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#1
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On Thursday, May 19, 2011 1:52:19 AM UTC+1, Patrick Turner wrote:
On May 18, 7:54*pm, mike s wrote: I have a couple of output transformers with separate feedback windings, and was hoping that one might be a suitable replacement for the failed transformer in the *RCA 82 C4 Monitor Amplifier. *Schematic herehttp://www..waltzingbear.com/Schematics/RCA/BA-4C.htm However it uses a high impedance winding *(1/6th of primary anode-anode), whereas the transformers I have both use low impedance windings, about 1:100. *I have a schematic for a British circuit by P J Baxandall *- last on this page *http://mike.wepoco.com/Home/retro-ge...-wire...*which places the feedback winding in series with the input pentode cathode, rather than the high impedance potential divider used in the RCA circuit. *Baxandall claimed to be using a transformer design patented by the BBC (Mayo, Tanner, Ellis). *I've seen the same arrangement used in a Marconi push-pull amplifier. I reckon I'm going to have to adopt the Baxandall arrangement, *but would be interested to learn what others think of these circuits. *Presumably it was amplifiers like this that inspired the rather quirky Quad 2 extra feedback to the output valves.http://www.drtube.com/schematics/quad/quad-22.gif Michael You won't easily find any "drop in" new replacement OPT which could be used in the the circuits by RCA, Peter Baxandall, or Peter Walker of Quad. Quirky old stuff is "my thing", so I stash away old transformers and the like. It does seem however that UK designers took a different tack when designing amps/transformers for this style of feedback. So the vintage British transformers I have won't simply "drop in". I'm OK with that. All these old circuits have serious shortcomings which are only made worse when someone tries to use an unsuitable OPT at home and they do not understand how NOT to build an oscillator while trying to build an amplifier. Hopefully whatever I pick won't be unsuitable, but is going to need some changes. Perhaps in the end the amount of NFB will need to be reduced. You say you "have a couple of OPT" without saying just exactly what their specification might be. To know what you are doing, you need to know every single bit of information which describes each tranny fully. You say your OPT in your RCA monitot amp has failed, so we assume it has an open primary or shorted turns. One half of the primary has failed, so the remaining half allows me to establish the ratios, and resistance, of all windings. So I can't use it - as is, but can learn enough about it to know what's needed. BREAKING NEWS - I've managed to un-pot it, so will post photos. I might even be able to repair it. Which would be nice. The Hammond 1650P OPT with 6k6 to 4, 8, 16 ohms will be OK with 6L6, 807, 1622 etc, but you will need to addapt the Hammond OPT to the RCA circuit you already seem to have. The RCA circuit has NFB loop with R24, 24k, and R8, 2k7, and we don't know how much voltage is generated at the NFB winding across OPT terminals 1 to 3. With the turns ratio I can figure that out, and knowing that with the 24k resistor it is unloaded, which I believe is the design principle of these amps - pure voltage feedback, with close to zero lag. The lazy dumb ****wits at RCA ommitted to provide us with a more clearly drawn schematic with all the working signal voltages for all electrodes and transformer windings. My guess is that It would have been nice to have the voltages, that's for sure. There are copies of the manual for these amps on the WWW so I might find more info. OPT terminal 3 produces a NFB voltage of about 50Vrms to be able to supply a high enough FB voltage at V3 6SN7 cathode, maybe 5Vrms, so that the amount of NFB is at least 12dB. I'll let you know when I've done the calcs, but these amps were claimed to have very low noise, so high NFB is to be expected. A normal OPT with no FB winding and just speaker secondaries could be used in the RCA circuit but you'd have to re-arrange the FB network to V3 cathode with R24 being a lower value. This will affect the way V3 cathode is biased; if R24 is made smaller it reduces the total value of Rk. Fso if R24 = 4k7, the R8 may need to be increased to 3k3 to give the equivalent of what is in the schematic, ie, 2k7 // 24k for the dc cathode current, ie, about 2k2. I'd never ever try Baxandall's circuit, and I'm no great fan of Quad, or ANY circuit which employs a "paraphase" input pair or uses the output of one triode to drive another as done on the RCA circuit. It is always better to employ a LONG TAIL PAIR as in many of my amplifier schematics which you may inspect at my website at http://www.turneraudio.com.au It's interesting that RCA and the BBC in the UK adopted these types of amplifier. Clearly broadcast engineers liked them. I wonder why? I've always used NFB applied to a cathode of SE triode input stage ahead of an LTP driver stage and this is better because the distortion of the input tube is included in the FB and thus reduced along with all following stages in the loop. The input tube works at low signal levels so second order products are minimal. The Williamson, Leak and Radford circuits emboby such principles as I do, but to make the amp unconditionally stable regardless of load reactance will challenge your abilities sorely unless you have a full understanding of what you are doing. Patrick Turner. |
