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#1
Posted to rec.audio.tubes
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Diodes on Cathode?
Hi,
what is the purpose of the reverse biased diodes on the OPT/Cathodes as in this Fender circuit? http://www.schematicheaven.com/fende...er_60_rack.pdf I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? Thanks, Gareth. |
#2
Posted to rec.audio.tubes
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Diodes on Cathode?
In article ,
"Gareth Magennis" wrote: Hi, what is the purpose of the reverse biased diodes on the OPT/Cathodes as in this Fender circuit? http://www.schematicheaven.com/fende...er_60_rack.pdf I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? I don't see any reversed biased diodes on the OPT/Cathodes, do you mean the reversed biased diodes on the OPT/Anodes? I believe the purpose of these diodes is to clamp the peak anode voltage at twice the B+ voltage, limiting the magnitude of the high voltage spikes that can be produced when the amp is overdriven into an inductive load. These voltage spikes can destroy tubes and transformers if not controlled. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#3
Posted to rec.audio.tubes
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Diodes on Cathode?
"Gareth Magennis" Hi, what is the purpose of the reverse biased diodes on the OPT/Cathodes as in this Fender circuit? ** They are not simple diodes, nor are they on the cathodes. They ARE diode strings wired from each plate to ground. http://www.schematicheaven.com/fende...er_60_rack.pdf ** The two " R3000 " prevent the plate voltage from swinging negative by more than 2 volts or so - something all tube amps do when overdriven into a reactive ( ie speaker) load. Peak negative plate voltage can exceed 2000 volts and damage the insulation of the tube, the socket or the OT. I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? ** The later is generally OK. Use " fast " diodes, if possible ........ Phil |
#4
Posted to rec.audio.tubes
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Diodes on Cathode?
"Gareth Magennis" said:
what is the purpose of the reverse biased diodes on the OPT/Cathodes as in this Fender circuit? http://www.schematicheaven.com/fende...er_60_rack.pdf I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? I use UF4007 or UF5408 diodes for that purpose, 3 in series. Don't use 1N4007 or 1N5408, they're too slow for this application. The reason why they are there is already explained by Phil Allison. -- - Maggies are an addiction for life. - |
#5
Posted to rec.audio.tubes
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Diodes on Cathode?
Gareth Magennis wrote: Hi, what is the purpose of the reverse biased diodes on the OPT/Cathodes as in this Fender circuit? http://www.schematicheaven.com/fende...er_60_rack.pdf I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? The diodes prevent the anodes ever swinging below 0V when no load ins connected and the volume is turnd up. Without the diodes, the OPT primary peak signal voltage can be up to say +2,000V on one side, and -1,000V on the other OPT side. This can cause arcing in the OPT and the windings to fuse. If you wish to use alternative diodes, i suggest 3 seriesed IN5408 which are each PIV rated for 1,000V, have 3amp rating, and you should have 1M resistances across each diode. Then the voltage max at one of the OPT can only be +1,000V and -2.1V at the other, and arcing won't happen. Patrick Turner. Thanks, Gareth. |
#6
Posted to rec.audio.tubes
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Diodes on Cathode?
John Byrns wrote: In article , "Gareth Magennis" wrote: Hi, what is the purpose of the reverse biased diodes on the OPT/Cathodes as in this Fender circuit? http://www.schematicheaven.com/fende...er_60_rack.pdf I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? I don't see any reversed biased diodes on the OPT/Cathodes, do you mean the reversed biased diodes on the OPT/Anodes? I believe the purpose of these diodes is to clamp the peak anode voltage at twice the B+ voltage, limiting the magnitude of the high voltage spikes that can be produced when the amp is overdriven into an inductive load. These voltage spikes can destroy tubes and transformers if not controlled. Regards, John Byrns Your'e right. Patrick Turner. -- Surf my web pages at, http://fmamradios.com/ |
#7
Posted to rec.audio.tubes
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Diodes on Cathode?
"Phil Allison" wrote in message ... "Gareth Magennis" Hi, what is the purpose of the reverse biased diodes on the OPT/Cathodes as in this Fender circuit? ** They are not simple diodes, nor are they on the cathodes. They ARE diode strings wired from each plate to ground. http://www.schematicheaven.com/fende...er_60_rack.pdf ** The two " R3000 " prevent the plate voltage from swinging negative by more than 2 volts or so - something all tube amps do when overdriven into a reactive ( ie speaker) load. Peak negative plate voltage can exceed 2000 volts and damage the insulation of the tube, the socket or the OT. I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? ** The later is generally OK. Use " fast " diodes, if possible Guitar amp technicians seem to refer to them as "flyback" diodes. This may or may not be a general term. Iain ....... Phil |
#8
Posted to rec.audio.tubes
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Diodes on Cathode?
