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#1
Posted to rec.audio.tubes
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Pioneer SM-B200A
"Alex" wrote in message ... Hello, You know those Japanese tuner-amplifiers of the 60's which have dual AM receivers, one possibly with FM and a stereo amplifier, typically powered by 6BM8 tubes... I got one of those, Pioneer SM-B200A. It is not working -- the tubes get hot and "burning". Most likely dead electrolytic and/or leaky interstage paper caps which bring positive bias to grids. I need to fix the beast before getting rid of it. I have no schematic, but tracing the wiring, realised that 6BM8s have fixed bias originating from some selenium rectifier. Cathodes of 6BM8s are grounded. More over, there are flinsy open (to dust) carbon trimpots to "balance" (!) bias of the output tubes. At the same time there is no way of knowing values of the currents in the tubes. There are no even small current sensing (measuring) resistors in the cathodes. That design does not make much sense. I would get rid of the fixed bias in favour of automatic bias. Automatic bias will be more robust and forgiving to tubes mismatch. It will not care if this selenium rectifier dies altogether. I would throw resistors from each cathode to ground, plus another between the cathodes to be able to check unbalance. Now my question is, from your experience, what should these resistor values be, to get a benign class AB? I would guess 300R from each cathode to ground and 100R between the cathodes? Regards, Alex Adding the 100R will only decrease the voltage imbalance between cathodes but will increase anode current imbalance. And that's not what you want. Datasheet Rk for ECL82/PCL82/UCL82 in AB-PP: B+ (V) 100 170 200 230 250 Rk (ohm) 140 125 170 200 220 Rk's above are spec'd in bean counter mode, Patrick would say. So, in single cathode resistor config the values must be doubled. More details: http://frank.pocnet.net/sheets/030/e/ECL82.pdf http://frank.pocnet.net/sheets/030/p/PCL82.pdf http://frank.pocnet.net/sheets/030/u/UCL82.pdf Equivalents: ECL82 = 6BM8 (Htr. 6.3V) PCL82 = 16A8 (Htr. 0.3A) UCL82 = 50BM8 (Htr. 0.1A) Finally ECL82 = PCL82 = UCL82 except for heater V/I and MINOR differences which are not relevant in the context of your question. Regards, Gio |
#2
Posted to rec.audio.tubes
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Pioneer SM-B200A
On 12/27/10 13:30, Alex so witilly quipped:
I have no schematic, but tracing the wiring, realised that 6BM8s have fixed bias originating from some selenium rectifier. Cathodes of 6BM8s are grounded. More over, there are flinsy open (to dust) carbon trimpots to "balance" (!) bias of the output tubes. At the same time there is no way of knowing values of the currents in the tubes. There are no even small current sensing (measuring) resistors in the cathodes. That design does not make much sense. I suggest pulling the tubes (except B+ rectifiers, if any), and checking the selenium rectifier output. As I recall they're not highly reliable, but I could be wrong. I suppose you could cheat and sub in a modern silicon rectifier. A bad bias rectifier (or bias filter cap) would explain "the symptoms". [Don't forget to discharge power supply caps after you power down since no tubes = no discharge current]. |
#3
Posted to rec.audio.tubes
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Pioneer SM-B200A
"Alex" wrote in message ... "GRe" wrote in message ... I would get rid of the fixed bias in favour of automatic bias. Automatic bias will be more robust and forgiving to tubes mismatch. It will not care if this selenium rectifier dies altogether. I would throw resistors from each cathode to ground, plus another between the cathodes to be able to check unbalance. Now my question is, from your experience, what should these resistor values be, to get a benign class AB? I would guess 300R from each cathode to ground and 100R between the cathodes? Regards, Alex Adding the 100R will only decrease the voltage imbalance between cathodes but will increase anode current imbalance. And that's not what you want. Thanks. Will it help to use independent