#2
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 20, 2:14*am, mike s wrote:
On Thursday, May 19, 2011 1:52:19 AM UTC+1, Patrick Turner wrote: On May 18, 7:54*pm, mike s wrote: I have a couple of output transformers with separate feedback windings, and was hoping that one might be a suitable replacement for the failed transformer in the *RCA 82 C4 Monitor Amplifier. *Schematic herehttp://www.waltzingbear.com/Schematics/RCA/BA-4C.htm However it uses a high impedance winding *(1/6th of primary anode-anode), whereas the transformers I have both use low impedance windings, about 1:100. *I have a schematic for a British circuit by P J Baxandall *- last on this page *http://mike.wepoco.com/Home/retro-ge...r-wire...which places the feedback winding in series with the input pentode cathode, rather than the high impedance potential divider used in the RCA circuit. *Baxandall claimed to be using a transformer design patented by the BBC (Mayo, Tanner, Ellis). *I've seen the same arrangement used in a Marconi push-pull amplifier. I reckon I'm going to have to adopt the Baxandall arrangement, *but would be interested to learn what others think of these circuits. *Presumably it was amplifiers like this that inspired the rather quirky Quad 2 extra feedback to the output valves.http://www.drtube.com/schematics/quad/quad-22.gif Michael You won't easily find any "drop in" new replacement OPT *which could be used in the the circuits by RCA, Peter Baxandall, or Peter Walker of Quad. Quirky old stuff is "my thing", so I stash away old transformers and the like. It does seem however that UK designers took a different tack when designing amps/transformers for this style of feedback. *So the vintage British transformers I have won't simply "drop in". *I'm OK with that. * There have been an amazing number of different amps designed by guys who yearned to be unique. "Yearning For Uniqueness" would have to an extremely vain pre-occupation of anyone's mind when we consider the **** of one man smells almost identically to the **** of the next man along. We all like redheads, brunnettes, and blondes, and what we do with them is remarkably similar, ie, the divorce rate in the US is about the same as in Britain. But the implementation of any one amplifier idea is often just as important as the idea itself. Ultralinear was the favourite in the US, and so was global NFB around 3 gain stages with FB froma speaker winding to cathode of V1. The Pomms showed how in the Williamson which was a PP triode thinge which quickly became UL if you wanted it to be for more power. The americans were fond of power, and their cars were bigger, waistlines longer, houses bigger and government more influential after WW2. Everything in the USA had to be bigger brighter, louder, et all while in Deare Olde England which was savaged by WW2 things remained GRIM with housing, rates of pay, and standard of living for far longer than in the US after WW2. With smaller houses and smaller budgets, only the rich in the UK could buy Quad-II amps with the luxury of powerful octal based tubes such as KT66. KT88 had to wait until 1957, 12 years after WW2. It wasa time when interest in hi-fi was seen by many as vain, vapid, absurd, and eccentric. In my observations of audiophiles, I can see that not much has changed. All these old circuits have serious shortcomings which are only made worse when someone tries to use an unsuitable OPT at home and they do not understand how NOT to build an oscillator while trying to build an amplifier. Hopefully whatever I pick won't be unsuitable, *but is going to need some changes. *Perhaps in the end the amount of NFB will need to be reduced.. The use of NFB from a dedicated NFB winding which uses fine wire because there is little current was something major makers never adopted. Baxandal and Williamson were the most ignored blokes of 1950. People wanted SIMPLE CHEAP solutions, and these two boffins offered neither cheap nor simple, and insisted on going the extra country mile to fetch a bucket full of real hi-fi. Baxandall was brilliant, and his analysis of the Quad ESL57 requirements of the step up trannies shows how damn dumb most consequent people were about basic audio LCR ideas. And many online DIY ppl who have tried to build ESL show how dumb they continue to be. Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. The Williamson is FAR BETTER amp design than anything with two pentode input tubes. But in 1950, many designers had a real bad case of pentoditis, the affliction where blokes chose a pentode instead of two low µ triodes which give much less THD and IMD. You say you "have a couple of OPT" without saying just exactly what their specification might be. To know what you are doing, you need to know every single bit of information which describes each tranny fully. You say your OPT in your RCA monitot amp has failed, so we assume it has an open primary or shorted turns. One half of the primary has failed, *so the remaining half allows me to establish the ratios, and resistance, of all windings. *So I can't use it - as is, but can learn enough about it to know what's needed. Many old OPT have several if not many things missing, which is why I am not delighted by what I find in old 1950s amps. Missing is enough primary turns, enough interleaving, thick enough primary wire, decent insulation with low dielectric constant, enough iron, and low winding losses. Once you know what could have been done and wasn't done in 1950 just so the boss could buy a decent car, then fascination tends to fade about OLD JUNK. BREAKING NEWS - *I've managed to un-pot it, so will post photos. *I might even be able to repair it. *Which would be nice. Yeah, maube the break is accessible, but most OPTs fuse a winding somewhere deep inside where windings get hot because someone forgot to build the amp so it shuts down WHEN an OP tube endures bias failure. The Hammond 1650P OPT with 6k6 to 4, 8, 16 ohms will be OK with 6L6, 807, 1622 etc, but you will need to addapt the Hammond OPT to the RCA circuit you already seem to have. The RCA circuit has NFB loop with R24, 24k, and R8, 2k7, and we don't know how much voltage is generated at the NFB winding across OPT terminals 1 to 3. With the turns ratio I can figure that out, and knowing that with the 24k resistor it is unloaded, which I believe is the design principle of these amps - pure voltage feedback, with close to zero lag. There will be phase shift depending on where the NFB winding is located relative to other windings. But if the NFB winding is tightly coupled to the primary, then possibly the phase shift of the secondary is avoided, especially if the sec load is a capacitance. So with a FB winding the amp should be easier to make unconditionally stable with NFB; no attempt is being made to correct for effects of high lekage inductance between P and speaker windings. Unless you know the real sequence and placement of all windings, you cannot be sure the NFB winding will work any better than just taking NFB from speaker sec (like everyone else) and to the cathode of V1. The lazy dumb ****wits at RCA ommitted to provide us with a more clearly drawn schematic with all the working signal voltages for all electrodes and transformer windings. My guess is that It would have been nice to have the voltages, that's for sure. *There are copies of the manual for these amps on the WWW so I might find more info.. * The manuals would help, but you can calculate all the loads and voltage gains based on 2 6L6 in PP class AB for a max of about 30W. Do your loadline analysis, and all shall be revealed. I guess about 12dB of NFB is applied, and that isn't much, and not enough, IMHO, but if you need about 1.5Vrms applied to an input pentode without NFB for clipping, then with 20dB NFB FB there will be 15V of input. The better way to do the business is to arrange the two input pentodes as a true LTP with CCS common cathode sink. This meant a negative supply and another pentode in 1955, so it was never done because my dad's generation were too cashed strapped raising the baby boomer generation, and they were parsimonious about any expenditure. But now we'd just use a bjt, but even with a large value R taken to -400V rails which are easy to make with Si diodes is OK. In 1955, there were no Si diodes. Freedom of circuit design was severely curtailed. But when solid state came along, everything changed. Sure they got freedom, and liberation from guilt about never quite ever building decent OPTs, but early SS was a horror story, and when dumper bins filled with ****ed tube amps by 1962, ppl were throwing the baby out with the bath water. OPT terminal 3 produces a NFB voltage of about 50Vrms to be able to supply a high enough FB voltage at V3 6SN7 cathode, maybe 5Vrms, so that the amount of NFB is at least 12dB. I'll let you know when I've done the calcs, but these amps were claimed to have very low noise, so high NFB is to be expected. I've made 55W SE amps with extremely low noise despite no use of loop NFB. My SE55 with 2 x 845 gave noise = 0.25mV with only 10dB global NFB. That's -98dB no weighting, referred to clip level. But noise should be low without regard to SNR, because if you have horn speakers you still want no audible hum from across the room, ie, less than 0.5mV. Low noise is easily achieved by using remote power supplies and dc to all input tube heaters, and to cathodes of all OP DHT, if used. The Williamson was renowned for being quiet. It was recommended that a remote PSU be used. When stero came along, ppl hated the expense of yet another two chassis with hot things on it. By then it was 1960, and they hated replacing 10 year old KT66, so the mono system got replaced by cool running transistor junk. A few makers kept the faith after 1960 and continued