing OT center tap?
I'm not sure it's a good idea; if the plate lose voltage whilst the screen grids remain energised, the screen grids may pass excessive current?
Best to check that out.
Of course if the screen grid supply etc is after the fuse then the above concern wouldn't be an issue.
Best to check that out.
Of course if the screen grid supply etc is after the fuse then the above concern wouldn't be an issue.
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Re: fusing OT center tap?
It depends on what you're trying to do.
Fuses have no inherent virtue just because they're there. They need a reason to justify their existence. Beyond that, you cannot size a fuse to do what it needs to do without accurately knowing at what current and time delay you want it to blow.
A fuse in the OT CT is likely to be for one of three purposes:
1. To protect the OT
2. To protect the output tubes
3. To protect the PT/rectifier/filter section.
Normally a fuse in the path into or out of the filter caps does 3. The preamp circuit is like a very small wart on the power going to the output stage, anyway.
1 and 2 require some thinking. It seems expedient to use one fuse in the CT to protect both OT and output tubes, but it makes choosing a value very hard. The transformer probably will withstand a short-term over current better than the tubes, but will die on long term minor overloading. The fuses to protect these cases are different.
Protecting strictly the transformer would best be done with *two* fuses, one in each tube plate lead. You cannot reliably protect a transformer by a fuse in a CT as opposed to each separate section of winding. Also, protecting each tube separately is at least marginally easier than protecting all of the OT and both tubes together.
This doesn't do a lot for the screens, of course, but protecting screens as opposed to plates and OTs is something that needs studied on its own.
There's a powerful urge in all of us to do one thing - a single fuse, for instance - to protect everything. Usually, it doesn't work out that well.
There are alternate things that could be done. I designed a current clamp for the kinds of voltages and currents in the output stage of an amp. It uses a high voltage MOSFET and a couple of bipolar transistors to actually monitor the current in a wire, and have the MOSFET start conducting less and less as current hits a resistor-sensed current. Sounds complicated, but it's really one TO-220 MOSFET, two TO-92 2N3904s, and half a dozen resistors. All that comes to about $2.00, not much more than the price of a fuse and holder.
The clamping current can be set to any value by the value of one resistor. I actually designed it to limit inrush current to the first filter cap when turning on the amplifier, or moving from standby to play. But it could easily enough perverted to clamp just the OT CT or just a tube, or just the screens.
There will be a predictable hue and cry from nothing-but-tubes-silicon-is-the-devil types, but I personally like the idea of many, many transistors slavishly making the world safe for precious hollow-state devices.
Fuses have no inherent virtue just because they're there. They need a reason to justify their existence. Beyond that, you cannot size a fuse to do what it needs to do without accurately knowing at what current and time delay you want it to blow.
A fuse in the OT CT is likely to be for one of three purposes:
1. To protect the OT
2. To protect the output tubes
3. To protect the PT/rectifier/filter section.
Normally a fuse in the path into or out of the filter caps does 3. The preamp circuit is like a very small wart on the power going to the output stage, anyway.
1 and 2 require some thinking. It seems expedient to use one fuse in the CT to protect both OT and output tubes, but it makes choosing a value very hard. The transformer probably will withstand a short-term over current better than the tubes, but will die on long term minor overloading. The fuses to protect these cases are different.
Protecting strictly the transformer would best be done with *two* fuses, one in each tube plate lead. You cannot reliably protect a transformer by a fuse in a CT as opposed to each separate section of winding. Also, protecting each tube separately is at least marginally easier than protecting all of the OT and both tubes together.
This doesn't do a lot for the screens, of course, but protecting screens as opposed to plates and OTs is something that needs studied on its own.
There's a powerful urge in all of us to do one thing - a single fuse, for instance - to protect everything. Usually, it doesn't work out that well.
There are alternate things that could be done. I designed a current clamp for the kinds of voltages and currents in the output stage of an amp. It uses a high voltage MOSFET and a couple of bipolar transistors to actually monitor the current in a wire, and have the MOSFET start conducting less and less as current hits a resistor-sensed current. Sounds complicated, but it's really one TO-220 MOSFET, two TO-92 2N3904s, and half a dozen resistors. All that comes to about $2.00, not much more than the price of a fuse and holder.
The clamping current can be set to any value by the value of one resistor. I actually designed it to limit inrush current to the first filter cap when turning on the amplifier, or moving from standby to play. But it could easily enough perverted to clamp just the OT CT or just a tube, or just the screens.
There will be a predictable hue and cry from nothing-but-tubes-silicon-is-the-devil types, but I personally like the idea of many, many transistors slavishly making the world safe for precious hollow-state devices.