Home |
Search |
Today's Posts |
#1
Posted to rec.audio.tubes
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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 . . . |
#7
Posted to rec.audio.tubes
|
|||
|
|||
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 |
#8
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
"Phil Allison" wrote in message ... ** Cos the usual RC network suppresses the spike sufficiently, along with the load. If a load is connected -- correct. If no load, then as I have mentioned you need microfarades + several kiloohms to absorb it, not a few nanofarades usually connected to dump HF ringing of the leakage inductance. Or is the assumption that no one will be changing tubes on a working amp? ** Well, not with no load connected. What about an amp being fool proof then? 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 ?? Only 250V. But I do not want to risk any possibility of insulation breakdown in an old lousy OPT. Probably a small varistor rated at 350...400Vdc and with less than 500pF of capacitance will be OK. The tread is also of general nature -- why do not we see varistors in the tube amp circuits. Probably you are right -- no one suffered serios damage of this particular stress mode. |
#9
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
On Sat, 09 Jul 2011 21:25:39 +1000, Alex Pogossov wrote:
"Phil Allison" wrote in message ... ** Cos the usual RC network suppresses the spike sufficiently, along with the load. If a load is connected -- correct. If no load, then as I have mentioned you need microfarades + several kiloohms to absorb it, not a few nanofarades usually connected to dump HF ringing of the leakage inductance. Or is the assumption that no one will be changing tubes on a working amp? ** Well, not with no load connected. What about an amp being fool proof then? 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 ?? Only 250V. But I do not want to risk any possibility of insulation breakdown in an old lousy OPT. Probably a small varistor rated at 350...400Vdc and with less than 500pF of capacitance will be OK. The tread is also of general nature -- why do not we see varistors in the tube amp circuits. Probably you are right -- no one suffered serios damage of this particular stress mode. It's probably traditional. :-) Small, effective varistors are a newer invention than valves. The two would never have met when the circuits were being developed. That, and engineers having it hammered into them that you *never* run a valve amp without a load. -- Mick (Working in a M$-free zone!) Web: http://www.nascom.info Filtering everything posted from googlegroups to kill spam. |
#10
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
In article ,
"Alex Pogossov" wrote: The tread is also of general nature -- why do not we see varistors in the tube amp circuits. Probably you are right -- no one suffered serios damage of this particular stress mode. It's interesting to note that 1950s, 1960s, and 1970s Telephones used to be full of varistors, both for protective purposes, and for regulating and controlling the speech level. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#11
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
On Jul 9, 9:25*pm, "Alex Pogossov" wrote:
"Phil Allison" wrote in message ... ** Cos the usual RC network suppresses the spike sufficiently, along with the load. If a load is connected -- correct. If no load, then as I have mentioned you need microfarades + several kiloohms to absorb it, not a few nanofarades usually connected to dump HF ringing of the leakage inductance. Or is the assumption that no one will be changing tubes on a working amp? ** Well, not with no load connected. What about an amp being fool proof then? 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 ?? Only 250V. But I do not want to risk any possibility of insulation breakdown in an old lousy OPT. Probably a small varistor rated at 350...400Vdc and with less than 500pF of capacitance will be OK. The tread is also of *general nature -- why do not we see varistors in the tube amp circuits. Probably you are right -- no one suffered serios damage of this particular stress mode. This issue of back emfs in unloaded tube amps was never worried about in the thousands of old radios with 1 x 6V6 used without any NFB and connected to OPT and a speaker with no Zobel damping network. The reactance of speaker inductance rises so that by 3kHz there is virtually no load on the 6V6 and the shunt C is low and leakage L also high. RDH4 does not mention that arcing in the tube would be a problem, and I've seen no evidence of it. Stored magnetic energy in bean couter designed radio OPTs is low; it uses a tiny core which is air gapped. B+ voltages in radios tend to be less than +280V, and even if anode goes to +560V, the insulation will survive. But in PP amps the B+ can be +500V