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#1
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B+ rectifier question
Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie |
#2
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B+ rectifier question
Edward Morris wrote:
Hello, I have a McIntosh C22 preamp. I'd like to replace the two diode B+ http://www.pmillett.com/file_downloa...sh/C22_sch.pdf Why do you need a voltage drop ? and by how much ? |
#3
Posted to rec.audio.tubes
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B+ rectifier question
"Edward Morris" wrote in message news Hello, I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge. |
#4
Posted to rec.audio.tubes
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B+ rectifier question
On Dec 12, 4:04*pm, ":-:Ghost Chip:-:" wrote:
"Edward Morris" wrote in message news Hello, * *I have a McIntosh C22 preamp. *I'd like to replace the two diode B+ selenium rectifiers. *One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. *One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. *Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text - - Show quoted text - Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of resistance replacing single-element selenium diodes with silicon. First, wall-plate voltages have climbed considerably here in the US in recent years from a typical ~115V to as much as 125V as a steady-state condition. Second, selenium diodes have a much greater voltage drop than silicon diodes. So, a little bit of additional resistance makes up for that reduced drop. Of course, one should check the loaded B+ after any diode replacement to make sure that it is well within operating specifications anyway. Others may have differing opinions. Peter Wieck Melrose Park, PA |
#5
Posted to rec.audio.tubes
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B+ rectifier question - C22 schematic
C22 schematic is at alt.binaries.schematics.electronic
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#6
Posted to rec.audio.tubes
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B+ rectifier question
"Peter Wieck" wrote in message ... On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote: "Edward Morris" wrote in message news Hello, I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text - - Show quoted text - Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of resistance replacing single-element selenium diodes with silicon. First, wall-plate voltages have climbed considerably here in the US in recent years from a typical ~115V to as much as 125V as a steady-state condition. Second, selenium diodes have a much greater voltage drop than silicon diodes. So, a little bit of additional resistance makes up for that reduced drop. Of course, one should check the loaded B+ after any diode replacement to make sure that it is well within operating specifications anyway. Others may have differing opinions. Peter Wieck Melrose Park, PA Yes. That's a good idea. |
#7
Posted to rec.audio.tubes
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B+ rectifier question
On 12 Dec, 22:00, Peter Wieck wrote:
On Dec 12, 4:04*pm, ":-:Ghost Chip:-:" wrote: "Edward Morris" wrote in message news Hello, * *I have a McIntosh C22 preamp. *I'd like to replace the two diode B+ selenium rectifiers. *One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. *One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. *Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text - - Show quoted text - Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of resistance replacing single-element selenium diodes with silicon. First, wall-plate voltages have climbed considerably here in the US in recent years from a typical ~115V to as much as 125V as a steady-state condition. *Second, selenium diodes have a much greater voltage drop than silicon diodes. So, a little bit of additional resistance makes up for that reduced drop. Of course, one should check the loaded B+ after any diode replacement to make sure that it is well within operating specifications anyway. Others may have differing opinions. Peter Wieck Melrose Park, PA- Hide quoted text - - Show quoted text - Agreed, I would measure the HT with the selenium rectifiers in place (If they still work). Then measure the curent in the red/yellow wire (be carefull if you do this). Use a bit of good ole Ohms law to determine what the equivalent load resistance on the supply is. Disconnect the HT from the rest of the circuit. bodge up a resistor combination of appropriate power rating and connect this to the now disconnected HT (the letters HT should be enough warning) leave the heaters intact for now as if you disconnect them the HT will rise. Now replace the rectifiers with whatever you choose, as long as the voltage/current ratings are ok. Measure the voltage, back to Ohms law again to work out your series resistance needed. Thats the long winded way. I think Peters 10-20R ballpark figure will be fine. There is another way to work it from the AC output from the transformer taking into account diode drop, ripple current etc. This is even more long winded and only really if you are designing from scratch. I just looked at the diagram. Cant believe I just typed all that, 10R will be fine with IN4007s or anything else for that matter. The only thing to watch is the heaters if you change to silicon. Especially as you guys have a higher than designed for mains. In the UK they were suposed to go down from 240V to 230V to get in with the rest of Europe. My mains is 254V in the evening dropping to 250 or so at "put the kettle on watch a crap soap time". I must admit that the distribution transformer is at the end of my garden so thats the reason. I work as a sparky by trade and I have seen as low as 200V in some rural locations. Even seen one house with glowing line taps. Matt. |
#8
Posted to rec.audio.tubes
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B+ rectifier question
Hello all,
Thanks for all your help. What a great bunch of people here! Would you recommend using a resistor between the red/yellow wire and the capacitor? Edward "bigwig" wrote in message ... On 12 Dec, 22:00, Peter Wieck wrote: On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote: "Edward Morris" wrote in message news Hello, I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text - - Show quoted text - Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of resistance replacing single-element selenium diodes with silicon. First, wall-plate voltages have climbed considerably here in the US in recent years from a typical ~115V to as much as 125V as a steady-state condition. Second, selenium diodes have a much greater voltage drop than silicon diodes. So, a little bit of additional resistance makes up for that reduced drop. Of course, one should check the loaded B+ after any diode replacement to make sure that it is well within operating specifications anyway. Others may have differing opinions. Peter Wieck Melrose Park, PA- Hide quoted text - - Show quoted text - Agreed, I would measure the HT with the selenium rectifiers in place (If they still work). Then measure the curent in the red/yellow wire (be carefull if you do this). Use a bit of good ole Ohms law to determine what the equivalent load resistance