On Mon, 11 Jun 2007 22:26:51 +0300, "Iain Churches"
wrote: Guitar amp technicians seem to refer to them as "flyback" diodes. This may or may not be a general term. That's an excellent choice of a name. It comes from old TV receiver practice, where the horizontal sweep output transformer was combined with a high-ratio step-up "secondary" and rectified to make the multi-KiloJolts needed for CRT anodes. The transformer itself (later with internal high voltage rectifiers) was called the "flyback" transformer, because its primary was shunted with a "reverse" biased diode, whose purpose was to fly the flying spot back from right to left. The vacuum valve diodes used for this gig in their day are very popular now as slow-heating rectifiers with amazingly large heater-cathode voltage ratings (unidirectional! arf.). They're commonly called damper diodes. Much thanks, as always, Chris Hornbeck |
#9
Posted to rec.audio.tubes
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Diodes on Cathode?
"Gareth Magennis" wrote in message ... Hi, what is the purpose of the reverse biased diodes on the OPT/Cathodes as in this Fender circuit? http://www.schematicheaven.com/fende...er_60_rack.pdf I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? Thanks, Gareth. Thanks for all the information and added confidence. (Incidentally, I had measured some good R3000's from another amp which showed a higher than single PN junction Vf - Phil has given the reason why). Cheers, Gareth. |
#10
Posted to rec.audio.tubes
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Diodes on Cathode?
"Chris Hornbeck" wrote in message ... On Mon, 11 Jun 2007 22:26:51 +0300, "Iain Churches" wrote: Guitar amp technicians seem to refer to them as "flyback" diodes. This may or may not be a general term. That's an excellent choice of a name. It comes from old TV receiver practice, where the horizontal sweep output transformer was combined with a high-ratio step-up "secondary" and rectified to make the multi-KiloJolts needed for CRT anodes. The transformer itself (later with internal high voltage rectifiers) was called the "flyback" transformer, because its primary was shunted with a "reverse" biased diode, whose purpose was to fly the flying spot back from right to left. The vacuum valve diodes used for this gig in their day are very popular now as slow-heating rectifiers with amazingly large heater-cathode voltage ratings (unidirectional! arf.). They're commonly called damper diodes. Chris. Interesting to know from where the term has originated. I was not sure if "flyback" was a general term,. as here in Scandinavia Swedish, Finnish and also Danish technicians seem to intersperse their vocabulary with English language expressions that may or may not mean the same thing in the US. Sometimes ago there was some confusion over the term "common cathode", for example. Best regards Iain |
#11
Posted to rec.audio.tubes
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Diodes on Cathode?
Iain wrote
Interesting to know from where the term has originated. I was not sure if "flyback" was a general term,. as here in Scandinavia Swedish, Finnish and also Danish technicians seem to intersperse their vocabulary with English language expressions that may or may not mean the same thing in the US. Sometimes ago there was some confusion over the term "common cathode", for example. Maybe in Europe other names were used for the TV circuit. See http://www.repairfaq.org/sam/flytest.htm#flyfly It now has common currency as a form of switch-mode power supply, which uses a circuit similar to the TV application. Possibly the origin outside the US and the UK seems mysterious. Ian |
#12
Posted to rec.audio.tubes
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Diodes on Cathode?
I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? Looks like the circuit is expecting the diode to start conducting when the voltage gets higher than the 3KV, so a generic replacement might not cut it. I've seen (in solid state "hot chassis" radios, yes, those were made back in the early 70's) neon bulbs (with a resistor about 5K in series) placed across the primary of the audio output transformer. Presumably to limit voltage excursions, to avoid blowing the output transistor up. |
#13
Posted to rec.audio.tubes
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Diodes on Cathode?
In article ,
robert casey wrote: I need to replace a faulty one and am having difficulty sourcing the part Fender use (R3000). Does this need to be a specific type of diode or can I make up the 3kV diode required using standard 1Kv rectifier types? Looks like the circuit is expecting the diode to start conducting when the voltage gets higher than the 3KV, so a generic replacement might not cut it. I've seen (in solid state "hot chassis" radios, yes, those were made back in the early 70's) neon bulbs (with a resistor about 5K in series) placed across the primary of the audio output transformer. These were not limited to the "early 70's", they were made at least as far back as the mid 1960s. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#14
Posted to rec.audio.tubes
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Diodes on Cathode?
"robert casey" Looks like the circuit is expecting the diode to start conducting when the voltage gets higher than the 3KV, ** If the R3000 diode (string) ever conducts in the reverse direction - that is the end of the diode. It is NOT a damn zener - you clot - the voltage limiting effect works by transformer action. BTW: There are in fact several ways in that Fender amp the R3000s CAN cop more than 3 kV in reverse, both at the same time. Then it is kaput for one of them. See if you can figure even one of them out. ......... Phil |
#15
Posted to rec.audio.tubes
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Diodes on Cathode?