and as we all know the anode swing can be to +/-1,500V, and thus threaten to arc. There's a simple answer which is well know and been around ever since Si diodes began to get good reverse voltage ratimgs, and that's to connect diodes from each end of the OPT to 0V with diode anode connected to tube anodes. I like to use 3 x 1N7007 in series with 1M strapped across each diode to equalise reverese voltages, and when one end of the OPT tries to rise to more than 2 x +B, the other end tries to go below 0V so diodes conduct, and thus the OPT voltage swing is CLAMPED. Voltage clamping with diodes has been around for ages. There is a schematic example at http://www.turneraudio.com.au/100w-monobloc2-2004.htm The amp also has Zobel networks across each 1/2 primary of the OPT to load the OPT at HF. The book I have with 17 designs of amps from 5W to 1,100 W written in anout 1960 has no provisions for dealing with excessive unloaded OPT voltages except on the high power types where an adjustable spark gap is used with a 10k0 load resistance in series between each end of the OPT. But arcing in OPTs is not confined to amps which have B+ of over 1kV or more. I once rescued a Geloso PA amp made in 1950s moments before a trundling bulldozer went over it after someone dumped it when people used to be able to take stuff to a tip and then raid each other's tip off's. This kind of amp was used at Italian weddings for an acordian player and singer. 2 x EL84 for about 15W max. The amp arced just sitting there doing nothing while turned on; there was a fault in insulation where B+ wires went into the winding and it arced between B + and the core. Some careful observation showed a blackened area and it had been doing is for awhile and probably because it had got DAMP at some time. I scraped out the carbon and wax in the area and dressed wire positions, heated the tranny to dry it, re-waxed it, and it never ever arced again. I then rewired it totally to give better guitar amp usability and sold it for $350, about 12 years ago, to a guy here who calls himself Dr Zot, and who dabbles in vintage electric guitars and amps. Patrick Turner. |
#12
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
"John Byrns" " It's interesting to note that 1950s, 1960s, and 1970s Telephones used to be full of varistors, both for protective purposes, and for regulating and controlling the speech level. ** More likely called " VDRs" ( Voltage Dependant Resistors ) and not varistors or MOVs. Although in the same general category, they are not identical. Most modern MOVs are made from Zinc oxide, which gives them very low leakage currents and sharper turn on characteristics compared to other types like the older Silicon carbide VDRs. ..... Phil |
#13
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
In article ,
Patrick Turner wrote: But in PP amps the B+ can be +500V and as we all know the anode swing can be to +/-1,500V, and thus threaten to arc. There's a simple answer which is well know and been around ever since Si diodes began to get good reverse voltage ratimgs, and that's to connect diodes from each end of the OPT to 0V with diode anode connected to tube anodes. I like to use 3 x 1N7007 in series with 1M strapped across each diode to equalise reverese voltages, and when one end of the OPT tries to rise to more than 2 x +B, the other end tries to go below 0V so diodes conduct, and thus the OPT voltage swing is CLAMPED. Voltage clamping with diodes has been around for ages. There is a schematic example at http://www.turneraudio.com.au/100w-monobloc2-2004.htm The amp also has Zobel networks across each 1/2 primary of the OPT to load the OPT at HF. There seems to be an inconsistency here, between what you said above, "connect diodes from each end of the OPT to 0V with diode anode connected to tube anodes", and the way the "schematic example" shows the diodes wired? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#14
|
|||
|
|||
This issue of back emfs in unloaded tube amps was never worried about
in the thousands of old radios with 1 x 6V6 used without any NFB and connected to OPT and a speaker with no Zobel damping network. The reactance of speaker inductance rises so that by 3kHz there is virtually no load on the 6V6 and the shunt C is low and leakage L also high. RDH4 does not mention that arcing in the tube would be a problem, and I've seen no evidence of it. Stored magnetic energy in bean couter designed radio OPTs is low; it uses a tiny core which is air gapped. B+ voltages in radios tend to be less than +280V, and even if anode goes to +560V, the insulation will survive. But in PP amps the B+ can be +500V and as we all know the anode swing can be to +/-1,500V, and thus threaten to arc. There's a simple answer which is well know and been around ever since Si diodes began to get good reverse voltage ratimgs, and that's to connect diodes