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#1
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Jones's servo bias
Following the 10c thread, I checked my slightly musty copy of the
quintessentially English Morgan Jones, "Valve Amplifiers", Newnes 1999. I was curious because I thought his circuit uses a zener to restrict the upswings of the signal from the cathode-current sensing resistor. Using a fixed reference isn't ideal if the bias current is adustable, because then the zener clipping point won't necessarily be correct. As it happens, he doesn't use a zener. This is what he does instead. First, he uses a 1R current sense resistor at the cathode of the valve, followed by an opamp set to a gain of 100. This minimises the effect of this part of the circuit on the operation of the valve. Each 10mA of cathode current thus results in 1V output from the opamp. He generates two reference voltages, one twice the other, from a convenient +ve voltage using a pot and two fixed resistors. The upper reference is used together with a transistor/diode clamp to limit the upswings of the signal from the opamp. A low-pass filter then smooths this to a steady DC before it is compared to the lower of the two reference voltages, using a second opamp. The ensuing difference signal is then used to alter the grid voltage via a second transistor. Because the signal to the filter is always limited to twice the set bias level, its AC component remains more or less symmetrical when the valve cuts off during AB operation, so it smooths to a steady DC level. With a triode, the servo will tend to very slightly raise bias current during extended periods of AB, which is better than reducing it as others tend to do. That's two opamps, two transistors, a diode, and a bunch of resistors and caps to maintain the fixed bias of one output valve. Morgan doesn't use the circuit in any of his amps AFAICS, so there's no example of how the scheme is extended to the one or more other output valves. It wouldn't be necessary to replicate the whole circuit for the other valves, because they need to be set at the same level, to balance the transformer current. The set reference voltages could be made common, and the rest of the circuit replicated. Alternatively the unclamped signal could be smoothed and used as the reference for the other servos, which then wouldn't need clamps. It's possible that reading the above will not result in a working diagram in your head. The actual circuit does work. Buy the book. It is British and we are short of money. Some thought, in the details, must be given to stability. Each servo is a feedback circuit, and all are related in various ways via various delays to themselves and to each other. Ian |
#2
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Jones's servo bias
On Nov 27, 9:55*pm, Ian Iveson wrote:
Following the 10c thread, I checked my slightly musty copy of the quintessentially English Morgan Jones, "Valve Amplifiers", Newnes 1999. I was curious because I thought his circuit uses a zener to restrict the upswings of the signal from the cathode-current sensing resistor. Using a fixed reference isn't ideal if the bias current is adustable, because then the zener clipping point won't necessarily be correct. As it happens, he doesn't use a zener. This is what he does instead. First, he uses a 1R current sense resistor at the cathode of the valve, followed by an opamp set to a gain of 100. This minimises the effect of this part of the circuit on the operation of the valve. Each 10mA of cathode current thus results in 1V output from the opamp. He generates two reference voltages, one twice the other, from a convenient +ve voltage using a pot and two fixed resistors. The upper reference is used together with a transistor/diode clamp to limit the upswings of the signal from the opamp. A low-pass filter then smooths this to a steady DC before it is compared to the lower of the two reference voltages, using a second opamp. The ensuing difference signal is then used to alter the grid voltage via a second transistor. Because the signal to the filter is always limited to twice the set bias level, its AC component remains more or less symmetrical when the valve cuts off during AB operation, so it smooths to a steady DC level. With a triode, the servo will tend to very slightly raise bias current during extended periods of AB, which is better than reducing it as others tend to do. That's two opamps, two transistors, a diode, and a bunch of resistors and caps to maintain the fixed bias of one output valve. Morgan doesn't use the circuit in any of his amps AFAICS, so there's no example of how the scheme is extended to the one or more other output valves. It wouldn't be necessary to replicate the whole circuit for the other valves, because they need to be set at the same level, to balance the transformer current. The set reference voltages could be made common, and the rest of the circuit replicated. Alternatively the unclamped signal could be smoothed and used as the reference for the other servos, which then wouldn't need clamps. It's possible that reading the above will not result in a working diagram in your head. The actual circuit does work. Buy the book. It is British and we are short of money. Some thought, in the details, must be given to stability. Each servo is a feedback circuit, and all are related in various ways via various delays to themselves and to each other. Ian Which edition is it ? |
#3
Posted to rec.audio.tubes
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Jones's servo bias
On Nov 28, 1:55*pm, Ian Iveson wrote:
Following the 10c thread, I checked my slightly musty copy of the quintessentially English Morgan Jones, "Valve Amplifiers", Newnes 1999. I was curious because I thought his circuit uses a zener to restrict the upswings of the signal from the cathode-current sensing resistor. Using a fixed reference isn't ideal if the bias current is adustable, because then the zener clipping point won't necessarily be correct. As it happens, he doesn't use a zener. This is what he does instead. First, he uses a 1R current sense resistor at the cathode of the valve, followed by an opamp set to a gain of 100. This minimises the effect of this part of the circuit on the operation of the valve. Each 10mA of cathode current thus results in 1V output from the opamp. He generates two reference voltages, one twice the other, from a convenient +ve voltage using a pot and two fixed resistors. The upper reference is used together with a transistor/diode clamp to limit the upswings of the signal from the opamp. A low-pass filter then smooths this to a steady DC before it is compared to the lower of the two reference voltages, using