on the supply is. Disconnect the HT from the rest of the circuit. bodge up a resistor combination of appropriate power rating and connect this to the now disconnected HT (the letters HT should be enough warning) leave the heaters intact for now as if you disconnect them the HT will rise. Now replace the rectifiers with whatever you choose, as long as the voltage/current ratings are ok. Measure the voltage, back to Ohms law again to work out your series resistance needed. Thats the long winded way. I think Peters 10-20R ballpark figure will be fine. There is another way to work it from the AC output from the transformer taking into account diode drop, ripple current etc. This is even more long winded and only really if you are designing from scratch. I just looked at the diagram. Cant believe I just typed all that, 10R will be fine with IN4007s or anything else for that matter. The only thing to watch is the heaters if you change to silicon. Especially as you guys have a higher than designed for mains. In the UK they were suposed to go down from 240V to 230V to get in with the rest of Europe. My mains is 254V in the evening dropping to 250 or so at "put the kettle on watch a crap soap time". I must admit that the distribution transformer is at the end of my garden so thats the reason. I work as a sparky by trade and I have seen as low as 200V in some rural locations. Even seen one house with glowing line taps. Matt. |
#9
Posted to rec.audio.tubes
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B+ rectifier question
On 13 Dec, 07:52, "Edward Morris" wrote:
Hello all, * * Thanks for all your help. *What a great bunch of people here! *Would you recommend using a resistor between the red/yellow wire and the capacitor? Edward "bigwig" wrote in message ... On 12 Dec, 22:00, Peter Wieck wrote: On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote: "Edward Morris" wrote in message news Hello, I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text - - Show quoted text - Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of resistance replacing single-element selenium diodes with silicon. First, wall-plate voltages have climbed considerably here in the US in recent years from a typical ~115V to as much as 125V as a steady-state condition. Second, selenium diodes have a much greater voltage drop than silicon diodes. So, a little bit of additional resistance makes up for that reduced drop. Of course, one should check the loaded B+ after any diode replacement to make sure that it is well within operating specifications anyway. Others may have differing opinions. Peter Wieck Melrose Park, PA- Hide quoted text - - Show quoted text - Agreed, * I would measure the HT with the selenium rectifiers in place (If they still work). Then measure the curent in the red/yellow wire (be carefull if you do this). Use a bit of good ole Ohms law to determine what the equivalent load resistance on the supply is. Disconnect the HT from the rest of the circuit. bodge up a resistor combination of appropriate power rating and connect this to the now disconnected HT (the letters HT should be enough warning) leave the heaters intact for now as if you disconnect them the HT will rise. Now replace the rectifiers with whatever you choose, as long as the voltage/current ratings are ok. Measure the voltage, back to Ohms law again to work out your series resistance needed. * Thats the long winded way. I think Peters 10-20R ballpark figure will be fine. There is another way to work it from the AC output from the transformer taking into account diode drop, ripple current etc. This is even more long winded and only really if you are designing from scratch. * I just looked at the diagram. Cant believe I just typed all that, 10R will be fine with IN4007s or anything else for that matter. The only thing to watch is the heaters if you change to silicon. Especially as you guys have a higher than designed for mains. In the UK they were suposed to go down from 240V to 230V to get in with the rest of Europe. My mains is 254V in the evening dropping to 250 or so at "put the kettle on watch a crap soap time". I must admit that the distribution transformer is at the end of my garden so thats the reason. I work as a sparky by trade and I have seen as low as 200V in some rural locations. Even seen one house with glowing line taps. * *Matt.- Hide quoted text - - Show quoted text - I would stick it in series with SR1. SR2 doesnt really need the series R. |
#10
Posted to rec.audio.tubes
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B+ rectifier question
In article ,
bigwig wrote: On 13 Dec, 07:52, "Edward Morris" wrote: Hello all, * * Thanks for all your help. *What a great bunch of people here! *Would you recommend using a resistor between the red/yellow wire and the capacitor? Edward "bigwig" wrote in message ... On 12 Dec, 22:00, Peter Wieck wrote: On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote: "Edward Morris" wrote in message news Hello, I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text - - Show quoted text - Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of resistance replacing single-element selenium diodes with silicon. First, wall-plate voltages have climbed considerably here in the US in recent years from a typical ~115V to as much as 125V as a steady-state condition. Second, selenium diodes have a much greater voltage drop than silicon diodes. So, a little bit of additional resistance makes up for that reduced drop. Of course, one should check the loaded B+ after any diode replacement to make sure that it is well within operating specifications anyway. Others may have differing opinions. Peter Wieck Melrose Park, PA- Hide quoted text - - Show quoted text - Agreed, * I would measure the HT with the selenium rectifiers in place (If they still work). Then measure the curent in the red/yellow wire (be carefull if you do this). Use a bit of good ole Ohms law to determine what the equivalent load resistance on the supply is. Disconnect the HT from the rest of the circuit. bodge up a resistor combination of appropriate power rating and connect this to the now disconnected HT (the letters HT should be enough warning) leave the heaters intact for now as if you disconnect them the HT will rise. Now replace the rectifiers with whatever you choose, as long as the voltage/current ratings are ok. Measure the voltage, back to Ohms law again to work out your series resistance needed. * Thats the long winded way. I think Peters 10-20R ballpark figure will be fine. There is another way to work it from the AC output from the transformer taking into account diode drop, ripple current etc. This is even more long winded and only really if you are designing from scratch. * I just looked at the diagram. Cant believe I just typed all that, 10R will be fine with IN4007s or anything else for that matter. The only thing to watch is the heaters if you change to silicon. Especially as you guys have a higher than designed for mains. In the UK they were suposed to go down from 240V to 230V to get in with the rest of Europe. My mains is 254V in the evening dropping to 250 or so at "put the kettle on watch a crap soap time". I must admit that the distribution transformer is at the end of my garden so thats the reason. I work as a sparky by trade and I have seen as low as 200V in some rural locations. Even seen one house with glowing line taps. * *Matt.- Hide quoted text - - Show quoted text - I would stick it in series with SR1. SR2 doesnt really need the series R. Why would SR1 need a series R any more, or any less, than SR2? The need, or lack thereof is equal for both diodes. If I were of a mind to use a series R I would connect it in series with the "RED" wire from the PT where it connects to the junction of SR1 & SR2. There does seem to be a minor drawing error in the schematic where a short piece of wire is missing. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#11