making tube amps, and many of these dreary things linger on, often with few saving graces. A normal OPT with no FB winding and just speaker secondaries could be used in the RCA circuit but you'd have to re-arrange the FB network to V3 cathode with R24 being a lower value. This will affect the way V3 cathode is biased; if R24 is made smaller it reduces the total value of Rk. Fso if R24 = 4k7, the R8 may need to be increased to 3k3 to give the equivalent of what is in the schematic, ie, 2k7 // 24k for the dc cathode current, ie, about 2k2. I'd never ever try Baxandall's circuit, and I'm no great fan of Quad, or ANY circuit which employs a "paraphase" input pair or uses the output of one triode to drive another as done on the RCA circuit. It is always better to employ a LONG TAIL PAIR as in many of my amplifier schematics which you may inspect at my website athttp://www.turneraudio..com.au It's interesting that RCA and the BBC in the UK adopted these types of amplifier. *Clearly broadcast engineers liked them. *I wonder why? But the BBC liked Quad-II and Leak and Radfords also, and didn't really favour any quirky damn amp. The engineers didn't have much of a say - what they got was what the accountants authorised. Engineers were a damn pest, always wanting bucket fulls of cash for "interesting solutions", eg, we need a 50W amp. Nope, youse can make do with a 20W amp. Patrick Turner. I've always used NFB applied to a cathode of SE triode input stage ahead of an LTP driver stage and this is better because the distortion of the input tube is included in the FB and *thus reduced along with all following stages in the loop. The input tube works at low signal levels so second order products are minimal. The Williamson, Leak and Radford circuits emboby such principles as I do, but to make the amp unconditionally stable regardless of load reactance will challenge your abilities sorely unless you have a full understanding of what you are doing. Patrick Turner.- Hide quoted text - - Show quoted text - |
#3
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On 05/20/11 03:22, Patrick Turner so wittily quipped:
Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. I hadn't heard the term 'paraphase' until you mentioned it, so I did a bit of online research, found out that I've seen things like that before and I never liked it done 'that way', but yeah, using PENTODES just makes everything worse for distortion and potential imbalance. Relying on the circuit gain characteristics of a tube for your design, or requiring a balance adjustment to make it work, is just silly. As for me, I prefer the use of a pair of triodes as a direct coupled 'cathodyne' setup (careful biasing and component choices gives you pretty good results), though a 'long tail pair with a constant current on the cathodes' (something you had in one of your amps I think) would be a good alternative (some additional research suggests the latter might actually work better in an overdriven guitar amp). The long tail pair has slight imbalance if you don't get close to a constant current on the cathode, so the simpler 'cathodyne' splitter gives you better balance with fewer parts (though it may require series resistors for downstream grids in case they draw current during overdrive on guitar amps). anyway, I found a couple of interesting web sites that describe the various types of phase splitters (among other things) http://www.freewebs.com/valvewizard/index.html (see index on the left) also more here http://www.bonavolta.ch/hobby/en/audio/split.htm But in 1950, many designers had a real bad case of pentoditis, the affliction where blokes chose a pentode instead of two low µ triodes which give much less THD and IMD. there's a general move to create something 'patentable' so that you have a portfolio of intellectual property, regardless of whether or not it is actually better. But in cases where another guy's patent steps on your design, it might be necessary to go a slightly inferior route to avoid paying royalties. By now all of those patents are expired. You say you "have a couple of OPT" without saying just exactly what their specification might be. To know what you are doing, you need to know every single bit of information which describes each tranny fully. You say your OPT in your RCA monitot amp has failed, so we assume it has an open primary or shorted turns. One half of the primary has failed, so the remaining half allows me to establish the ratios, and resistance, of all windings. So I can't use it - as is, but can learn enough about it to know what's needed. Many old OPT have several if not many things missing, which is why I am not delighted by what I find in old 1950s amps. Missing is enough primary turns, enough interleaving, thick enough primary wire, decent insulation with low dielectric constant, enough iron, and low winding losses. Once you know what could have been done and wasn't done in 1950 just so the boss could buy a decent car, then fascination tends to fade about OLD JUNK. old junk indeed. reminds me of those 'low iron' class 'A' transformers that I swapped the phase on (both sides of course) and saw output where none was before, apparently due to hysteresis problems, on a late 50's or early 1960's console stereo system. Funny, though, 2W per channel just wasn't enough so I bought an old 15W/channel (tube) stereo amp in a caged chassis and just wired it up inside the cabinet. /me wonders if you could use a 70V 'line' output as the NFB source with a few mods to the circuit. |
