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#1
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Valve amp HV switches
I have a fairly vintage valve amplifier with a dead HT (standby) switch.
It's this kind of format, SPST rocker: http://uk.farnell.com/arcolectric/c1...ack/dp/2068581 Now, this switches the rectified HT winding from the transformer, which is around the usual 500v with no valves in place. I've been looking at replacements, but have not found any such switch that has a rating of 500v DC. Here's a typical search result: http://uk.farnell.com/webapp/wcs/sto... 110173704&vw= Many of these seem to be only rated,or at least tested, at 24v DC (?) Nothing I have found has been anywhere near 500v. And, here is a Carling switch typically found in Fenders, Mesa etc, to switch the typical 500v HT. http://www.hotroxuk.com/carling-spst...-sw110-63.html It has a maximum DC voltage rating of 250v, way higher than anything else I have managed to find, but definitely not 500v rated. I'm particularly concerned that the switch originally fitted, and similar generic types available from Farnell only seem to be rated to 24v DC. No wonder it is faulty. Unless I am missing something here. Gareth. |
#2
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Valve amp HV switches
On 10/08/2016 20:07, Gareth Magennis wrote:
I have a fairly vintage valve amplifier with a dead HT (standby) switch. It's this kind of format, SPST rocker: http://uk.farnell.com/arcolectric/c1...ack/dp/2068581 Now, this switches the rectified HT winding from the transformer, which is around the usual 500v with no valves in place. I've been looking at replacements, but have not found any such switch that has a rating of 500v DC. Here's a typical search result: http://uk.farnell.com/webapp/wcs/sto... 110173704&vw= Many of these seem to be only rated,or at least tested, at 24v DC (?) Nothing I have found has been anywhere near 500v. And, here is a Carling switch typically found in Fenders, Mesa etc, to switch the typical 500v HT. http://www.hotroxuk.com/carling-spst...-sw110-63.html It has a maximum DC voltage rating of 250v, way higher than anything else I have managed to find, but definitely not 500v rated. I'm particularly concerned that the switch originally fitted, and similar generic types available from Farnell only seem to be rated to 24v DC. No wonder it is faulty. Unless I am missing something here. Gareth. Is it a problem with the make or break? any HV cap over the contacts? Parallel up a DPST switch, perhaps NOS would be better |
#3
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Valve amp HV switches
"N_Cook" wrote in message ... On 10/08/2016 20:07, Gareth Magennis wrote: I have a fairly vintage valve amplifier with a dead HT (standby) switch. It's this kind of format, SPST rocker: http://uk.farnell.com/arcolectric/c1...ack/dp/2068581 Now, this switches the rectified HT winding from the transformer, which is around the usual 500v with no valves in place. I've been looking at replacements, but have not found any such switch that has a rating of 500v DC. Here's a typical search result: http://uk.farnell.com/webapp/wcs/sto... 110173704&vw= Many of these seem to be only rated,or at least tested, at 24v DC (?) Nothing I have found has been anywhere near 500v. And, here is a Carling switch typically found in Fenders, Mesa etc, to switch the typical 500v HT. http://www.hotroxuk.com/carling-spst...-sw110-63.html It has a maximum DC voltage rating of 250v, way higher than anything else I have managed to find, but definitely not 500v rated. I'm particularly concerned that the switch originally fitted, and similar generic types available from Farnell only seem to be rated to 24v DC. No wonder it is faulty. Unless I am missing something here. Gareth. Is it a problem with the make or break? any HV cap over the contacts? Parallel up a DPST switch, perhaps NOS would be better This is a vintage(ish) amp that has snap-in rectangular Mains and standby switches. The Mains DPDT double width, the Standby SPST, single width. I don't really want to be drilling holes for another switch that isn't even rated to 500v. Gareth. |
#4
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Valve amp HV switches
On Wednesday, August 10, 2016 at 3:07:10 PM UTC-4, Gareth Magennis wrote:
I have a fairly vintage valve amplifier with a dead HT (standby) switch. It's this kind of format, SPST rocker: http://uk.farnell.com/arcolectric/c1...ack/dp/2068581 Now, this switches the rectified HT winding from the transformer, which is around the usual 500v with no valves in place. I've been looking at replacements, but have not found any such switch that has a rating of 500v DC. Here's a typical search result: http://uk.farnell.com/webapp/wcs/sto... 110173704&vw= Many of these seem to be only rated,or at least tested, at 24v DC (?) Nothing I have found has been anywhere near 500v. And, here is a Carling switch typically found in Fenders, Mesa etc, to switch the typical 500v HT. http://www.hotroxuk.com/carling-spst...-sw110-63.html It has a maximum DC voltage rating of 250v, way higher than anything else I have managed to find, but definitely not 500v rated. This gets you to 400VDC: http://76.12.210.115/pdf/s-toggle-sw-rocker-400v-dc.pdf and then, if you use a relay, you do not risk arc-welding the switch: http://www.vsholding.com/datasheets/7107%20D3F500D6.PDF Hope it helps. Keep in mind that at higher voltages, arcing is the concern. But you knew that already or you would not be asking the question. Peter Wieck Melrose Park, PA |