resistors from each cathode to ground and then a large non-polarised electrolytic between the cathodes? I would think this will give the best DC current balance, highest gain while in class A (low volume) -- hence lowest output impedance, but might result in higher distortion in transition from A to B (at higher volume), but who cares? I was only commenting on the 100R cathode to cathode resistor. Forget about the C to C resistor and the common cathode resistor configuration. For best anode current balance the single cathode resistors configuration will help. The value of them depend on your B+, see the datasheets and the table below. Regards, Gio Datasheet Rk for ECL82/PCL82/UCL82 in AB-PP: B+ (V) 100 170 200 230 250 Rk (ohm) 140 125 170 200 220 Rk's above are spec'd in bean counter mode, Patrick would say. So, in single cathode resistor config the values must be doubled. More details: http://frank.pocnet.net/sheets/030/e/ECL82.pdf http://frank.pocnet.net/sheets/030/p/PCL82.pdf http://frank.pocnet.net/sheets/030/u/UCL82.pdf Equivalents: ECL82 = 6BM8 (Htr. 6.3V) PCL82 = 16A8 (Htr. 0.3A) UCL82 = 50BM8 (Htr. 0.1A) Finally ECL82 = PCL82 = UCL82 except for heater V/I and MINOR differences which are not relevant in the context of your question. Regards, Gio |
#4
Posted to rec.audio.tubes
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Pioneer SM-B200A
Hello,
You know those Japanese tuner-amplifiers of the 60's which have dual AM receivers, one possibly with FM and a stereo amplifier, typically powered by 6BM8 tubes... I got one of those, Pioneer SM-B200A. It is not working -- the tubes get hot and "burning". Most likely dead electrolytic and/or leaky interstage paper caps which bring positive bias to grids. I need to fix the beast before getting rid of it. I have no schematic, but tracing the wiring, realised that 6BM8s have fixed bias originating from some selenium rectifier. Cathodes of 6BM8s are grounded. More over, there are flinsy open (to dust) carbon trimpots to "balance" (!) bias of the output tubes. At the same time there is no way of knowing values of the currents in the tubes. There are no even small current sensing (measuring) resistors in the cathodes. That design does not make much sense. I would get rid of the fixed bias in favour of automatic bias. Automatic bias will be more robust and forgiving to tubes mismatch. It will not care if this selenium rectifier dies altogether. I would throw resistors from each cathode to ground, plus another between the cathodes to be able to check unbalance. Now my question is, from your experience, what should these resistor values be, to get a benign class AB? I would guess 300R from each cathode to ground and 100R between the cathodes? Regards, Alex |
#5
Posted to rec.audio.tubes
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Pioneer SM-B200A
"flipper" wrote in message ... On Mon, 27 Dec 2010 13:30:43 -0800, "Alex" wrote: Hello, You know those Japanese tuner-amplifiers of the 60's which have dual AM receivers, one possibly with FM and a stereo amplifier, typically powered by 6BM8 tubes... I got one of those, Pioneer SM-B200A. It is not working -- the tubes get hot and "burning". Most likely dead electrolytic and/or leaky interstage paper caps which bring positive bias to grids. I need to fix the beast before getting rid of it. I have no schematic, I found one. If you would like I can post it to alt.binaries.schematics.electronic Thank you very-very much for the offer. Schematic will help. Could you please e-mail it to alex-at-microcoin-dot-com ? Regards, Alex |
#6
Posted to rec.audio.tubes
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Pioneer SM-B200A