a second opamp. The ensuing difference signal is then used to alter the grid voltage via a second transistor. Because the signal to the filter is always limited to twice the set bias level, its AC component remains more or less symmetrical when the valve cuts off during AB operation, so it smooths to a steady DC level. With a triode, the servo will tend to very slightly raise bias current during extended periods of AB, which is better than reducing it as others tend to do. That's two opamps, two transistors, a diode, and a bunch of resistors and caps to maintain the fixed bias of one output valve. Morgan doesn't use the circuit in any of his amps AFAICS, so there's no example of how the scheme is extended to the one or more other output valves. It wouldn't be necessary to replicate the whole circuit for the other valves, because they need to be set at the same level, to balance the transformer current. The set reference voltages could be made common, and the rest of the circuit replicated. Alternatively the unclamped signal could be smoothed and used as the reference for the other servos, which then wouldn't need clamps. It's possible that reading the above will not result in a working diagram in your head. The actual circuit does work. Buy the book. It is British and we are short of money. Some thought, in the details, must be given to stability. Each servo is a feedback circuit, and all are related in various ways via various delays to themselves and to each other. Ian There have been many schemes published over the years to adjust grid bias to lessen the effects of rising Ek across cathode RC bias networks, and all are no good if Ek is allowed to rise more than about 10% from the idle value. Some published schemes have sought to use a differential amp which accepts the two Vdc signals across the 1ohm current sensors after filtering out the Vac. I've tried out a few schemes like this, and the simplest is done using two darlington pairs of PNP bjts for an LTP set up with emitter CCS so that rise in Ek does not cause any increase in grid bias voltage, but any difference in the Ek of either PP tube is corrected. The LTP has good CMRR, and DC gain can easily be up to 1,000, which too much, so 100 is probably OK. I found such a circuit worked fabulously to keep the Idc of each tube equal even in extreme class AB, say with a 1 ohm connected across the 8 ohm labelled outlet. In other words, it worked with the worst of circumstances. But then when global NFB was connected, I could not stop LF oscillations no matter how much open loop gain tailoring and network tweaking I did. So I went right off the idea of having anything to do with grid bias swinging to force the OPT stage to have balanced Idc on both halves of the OPT. Electronics World even published a circuit using opamps to perform exactly what I have described and the author said that with a toroidal OPT, the iron permeability of the core could easily be 40,000, so it is critical to keep Idc balance in the OPT core or else the core saturates all too easily even with say only 10mA dc imbalance. If the OPT design has been done right, there will always be enough primary inductance at any Va-a level above 5Vrms, and Fsat at full power should be under 20Hz. Usually when these conditions are met, the iron µ need rarely ever be more than 2,500, which is ideally obtained with the use of GOSS C-cores with a small air gap, or E&I laminations assembled with partial air gapping using bundles of Es and Is facing same way, so that in a 50mm stack there may be 17 reverse direction bundles interleaved. So the high µ of the GOSS is reduced, and the saturation due to Idc imbalance becomes negligible. This being the case, the bias servo isn't needed, and all because of issues not raised in trendy magazines and books, and all because of good basic knowledge within my father's generation. The brightest of them knew all about air gapping PP OPTs, and using enough turns and large enough cores to avoid the problems of balanced operation with transformers, all of which tend to be unbalanced at some occasion as fixed bias drifts away from its ideal setting as tube age. But my father's generation never sought to have the convenience of cathode bias while also triying to have Ek stay constant to allow AB operation without mis-biasing. I found my Dynamic Bias Stabilizer seemed to work well in the presence of the CFB of the OP stage plus the global NFB. Ian likes to pour buckets of poop on all my ideas, but we all must remember he rarely ever solders up a circuit and spends days analyising it, then publishing his consise findings. He has Flipper's disease in a big way, and relies on simulation for his self validation of righteousness without doubt. Well, I await the result of Ian's soldering and measuring efforts. Nothing I've seen in Morgan Jones makes me wanna copy anything Morgan suggests with regard to fiddling with grid bias. A number of amp makers have used toroidal OPTs over the last 15 years, and those I have rewired because of smoke and unreliability all had fixed bias and suffered because of attrocious methods used to adjust bias and monitor bias as it is adjusted, thus leading owners to be their own worst enemy. But not of their own intentions. Owners much prefer cathode bias, just plug in OP tubes and let the RC do the business. In a pair of Quad-II-Forty I have just completed re-wiring, samples of Red Lion KT88 have given 4 different Ek voltages between 46Vdc and 37Vdc across the 630 ohm I have put in to replace the Rk = 390r originals which allowed the KT88 to overheat all to often and all too easily. The range of Ek differences gives a maximum 15mA Idc difference. Cores in Chinese made OPTs designed by Andy Grove are probably E&I GOSS, and the Lp seems to be rather low, even though Fsat at full PO is at 25Hz, 33Watts. So iron µ is maybe low, and whatever it is I can't change that, but despite the maximum Idc imbalance the core sat character as F is reduced is not alarming as it would be in an ungapped strip-wound toroidal core. The answer to such uneven matching amoung a given batch of supposedly NOS Red Lion KT88 is to have some additional bias balancing adjust method as seen in another pair of Quad-II I rewired where I used EHKT90 instead of KT66, see http://www.turneraudio.com.au/quad2powerampmods.htm and proceed to about 1/2 way down the page to the 2005 schematic. There are more ways than one of doing something. Anyway, although the Ek rises alarmingly in Quad-II amps when a sine wave is used up to clipping level when say a 4 ohm load is connected while the OPT is strapped for "8 ohms", most listeners only use a watt of average power, so the load mismatch is well tolerated and Ek stays