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Old 5th September 2012, 10:01 PM   #1
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Default 6550 tube protection socket

What to assemble a bunch of octal sockets that would go between the power tube and eixisting amp socket. Use resisters or diodes inside the socket that would blow or protect an individual tube's supporting circutry . Problem I am trying to solve is in the case of a large amount of power tubes...relying on a B+ fuse doesn't always work....since the current level needed to blow a B+ fuse in a large amp is always higher than what would blow circuitry immediately adjacent to a power tube. My impressions are that most B+ fuses really are there to protect output transformers...not really parts in a tubes local circut. so what I am looking for are the recommendations for wattage ratings of 1 ohm resisters I could wire into an octal socket, and insert that socket in line to the tube/existing amp sockets. I have heard of folks using resisters as fuses and also heard of folks using diodes. Any constructive help would be appreciated.

Last edited by speakerfritz; 5th September 2012 at 10:04 PM.
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Old 5th September 2012, 10:25 PM   #2
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Resistors make bad fuses. Yes, when run grossly over their power ratings they do evaporate quite spectacularly, but generally they're no fuses.

Consider Ohm's law; U = R x I and P = U x I. For a 1 ohm resistor an amp has to flow to make a watt of power. If you needed to protect a KT88 anode you'd need something like a 200mW rated resistor, and even then it might not burn fast enough to work as a fuse.

Diodes, on the other hand, can be used as protection devices. Just not in the way you seem to be proposing. They have to be designed in the circuit in the right place and in the right way. They're even less usable as fuses than resistors.

My advice? Design your circuit with the worst case scenario - catastrophic tube failure for example - in mind.
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Old 5th September 2012, 11:37 PM   #3
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Quote:
Originally Posted by Adolf Corkscrew View Post
Resistors make bad fuses. Yes, when run grossly over their power ratings they do evaporate quite spectacularly, but generally they're no fuses.

Consider Ohm's law; U = R x I and P = U x I. For a 1 ohm resistor an amp has to flow to make a watt of power. If you needed to protect a KT88 anode you'd need something like a 200mW rated resistor, and even then it might not burn fast enough to work as a fuse.

Diodes, on the other hand, can be used as protection devices. Just not in the way you seem to be proposing. They have to be designed in the circuit in the right place and in the right way. They're even less usable as fuses than resistors.

My advice? Design your circuit with the worst case scenario - catastrophic tube failure for example - in mind.
we are not designing an amp....the amp weights 250 pounds and I do not want to move it around to attempt to find blown resisters when ever a tube flashes over.

If we re-read my post...the goal is to protect the tubes supporting components...assumption is the tube shorts out, flashers, etc. so, what I will do is use resister values that are smaller than the values of the supporting circut under the premise that a smaller resister will blow before the larger one. I can much easier replace a smaller resister in a tube socket saver than to open a very heavey amp and dig and find parts that would lift solder traces to remove and replace. So, having said that....in the amp in question there are 5 watt resisters used in the bias circut and one of the grids as an example....so I'll put in a 3 watt one in the tube socket saver.
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Old 6th September 2012, 12:20 AM   #4
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I would think that leaded picofuses with their low thermal mass might be reasonably good protective devices. You can place one in the cathode circuit, probably something that blows at about 200mA. (Check voltage rating, possible that under fault conditions the voltage here may be higher than one of these devices can safely interrupt)

Resistors are not particularly effective protective devices although fusing resistors might be helpful - in this case a 1/2W - 1W 100 ohm in the screen circuit. (Check voltage rating!)

My concern with these schemes is that as these components open up there is the possibility of an arc occurring which will not be quenched until something else lets go.

Diodes frequently fail shorted so not a good choice except as a clamp on the primary of the output transformer, but again make sure you choose a device with a PIV at least twice the highest voltage you expect in operation. (Say 2X the plate supply and double this)

Add stopper resistors on the screen and control grids between the plug and socket to help prevent parasitic oscillations due to the additional wire length.

What are the tubes in question and the application? Some types and brands are more rugged than others. Perhaps there other angles to look at here as well.
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