"GRe" wrote in message ... I would get rid of the fixed bias in favour of automatic bias. Automatic bias will be more robust and forgiving to tubes mismatch. It will not care if this selenium rectifier dies altogether. I would throw resistors from each cathode to ground, plus another between the cathodes to be able to check unbalance. Now my question is, from your experience, what should these resistor values be, to get a benign class AB? I would guess 300R from each cathode to ground and 100R between the cathodes? Regards, Alex Adding the 100R will only decrease the voltage imbalance between cathodes but will increase anode current imbalance. And that's not what you want. Thanks. Will it help to use independent resistors from each cathode to ground and then a large non-polarised electrolytic between the cathodes? I would think this will give the best DC current balance, highest gain while in class A (low volume) -- hence lowest output impedance, but might result in higher distortion in transition from A to B (at higher volume), but who cares? Alex Datasheet Rk for ECL82/PCL82/UCL82 in AB-PP: B+ (V) 100 170 200 230 250 Rk (ohm) 140 125 170 200 220 Rk's above are spec'd in bean counter mode, Patrick would say. So, in single cathode resistor config the values must be doubled. More details: http://frank.pocnet.net/sheets/030/e/ECL82.pdf http://frank.pocnet.net/sheets/030/p/PCL82.pdf http://frank.pocnet.net/sheets/030/u/UCL82.pdf Equivalents: ECL82 = 6BM8 (Htr. 6.3V) PCL82 = 16A8 (Htr. 0.3A) UCL82 = 50BM8 (Htr. 0.1A) Finally ECL82 = PCL82 = UCL82 except for heater V/I and MINOR differences which are not relevant in the context of your question. Regards, Gio |
#7
Posted to rec.audio.tubes
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Pioneer SM-B200A
On Dec 27 2010, 10:11*am, flipper wrote:
On Mon, 27 Dec 2010 23:24:44 -0800, "Alex" wrote: "GRe" wrote in message .. . I would get rid of the fixed bias in favour of automatic bias. Automatic bias will be more robust and forgiving to tubes mismatch. It will not care if this selenium rectifier dies altogether. I would throw resistors from each cathode to ground, plus another between the cathodes to be able to check unbalance. Now my question is, from your experience, what should these resistor values be, to get a benign class AB? I would guess 300R from each cathode to ground and 100R between the cathodes? Regards, Alex Adding the 100R will only decrease the voltage imbalance between cathodes but will increase anode current imbalance. *And that's not what you want. Thanks. Will it help to use independent resistors from each cathode to ground and then a large non-polarised electrolytic between the cathodes? I would think this will give the best DC current balance, highest gain while in class A (low volume) -- hence lowest output impedance, but might result in higher distortion in transition from A to B (at higher volume), but who cares? Personally I'd restore it to stock and if you wanted to improve fault tolerance put some fuses in the output circuit but if you're dead set on self bias then separate Rks each with their own bypass cap from cathode to ground will give you the best idle balance. But fixed bias is technically 'better' since bias will not shift when it enters Class B and, as I mentioned earlier, I'm sure the existing balance pot was for dynamic balance (which is what matters), not idle current. Alex Datasheet Rk for ECL82/PCL82/UCL82 in AB-PP: B+ (V) * 100 *170 *200 *230 *250 Rk (ohm) 140 *125 *170 *200 *220 Rk's above are spec'd in bean counter mode, Patrick would say. So, in single cathode resistor config the values must be doubled. More details: http://frank.pocnet.net/sheets/030/e/ECL82.pdf http://frank.pocnet.net/sheets/030/p/PCL82.pdf http://frank.pocnet.net/sheets/030/u/UCL82.pdf Equivalents: ECL82 = 6BM8 *(Htr. 6.3V) PCL82 = 16A8 *(Htr. 0.3A) UCL82 = 50BM8 (Htr. 0.1A) Finally ECL82 = PCL82 = UCL82 except for heater V/I and MINOR differences which are not relevant in the context of your question. Regards, Gio There's another reason to keep the fixed bias... you get to keep an extra 16 volts or so of B+. The SM-B200 has quite a low B+... only some 240 VDC comes to mind. Also, the negative bias supply also feeds the transistorized phono preamp. If the selenium bias rectifier is dead replace it with a 1N4007 diode. I've refurbished two of these units (one a basket case)... not easy to work on. Make sure you replace all the grey, Suzuki 0.05 "oil filled" caps - most will be leaky, including the coupling caps to the 6BM8's - wreaks havok on the bias! The weird dual eye-tube is close to "unobtanium" these days, and most are burned out by now, but you can sub a single eye-tube and switch the AVC line between the two receivers (I plant to use the now disconnected speaker reverse switch for this.) Good luck! Cheers, Roger |