#5
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Valve amp HV switches
wrote in message ... On Wednesday, August 10, 2016 at 3:07:10 PM UTC-4, Gareth Magennis wrote: I have a fairly vintage valve amplifier with a dead HT (standby) switch. It's this kind of format, SPST rocker: http://uk.farnell.com/arcolectric/c1...ack/dp/2068581 Now, this switches the rectified HT winding from the transformer, which is around the usual 500v with no valves in place. I've been looking at replacements, but have not found any such switch that has a rating of 500v DC. Here's a typical search result: http://uk.farnell.com/webapp/wcs/sto... 110173704&vw= Many of these seem to be only rated,or at least tested, at 24v DC (?) Nothing I have found has been anywhere near 500v. And, here is a Carling switch typically found in Fenders, Mesa etc, to switch the typical 500v HT. http://www.hotroxuk.com/carling-spst...-sw110-63.html It has a maximum DC voltage rating of 250v, way higher than anything else I have managed to find, but definitely not 500v rated. This gets you to 400VDC: http://76.12.210.115/pdf/s-toggle-sw-rocker-400v-dc.pdf and then, if you use a relay, you do not risk arc-welding the switch: http://www.vsholding.com/datasheets/7107%20D3F500D6.PDF Hope it helps. Keep in mind that at higher voltages, arcing is the concern. But you knew that already or you would not be asking the question. Peter Wieck Melrose Park, PA Interesting that the switch you linked to uses a magnet to steer the arc, as used on some Speaker Relays. I'm guessing that valve amp manufacturers have used underated HT switches for decades now, in much the same way that they have often put way higher voltages on pre-amp valves than they are designed to cope with. (Fender, Mesa reverb drive circuits for instance). So I guess I shouldn't be so shocked at this revelation. Gareth. |
#6
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Valve amp HV switches
On Wednesday, August 10, 2016 at 12:07:10 PM UTC-7, Gareth Magennis wrote:
I have a fairly vintage valve amplifier with a dead HT (standby) switch. Now, this switches the rectified HT winding from the transformer, which is around the usual 500v with no valves in place. I'm particularly concerned that the switch originally fitted, and similar generic types available from Farnell only seem to be rated to 24v DC. No wonder it is faulty. I'd be tempted to rewire the unit to activate a high-side PMOS power FET switch. The plastic switch could bias it from any positive or negative low voltage with a NPN level translator (emitter driven from low voltage DC on the replacement switch). Depending on the current you're switching, even an optoisolator might suffice. |
#7
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Valve amp HV switches
Gareth Magennis wrote:
I have a fairly vintage valve amplifier with a dead HT (standby) switch. ** Is the failure mechanical or are the contacts burned up ? It's this kind of format, SPST rocker: http://uk.farnell.com/arcolectric/c1...ack/dp/2068581 Now, this switches the rectified HT winding from the transformer, ** Where is the switch connected - in the AC feed ? which is around the usual 500v with no valves in place. I've been looking at replacements, but have not found any such switch that has a rating of 500v DC. ** And you won't. Many of these seem to be only rated,or at least tested, at 24v DC (?) Nothing I have found has been anywhere near 500v. And, here is a Carling switch typically found in Fenders, Mesa etc, to switch the typical 500v HT. http://www.hotroxuk.com/carling-spst...-sw110-63.html It has a maximum DC voltage rating of 250v, way higher than anything else I have managed to find, but definitely not 500v rated. I'm particularly concerned that the switch originally fitted, and similar generic types available from Farnell only seem to be rated to 24v DC. No wonder it is faulty. Unless I am missing something here. ** You have missed the fact that the breaking capacity of a switch depends on the current flowing at the time. That rocker switch will break 24V DC and 250V AC all day long as long as the current does not exceed 16 amps. In the Standby position of a valve amp, it maybe has to break 0.4 amps at most - so the voltage rating goes way up and it can be expected to do the job fine. Plus any 250VAC mains switch is insulated to way more than the nominal voltage to allow for spikes etc - so that is OK too. The best idea is to have the switch in the transformer wiring, breaking the AC voltage rather than the DC after the rectifier. Marshall did this in most models, using a double pole rocker. But Fender got it *horribly* wrong in a great many models - like the Bassman 100 - connecting the switch in the one spot it should never be. http://www.thevintagesound.com/ffg/s..._100_schem.gif Imagine the switch is operated when the amp is on and running at high volume - maybe because of a sudden acoustic feedback. Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now in SERIES with the filter choke as the main electros are disconnected. Audio frequency current flows in the choke and generates a very large AC voltage pulse - making the switch contacts arc and the same voltage also appears on the output valve plates. I have measured this voltage at more than 3kV rms. A similar thing happens when the HT fuse in a70s or 80s Marshall blows, since it is wired in the same place. Explains a lot of arc burnt valves and octal sockets. ..... Phil |