#4
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
In article ,
Big Bad Bob wrote: On 05/20/11 03:22, Patrick Turner so wittily quipped: Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. I hadn't heard the term 'paraphase' until you mentioned it, so I did a bit of online research, found out that I've seen things like that before and I never liked it done 'that way', but yeah, using PENTODES just makes everything worse for distortion and potential imbalance. Relying on the circuit gain characteristics of a tube for your design, or requiring a balance adjustment to make it work, is just silly. I prefer the floating paraphase phase inverter as it has many advantages over other forms of phase inverter. If the inverter tube has sufficient gain, balance is excellent and no balance adjustment is necessary. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#5
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On 05/20/11 19:21, flipper so wittily quipped:
On Fri, 20 May 2011 06:54:32 -0700, Big Bad Bob wrote: On 05/20/11 03:22, Patrick Turner so wittily quipped: Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. I hadn't heard the term 'paraphase' until you mentioned it, so I did a bit of online research, found out that I've seen things like that before and I never liked it done 'that way', but yeah, using PENTODES just makes everything worse for distortion and potential imbalance. Relying on the circuit gain characteristics of a tube for your design, or requiring a balance adjustment to make it work, is just silly. Yes, well, the original Williamson called for a balance control on the separate triode voltage amps because their gain is what it is. The Williamson looks balanced but unless the triodes are identical, fat chance, it isn't. (He later decided GNFB was enough to not need the balance pot). The Baxandall is a floating paraphase, which uses negative feedback. Look at it again. The second pentode is driven by the difference of the two signal paths, the two 470k summing 'sense' resistors, and is not operating open loop like the Williamson voltage amp triodes are. I used a similar floating paraphase on my "Batman" battery guitar amp but used triodes and since they were low gain I 'predisposed' the balance to nominal, and let the feedback work around that, but with pentodes you have lots of gain for NFB around the phase splitter. curious, which of these designs gives you the best overall gain, transient response, overdrive characteristics, and lowest cost (or 'part count')? Down side for cathodyne is that you have to make sure that the Rp-Rk never drops below around 50V or so (depending on the tube) in the worst case voltage swing. Alternative is to put a 2nd set of triodes between the splitter and the output tubes, but not sure if that adds potential imbalance or not (you'd have to match more parts again). At that point I'd do the 'long tail' thing and match the Rp's, and use a constant current source. |
#6
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 20, 11:54*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote: On 05/20/11 03:22, Patrick Turner so wittily quipped: Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. I hadn't heard the term 'paraphase' until you mentioned it, so I did a bit of online research, found out that I've seen things like that before and I never liked it done 'that way', but yeah, using PENTODES just makes everything worse for distortion and potential imbalance. *Relying on the circuit gain characteristics of a tube for your design, or requiring a balance adjustment to make it work, is just silly. *As for me, I prefer the use of a pair of triodes as a direct coupled 'cathodyne' setup (careful biasing and component choices gives you pretty good results), though a 'long tail pair with a constant current on the cathodes' (something you had in one of your amps I think) would be a good alternative (some additional research suggests the latter might actually work better in an overdriven guitar amp). *The long tail pair has slight imbalance if you don't get close to a constant current on the cathode, so the simpler 'cathodyne' splitter gives you better balance with fewer parts (though it may require series resistors for downstream grids in case they draw current during overdrive on guitar amps). anyway, I found a couple of interesting web sites that describe the various types of phase splitters (among other things) Quad-II used a kind of floating paraphase. Nobody much understands exactly how it works but fact is that the output of one EF86 is reduced by resistance divider and fed to the grid of another EF86 and both thus have a stage which