#8
Posted to rec.audio.tubes
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Pioneer SM-B200A
On Jan 1, 3:03*pm, Engineer wrote:
On Dec 27 2010, 10:11*am, flipper wrote: On Mon, 27 Dec 2010 23:24:44 -0800, "Alex" wrote: "GRe" wrote in message .. . I would get rid of the fixed bias in favour of automatic bias. Automatic bias will be more robust and forgiving to tubes mismatch. It will not care if this selenium rectifier dies altogether. I would throw resistors from each cathode to ground, plus another between the cathodes to be able to check unbalance. Now my question is, from your experience, what should these resistor values be, to get a benign class AB? I would guess 300R from each cathode to ground and 100R between the cathodes? Regards, Alex Adding the 100R will only decrease the voltage imbalance between cathodes but will increase anode current imbalance. *And that's not what you want. Thanks. Will it help to use independent resistors from each cathode to ground and then a large non-polarised electrolytic between the cathodes? I would think this will give the best DC current balance, highest gain while in class A (low volume) -- hence lowest output impedance, but might result in higher distortion in transition from A to B (at higher volume), but who cares? Personally I'd restore it to stock and if you wanted to improve fault tolerance put some fuses in the output circuit but if you're dead set on self bias then separate Rks each with their own bypass cap from cathode to ground will give you the best idle balance. But fixed bias is technically 'better' since bias will not shift when it enters Class B and, as I mentioned earlier, I'm sure the existing balance pot was for dynamic balance (which is what matters), not idle current. Alex Datasheet Rk for ECL82/PCL82/UCL82 in AB-PP: B+ (V) * 100 *170 *200 *230 *250 Rk (ohm) 140 *125 *170 *200 *220 Rk's above are spec'd in bean counter mode, Patrick would say. So, in single cathode resistor config the values must be doubled. More details: http://frank.pocnet.net/sheets/030/e/ECL82.pdf http://frank.pocnet.net/sheets/030/p/PCL82.pdf http://frank.pocnet.net/sheets/030/u/UCL82.pdf Equivalents: ECL82 = 6BM8 *(Htr. 6.3V) PCL82 = 16A8 *(Htr. 0.3A) UCL82 = 50BM8 (Htr. 0.1A) Finally ECL82 = PCL82 = UCL82 except for heater V/I and MINOR differences which are not relevant in the context of your question. Regards, Gio There's another reason to keep the fixed bias... you get to keep an extra 16 volts or so of B+. * *The SM-B200 has quite a low B+... only some 240 VDC comes to mind. *Also, the negative bias supply also feeds the transistorized phono preamp. *If the selenium bias rectifier is dead replace it with a 1N4007 diode. I've refurbished two of these units (one a basket case)... not easy to work on. *Make sure you replace all the grey, Suzuki 0.05 "oil filled" caps - most will be leaky, including the coupling caps to the 6BM8's - wreaks havok on the bias! *The weird dual eye-tube is close to "unobtanium" these days, and most are burned out by now, but you can sub a single eye-tube and switch the AVC line between the two receivers (I plant to use the now disconnected speaker reverse switch for this.) Good luck! Cheers, Roger Forgot to mention... these chassis have a neat centre channel or mono option. The two stereo channels ground their respective 4 ohm OPT taps (not the zero ohm tap as is conventional.) The number of OPT secondary turns is the same from "4 ohms to zero ohms" as from "16 ohms to 4 ohms", both being a 4 ohm impedance match (as we know, impedance is proportional to the square of the turns.) But now the L- channel "16 ohm tap" is 180 degrees out of phase with the R-channel "zero ohm" tap giving a 8 ohm match between the two of them, both channels driven (needs an 8 ohm mono speaker, of course). No tricky amplifier bridging necessary to combine the two output channels. Draw a picture if this is not clear! Cheers, Roger |
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