fairly constant, and with only mild variations in the Idc in each 1/2 of the OPT. I have a pair of Quad-II amps awaiting total re-wire, and this time I will remove the OPTs from their cans and re-arrange the OPT sec windings to give waste free connection to 4 ohms so that winding losses will be optimal as they are when the OPT is strapped for 16 ohms. Kieth Snook's website explains what is inside a Quad-II OPT can. Peter Walker could so easily have provided two extra board terminals on his OPTs but he must have thought such a good thing would have confused most ppl in 1955, most of whom didn't have any 4 ohm speakers. So, Quad-II as they are in original condtion do not work well with 4 ohms. Even with 8 ohms they work in AB, and the better load for a decent 20 watts is 9ka-a, not bloomin 3k8. Quad-II work best with OPTs set for 8 ohms, and speaker RL = 16 ohms. I also have a pair of better OPTs wound especially for my pair of Quads so the original Quad-II with correctly arranged OPT secs can be used in another amp to replaced even worse OPTs. Ive tried my DBS scheme 3 years ago in yet another pair of Quads with KT88 outputs. Worked just fine, and with sine waves at clipping PO is 34Watts, low THD, and excellent sound. Ian can hate my scheme as much as he likes, but my customers wouldn't agree with him. Morgan Jones's book had several editions. The early editions were riddled with frightful mistakes on nearly every page, most of which the average groovy non technically trained audiophile never ever would see. Later editions are better, but my copy sure has some real clangers. So, with Morgan, you need be aware of his mistakes, the sort of things that never would have been seen in RDH4, because there was so much better proof reading and self criticism prior to publication. Patrick Turner. |
#4
Posted to rec.audio.tubes
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Jones's servo bias
On Nov 28, 5:10*am, wrote:
On Nov 27, 9:55*pm, Ian Iveson wrote: Following the 10c thread, I checked my slightly musty copy of the quintessentially English Morgan Jones, "Valve Amplifiers", Newnes 1999. I was curious because I thought his circuit uses a zener to restrict the upswings of the signal from the cathode-current sensing resistor. Using a fixed reference isn't ideal if the bias current is adustable, because then the zener clipping point won't necessarily be correct. As it happens, he doesn't use a zener. This is what he does instead. First, he uses a 1R current sense resistor at the cathode of the valve, followed by an opamp set to a gain of 100. This minimises the effect of this part of the circuit on the operation of the valve. Each 10mA of cathode current thus results in 1V output from the opamp. He generates two reference voltages, one twice the other, from a convenient +ve voltage using a pot and two fixed resistors. The upper reference is used together with a transistor/diode clamp to limit the upswings of the signal from the opamp. A low-pass filter then smooths this to a steady DC before it is compared to the lower of the two reference voltages, using a second opamp. The ensuing difference signal is then used to alter the grid voltage via a second transistor. Because the signal to the filter is always limited to twice the set bias level, its AC component remains more or less symmetrical when the valve cuts off during AB operation, so it smooths to a steady DC level. With a triode, the servo will tend to very slightly raise bias current during extended periods of AB, which is better than reducing it as others tend to do. That's two opamps, two transistors, a diode, and a bunch of resistors and caps to maintain the fixed bias of one output valve. Morgan doesn't use the circuit in any of his amps AFAICS, so there's no example of how the scheme is extended to the one or more other output valves. It wouldn't be necessary to replicate the whole circuit for the other valves, because they need to be set at the same level, to balance the transformer current. The set reference voltages could be made common, and the rest of the circuit replicated. Alternatively the unclamped signal could be smoothed and used as the reference for the other servos, which then wouldn't need clamps. It's possible that reading the above will not result in a working diagram in your head. The actual circuit does work. Buy the book. It is British and we are short of money. Some thought, in the details, must be given to stability. Each servo is a feedback circuit, and all are related in various ways via various delays to themselves and to each other. Ian Which edition is it ?- Hide quoted text - - Show quoted text - Second. Circuit is on p294. Ian |
#5
Posted to rec.audio.tubes
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Jones's servo bias
"Patrick Turner" wrote in message ... [...] The answer to such uneven matching amoung a given batch of supposedly NOS Red Lion KT88 is to have some additional bias balancing adjust method as seen in another pair of Quad-II I rewired where I used EHKT90 instead of KT66, see http://www.turneraudio.com.au/quad2powerampmods.htm and proceed to about 1/2 way down the page to the 2005 schematic. What's the reason you chose 560K for R11 while R10= 470K? Rgds, Gio Patrick Turner. |
#6
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Quote:
Full loop NFB from the OPT secodary to the Grid 2 of the input dual triode as is customary on these kind of amp. Cheers, John |
#7
Posted to rec.audio.tubes
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Jones's servo bias
On Nov 29, 8:31*am, "GRe" wrote:
"Patrick Turner" wrote in message ... [...] The answer to such uneven matching amoung a given batch of supposedly NOS Red Lion KT88 is to have some additional bias balancing adjust method as seen in another pair of Quad-II I rewired where I used EHKT90 instead of KT66, see http://www.turneraudio.com.au/quad2powerampmods.htmand proceed to about 1/2 way down the page to the 2005 schematic. What's the reason you chose 560K for R11 while R10= 470K? Rgds, Gio To achieve Vac drive balance to each OP tube where there is an LTP with common Rk resistance, the RLa seen by each gain tube of the LTP should be slightly dissimilar to compensate for the unbalancing effect of the common Rk value. If Rk was a CCS, then the total cap coupled RLa on each side of the LTP needs to be the same for perfect balance, even if the tube types used are quite different, eg, see the Radford schematics. But where you have two pentodes as LTP gain tubes, gain is close to gm x RL, and quite high, and the current change in one equals the current cjange in the other plus the change of current in the common Rk. You should be able to do the analysis of all working voltages