Posted to rec.audio.tubes
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B+ rectifier question
On 13 Dec, 18:49, John Byrns wrote:
In article , *bigwig wrote: On 13 Dec, 07:52, "Edward Morris" wrote: Hello all, * * Thanks for all your help. *What a great bunch of people here! *Would you recommend using a resistor between the red/yellow wire and the capacitor? Edward "bigwig" wrote in message .... On 12 Dec, 22:00, Peter Wieck wrote: On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote: "Edward Morris" wrote in message news Hello, I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors.. Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text - - Show quoted text - Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of resistance replacing single-element selenium diodes with silicon. First, wall-plate voltages have climbed considerably here in the US in recent years from a typical ~115V to as much as 125V as a steady-state condition. Second, selenium diodes have a much greater voltage drop than silicon diodes. So, a little bit of additional resistance makes up for that reduced drop. Of course, one should check the loaded B+ after any diode replacement to make sure that it is well within operating specifications anyway. Others may have differing opinions. Peter Wieck Melrose Park, PA- Hide quoted text - - Show quoted text - Agreed, * I would measure the HT with the selenium rectifiers in place (If they still work). Then measure the curent in the red/yellow wire (be carefull if you do this). Use a bit of good ole Ohms law to determine what the equivalent load resistance on the supply is. Disconnect the HT from the rest of the circuit. bodge up a resistor combination of appropriate power rating and connect this to the now disconnected HT (the letters HT should be enough warning) leave the heaters intact for now as if you disconnect them the HT will rise. Now replace the rectifiers with whatever you choose, as long as the voltage/current ratings are ok. Measure the voltage, back to Ohms law again to work out your series resistance needed. * Thats the long winded way. I think Peters 10-20R ballpark figure will be fine. There is another way to work it from the AC output from the transformer taking into account diode drop, ripple current etc. This is even more long winded and only really if you are designing from scratch. * I just looked at the diagram. Cant believe I just typed all that, 10R will be fine with IN4007s or anything else for that matter. The only thing to watch is the heaters if you change to silicon. Especially as you guys have a higher than designed for mains. In the UK they were suposed to go down from 240V to 230V to get in with the rest of Europe. My mains is 254V in the evening dropping to 250 or so at "put the kettle on watch a crap soap time". I must admit that the distribution transformer is at the end of my garden so thats the reason. I work as a sparky by trade and I have seen as low as 200V in some rural locations. Even seen one house with glowing line taps. * *Matt.- Hide quoted text - - Show quoted text - I would stick it in series with SR1. SR2 doesnt really need the series R. Why would SR1 need a series R any more, or any less, than SR2? *The need, or lack thereof is equal for both diodes. *If I were of a mind to use a series R I would connect it in series with the "RED" wire from the PT where it connects to the junction of SR1 & SR2. There does seem to be a minor drawing error in the schematic where a short piece of wire is missing. -- Regards, John Byrns Surf my web pages at, *http://fmamradios.com/- Hide quoted text - - Show quoted text - oops yes both diodes. You could just put one in at the junction of the two. |
#12
Posted to rec.audio.tubes
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B+ rectifier question
"John Byrns" wrote in message ... In article , bigwig wrote: On 13 Dec, 07:52, "Edward Morris" wrote: Hello all, Thanks for all your help. What a great bunch of people here! Would you recommend using a resistor between the red/yellow wire and the capacitor? Edward "bigwig" wrote in message ... On 12 Dec, 22:00, Peter Wieck wrote: On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote: "Edward Morris" wrote in message news Hello, I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. Does anyone have a ballpark guess what value of resistors I need for the voltage drop. Thanks in advance, Eddie Refer to the schematic. Replace SR1 and SR2 with 1n4007 diodes. Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted text - - Show quoted text - Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of resistance replacing single-element selenium diodes with silicon. First, wall-plate voltages have climbed considerably here in the US in recent years from a typical ~115V to as much as 125V as a steady-state condition. Second, selenium diodes have a much greater voltage drop than silicon diodes. So, a little bit of additional resistance makes up for that reduced drop. Of course, one should check the loaded B+ after any diode replacement to make sure that it is well within operating specifications anyway. Others may have differing opinions. Peter Wieck Melrose Park, PA- Hide quoted text - - Show quoted text - Agreed, I would measure the HT with the selenium rectifiers in place (If they still work). Then measure the curent in the red/yellow wire (be carefull if you do this). Use a bit of good ole Ohms law to determine what the equivalent load resistance on the supply is. Disconnect the HT from the rest of the circuit. bodge up a resistor combination of appropriate power rating and connect this to the now disconnected HT (the letters HT should be enough warning) leave the heaters intact for now as if you disconnect them the HT will rise. Now replace the rectifiers with whatever you choose, as long as the voltage/current ratings are ok. Measure the voltage, back to Ohms law again to work out your series resistance needed. Thats the long winded way. I think Peters 10-20R ballpark figure will be fine. There is another way to work it from the AC output from the transformer taking into account diode drop, ripple current etc. This is even more long winded and only really if you are designing from scratch. I just looked at the diagram. Cant believe I just typed all that, 10R will be fine with IN4007s or anything else for that matter. The only thing to watch is the heaters if you change to silicon. Especially as you guys have a higher than designed for mains. In the UK they were suposed to go down from 240V to 230V to get in with the rest of Europe. My mains is 254V in the evening dropping to 250 or so at "put the kettle on watch a crap soap time". I must admit that the distribution transformer is at the end of my garden so thats the reason. I work as a sparky by trade and I have seen as low as 200V in some rural locations. Even seen one house with glowing line taps. Matt.- Hide quoted text - - Show quoted text - I would stick it in series with SR1. SR2 doesnt really need the series R. Why would SR1 need a series R any more, or any less, than SR2? The need, or lack thereof is equal for both diodes. If I were of a mind to use a series R I would connect it in series with the "RED" wire from the PT where it connects to the junction of SR1 & SR2. There does seem to be a minor drawing error in the schematic where a short piece of wire is missing. -- Regards, John Byrns Wow. What good eyes you have. I had to zoom to 400 to see the little "poor solder joint." Good idea; 1 resistor in the red wire will do nicely. This circuit is a voltage doubler riding on top of the rectified 27.6 volts for the filaments. There's more than one way to get HV. Ghost |
