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#1
Posted to rec.audio.tubes
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Overvoltage protection
If you are experimenting on a running tube amp, you might be at some stage
pulling a hot output tube from its socket while the amp is running. Alternatively you might accidentally send a high negative pilse to a grid of the tube while working on the circuit. In any case, an abrupt interruption of the plate current will cause a voltage spike on the OPT primary. The stored magnetisation energy of Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent amp. If a load is connected, then the energy will be dissipated in the load giving you a loud crack in the speaker. A residual smaller energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in a snubber of say 2200pF+5K usually connected in parallel to the OPT primary. But what is the load is not connected? Then the huge magnetisation energy will either cause: - arcing in the tube; - breakdown in the OPT winding insulation; - breakdown of the subber capacitor (say 2200pF); - arcing elsewhere in the wiring.or in the tube socket. Would it be a good idea to placa a varistor rated slightly above the +B across the primary? Or a spark gap of some sort, or gas discharge surge arrester tube (the later have miniscule capacitance and very reliable). I remember in vertical deflection stages of old TVs such varistors were a must, since the abrupt cutoff of the current (during flyback) in EL84 often used for this purpose was the mode of operation. However I have never seen any discussions on the varistor protection issue on this site. What is your opinion? Regards, Alex |
#2
Posted to rec.audio.tubes
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Overvoltage protection
"Alex Pogossov" In any case, an abrupt interruption of the plate current will cause a voltage spike on the OPT primary. The stored magnetisation energy of Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent amp. If a load is connected, then the energy will be dissipated in the load giving you a loud crack in the speaker. A residual smaller energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in a snubber of say 2200pF+5K usually connected in parallel to the OPT primary. But what is the load is not connected? Then the huge magnetisation energy ** Why huge ??? If Lp is say 100H and Ia = 0.1A then how many Joules is that ? ..... Phil |
#3
Posted to rec.audio.tubes
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Overvoltage protection
"Phil Allison" wrote in message ... "Alex Pogossov" In any case, an abrupt interruption of the plate current will cause a voltage spike on the OPT primary. The stored magnetisation energy of Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent amp. If a load is connected, then the energy will be dissipated in the load giving you a loud crack in the speaker. A residual smaller energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in a snubber of say 2200pF+5K usually connected in parallel to the OPT primary. But what is the load is not connected? Then the huge magnetisation energy ** Why huge ??? If Lp is say 100H and Ia = 0.1A then how many Joules is that ? Apparently 0.5J. It is the same as charging a 10uF to 315V or 47uF to 160V. If you are in Europe or Australia, try charging 10uF from the grid through a diode and then discharging it with a screwdriver... The closer you keep the cap to your face when discharging -- the better... Then you will see if it is huge or not... It is all subjective after all... If you are in the US, use 47uF for a similar experiment. Enjoy Alex |
#4
Posted to rec.audio.tubes
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Overvoltage protection
"Alex Pogossov" "Phil Allison" "Alex Pogossov" In any case, an abrupt interruption of the plate current will cause a voltage spike on the OPT primary. The stored magnetisation energy of Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent amp. If a load is connected, then the energy will be dissipated in the load giving you a loud crack in the speaker. A residual smaller energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in a snubber of say 2200pF+5K usually connected in parallel to the OPT primary. But what is the load is not connected? Then the huge magnetisation energy ** Why huge ??? If Lp is say 100H and Ia = 0.1A then how many Joules is that ? Apparently 0.5J. ** IOW - **** all. Even the smallest Varistor you can buy will absorb it with ease. Like this one for $ 1.58 + gst http://au.element14.com/epcos/b72205...9?Ntt=100+4399 ..... Phil |
#5
Posted to rec.audio.tubes
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Overvoltage protection