and currents in my circuit as easily as in any other. In fact, the use of 560k and 470k is a bit high for Rg bias values, because reverse grid current in ageing tubes can cause Eg1 to become several volts more positive than the actual bias voltage at the end of Rg away from the grid. In my example, if that happens, it unbalances the Idc of the OP tubes thus causing the balance monitoring LEDs to change from normal OK with both equally bright to having one brighter than the other. An owner will turn the adjust pot with a thumbnail or a dinner knife until Idc balance is restored, but if the problem of positve grids gets so bad the balance pot cannot be adjusted to get equal LED brightness, then he knows he has a tube problem, and needs new tubes. For this reason, the high Rg values are not so bad, bearing in mind the amp has cathode bias with individual R&C networks in each OP tube cathode circuit. In more recent rewiring of Quad-II-Forty amps completed last week, I used 6SH7 in triode for V1, then used a 6SN7 for LTP with a CCS with MJE340. RLa to B+ was 50k per 1/2 6SN7, and Rg = 120k taken to to the OPT CFB tranny winding to partially bootstrap the Rg to have it appear to to the 6SN7 as about 300k. The dc balance will remain fair but not excellent with individual R&C bias networks, and with Rg at 120k, the reverse positive grid current in ageing tubes won't cause too much trouble. In two samples of the Red Lion KT88 in one Quad-II-Forty monobloc, Ek = +38Vdc in one tube and +44Vdc in the other. Rk = 630 ohms, so Il for the two OP tubes is 60.3mAdc and 69.8mAdc, a difference of 9.5mAdc, a considerable difference. But effects of the Idc imbalance in Chinese made OPTs is minimal because although the OPT cores seem large enough, the number of turns is dissapointing. But the paucity of primary turns means the Np x Idc product in the equation governing the Bdc within the OPT core remains fairly low, thus allowing a high ammount of additive Bac before saturation occurs at full power at about 25Hz, good enough. But Quad-II amps from the 1950s have more Np turns and smaller core, and the use of a balancing pot for Idc in each OPT tube isn't a bad idea. My customer who had me re-wire his Quad-II-Forty amps has given me a dozen Red Lion KT88 to test, all with unknown condition. I should be able to find close pairs among them all, and then if he had an uncle, his name would be Bob. Generally, whatever I do in amps is done to help the tubes give their best in terms of distortion, bandwidth, output resistance and stability. I don't include ALL techniques in every pair of amps; not all amps of mine have the same range of techniques. They don't need to. Providing the loading of tubes in tube amplifiers is technically optimised, and the other 4 items done properly, with optimal DC biasing, music will flow well without it becoming insiduously grubby as the tube age, which is so common in Quad-II amps where their Uncle Scrooge owners cannot bring themselves to replace 20 year old KT66. Patrick Turner. |
#8
Posted to rec.audio.tubes
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Jones's servo bias
On Nov 29, 5:22*pm, John L Stewart John.L.Stewart.
wrote: 'Ian Iveson[_2_ Wrote: ;943887']Following the 10c thread, I checked my slightly musty copy of the quintessentially English Morgan Jones, "Valve Amplifiers", Newnes 1999. I was curious because I thought his circuit uses a zener to restrict the upswings of the signal from the cathode-current sensing resistor. Using a fixed reference isn't ideal if the bias current is adustable, because then the zener clipping point won't necessarily be correct. As it happens, he doesn't use a zener. This is what he does instead. First, he uses a 1R current sense resistor at the cathode of the valve, followed by an opamp set to a gain of 100. This minimises the effect of this part of the circuit on the operation of the valve. Each 10mA of cathode current thus results in 1V output from the opamp. He generates two reference voltages, one twice the other, from a convenient +ve voltage using a pot and two fixed resistors. The upper reference is used together with a transistor/diode clamp to limit the upswings of the signal from the opamp. A low-pass filter then smooths this to a steady DC before it is compared to the lower of the two reference voltages, using a second opamp. The ensuing difference signal is then used to alter the grid voltage via a second transistor. Because the signal to the filter is always limited to twice the set bias level, its AC component remains more or less symmetrical when the valve cuts off during AB operation, so it smooths to a steady DC level. With a triode, the servo will tend to very slightly raise bias current during extended periods of AB, which is better than reducing it as others tend to do. That's two opamps, two transistors, a diode, and a bunch of resistors and caps to maintain the fixed bias of one output valve. Morgan doesn't use the circuit in any of his amps AFAICS, so there's no example of how the scheme is extended to the one or more other output valves. It wouldn't be necessary to replicate the whole circuit for the other valves, because they need to be set at the same level, to balance the transformer current. The set reference voltages could be made common, and the rest of the circuit replicated. Alternatively the unclamped signal could be smoothed and used as the reference for the other servos, which then wouldn't need clamps. It's possible that reading the above will not result in a working diagram in your head. The actual circuit does work. Buy the book. It is British and we are short of money. Some thought, in the details, must be given to stability. Each servo is a feedback circuit, and all are related in various ways via various delays to themselves and to each other. Ian Here is a successful auto bias cct used by Kevin O'Conner, published in one of his books on guitar amps. Output is 4X 5881 running into a Plitron OPT (toroid). The cathode current sampling resistor is 10R driving point B on the schema. Each 5881 has an indepedant bias set cct. Point A drives the OP tube grid bias. Full loop NFB from the OPT secodary to the Grid 2 of the input dual triode as is customary on these kind of amp. Cheers, John +-------------------------------------------------------------------+ |Filename: Kevin OConnor Auto Bias Control.jpg * * * * * * * * * * *| |Download:http://www.audiobanter.com/attachment.php?attachmentid=250| +-------------------------------------------------------------------+ -- John L Stewart- Hide quoted text - - Show quoted text - Hi John, good to see you're still here. I see this thread is quite a close replica of one a long time ago. I don't see how the Kevin O'Connor circuit works. It looks like the intention is to force symmetry of the AC component of the current sense signal by clipping both positive and negative-going excursions, but hasn't the signal already been smoothed? If so, then the one diode limits the DC drift but doesn't prevent it, and the other diode is redundant. I wish I was better at understanding transistor and opamp circuits. Looking at the other end of the servo, I don't see how that open collector works. Doesn't it need a pull-down resistor to the negative supply? And there's a resistor with no value shown...where's Flipper? He seems to know about solid state stuff. I guess Morgan got carried away with the elegance of his idea. Clipping both ways isn't so clever, but it's a better solution I think. The more the AC is chopped, the less the drift caused by distortion. Toroid in a guitar amp? Ian |