from each end of the OPT to 0V with diode anode connected to tube anodes. I like to use 3 x 1N7007 in series with 1M strapped across each diode to equalise reverese voltages, and when one end of the OPT tries to rise to more than 2 x +B, the other end tries to go below 0V so diodes conduct, and thus the OPT voltage swing is CLAMPED. Voltage clamping with diodes has been around for ages. There is a schematic example at http://www.turneraudio.com.au/100w-monobloc2-2004.htm The amp also has Zobel networks across each 1/2 primary of the OPT to load the OPT at HF. The book I have with 17 designs of amps from 5W to 1,100 W written in anout 1960 has no provisions for dealing with excessive unloaded OPT voltages except on the high power types where an adjustable spark gap is used with a 10k0 load resistance in series between each end of the OPT. But arcing in OPTs is not confined to amps which have B+ of over 1kV or more. I once rescued a Geloso PA amp made in 1950s moments before a trundling bulldozer went over it after someone dumped it when people used to be able to take stuff to a tip and then raid each other's tip off's. This kind of amp was used at Italian weddings for an acordian player and singer. 2 x EL84 for about 15W max. The amp arced just sitting there doing nothing while turned on; there was a fault in insulation where B+ wires went into the winding and it arced between B + and the core. Some careful observation showed a blackened area and it had been doing is for awhile and probably because it had got DAMP at some time. I scraped out the carbon and wax in the area and dressed wire positions, heated the tranny to dry it, re-waxed it, and it never ever arced again. I then rewired it totally to give better guitar amp usability and sold it for $350, about 12 years ago, to a guy here who calls himself Dr Zot, and who dabbles in vintage electric guitars and amps. Patrick Turner.[/quote] Before 1960 the Selenium Thyrector Diode by GE & others was available to circuit designers. These were meant for power applications & came in both single ended & back to back stacks in several voltage ratings. Reverse leakage current specs were low enough so as not to adversley effect the questionable fidelity of those Class AB & B PA amplifiers. In a guitar amp they might add something. Cheers, John If the designer chose to he could have selected a pair of back to back devices attached across the OPT primary. But caps & resistors were less money. |
#15
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
On Jul 11, 1:00*am, John Byrns wrote:
In article , *Patrick Turner wrote: But in PP amps the B+ can be +500V and as we all know the anode swing can be to +/-1,500V, and thus threaten to arc. There's a simple answer which is well know and been around ever since Si diodes began to get good reverse voltage ratimgs, and that's to connect diodes from each end of the OPT to 0V with diode anode connected to tube anodes. I like to use 3 x 1N7007 in series with 1M strapped across each diode to equalise reverese voltages, and when one end of the OPT tries to rise to more than 2 x +B, the other end tries to go below 0V so diodes conduct, and thus the OPT voltage swing is CLAMPED. Voltage clamping with diodes has been around for ages. There is a schematic example at * http://www.turneraudio.com.au/100w-monobloc2-2004.htm The amp also has Zobel networks across each 1/2 primary of the OPT to load the OPT at HF. There seems to be an inconsistency here, between what you said above, "connect diodes from each end of the OPT to 0V with diode anode connected to tube anodes", and the way the "schematic example" shows the diodes wired? Gee, I thought you slept through most days. But you are correct and the schematic IS RIGHT, and what I said here in the post WAS WRONG. Readers take note ! But the diodes work excellently to limit or clamp the Va swings to no more than +/- B+ at each anode. Now the same scheme would be impossible to implement on an SE amp because there's only one phase of voltage and nothing much to stop Ea rising enormously when no load is present and the tube cuts off when in pentode or beam tetrode mode where grid current usually has no limiting effect on Ea swings and voltage gain approaches gm x Ra, or perhaps 130 for an EL34 or 170 for KT88. But one could rig a voltage doubler supply to create a rail at 2 x B+, and run SI diodes with their cathodes taken to tube anodes, and diode anodes taken to 2 x B+, say +800V, so that when and not if Ea rises that far the diodes conduct, and Ea is clipped. There will be a negligible rise in the B+ rail as a a result. Diodes may be also placed between tube anode and 0V to prevent Ea ever going negative. I find that SE amps don't tend to exhibit the excessive Ea problems as much as PP pentode/tetrode amps, and despite building and selling quite a few SE amps I've never seen a problem