effectively has 6dB of local positive FB. Both EF86 have to make up to about 40Vrms for each KT66 grid, ( depending on the load ). I've tried using KT88 in triode with fixed bias in Quad- II with an all triode input stage using low µ SE input and LTP driver with CCS cathode tail. There is definately less high order HD products but the measurements don't get dramatically better than the original Quad-II. I've also tried using EF86 set up as a true LTP rather than paraphase and methinks all things that I have tried are better measuring and sounding than anything concocted for a quid by Peter Walker, ( God luv 'im ). See http://www.turneraudio.com.au/quad2powerampmods.htm GE published a book giving 17 amps from 5W to 1,100W and in it they recommended both Williamson and paraphase circuits. The best paraphase circuits are done using low µ triodes with a following balanced amp so that the paraphasing is done at low signal levels where the positive FB doesn't increase HD very much because it is so low to begin with. there's a general move to create something 'patentable' so that you have a portfolio of intellectual property, regardless of whether or not it is actually better. *But in cases where another guy's patent steps on your design, it might be necessary to go a slightly inferior route to avoid paying royalties. *By now all of those patents are expired. Indeed. Patrick Turner. |
#7
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 21, 10:07*am, John Byrns wrote:
In article , *Big Bad Bob wrote: On 05/20/11 03:22, Patrick Turner so wittily quipped: Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. I hadn't heard the term 'paraphase' until you mentioned it, so I did a bit of online research, found out that I've seen things like that before and I never liked it done 'that way', but yeah, using PENTODES just makes everything worse for distortion and potential imbalance. *Relying on the circuit gain characteristics of a tube for your design, or requiring a balance adjustment to make it work, is just silly. I prefer the floating paraphase phase inverter as it has many advantages over other forms of phase inverter. *If the inverter tube has sufficient gain, balance is excellent and no balance adjustment is necessary. I've seen several versions of floating paraphase, usually you need to have output tube grids biased at 0Vdc so that the feed from R divider to one input tube grid is also at 0Vdc like the V1 tube. Using fixed bias with adjust pots for the two output tubes complicates the design unecessarily. But I've always found them slow, ie, their bandwidth is lower than other methods so amp stabilization at HF for unconditional stability even with a pure 0.22 uF load is more difficult to achieve, so I never ever have used paraphase in any amp I've built. Many other methods are better. The best natural balance is achieved with a LTP in which the tubes on each side do not need to be matched, and the drive to each OP tube is dependant on the equality of the resistance loads on the two tubes of the LTP. 1% metal film resistors have been routinely available for many years now and they don't change value like the crappy old carbon composition types. Patrick Turner. |
#8
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 21, 12:21*pm, flipper wrote:
On Fri, 20 May 2011 06:54:32 -0700, Big Bad Bob wrote: On 05/20/11 03:22, Patrick Turner so wittily quipped: Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. I hadn't heard the term 'paraphase' until you mentioned it, so I did a bit of online research, found out that I've seen things like that before and I never liked it done 'that way', but yeah, using PENTODES just makes everything worse for distortion and potential imbalance. *Relying on the circuit gain characteristics of a tube for your design, or requiring a balance adjustment to make it work, is just silly. Yes, well, the original Williamson called for a balance control on the separate triode voltage amps because their gain is what it is. The Williamson looks balanced but unless the triodes are identical, fat chance, it isn't. (He later decided GNFB was enough to not need the balance pot). Williamson fell under the spell of the bean counters who had declared war upon anyone who ever used one more resistor than necessary acording to the rules laid down by the Society Of Bean Counters. So he used that stupid arrangement with his KT66 cathode circuit involving pots to balance ****. Balance it did, until a tube over heated, and in EVERY single Williamson sample I have had to repair, guess what - fried pots where thy just should *never ever* be used. Leak's idea of cathode bias with two R&C bias networks was best, and **** the rest. Its because the dual R gave the best bias regulation. The balance of the Willy *short* tail pair, STP, was poor because the tail R was such a low value. But the concertina as per Willy gave excellent balance providing anode and cathode R were equal. This balance is maintained well enough in a Willy amp, but can be be much improved if a 4k7 is put in between 0V and the 470R which is in the orriginal. THEN