#9
Posted to rec.audio.tubes
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Jones's servo bias
On Thu, 1 Dec 2011 08:43:21 -0800 (PST), Ian Iveson
wrote: On Nov 29, 5:22*pm, John L Stewart John.L.Stewart. wrote: Here is a successful auto bias cct used by Kevin O'Conner, published in one of his books on guitar amps. Output is 4X 5881 running into a Plitron OPT (toroid). The cathode current sampling resistor is 10R driving point B on the schema. Each 5881 has an indepedant bias set cct. Point A drives the OP tube grid bias. Full loop NFB from the OPT secodary to the Grid 2 of the input dual triode as is customary on these kind of amp. Cheers, John +-------------------------------------------------------------------+ |Filename: Kevin OConnor Auto Bias Control.jpg * * * * * * * * * * *| |Download:http://www.audiobanter.com/attachment.php?attachmentid=250| +-------------------------------------------------------------------+ -- John L Stewart- Hide quoted text - - Show quoted text - Hi John, good to see you're still here. I see this thread is quite a close replica of one a long time ago. I don't see how the Kevin O'Connor circuit works. It looks like the intention is to force symmetry of the AC component of the current sense signal by clipping both positive and negative-going excursions, but hasn't the signal already been smoothed? If so, then the one diode limits the DC drift but doesn't prevent it, and the other diode is redundant. I wish I was better at understanding transistor and opamp circuits. Looking at the other end of the servo, I don't see how that open collector works. Doesn't it need a pull-down resistor to the negative supply? And there's a resistor with no value shown...where's Flipper? He seems to know about solid state stuff. I think your assessment is correct. For the diodes to work as you presume they were intended to you'd need to isolate them from the filter. Like, for example, stick another 100k between them and the -input so they can clip and then you average the result. Not saying that would be ideal, just an example. The circuit's application is also limited because it depends on bias being in the right range for the diodes although one might be able to fix that with a gain buffer off the sense resistor. Well, you could vary the sense resistor too but you usually want that as low as possible and a buffer amp let's you do that and then gain to suit. (That's 'modern times' solid state thinking, you see. What the heck, throw in an opamp, they're cheap). I presumed the open collector went into something else because it doesn't do anything all by it's lonesome, unless he's depending on grid leakage for some nominal current. Looks upside down to me anyway because I'd end with a low impedance follower. I guess Morgan got carried away with the elegance of his idea. Clipping both ways isn't so clever, but it's a better solution I think. The more the AC is chopped, the less the drift caused by distortion. Toroid in a guitar amp? Ian |
#10
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Jones's servo bias
"Ian Iveson" wrote in message ... On Nov 29, 5:22 pm, John L Stewart John.L.Stewart. wrote: 'Ian Iveson[_2_ Wrote: [...] Here is a successful auto bias cct used by Kevin O'Conner, published in one of his books on guitar amps. Output is 4X 5881 running into a Plitron OPT (toroid). The cathode current sampling resistor is 10R driving point B on the schema. Each 5881 has an indepedant bias set cct. Point A drives the OP tube grid bias. Full loop NFB from the OPT secodary to the Grid 2 of the input dual triode as is customary on these kind of amp. Cheers, John +-------------------------------------------------------------------+ |Filename: Kevin OConnor Auto Bias Control.jpg | |Download:http://www.audiobanter.com/attachment.php?attachmentid=250| +-------------------------------------------------------------------+ -- John L Stewart- Hide quoted text - - Show quoted text - Hi John, good to see you're still here. I see this thread is quite a close replica of one a long time ago. I don't see how the Kevin O'Connor circuit works. It looks like the intention is to force symmetry of the AC component of the current sense signal by clipping both positive and negative-going excursions, but hasn't the signal already been smoothed? If so, then the one diode limits the DC drift but doesn't prevent it, and the other diode is redundant. I wish I was better at understanding transistor and opamp circuits. Looking at the other end of the servo, I don't see how that open collector works. Doesn't it need a pull-down resistor to the negative supply? And there's a resistor with no value shown...where's Flipper? He seems to know about solid state stuff. I guess Morgan got carried away with the elegance of his idea. Clipping both ways isn't so clever, but it's a better solution I think. The more the AC is chopped, the less the drift caused by distortion. Toroid in a guitar amp? Ian I don't know the circuit either and some info is missing in John's post. But you can make a few assumptions and see if that works out. Assume Vref (see schematic in John's post) is just above 400mV(say 410mV), -Vb is 40V and Rx= Ry Rg. Also assume the -Vg/G1 drive is as in the schematic below but other configurations possible/imagineable. Points "A" and "B" in the ASCII schematic go to the identical points in John's schematic. If (one of) the 5881 idle's at Ic= 40mA than V-Rc= 400mV, so Vout of OP-A1 is positive(assuming bipolar supply) resulting in T1(MPSA56) turned off and T2(MPSA92) turned on, latter resulting in -Vg= ~1/2-Vb= 20V. Now, if Ic rises above 41mA, resulting in V-Rc 410mV, OP-A1/Vout will ramp negative* until T1 starts to conduct thereby robbing base-current from T2 which again results in a -Vg value that is somewhat more negative than before. Et Voilà. My 2 euro-cents (that's even less than before). +--- OPT | | ===== 5881 ----- ----- Vin ---+---- -- -- G1 | O | | +-+ V Ic | |Rg | +-+ +--- B | | +-------+ -Vg | --- | | | ^ +-+ +-+ +-+ | | |Ry | |Rx | | Rc 10R |V-Rc +-+ +-+ +-+ | | | | | | | | | | | --- GND --- ^ | A -Vb It would be nice to know a few more details of John's schematic. - Vref - -Vg/G1 drive circuit details - -Vb - The missing value resistor - The supply voltages* of A1 (bipolar, bipolar a-symmetric, unipolar) Do you have that information John? Gio *Ramp-negative to T1-conduct time, as well as ramp-positive to T1-off time, will depend on the supply voltage of A1(besides V-Rc). |