In article ,
"Phil Allison" wrote: "robert casey" Looks like the circuit is expecting the diode to start conducting when the voltage gets higher than the 3KV, ** If the R3000 diode (string) ever conducts in the reverse direction - that is the end of the diode. It is NOT a damn zener - you clot - the voltage limiting effect works by transformer action. BTW: There are in fact several ways in that Fender amp the R3000s CAN cop more than 3 kV in reverse, both at the same time. Then it is kaput for one of them. See if you can figure even one of them out. Leakage inductance between the two primary halves is one of them, I am curious what the others are? Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#16
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Diodes on Cathode?
On Wed, 13 Jun 2007 02:44:09 GMT, John Byrns
wrote: Leakage inductance between the two primary halves is one of them, I am curious what the others are? dI/dT can make some serious voltage spikes, but might require either major overdrive or oscillation. In a gitfiddle amplifier? Nah... Never happen. Much thanks, as always, Chris Hornbeck |
#17
Posted to rec.audio.tubes
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Diodes on Cathode?
** If the R3000 diode (string) ever conducts in the reverse direction - that is the end of the diode. It is NOT a damn zener - you clot - the voltage limiting effect works by transformer action. Duh! Of course! It's the diode on the other side of the transformer that does the clamping by going forward conduction. :-) |
#18
Posted to rec.audio.tubes
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Diodes on Cathode?
"robert casey" ** If the R3000 diode (string) ever conducts in the reverse direction - that is the end of the diode. It is NOT a damn zener - you clot - the voltage limiting effect works by transformer action. Duh! Of course! It's the diode on the other side of the transformer that does the clamping by going forward conduction. :-) ** I can see that light bulb on top of your head is now brightly lit !! Or is it maybe in your mouth, like " Uncle Fester ". ........ Phil |
#19
Posted to rec.audio.tubes
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Diodes on Cathode?
John Byrns wrote: In article , "Phil Allison" wrote: "robert casey" Looks like the circuit is expecting the diode to start conducting when the voltage gets higher than the 3KV, ** If the R3000 diode (string) ever conducts in the reverse direction - that is the end of the diode. It is NOT a damn zener - you clot - the voltage limiting effect works by transformer action. BTW: There are in fact several ways in that Fender amp the R3000s CAN cop more than 3 kV in reverse, both at the same time. Then it is kaput for one of them. See if you can figure even one of them out. Leakage inductance between the two primary halves is one of them, I am curious what the others are? Speakers are inductive loads at HF, especially in guitar amps where there is never any attempt made to place an impedance equalization Zobel across the speaker. A pair of Zobel networks are never placed network where they are usually most effective, across each 1/2 of the primary on the OPT. So a stray bit of noise with F content up to 20kHz can produce high voltages at the anodes of a guitar amp in pentode mode because the load at the sec becomes high, as well as being decoupled from the primary because of the leakage L. However, the high Ra of the pentoads and the shunt C of the OPT DOES tend to shunt the extreme HF gain of the pentoad. I like to see a pair of Zobels beginning to load the output tubes at 20kHz at least. I have often included the anode to 0V diodes to limit transformer primary voltage excursions with no load and have had no trouble with 3 seriesed diodes with PIV = 1,000V each. The highest voltages geberated in an unloaded OPT seem to be with pentoad amps, and about the highest voltages I have seen are up to about 3 times the maximum Va-a with rated load at clipping. So with an amp making 100 watts into 5k, this is 707Vrms, and with no load I have seen maybe 2,200Vrms a-a. UL will also produce high no load voltages, and triodes not so much. The amount of loading needed to prevent the strange non-commonsense phenomena observed when an amp continues to produce a really high output voltage with no load isn't anything like the rated load; it appears the load to prevent more than 10% above high back-emf no load voltages is an RL of many more ohm than the rated normal load, but just what the general rule for such a load is I don't know. I've never made the trials to find out. Some older amp schematics show what look to be a silly amount of R across the output, say 100 ohms, for where the load might normally be 16 ohms. There is never a reason given for the 100 ohms, but perhaps its is to guard against the back emf voltage production with no load caused by the cut off of current in one pentode slowly turning off while the other one which conducts until the anode Va goes down to 0V and then it too cuts off sharply because its anode becomes negative. A loading of 100 ohms would not cause a huge power loss. 100W into 16 ohms = 40Vrms, and this voltage across 100 ohms = 16 watts. Perhaps 220 ohms would be OK, and the extra reliablity would be much worth the few watts wasted in the permamnent high ohm load across the output. A better place to put such a load of say 33k would be across each 1/2 primary of the OPT. People are welcome to investigate further, **cautiously!** and perhaps not have use any diodes. But because the diodes don't waste any power, they are in favour. Patrick Turner. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#20
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Diodes on Cathode?
Or is it maybe in your mouth, like " Uncle Fester ". :-) Good one! |
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