#8
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Valve amp HV switches
"Phil Allison" wrote in message ... Gareth Magennis wrote: I have a fairly vintage valve amplifier with a dead HT (standby) switch. ** Is the failure mechanical or are the contacts burned up ? It's this kind of format, SPST rocker: http://uk.farnell.com/arcolectric/c1...ack/dp/2068581 Now, this switches the rectified HT winding from the transformer, ** Where is the switch connected - in the AC feed ? which is around the usual 500v with no valves in place. I've been looking at replacements, but have not found any such switch that has a rating of 500v DC. ** And you won't. Many of these seem to be only rated,or at least tested, at 24v DC (?) Nothing I have found has been anywhere near 500v. And, here is a Carling switch typically found in Fenders, Mesa etc, to switch the typical 500v HT. http://www.hotroxuk.com/carling-spst...-sw110-63.html It has a maximum DC voltage rating of 250v, way higher than anything else I have managed to find, but definitely not 500v rated. I'm particularly concerned that the switch originally fitted, and similar generic types available from Farnell only seem to be rated to 24v DC. No wonder it is faulty. Unless I am missing something here. ** You have missed the fact that the breaking capacity of a switch depends on the current flowing at the time. That rocker switch will break 24V DC and 250V AC all day long as long as the current does not exceed 16 amps. In the Standby position of a valve amp, it maybe has to break 0.4 amps at most - so the voltage rating goes way up and it can be expected to do the job fine. Plus any 250VAC mains switch is insulated to way more than the nominal voltage to allow for spikes etc - so that is OK too. The best idea is to have the switch in the transformer wiring, breaking the AC voltage rather than the DC after the rectifier. Marshall did this in most models, using a double pole rocker. But Fender got it *horribly* wrong in a great many models - like the Bassman 100 - connecting the switch in the one spot it should never be. http://www.thevintagesound.com/ffg/s..._100_schem.gif Imagine the switch is operated when the amp is on and running at high volume - maybe because of a sudden acoustic feedback. Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now in SERIES with the filter choke as the main electros are disconnected. Audio frequency current flows in the choke and generates a very large AC voltage pulse - making the switch contacts arc and the same voltage also appears on the output valve plates. I have measured this voltage at more than 3kV rms. A similar thing happens when the HT fuse in a70s or 80s Marshall blows, since it is wired in the same place. Explains a lot of arc burnt valves and octal sockets. ..... Phil Thanks, Phil, that makes a little more sense now. This switch is connected to the main filter caps, as in the Bassman schematic, though I don't think it has a choke, it's a pretty sparse 50W EL34 design (Roost, made in UK). One Mullard EL34 is toast, the other is OK. It also had a shorted cap in the tone stack totally crippling the whole amp. I haven't yet managed to get the switch out of the chassis, it's a tight *******, was going to see if I could repair the contacts, but to do so it needs to come out intact, or I need to source a replacement. Cheers, Gareth. |
#9
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Valve amp HV switches
Gareth Magennis wrote:
** You have missed the fact that the breaking capacity of a switch depends on the current flowing at the time. That rocker switch will break 24V DC and 250V AC all day long as long as the current does not exceed 16 amps. In the Standby position of a valve amp, it maybe has to break 0.4 amps at most - so the voltage rating goes way up and it can be expected to do the job fine. Plus any 250VAC mains switch is insulated to way more than the nominal voltage to allow for spikes etc - so that is OK too. The best idea is to have the switch in the transformer wiring, breaking the AC voltage rather than the DC after the rectifier. Marshall did this in most models, using a double pole rocker. But Fender got it *horribly* wrong in a great many models - like the Bassman 100 - connecting the switch in the one spot it should never be. http://www.thevintagesound.com/ffg/s..._100_schem.gif Imagine the switch is operated when the amp is on and running at high volume - maybe because of a sudden acoustic feedback. Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now in SERIES with the filter choke as the main electros are disconnected. Audio frequency current flows in the choke and generates a very large AC voltage pulse - making the switch contacts arc and the same voltage also appears on the output valve plates. I have measured this voltage at more than 3kV rms. A similar thing happens when the HT fuse in a70s or 80s Marshall blows, since it is wired in the same place. Explains a lot of arc burnt valves and octal sockets. Thanks, Phil, that makes a little more sense now. This switch is connected to the main filter caps, as in the Bassman schematic, though I don't think it has a choke, it's a pretty sparse 50W EL34 design (Roost, made in UK). One Mullard EL34 is toast, the other is OK. ** The bad EL34 may be responsible for the burnt switch. The owner may have tried to use the standby switch when the valve was passing a large DC current - like a couple of amps. At that level, a continuous arc would form and quickly destroy the contacts. I see there is no DC choke in the schem but the fusing would allow that sort of current to flow. http://www.andy-duke.co.uk/roost/Non...2006_05_14.png Maybe when you replace the switch, rewire it in series with the HT fuse - that should allow arcs to extinguish at switch off. ..... Phil |
#10
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Valve amp HV switches
On 11/08/2016 18:50, Phil Allison wrote:
Gareth Magennis wrote: ** You have missed the fact that the breaking capacity of a switch depends on the current flowing at the time. That rocker switch will break 24V DC and 250V AC all day long as long as the current does not exceed 16 amps. In the Standby position of a valve amp, it maybe has to break 0.4 amps at most - so the voltage rating goes way up and it can be expected to do the job fine. Plus any 250VAC mains switch is insulated to way more than the nominal voltage to allow for spikes etc - so that is OK too. The best idea is to have the switch in the transformer wiring, breaking the AC voltage rather than the DC after the rectifier. Marshall did this in most models, using a double pole rocker. But Fender got it *horribly* wrong in a great many models - like the Bassman 100 - connecting the switch in the one spot it should never be. http://www.thevintagesound.com/ffg/s..._100_schem.gif Imagine the switch is operated when the amp is on and running at high volume - maybe because of a sudden acoustic feedback. Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now in SERIES with the filter choke as the main electros are disconnected. Audio frequency current flows in the choke and generates a very large AC voltage pulse - making the switch contacts arc and the same voltage also appears on the output valve plates. I have measured this voltage at more than 3kV rms. A similar thing happens when the HT fuse in a70s or 80s Marshall blows, since it is wired in the same place. Explains a lot of arc burnt valves and octal sockets. Thanks, Phil, that makes a little more sense now. This switch is connected to the main filter caps, as in the Bassman schematic, though I don't think it has a choke, it's a pretty sparse 50W EL34 design (Roost, made in UK). One Mullard EL34 is toast, the other is OK. ** The bad EL34 may be responsible for the burnt switch. The owner may have tried to use the standby switch when the valve was passing a large DC current - like a couple of amps. At that level, a continuous arc would form and quickly destroy the contacts. I see there is no DC choke in the schem but the fusing would allow that sort of current to flow. http://www.andy-duke.co.uk/roost/Non...2006_05_14.png Maybe when you replace the switch, rewire it in series with the HT fuse - that should allow arcs to extinguish at switch off. .... Phil Also, if you can get a 2-pole switch that fits ok, then wiring the two sets of contacts in series ought to improve the ability to extinguish the arc, (for a given contact separation etc.). |
#11
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Valve amp HV switches
Chris Jones wrote:
http://www.andy-duke.co.uk/roost/Non...2006_05_14.png Maybe when you replace the switch, rewire it in series with the HT fuse - that should allow arcs to extinguish at switch off. .... Phil Also, if you can get a 2-pole switch that fits ok, then wiring the two sets of contacts in series ought to improve the ability to extinguish the arc, (for a given contact separation etc.). ** If you look at the schem, the HT fuse is in the rectifier ground path where the voltage and current levels drop to zero at twice the supply frequency. So for a couple of mS each 10mS, there is not enough of either to sustain an arc. .... Phil |
#12
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Valve amp HV switches
On 11/08/2016 23:54, Phil Allison wrote:
Chris Jones wrote: http://www.andy-duke.co.uk/roost/Non...2006_05_14.png Maybe when you replace the switch, rewire it in series with the HT fuse - that should allow arcs to extinguish at switch off. .... Phil Also, if you can get a 2-pole switch that fits ok, then wiring the two sets of contacts in series ought to improve the ability to extinguish the arc, (for a given contact separation etc.). ** If you look at the schem, the HT fuse is in the rectifier ground path where the voltage and current levels drop to zero at twice the supply frequency. So for a couple of mS each 10mS, there is not enough of either to sustain an arc. Yes, I noticed that. It is much better than the original arrangement. If C23 and C24 are in good condition and the B supply rail doesn't get shorted, then the fuse (and new switch) should only be carrying current for a quite small fraction of the time, so arcs should extinguish well, perhaps even better than with a resistive load on AC. |
#13
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Valve amp HV switches
"Chris Jones" wrote in message ... On 11/08/2016 23:54, Phil Allison wrote: Chris Jones wrote: http://www.andy-duke.co.uk/roost/Non...2006_05_14.png Maybe when you replace the switch, rewire it in series with the HT fuse - that should allow arcs to extinguish at switch off. .... Phil Also, if you can get a 2-pole switch that fits ok, then wiring the two sets of contacts in series ought to improve the ability to extinguish the arc, (for a given contact separation etc.). ** If you look at the schem, the HT fuse is in the rectifier ground path where the voltage and current levels drop to zero at twice the supply frequency. So for a couple of mS each 10mS, there is not enough of either to sustain an arc. Yes, I noticed that. It is much better than the original arrangement. If C23 and C24 are in good condition and the B supply rail doesn't get shorted, then the fuse (and new switch) should only be carrying current for a quite small fraction of the time, so arcs should extinguish well, perhaps even better than with a resistive load on AC. Thanks Chris and Phil, that's very interesting. Why wasn't this practice adopted decades ago? Were Fender (Bassman) really that clueless? Beggars belief. Cheers, Gareth. |
#14
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Valve amp HV switches
Gareth Magennis wrote:
I have a fairly vintage valve amplifier with a dead HT (standby) switch. [...] Now, this switches the rectified HT winding from the transformer, which is around the usual 500v with no valves in place. Don't make the mistake which comes around time and again* of switching a charged capacitor directly onto a discharged one, the switch won't last long. Use a resistor to limit the surge current. * The AR88 communications receiver and a Philips transistor car radio (40 years apart) spring immediately to mind. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#15
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Valve amp HV switches
Gareth Magennis wrote:
** If you look at the schem, the HT fuse is in the rectifier ground path where the voltage and current levels drop to zero at twice the supply frequency. So for a couple of mS each 10mS, there is not enough of either to sustain an arc. Yes, I noticed that. It is much better than the original arrangement. If C23 and C24 are in good condition and the B supply rail doesn't get shorted, then the fuse (and new switch) should only be carrying current for a quite small fraction of the time, so arcs should extinguish well, perhaps even better than with a resistive load on AC. Thanks Chris and Phil, that's very interesting. ** Nice example of how so often, the devil is in the detail ..... Why wasn't this practice adopted decades ago? Were Fender (Bassman) really that clueless? ** Fender originally placed the Standby switch after the main filter electros in consideration of the valve rectifiers being used. Placing it earlier creates a large current surge every time the switch closes while the electros charge. Such surges well exceed the maximum ratings for 5U4s or GZ34s and will drastically shorten their life spans. When Fender changed to using strings of silicon diodes, the problem vanished and they ought to have reconsidered the standby switch scheme as Marshall did. Ferder's "The Twin" model ( 1993) finally has a double pole switch in the AC like Marshall. ..... Phil |
#16
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Valve amp HV switches
Adrian Tuddenham wrote:
Don't make the mistake which comes around time and again* of switching a charged capacitor directly onto a discharged one, the switch won't last long. ** You mean like the Mesa Boogie MkI model does ?? http://bmamps.com/Schematics/Mesa_Bo..._Schematic.pdf Note the 60F, 500V electro fitted AFTER the standby switch !! The main electro bank is inadequate ( 2 x 60uF, 350V in series )so someone decided to add an extra cap. There is a loud *snap* whenever the standby is closed and it soon burns the hell out of it. Later versions used 2 x 220uF in the main bank and a 0.1uF 600V film cap after the switch ( so from OT centre tap to ground ) which sometimes fails short. For sheer stupidity it is hard to beat early 70s Marshalls that switched OFF the bias supply with the standby. http://bmamps.com/Schematics/marshal..._year_1970.gif While the 400V HT come up instantly, output valve grid bias take 3 or 4 seconds to arrive !! ..... Phil |
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