#13
Posted to rec.audio.tubes
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B+ rectifier question
Edward Morris wondered:
I have a McIntosh C22 preamp. I'd like to replace the two diode B+ selenium rectifiers. One lead from the transformer attaches to the + and - leads together in the middle of the rectifiers and the other lead attaches to a 40mfd capacitor. One outside rectifier leg attaches to the bridge rectifier that powers the filaments and the other leg ties into a common tie with a number of resistors that tie into a number of capacitors. Does anyone have a ballpark guess what value of resistors I need for the voltage drop Just been trying to find out about selenium rectifiers. It seems they can have several cells in series, and each cell can vary in several ways. The voltage drop can be measured, or could have been estimated from an appropriate data sheet which is unlikely to be available now. If you can measure the drop across and current through each one, you can get an idea of what resistor you would need in series with each, using ohm's law. The waveforms aren't sinusoidal, though, so you would need to measure the two final voltages and adjust to suit, depending on how exact they need to be. You would also need to be careful with power ratings, for the same reason. A much easier way is to measure the resistance and open-circuit voltage of the transformer secondaries, and then simulate the circuit with SPICE. Especially if you don't happen to have a stock of real resistors to try. If you can measure but are unable to simulate, someone here might do that for you. Or perhaps someone has already made a near-enough guess? There is a question of strategy. Would it be better to add resistance to the heater circuit after the feed to the HT, and to add no resistance to that feed, thus raising the floor of the HT circuit. Then add resistance to the HT after the doubler? It would be a bit interesting to find the optimum arrangement. I wonder how you should dispose of the old rectifiers? Authorities here are, quite rightly I suppose, getting very touchy about what goes in the bin. I noticed, passing a so called "health food" shop the other day, that selenium is essential for fitness, so perhaps you could grind them up and eat them? Ian |
#14
Posted to rec.audio.tubes
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B+ rectifier question
On Dec 15, 9:17*am, "Ian Iveson"
wrote: Just been trying to find out about selenium rectifiers. It seems they can have several cells in series, and each cell can vary in several ways. The voltage drop can be measured, or could have been estimated from an appropriate data sheet which is unlikely to be available now. If you can measure the drop across and current through each one, you can get an idea of what resistor you would need in series with each, using ohm's law. The waveforms aren't sinusoidal, though, so you would need to measure the two final voltages and adjust to suit, depending on how exact they need to be. You would also need to be careful with power ratings, for the same reason. A much easier way is to measure the resistance and open-circuit voltage of the transformer secondaries, and then simulate the circuit with SPICE. Especially if you don't happen to have a stock of real resistors to try. If you can measure but are unable to simulate, someone here might do that for you. Or perhaps someone has already made a near-enough guess? There is a question of strategy. Would it be better to add resistance to the heater circuit after the feed to the HT, and to add no resistance to that feed, thus raising the floor of the HT circuit. Then add resistance to the HT after the doubler? It would be a bit interesting to find the optimum arrangement. I wonder how you should dispose of the old rectifiers? Authorities here are, quite rightly I suppose, getting very touchy about what goes in the bin. I noticed, passing a so called "health food" shop the other day, that selenium is essential for fitness, so perhaps you could grind them up and eat them? WOW ------ WAY, way to complicated. Selenium rectifiers have a much higher voltage drop than silicon diodes. In *some* (but not all) circuits where the down-line voltage is critical, that drop must be replaced if the selenium diodes are replaced with silicon. So far, so good. Not all selenium rectifiers are created equal. Some are single- element, some are stacks of 2 - 20-or-more depending on the current they are to carry. The specific SIZE of the diode does not matter, the NUMBER OF ELEMENTS (stack) does matter relative to this question. Rule-of-thumb, all other things being equal (Ed's ballpark) is 10 ohms per element. After which additional resistance *may* be desired to further reduce B+ (or C- if a bias circuit) to make up for higher line (wallplate) voltage common in some areas these days. Naturally, this is to be verified in the field by actual measurement. Ed was requesting ball-park numbers for a single-element system. 10 - 20 ohms covers the from home plate to far-away center field. Close enough. Sometimes a small brass hammer in the hands of a Vermont Blacksmith is all that is necessary. In our township, one may drop "hazardous waste" at the township transfer station for separate disposal without additional charge. In general, I collect this sort of stuff in an empty (clean) paint can - when it gets nearly full (perhaps once per year), I drop it off at the transfer station about 2 miles away. From there it is taken to a certified hazardous-waste landfill for proper disposal. Peter Wieck Melrose Park, PA |
#15
Posted to rec.audio.tubes
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B+ rectifier question
Peter Wieck wrote:
Just been trying to find out about selenium rectifiers. It seems they can have several cells in series, and each cell can vary in several ways. The voltage drop can be measured, or could have been estimated from an appropriate data sheet which is unlikely to be available now. If you can measure the drop across and current through each one, you can get an idea of what resistor you would need in series with each, using ohm's law. The waveforms aren't sinusoidal, though, so you would need to measure the two final voltages and adjust to suit, depending on how exact they need to be. You would also need to be careful with power ratings, for the same reason. A much easier way is to measure the resistance and open-circuit voltage of the transformer secondaries, and then simulate the circuit with SPICE. Especially if you don't happen to have a stock of real resistors to try. If you can measure but are unable to simulate, someone here might do that for you. Or perhaps someone has already made a near-enough guess? There is a question of strategy. Would it be better to add resistance to the heater circuit after the feed to the HT, and to add no resistance to that feed, thus raising the floor of the HT circuit. Then add resistance to the HT after the doubler? It would be a bit interesting to find the optimum arrangement. *** WOW ------ WAY, way to complicated. Selenium rectifiers have a much higher voltage drop than silicon diodes. In *some* (but not all) circuits where the down-line voltage is critical, that drop must be replaced if the selenium diodes are replaced with silicon. So far, so good. Not all selenium rectifiers are created equal. Some are single- element, some are stacks of 2 - 20-or-more depending on the current they are to carry. The specific SIZE of the diode does not matter, the NUMBER OF ELEMENTS (stack) does matter relative to this question. Rule-of-thumb, all other things being equal (Ed's ballpark) is 10 ohms per element. After which additional resistance *may* be desired to further reduce B+ (or C- if a bias circuit) to make up for higher line (wallplate) voltage common in some areas these days. Naturally, this is to be verified in the field by actual measurement. Ed was requesting ball-park numbers for a single-element system. 10 - 20 ohms covers the from home plate to far-away center field. Close enough. Sometimes a small brass hammer in the hands of a Vermont Blacksmith is all that is necessary. *** The situation is no less complicated than I wrote. You may believe that the complications don't matter, and Edward may not care. We don't have ball-parks here in England, but anyway, if Edward asks what's the right ball-park voltage for his heaters, would 35V be in the right field? How close is close enough for Edward? We don't even know the voltage ratings of his capacitors. If the voltage drop across a selenium diode is truly "much" greater than that of a silicon diode, and particularly if it is a true diode drop rather than the result of inherent series resistance, then the resistance necessary to restore it will depend on the current through the diode. So if, say, the current is 1A, then 10 ohms will result in a 10V reduction, whereas if the current is 10mA, then 10 ohms will result in a drop of only 100mV, each assuming DC or sinusoidal current. Now, the circuit in question includes a relatively high-current heater supply, and a low-current HT supply, and an even-lower-current link between the two. Further, the heater supply uses a bridge rectifier, whereas the other two are half-wave. Further still, the high-current supply is relatively low voltage, so the greater voltage difference will be a much more significant proportion of the supply voltage. What is more, we don't know the specifications of Edwards selenium diodes, although the knowledgeable may be able to divine them from the voltages. If 10-20 ohms is a good guess for one, it is unlikely to be a good guess for the others, surely? Included in your answer is the assumption that Edward doesn't care too much about the consequent voltages. Maybe you're right, and maybe they will be close enough for reasonable operation. In which case, considering 20 ohms will have so little effect in the two lower-current situations, he could leave them out and concentrate on getting the heater voltage right. What power rating should the added resistor(s) be, there, and exactly where in the circuit should he put it, or them, do you think? A simulation would not be complicated at all, BTW, unless an exact mean power dissipation is required because that's awkward to do. Perhaps Edward will report the results of his experiments. I'm quite interested. I wonder how you should dispose of the old rectifiers? Authorities here are, quite rightly I suppose, getting very touchy about what goes in the bin. I noticed, passing a so called "health food" shop the other day, that selenium is essential for fitness, so perhaps you could grind them up and eat them? *** In our township, one may drop "hazardous waste" at the township transfer station for separate disposal without additional charge. In general, I collect this sort of stuff in an empty (clean) paint can - when it gets nearly full (perhaps once per year), I drop it off at the transfer station about 2 miles away. From there it is taken to a certified hazardous-waste landfill for proper disposal. *** Probably the same here. I haven't managed to fill my jar yet...just one dead valve and one extremely dead tantalum capacitor. If there were a damaged selenium diode in there I would want to get rid of it sooner. Ian |
#16
Posted to rec.audio.tubes
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B+ rectifier question
On Dec 15, 12:10 pm, "Ian Iveson"
wrote: The situation is no less complicated than I wrote. You may believe that the complications don't matter, and Edward may not care. Um... um... But, most of the parameters are already given by the nature of the beast in question - single-element selenium diode(s), in a McIntosh C22 pre-amp. So, the application is low-current, single-element diodes. Wattage on the resistor will be 2W or less, thereabouts. I would counsel a 5- watter as they are certainly cheap enough for the insurance. And 10 ohms/element by l-o-n-g experience in low-current applications of this nature (lots-O-vintage radios have selenium diodes in various applications as well as a fair amount of vintage audio stuff) if straight-up voltage-drop replacement is required. Beyond that, one must experiment (or, Heaven forfend!! *Calculate*) the additional or lower/none resistance wished. Were this a piece of unknown antecedents, brand or application, I would not have been so glib. Sometimes, a Cigar is just a Cigar. Peter Wieck Melrose Park, PA |
#17
Posted to rec.audio.tubes
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B+ rectifier question
Hey guys,