with arcing tubes or OPT insulations. The other way to clamp output voltage and thus clamp the anode swing voltages in a tube amp with OPT is to use two networks of 5W rated Zener diodes in series with a normal 1N5404 3 amp Si diode between the OPT secondary outlet and 0V so that if the Vo exceeds the rated Vo for maximum rated power by say +3dB, then the zeners+diodes conduct to clamp the voltage. Such clamping of outpur voltage is done routinely on many SS amps where a diode such as 1N5404 is taken to each + and - rail so that if the Vo ever exceeds the rail voltages then the diodes conduct. Back emfs generated in speakers and inductiove loads are thus prevented from flow *backwards* through output SS devices which will destroy themselves instantly is the Ve-b back flow V exceeds about 7Vpk. The same sort of diode clamping is used routinely in SS amps with mosfet OP devices to limit the Vg-s, and thus not exceed a rated Vg-s which would punctuture gate insulation which is very thin indeed. Voltage clamping is thus very easy for anyone to achieve. Patrick Turner. -- Regards, John Byrns Surf my web pages at, *http://fmamradios.com/ |
#16
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
Two Zener diodes connected facing in opposite directions to each other in parallel ? And a voltage dropping resistor in series . . .- Hide quoted text - But not in parallel, because zeners act like ordinary diodes in one direction of current, so the zeners must be in series, and then it clamps. Using a series R might be OK to give a higher load once clamping limiting begins to avoid the low Z when zeners turn on. It should not matter in a tube amp though because the zener voltage should just exceed the likely peak Vo max when normally loaded. The zeners have some capacitance and are accused of being slow to act, but they'll act quite fast enough, and the capacitance is orders of magnitude below what might cause HF instability because of capacitive loading of the amp. I have used about ten 68V x 5W zener diodes in parallel as diode vari- caps to generate +/- 40kHz of FM signal deviation of a 455kHz RF signal. The modulation applied to the diodes can be a saw tooth, and this gives a "wobbulated" IF signal which allows the selectivity curve of a radio set to be seen on a CRO, so that when one aligns an AM radio IFTs, or RF sections, or experiments with an IFT teriary winding, the effects on pass bandwidth and skirt selectivity can be seen easily. The amount of capacitance with each zener is only tens of pF maximum. This form of FM modulation would be most difficult to otherwise achieve unless one had a tuning gang which was driven at say 40Hz rotation by a motor. The electronic method of wobbulation has no mechanical parts which tend to wear out. Patrick Turner. |
#17
Posted to rec.audio.tubes
|
|||
|
|||
Overvoltage protection
On Jul 12, 4:47*am, Patrick Turner wrote:
Two Zener diodes connected facing in opposite directions to each other in parallel ? And a voltage dropping resistor in series . . .- Hide quoted text - But not in parallel, because zeners act like ordinary diodes in one direction of current, so the zeners must be in series, and then it clamps. Using a series R might be OK to give a higher load once clamping limiting begins to avoid the low Z when zeners turn on. It should not matter in a tube amp though because the zener voltage should just exceed the likely peak Vo max when normally loaded. The zeners have some capacitance and are accused of being slow to act, but they'll act quite fast enough, and the capacitance is orders of magnitude below what might cause HF instability because of capacitive loading of the amp. I have used about ten 68V x 5W zener diodes in parallel as diode vari- caps to generate +/- 40kHz of FM signal deviation of a 455kHz RF signal. The modulation applied to the diodes can be a saw tooth, and this gives a "wobbulated" IF signal which allows the selectivity curve of a radio set to be seen on a CRO, so that when one aligns an AM radio IFTs, or RF sections, or experiments with an IFT teriary winding, the effects on pass bandwidth and skirt selectivity can be seen easily. The amount of capacitance with each zener is only tens of pF maximum. *This form of FM modulation would be most difficult to otherwise achieve unless one had a tuning gang which was driven at say 40Hz rotation by a motor. The electronic method of wobbulation has no mechanical parts which tend to wear out. Patrick Turner. Ooh, yes, the Zeners have to be connected in series . Absolutely and positively . |
#18
Posted to rec.audio.tubes
|
|||
|
|||
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 |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Protection of investment | Vacuum Tubes | |||
protection | Pro Audio | |||
protection | Pro Audio | |||
Amp protection light | Car Audio | |||
Hearing Protection | General |