"Phil Allison" wrote in message ... "Alex Pogossov" "Phil Allison" "Alex Pogossov" In any case, an abrupt interruption of the plate current will cause a voltage spike on the OPT primary. The stored magnetisation energy of Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent amp. If a load is connected, then the energy will be dissipated in the load giving you a loud crack in the speaker. A residual smaller energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in a snubber of say 2200pF+5K usually connected in parallel to the OPT primary. But what is the load is not connected? Then the huge magnetisation energy ** Why huge ??? If Lp is say 100H and Ia = 0.1A then how many Joules is that ? Apparently 0.5J. ** IOW - **** all. Even the smallest Varistor you can buy will absorb it with ease. Like this one for $ 1.58 + gst http://au.element14.com/epcos/b72205...9?Ntt=100+4399 Agree, it is no problem to clamp the surges, but the point is: why I have not seen varistors across the OPT primaries in various schematics? Is this sort of protection not needed? Or is the assumption that no one will be changing tubes on a working amp? Or occasional arcing and sparking is not a problem? |
#6
Posted to rec.audio.tubes
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Overvoltage protection
"Alex Pogossov" "Phil Allison" But what is the load is not connected? Then the huge magnetisation energy ** Why huge ??? If Lp is say 100H and Ia = 0.1A then how many Joules is that ? Apparently 0.5J. ** IOW - **** all. Even the smallest Varistor you can buy will absorb it with ease. Like this one for $ 1.58 + gst http://au.element14.com/epcos/b72205...9?Ntt=100+4399 Agree, it is no problem to clamp the surges, but the point is: why I have not seen varistors across the OPT primaries in various schematics? ** Cos the usual RC network suppresses the spike sufficiently, along with the load. Or is the assumption that no one will be changing tubes on a working amp? ** Well, not with no load connected. Or occasional arcing and sparking is not a problem? ** The Aussie made 6V6GT used in my first valve amp liked to spark internally with no load. Never did it, the A&R OP tranny or the socket any permanent harm. But if you wanna go belt and braces - fine with me.... BTW - how much HT have you got ?? ..... Phil |
#7
Posted to rec.audio.tubes
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Overvoltage protection
On Jul 8, 11:16*pm, "Alex Pogossov" wrote:
"Phil Allison" wrote in message ... "Alex Pogossov" In any case, an abrupt interruption of the plate current will cause a voltage spike on the OPT primary. The stored magnetisation energy of Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent amp. If a load is connected, then the energy will be dissipated in the load giving you a loud crack in the speaker. A residual smaller energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in a snubber of say 2200pF+5K usually connected in parallel to the OPT primary. But what is the load is not connected? Then the huge magnetisation energy ** Why huge ??? If Lp is say 100H and *Ia = 0.1A *then how many Joules is that ? Apparently 0.5J. It is the same as charging a 10uF to 315V or 47uF to 160V. If you are in Europe or Australia, try charging 10uF from the grid through a diode and then discharging it with a screwdriver... The closer you keep the cap to your face when discharging -- the better... Then you will see if it is huge or not... It is all subjective after all... If you are in the US, use 47uF for a similar experiment. Enjoy Alex- Hide quoted text - - Show quoted text - We used to use 100 uF@ 500VDC in my highschool to play jokes in the electronics shop. Gives a very nice wake-up jolt , especially to those with sweaty palms . . . |
#8
Posted to rec.audio.tubes
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Overvoltage protection
On Jul 9, 3:08*am, "Alex Pogossov" wrote:
If you are experimenting on a running tube amp, you might be at some stage pulling a hot output tube from its socket while the amp is running. Alternatively you might accidentally send a high negative pilse to a grid of the tube while working on the circuit. In any case, an abrupt interruption of the plate current will cause a voltage spike on the OPT primary. The stored magnetisation energy of Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a decent amp. If a load is connected, then the energy will be dissipated in the load giving you a loud crack in the speaker. A residual smaller energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be safely dissipated in a snubber of say 2200pF+5K usually connected in parallel to the OPT primary. But what is the load is not connected? Then the huge magnetisation energy will either cause: - arcing in the tube; - breakdown in the OPT winding insulation; - breakdown of the subber capacitor (say 2200pF); - arcing elsewhere in the wiring.or in the tube socket. Would it be a good idea to placa a varistor rated slightly above the +B across the primary? Or a spark gap of some sort, or gas discharge surge arrester tube (the later have miniscule capacitance and very reliable). I remember in vertical deflection stages of old TVs such varistors were a must, since the abrupt cutoff of the current (during flyback) in EL84 often used for this purpose was the mode of operation. However I have never seen any discussions on the varistor protection issue on this site. What is your opinion? Regards, Alex I've seen spark gaps on the primary on amps occasionally. But not often. NT |
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