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Kevin O'Connor is adventurous & perhaps does some things others may not approve of. Thus a toroidal OPT in a guitar amp. No problem with servo bias holding the lid down. This one not as elegant as that found in the Brook, easily the best I've seen in my very long career. It is unique & sure makes a lot of sense. How else to get 30 Watts at low distortion out of a pair of 2A3's in AB2? I got interested in servo bias some time ago while doing my thing with 6AS7/6080 PP circuits. Some may remember the bootstrapped driver schemes with & without the UL OPT. But the heat from the cathode bias resistors under the chassis is a serious problem. So lets try somekind of servo bias. Perhaps this coming Winter. I've been busy on the bicycle (like Patrick), more than 4000 Km since May 1st. And a lot of physical labor here on the acres. I would rather be outside than slaving over a hot soldering iron! For anyone who would like a complete schema of the Kevin O'Connor guitar amp referenced email me direct as follows- jh dot stewart at sympatico dot ca Gotta go now. Today its time to service the summer equipment & put it away till Spring! And here is that easy to drive 6AS7/6080 amp for the curious. Cheers, John |
#12
Posted to rec.audio.tubes
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Jones's servo bias
"Patrick Turner" wrote in message ... On Nov 29, 8:31 am, "GRe" wrote: "Patrick Turner" wrote in message ... [...] The answer to such uneven matching amoung a given batch of supposedly NOS Red Lion KT88 is to have some additional bias balancing adjust method as seen in another pair of Quad-II I rewired where I used EHKT90 instead of KT66, see http://www.turneraudio.com.au/quad2powerampmods.htmand proceed to about 1/2 way down the page to the 2005 schematic. What's the reason you chose 560K for R11 while R10= 470K? Rgds, Gio To achieve Vac drive balance to each OP tube where there is an LTP with common Rk resistance, the RLa seen by each gain tube of the LTP should be slightly dissimilar to compensate for the unbalancing effect of the common Rk value. If Rk was a CCS, then the total cap coupled RLa on each side of the LTP needs to be the same for perfect balance, even if the tube types used are quite different, eg, see the Radford schematics. But where you have two pentodes as LTP gain tubes, gain is close to gm x RL, and quite high, and the current change in one equals the current cjange in the other plus the change of current in the common Rk. You should be able to do the analysis of all working voltages and currents in my circuit as easily as in any other. In fact, the use of 560k and 470k is a bit high for Rg bias values, because reverse grid current in ageing tubes can cause Eg1 to become several volts more positive than the actual bias voltage at the end of Rg away from the grid. In my example, if that happens, it unbalances the Idc of the OP tubes thus causing the balance monitoring LEDs to change from normal OK with both equally bright to having one brighter than the other. An owner will turn the adjust pot with a thumbnail or a dinner knife until Idc balance is restored, but if the problem of positve grids gets so bad the balance pot cannot be adjusted to get equal LED brightness, then he knows he has a tube problem, and needs new tubes. For this reason, the high Rg values are not so bad, bearing in mind the amp has cathode bias with individual R&C networks in each OP tube cathode circuit. In more recent rewiring of Quad-II-Forty amps completed last week, I used 6SH7 in triode for V1, then used a 6SN7 for LTP with a CCS with MJE340. RLa to B+ was 50k per 1/2 6SN7, and Rg = 120k taken to to the OPT CFB tranny winding to partially bootstrap the Rg to have it appear to to the 6SN7 as about 300k. The dc balance will remain fair but not excellent with individual R&C bias networks, and with Rg at 120k, the reverse positive grid current in ageing tubes won't cause too much trouble. In two samples of the Red Lion KT88 in one Quad-II-Forty monobloc, Ek = +38Vdc in one tube and +44Vdc in the other. Rk = 630 ohms, so Il for the two OP tubes is 60.3mAdc and 69.8mAdc, a difference of 9.5mAdc, a considerable difference. But effects of the Idc imbalance in Chinese made OPTs is minimal because although the OPT cores seem large enough, the number of turns is dissapointing. But the paucity of primary turns means the Np x Idc product in the equation governing the Bdc within the OPT core remains fairly low, thus allowing a high ammount of additive Bac before saturation occurs at full power at about 25Hz, good enough. But Quad-II amps from the 1950s have more Np turns and smaller core, and the use of a balancing pot for Idc in each OPT tube isn't a bad idea. My customer who had me re-wire his Quad-II-Forty amps has given me a dozen Red Lion KT88 to test, all with unknown condition. I should be able to find close pairs among them all, and then if he had an uncle, his name would be Bob. Generally, whatever I do in amps is done to help the tubes give their best in terms of distortion, bandwidth, output resistance and stability. I don't include ALL techniques in every pair of amps; not all amps of mine have the same range of techniques. They don't need to. Providing the loading of tubes in tube amplifiers is technically optimised, and the other 4 items done properly, with optimal DC biasing, music will flow well without it becoming insiduously grubby as the tube age, which is so common in Quad-II amps where their Uncle Scrooge owners cannot bring themselves to replace 20 year old KT66. Patrick Turner. Hi Patrick, It took me some time to digest the comprehensive info, especially when I started reading the QuadII mod's article on your site, hence the delayed answer. I don't think I digested it all, yet I learned something, thanks. B.t.w., do yourself a favor, leave the narcissists doing what they do best; gazing at their own belly button. Gio. |
#13
Posted to rec.audio.tubes
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Jones's servo bias
On Dec 5, 4:44*pm, John L Stewart John.L.Stewart.