I appreciate all the input. I replaced all the bumble bee capacitors. I replaced all the electrolytic capacitors (cans if possible). I couldn't find the can for the filament circuit so I had to go with 3 separate capacitors for that circuit. I went with keeping the capacitor voltages the same as the originals or at least close. I replaced the resistors in the power supply as well as the filament bridge which was originally 4 separate diodes mounted on a strip and I replaced the original 2 diodes for the B+. I have to run the preamp on a variac at 110v because the filament voltage tested at each tube is right at 12 volts. My goal is to get the resistance added necessary to be able to plug the preamp into the wall outlet about 123V but I also don't want to add too much more heat to the innards. The filament circuit pulls a lot of current. I've got a 6 amp bridge for the filament circuit and it runs warm. The resistors (2-7.5 ohm 5 watt) run so hot you can even touch them. But they were that way before the new parts. The preamp at this point sounds great. I've used it running from the variac for a number of months now with no problems and the PT runs just a little warm. I have a number of resistors to be able to experiment. It's just getting the thing unhooked and out of my cabinet. I know I sound lazy. I guess I just don't like the idea of getting it out again. It was a lot of work at time to (rebuild it). You guys know what I mean? I have the schematic so I know the recommended voltages. I know just about enough about electronics to be dangerous. If you go to www.nosvalves.com/C22.htm you can see the 2 grey B+ diodes at the left of the pictures. I still have them but they don't have stacks per se like I've seen some other selenium rectifiers have. Do you recommend for the B+ using resistance before the diodes or after? Also the 2 HT wires Red and Red/Yellow; do I need to install a resistor between the capacitor and the Red/Yellow wire; the voltage is running a little high, or will reducing the B+ voltage take care of that? Thanks again, Edward Morris "Ian Iveson" wrote in message ... Peter Wieck wrote: Just been trying to find out about selenium rectifiers. It seems they can have several cells in series, and each cell can vary in several ways. The voltage drop can be measured, or could have been estimated from an appropriate data sheet which is unlikely to be available now. If you can measure the drop across and current through each one, you can get an idea of what resistor you would need in series with each, using ohm's law. The waveforms aren't sinusoidal, though, so you would need to measure the two final voltages and adjust to suit, depending on how exact they need to be. You would also need to be careful with power ratings, for the same reason. A much easier way is to measure the resistance and open-circuit voltage of the transformer secondaries, and then simulate the circuit with SPICE. Especially if you don't happen to have a stock of real resistors to try. If you can measure but are unable to simulate, someone here might do that for you. Or perhaps someone has already made a near-enough guess? There is a question of strategy. Would it be better to add resistance to the heater circuit after the feed to the HT, and to add no resistance to that feed, thus raising the floor of the HT circuit. Then add resistance to the HT after the doubler? It would be a bit interesting to find the optimum arrangement. *** WOW ------ WAY, way to complicated. Selenium rectifiers have a much higher voltage drop than silicon diodes. In *some* (but not all) circuits where the down-line voltage is critical, that drop must be replaced if the selenium diodes are replaced with silicon. So far, so good. Not all selenium rectifiers are created equal. Some are single- element, some are stacks of 2 - 20-or-more depending on the current they are to carry. The specific SIZE of the diode does not matter, the NUMBER OF ELEMENTS (stack) does matter relative to this question. Rule-of-thumb, all other things being equal (Ed's ballpark) is 10 ohms per element. After which additional resistance *may* be desired to further reduce B+ (or C- if a bias circuit) to make up for higher line (wallplate) voltage common in some areas these days. Naturally, this is to be verified in the field by actual measurement. Ed was requesting ball-park numbers for a single-element system. 10 - 20 ohms covers the from home plate to far-away center field. Close enough. Sometimes a small brass hammer in the hands of a Vermont Blacksmith is all that is necessary. *** The situation is no less complicated than I wrote. You may believe that the complications don't matter, and Edward may not care. We don't have ball-parks here in England, but anyway, if Edward asks what's the right ball-park voltage for his heaters, would 35V be in the right field? How close is close enough for Edward? We don't even know the voltage ratings of his capacitors. If the voltage drop across a selenium diode is truly "much" greater than that of a silicon diode, and particularly if it is a true diode drop rather than the result of inherent series resistance, then the resistance necessary to restore it will depend on the current through the diode. So if, say, the current is 1A, then 10 ohms will result in a 10V reduction, whereas if the current is 10mA, then 10 ohms will result in a drop of only 100mV, each assuming DC or sinusoidal current. Now, the circuit in question includes a relatively high-current heater supply, and a low-current HT supply, and an even-lower-current link between the two. Further, the heater supply uses a bridge rectifier, whereas the other two are half-wave. Further still, the high-current supply is relatively low voltage, so the greater voltage difference will be a much more significant proportion of the supply voltage. What is more, we don't know the specifications of Edwards selenium diodes, although the knowledgeable may be able to divine them from the voltages. If 10-20 ohms is a good guess for one, it is unlikely to be a good guess for the others, surely? Included in your answer is the assumption that Edward doesn't care too much about the consequent voltages. Maybe you're right, and maybe they will be close enough for reasonable operation. In which case, considering 20 ohms will have so little effect in the two lower-current situations, he could leave them out and concentrate on getting the heater voltage right. What power rating should the added resistor(s) be, there, and exactly where in the circuit should he put it, or them, do you think? A simulation would not be complicated at all, BTW, unless an exact mean power dissipation is required because that's awkward to do. Perhaps Edward will report the results of his experiments. I'm quite interested. I wonder how you should dispose of the old rectifiers? Authorities here are, quite rightly I suppose, getting very touchy about what goes in the bin. I noticed, passing a so called "health food" shop the other day, that selenium is essential for fitness, so perhaps you could grind them up and eat them? *** In our township, one may drop "hazardous waste" at the township transfer station for separate disposal without additional charge. In general, I collect this sort of stuff in an empty (clean) paint can - when it gets nearly full (perhaps once per year), I drop it off at the transfer station about 2 miles away. From there it is taken to a certified hazardous-waste landfill for proper disposal. *** Probably the same here. I haven't managed to fill my jar yet...just one dead valve and one extremely dead tantalum capacitor. If there were a damaged selenium diode in there I would want to get rid of it sooner. Ian |
#18
Posted to rec.audio.tubes
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B+ rectifier question
Ed:
Please note the interpolations: On Dec 15, 2:26*pm, "Edward Morris" wrote: Hey guys, * * I appreciate all the input. *I replaced all the bumble bee capacitors. I replaced all the electrolytic capacitors (cans if possible). *I couldn't find the can for the filament circuit so I had to go with 3 separate capacitors for that circuit. *I went with keeping the capacitor voltages the same as the originals or at least close. *I replaced the resistors in the power supply as well as the filament bridge which was originally 4 separate diodes mounted on a strip and I replaced the original 2 diodes for the B+.. I have to run the preamp on a variac at 110v because the filament voltage tested at each tube is right at 12 volts. Actually, the filaments for a 12AX7 are 12.6V nominal. Running at 12V will help tube life somewhat, and not materially affect peformance. *My goal is to get the resistance added necessary to be able to plug the preamp into the wall outlet about 123V but I also don't want to add too much more heat to the innards. *The filament circuit pulls a lot of current. *I've got a 6 amp bridge for the filament circuit and it runs warm. *The resistors (2-7.5 ohm 5 watt) run so hot you can even touch them. *But they were that way before the new parts. Too hot! Nothing should get that hot inside that amp. I know that in many cases OEM equipment does have dropping resistors that do run seriously hot - my preference would be to replace them with a higher wattage resistors - but in this case, pay _special_ attention to fusing. Heat and equipment are enemies. Heat and phenolic circuit boards are enemies. So, if you have put in silicon diodes and are running at 110V to get 12v at the filaments with 7.5 ohm resistors, I would *suggest* you go with 20 ohms (additional 12.5 ohms) at 5-watts and see what you get at 124V. Again, you are working with single-element (originally) diodes, so a massive amount of additional resistance over the OEM 7.5ohms will not be needed. The preamp at this point sounds great. *I've used it running from the variac for a number of months now with no problems and the PT runs just a little warm. What is "just a little"? Egg-frying hot after a few hours/minutes/ ever? Or warm-to-the-touch-but-no-more forever? T The latter is what you want. *I have a number of resistors to be able to experiment. *It's just getting the thing unhooked and out of my cabinet. *I know I sound lazy. *I guess I just don't like the idea of getting it out again. *It was a lot of work at time to (rebuild it). *You guys know what I mean? *I have the schematic so I know the recommended voltages. *I know just about enough about electronics to be dangerous. *If you go towww.nosvalves.com/C22.htm you can see the 2 grey B+ diodes at the left of the pictures. *I still have them but they don't have stacks per se like I've seen some other selenium rectifiers have. *Do you recommend for the B+ using resistance before the diodes or after? Always _after_ the diodes. *Also the 2 HT wires Red and Red/Yellow; do I need to install a resistor between the capacitor and the Red/Yellow wire; the voltage is running a little high, or will reducing the B+ voltage take care of that? It should. But when you finally walk away from the amp, the voltage on the caps should be at/slightly below the listed voltage(s), and B+ should be at or slightly below the listed voltage(s). But you would like to work to as few points as possble, not have new resistors spread all over the place. Further, be sure you document what you have done so you or your successors do not have to re-invent the wheel each time you service the amp in the future. Peter Wieck Melrose Park, PA |
#19
Posted to rec.audio.tubes
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B+ rectifier question
Hi Peter,
I can leave the preamp on for hours on end and the PT only gets slightly warm. You can hold your hand over it and it's not uncomfortable at all. I'll do what you suggest and give you an update. I used an volt meter and measured voltage from the chassis to ground. It's pretty low (around 8 volts) and just a little difference when changing the position of the plug in the outlet. The preamp had 6 Telefunken 12AX7's and I replaced them with Sovtek 12AX7LPS. Do you think the Sovtek's require more filament current than the Telefunken? I guess now it a matter of getting the voltages lower so I can safely use it without a variac. Thank to all, Edward Morris "Peter Wieck" wrote in message ... Ed: Please note the interpolations: On Dec 15, 2:26 pm, "Edward Morris" wrote: Hey guys, I appreciate all the input. I replaced all the bumble bee capacitors. I replaced all the electrolytic capacitors (cans if possible). I couldn't find the can for the filament circuit so I had to go with 3 separate capacitors for that circuit. I went with keeping the capacitor voltages the same as the originals or at least close. I replaced the resistors in the power supply as well as the filament bridge which was originally 4 separate diodes mounted on a strip and I replaced the original 2 diodes for the B+. I have to run the preamp on a variac at 110v because the filament voltage tested at each tube is right at 12 volts. Actually, the filaments for a 12AX7 are 12.6V nominal. Running at 12V will help tube life somewhat, and not materially affect peformance. My goal is to get the resistance added necessary to be able to plug the preamp into the wall outlet about 123V but I also don't want to add too much more heat to the innards. The filament circuit pulls a lot of current. I've got a 6 amp bridge for the filament circuit and it runs warm. The resistors (2-7.5 ohm 5 watt) run so hot you can even touch them. But they were that way before the new parts. Too hot! Nothing should get that hot inside that amp. I know that in many cases OEM equipment does have dropping resistors that do run seriously hot - my preference would be to replace them with a higher wattage resistors - but in this case, pay _special_ attention to fusing. Heat and equipment are enemies. Heat and phenolic circuit boards are enemies. So, if you have put in silicon diodes and are running at 110V to get 12v at the filaments with 7.5 ohm resistors, I would *suggest* you go with 20 ohms (additional 12.5 ohms) at 5-watts and see what you get at 124V. Again, you are working with single-element (originally) diodes, so a massive amount of additional resistance over the OEM 7.5ohms will not be needed. The preamp at this point sounds great. I've used it running from the variac for a number of months now with no problems and the PT runs just a little warm. What is "just a little"? Egg-frying hot after a few hours/minutes/ ever? Or warm-to-the-touch-but-no-more forever? T The latter is what you want. I have a number of resistors to be able to experiment. It's just getting the thing unhooked and out of my cabinet. I know I sound lazy. I guess I just don't like the idea of getting it out again. It was a lot of work at time to (rebuild it). You guys know what I mean? I have the schematic so I know the recommended voltages. I know just about enough about electronics to be dangerous. If you go towww.nosvalves.com/C22.htm you can see the 2 grey B+ diodes at the left of the pictures. I still have them but they don't have stacks per se like I've seen some other selenium rectifiers have. Do you recommend for the B+ using resistance before the diodes or after? Always _after_ the diodes. Also the 2 HT wires Red and Red/Yellow; do I need to install a resistor between the capacitor and the Red/Yellow wire; the voltage is running a little high, or will reducing the B+ voltage take care of that? It should. But when you finally walk away from the amp, the voltage on the caps should be at/slightly below the listed voltage(s), and B+ should be at or slightly below the listed voltage(s). But you would like to work to as few points as possble, not have new resistors spread all over the place. Further, be sure you document what you have done so you or your successors do not have to re-invent the wheel each time you service the amp in the future. Peter Wieck Melrose Park, PA |