wrote: Hi Ian & Others- Yeh, I'm back again after another 5 year stint in hitech sales. But I guess for the last time at my age! Kevin O'Connor is adventurous & perhaps does some things others may not approve of. Thus a toroidal OPT in a guitar amp. No problem with servo bias holding the lid down. That might be hotly disputed by those who feel that a softly- saturating OPT is a desirable tone option. This one not as elegant as that found in the Brook, easily the best I've seen in my very long career. It is unique & sure makes a lot of sense. How else to get 30 Watts at low distortion out of a pair of 2A3's in AB2? I got interested in servo bias some time ago while doing my thing with 6AS7/6080 PP circuits. Some may remember the bootstrapped driver schemes with & without the UL OPT. But the heat from the cathode bias resistors under the chassis is a serious problem. So lets try somekind of servo bias. Perhaps this coming Winter. I've been busy on the bicycle (like Patrick), more than 4000 Km since May 1st. And a lot of physical labor here on the acres. I would rather be outside than slaving over a hot soldering iron! I quite fancy a little rocket. Bradford is full of hills and dangerous drivers with no insurance. Even walking is scary. I don't have room for a helicopter. For anyone who would like a complete schema of the Kevin O'Connor guitar amp referenced email me direct as follows- jh dot stewart at sympatico dot ca I replaced Jones's clamp with O'Connor's. Perfect. Gotta go now. Today its time to service the summer equipment & put it away till Spring! And here is that easy to drive 6AS7/6080 amp for the curious. Summer equipment? In Europe, we have a fire in the sky controlled by druids with henges. Every time the druids get locked up on drug charges, the weather falls apart. +-------------------------------------------------------------------+ |Filename: 6BQ7 6080 Amp.jpg * * * * * * * * * * * * * * * * * * * *| |Download:http://www.audiobanter.com/attachment.php?attachmentid=251| +-------------------------------------------------------------------+ Thanks. Interesting. I guess there's some maths that goes with the weirdness? Like the load seen by, and output resistance of, the short- tailed pair's anodes. Ian |
#14
Posted to rec.audio.tubes
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Jones's servo bias
On Dec 6, 11:30*pm, "GRe" wrote:
"Patrick Turner" wrote in message ... On Nov 29, 8:31 am, "GRe" wrote: "Patrick Turner" wrote in message .... [...] The answer to such uneven matching amoung a given batch of supposedly NOS Red Lion KT88 is to have some additional bias balancing adjust method as seen in another pair of Quad-II I rewired where I used EHKT90 instead of KT66, see http://www.turneraudio.com.au/quad2p....htmandproceed to about 1/2 way down the page to the 2005 schematic. What's the reason you chose 560K for R11 while R10= 470K? Rgds, Gio To achieve Vac drive balance to each OP tube where there is an LTP with common Rk resistance, the RLa seen by each gain tube of the LTP should be slightly dissimilar to compensate for the unbalancing effect of the common Rk value. If Rk was a CCS, then the total cap coupled RLa on each side of the LTP needs to be the same for perfect balance, even if the tube types used are quite different, eg, see the Radford schematics. But where you have two pentodes as LTP gain tubes, gain is close to gm x RL, and quite high, and the current change in one equals the current cjange in the other plus the change of current in the common Rk. You should be able to do the analysis of all working voltages and currents in my circuit as easily as in any other. In fact, the use of 560k and 470k is a bit high for Rg bias values, because reverse grid current in ageing tubes can cause Eg1 to become several volts more positive than the actual bias voltage at the end of Rg away from the grid. In my example, if that happens, it unbalances the Idc of the OP tubes thus causing the balance monitoring LEDs to change from normal OK with both equally bright to having one brighter than the other. An owner will turn the adjust pot with a thumbnail or a dinner knife until Idc balance is restored, but if the problem of positve grids gets so bad the balance pot cannot be adjusted to get equal LED brightness, then he knows he has a tube problem, and needs new tubes. For this reason, the high Rg values are not so bad, bearing in mind the amp has cathode bias with individual R&C networks in each OP tube cathode circuit. In more recent rewiring of Quad-II-Forty amps completed last week, I used 6SH7 in triode for V1, then used a 6SN7 for LTP with a CCS with MJE340. RLa to B+ was 50k per 1/2 6SN7, and Rg = 120k taken to to the OPT CFB tranny winding to partially bootstrap the Rg to have it appear to to the 6SN7 as about 300k. The dc balance will remain fair but not excellent with individual R&C bias networks, and with Rg at 120k, the reverse positive grid current in ageing tubes won't cause too much trouble. *In two samples of the Red Lion KT88 in one Quad-II-Forty monobloc, Ek = +38Vdc in one tube and +44Vdc in the other. Rk = 630 ohms, so Il for the two OP tubes is 60.3mAdc and 69.8mAdc, a difference of 9.5mAdc, a considerable difference. But effects of the Idc imbalance in Chinese made OPTs is minimal because although the OPT cores seem large enough, the number of turns is dissapointing. But the paucity of primary turns means the Np x Idc product in the equation governing the Bdc within the OPT core remains fairly low, thus allowing a high ammount of additive Bac before saturation occurs at full power at about 25Hz, good enough. But Quad-II amps from the 1950s have more Np turns and smaller core, and the use of a balancing pot for Idc in each OPT tube isn't a bad idea. My customer who had me re-wire his Quad-II-Forty amps has given me a dozen Red Lion KT88 to test, all with unknown condition. I should be able to find close pairs among them all, and then if he had an uncle, his name would be Bob. Generally, whatever I do in amps is done to help the tubes give their