#20
Posted to rec.audio.tubes
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B+ rectifier question
Hello,
I used 330 ohm resistors for each of the replacement B+ diodes and the highest B+ voltage is just under 4 volts from the schematic and I replaced a 7.5 ohm 5 watt with a 14 ohm 5 watt resistor in the filament supply and the filaments are running 12 volts. Thanks everyone for all your help, Edward "flipper" wrote in message ... On Mon, 15 Dec 2008 17:10:28 -0000, "Ian Iveson" wrote: Peter Wieck wrote: Just been trying to find out about selenium rectifiers. It seems they can have several cells in series, and each cell can vary in several ways. The voltage drop can be measured, or could have been estimated from an appropriate data sheet which is unlikely to be available now. If you can measure the drop across and current through each one, you can get an idea of what resistor you would need in series with each, using ohm's law. The waveforms aren't sinusoidal, though, so you would need to measure the two final voltages and adjust to suit, depending on how exact they need to be. You would also need to be careful with power ratings, for the same reason. A much easier way is to measure the resistance and open-circuit voltage of the transformer secondaries, and then simulate the circuit with SPICE. Especially if you don't happen to have a stock of real resistors to try. If you can measure but are unable to simulate, someone here might do that for you. Or perhaps someone has already made a near-enough guess? There is a question of strategy. Would it be better to add resistance to the heater circuit after the feed to the HT, and to add no resistance to that feed, thus raising the floor of the HT circuit. Then add resistance to the HT after the doubler? It would be a bit interesting to find the optimum arrangement. *** WOW ------ WAY, way to complicated. Selenium rectifiers have a much higher voltage drop than silicon diodes. In *some* (but not all) circuits where the down-line voltage is critical, that drop must be replaced if the selenium diodes are replaced with silicon. So far, so good. Not all selenium rectifiers are created equal. Some are single- element, some are stacks of 2 - 20-or-more depending on the current they are to carry. The specific SIZE of the diode does not matter, the NUMBER OF ELEMENTS (stack) does matter relative to this question. Rule-of-thumb, all other things being equal (Ed's ballpark) is 10 ohms per element. After which additional resistance *may* be desired to further reduce B+ (or C- if a bias circuit) to make up for higher line (wallplate) voltage common in some areas these days. Naturally, this is to be verified in the field by actual measurement. Ed was requesting ball-park numbers for a single-element system. 10 - 20 ohms covers the from home plate to far-away center field. Close enough. Sometimes a small brass hammer in the hands of a Vermont Blacksmith is all that is necessary. *** The situation is no less complicated than I wrote. You may believe that the complications don't matter, and Edward may not care. We don't have ball-parks here in England, but anyway, if Edward asks what's the right ball-park voltage for his heaters, would 35V be in the right field? How close is close enough for Edward? We don't even know the voltage ratings of his capacitors. If the voltage drop across a selenium diode is truly "much" greater than that of a silicon diode, and particularly if it is a true diode drop rather than the result of inherent series resistance, then the resistance necessary to restore it will depend on the current through the diode. So if, say, the current is 1A, then 10 ohms will result in a 10V reduction, whereas if the current is 10mA, then 10 ohms will result in a drop of only 100mV, each assuming DC or sinusoidal current. Now, the circuit in question includes a relatively high-current heater supply, and a low-current HT supply, and an even-lower-current link between the two. Further, the heater supply uses a bridge rectifier, whereas the other two are half-wave. Further still, the high-current supply is relatively low voltage, so the greater voltage difference will be a much more significant proportion of the supply voltage. What is more, we don't know the specifications of Edwards selenium diodes, although the knowledgeable may be able to divine them from the voltages. If 10-20 ohms is a good guess for one, it is unlikely to be a good guess for the others, surely? Included in your answer is the assumption that Edward doesn't care too much about the consequent voltages. Maybe you're right, and maybe they will be close enough for reasonable operation. In which case, considering 20 ohms will have so little effect in the two lower-current situations, he could leave them out and concentrate on getting the heater voltage right. What power rating should the added resistor(s) be, there, and exactly where in the circuit should he put it, or them, do you think? A simulation would not be complicated at all, BTW, unless an exact mean power dissipation is required because that's awkward to do. Perhaps Edward will report the results of his experiments. I'm quite interested. Sounds to me like there are a number of assumptions but, first... Individual selenium cells are only good for, depending on the particular unit, between 20V and 45V reverse voltage and that's the reason for the stacking. I think 20V was the most common value back then. The metal plate inherent to the construction acts as a heatsink and the 'rule of thumb' I heard was about 500mA per sq inch. The break over knee actually looks a bit lower per cell (about .45V) than your typical silicon diode but, then, you seldom have one cell in HV rectifiers because of the low reverse breakdown voltage. Impedance in the 500mA area looks to be between .67 and 1 ohm per cell. Also, by silicon standards they're quite leaky, passing as much as 6mA in reverse at the rated voltage. I wonder how you should dispose of the old rectifiers? Authorities here are, quite rightly I suppose, getting very touchy about what goes in the bin. I noticed, passing a so called "health food" shop the other day, that selenium is essential for fitness, so perhaps you could grind them up and eat them? *** In our township, one may drop "hazardous waste" at the township transfer station for separate disposal without additional charge. In general, I collect this sort of stuff in an empty (clean) paint can - when it gets nearly full (perhaps once per year), I drop it off at the transfer station about 2 miles away. From there it is taken to a certified hazardous-waste landfill for proper disposal. *** Probably the same here. I haven't managed to fill my jar yet...just one dead valve and one extremely dead tantalum capacitor. If there were a damaged selenium diode in there I would want to get rid of it sooner. Ian |
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