best in terms of distortion, bandwidth, output resistance and stability. I don't include ALL techniques in every pair of amps; not all amps of mine have the same range of techniques. They don't need to. Providing the loading of tubes in tube amplifiers is technically optimised, and the other 4 items done properly, with optimal DC biasing, music will flow well without it becoming insiduously grubby as the tube age, which is so common in Quad-II amps where their Uncle Scrooge owners cannot bring themselves to replace 20 year old KT66. Patrick Turner. Hi Patrick, It took me some time to digest the comprehensive info, especially when I started reading the QuadII mod's article on your site, hence the delayed answer. I don't think I digested it all, yet I learned something, thanks. B.t.w., do yourself a favor, leave the narcissists doing what they do best; gazing at their own belly button. Gio If ppl ask many more questions after reading my website then all is very well indeed. But if they just throw rocks at me, what's the point? I just done a pair of Quad-II-Forty amps with a basic 5050 schematic instead of the original Quad horror. 6SH7 is a really good pentode when Ia becomes high enough, ie, much more than in any Quad. But the 6SH7 becomes a really super dooper triode with high gain and low Ra, and it works with 6SN7 LTP very well. During the same months I did tha last Quad mods, I also did a re-wire of an Earl Weston amp which had about the worst damn schematic I have ever seen in any amp, but after my singing lessons with a 5050 type schematic, the owner said it sounded better than the Quads. Earl is a bloke in Victoria making more amps than I ever will, but man, although his work looks so pretty, there's a mess under the bonnet. My customer got lotsa smoke instead of music, I could not just repair it; it needed complete gutting and re-wiring. Earl's transformers were quite OK and I could give the amp unconditional stability with just the right phase tweaking networks. Tube line up ended up 1/2 6SN7 for input SET and then 6SN7 for LTP, then chinese KT66 with 40% UL operation, it gave about 30Watts, very nice. When I get time, all the schematics of these mods to amps will be posted at my site. At the CERN accelerator, they are looking for a Higs Boson or somink like that, and today I read a whole page in the newspaper about it. I didn't understand a word. But, maybe it'd be nice to know what particle causes Time, and Mass, as then we might manipulate it, and fly to elswhere at a speed of C squared, or C to the power of infinity, and we might amble up to God, and ask him how the universe doth go, and he might tell us he's busy, because he has to look after an infinite number of universes besides our little one we find ourselves within. It would not seem unreasonable to think that if some God does exist who we might comprehend, then He, She, or It might just know NOW what's going on, and that means light must travel infinitely faster than it does, and just why it seems like it is stuck at about 300,000km/sec and restricted to this speed is a deep question indeed. I also read today about nuclear power and how Green Politics is often so backward in thinking, like they never wanna give up old beliefs based on BS rather than answers to questions based on facts observed. As a result of much questioning, blokes have come up with IFRs, Integral Fast Reactors which use up leftover waste from old fashioned nuclear power plants. With IFRs, we could avoid greenhouse effects, and get off fossil fuels, thus be able to use fossil deposits sensibly without burning them for transport, hot water, cooking etc. Of course, if we had cheap electricity and no greenhouse effect, nothing would stop economic growth and population growth so we'd have to farewell the environment anyway. Food production has to double in 35 years, well, its what ppl will want, and anyway, lots more questions will need answering as we muddle along, fixin' one problem, only to find we have another. But I digress. Patrick Turner. |
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Posted to rec.audio.tubes
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Jones's servo bias
Of course, if we had cheap electricity and no greenhouse effect, nothing would stop economic growth and population growth so we'd have to farewell the environment anyway. Food production has to double in 35 years, well, its what ppl will want, and anyway, lots more questions will need answering as we muddle along, fixin' one problem, only to find we have another. One word: Eugenics. |
#16
Posted to rec.audio.tubes
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Jones's servo bias
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#17
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Jones's servo bias
On Dec 15, 5:15*pm, wrote:
Of course, if we had cheap electricity and no greenhouse effect, nothing would stop economic growth and population growth so we'd have to farewell the environment anyway. Food production has to double in 35 years, well, its what ppl will want, and anyway, lots more questions will need answering as we muddle along, fixin' one problem, only to find we have another. *One word: Eugenics. Somethin's gotta give. Nothing can repeat forever. I can't know the future, and I don't care about it; I won't be in it. If humans alter their genes to cope with future circumstances ( if they can afford the therapy under USA style medical system ) then that's their choice then, not mine now. Meanwhile, the radioactive **** from Fukashima power plants circles the Earth in its oceans, and that **** will be joined in future by lots more ****, because everyone knows **** happens. It seems most unfortunate that the Japanese bore the brunt of nuclear madness after copping a dose of genocidal atomic bombings in WW2. ( Boy, if thay don't start a flame war, nothing will.) "Sir, would you like sunglasses to eat your tuna caught last Friday? We do realise they glow in the dark..." Patrick Turner. |
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