the natural balance of the STP becomes good enough to never wish for more. But with that 4k7 added, the STP won't give very good balance if driven on only one side, like the Leak circuit and so many others, Mullard 520 etc. The simple answer to good balance and simplicity is to use an LTP with CCS tail. And if one don't like the 2H generated in V1 SE stage from ruining music then use an input LTP with 6SN7/6CG7/6DJ8/ etc for V1 and V2, with CCS tail, then use a second LTP for V3 and V4 with 6SN7/6CG7/12BH7/EL84/EL86/ etc, which has a tail R taken to -150Vdc. The amp input is taken to V1 grid and GNFB is applied to V2 grid, and the whole input-driver is thus beautifully balanced. I recall I saw a version in RDH4, but have forgotten where, and it don't matter because in 1955 they didn't have handy things like an MJE340 which makes a splendid CCS. The Baxandall is a floating paraphase, which uses negative feedback. Look at it again. The second pentode is driven by the difference of the two signal paths, the two 470k summing 'sense' resistors, and is not operating open loop like the Williamson voltage amp triodes are. True. What distracted me was that the SP61 have different anode RL for Idc, input has 47k, and second tube has 4k7, and this looks like a typo. Let's assume both SP61 have RLdc = 47k. But you'll find balance only fair enough. The higher the gain of such signal pentodes, the better the balance becomes. I don't like seeing any grid circuit operating from a high impedance like the two 470k. Isn't the Baxandall circuit almost identical to the Schmitt? p526 RDH4. But ARC d40 used a circuit where the input signal came to one input tube with cathode FB and also to a unity gain amp to make the second phase which powered a second input tube with an oppositely phased FB signal. The OPT had a CT and there were balanced GNFB loops of opposite phase. Too complex of course. But the d40 was better IMHO than later ARC which became a ****in nightmare of complexity. I used a similar floating paraphase on my "Batman" battery guitar amp but used triodes and since they were low gain I 'predisposed' the balance to nominal, and let the feedback work around that, but with pentodes you have lots of gain for NFB around the phase splitter. The floating paraphase has a different HF roll off on the two paths but, then, so does a concertina. Also true, but the HF output roll off of the two phases in a Williamson are both at such a high F that the THD thus generated is well above 20kHz and not important. I found that the the HF balance from a concertina could be made near equal if about 15pF or other low C value is across the cathode R of the concertina to slightly boost the gain from the anode. None of them are 'perfect'. In RDH4, P344, there is a 30W beam tetrode amp with 807 with SE pentode input, pentode concertina. Page 344 has the triode Williamson with 807. P353 is interesting though, positive and negative FB loops are used. This is PURE QUIRKINESS for those who must have quirks in their amplifier collection. I have never seen any commercial design with PFB and NFB - too hard to get right. But most amps do have quite a few quarks lurking about. Nothing is ever perfect. Patrick Turner. |
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NFB windings, was there a US style and UK style?
On May 21, 7:03*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote: On 05/20/11 19:21, flipper so wittily quipped: On Fri, 20 May 2011 06:54:32 -0700, Big Bad Bob *wrote: On 05/20/11 03:22, Patrick Turner so wittily quipped: Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. I hadn't heard the term 'paraphase' until you mentioned it, so I did a bit of online research, found out that I've seen things like that before and I never liked it done 'that way', but yeah, using PENTODES just makes everything worse for distortion and potential imbalance. *Relying on the circuit gain characteristics of a tube for your design, or requiring a balance adjustment to make it work, is just silly. Yes, well, the original Williamson called for a balance control on the separate triode voltage amps because their gain is what it is. The Williamson looks balanced but unless the triodes are identical, fat chance, it isn't. (He later decided GNFB was enough to not need the balance pot). The Baxandall is a floating paraphase, which uses negative feedback. Look at it again. The second pentode is driven by the difference of the two signal paths, the two 470k summing 'sense' resistors, and is not operating open loop like the Williamson voltage amp triodes are. I used a similar floating paraphase on my "Batman" battery guitar amp but used triodes and since they were low gain I 'predisposed' the balance to nominal, and let the feedback work around that, but with pentodes you have lots of gain for NFB around the phase splitter. curious, which of these designs gives you the best overall gain, transient response, overdrive characteristics, and lowest cost (or 'part count')? *Down side for cathodyne is that you have to make sure that the Rp-Rk never drops below around 50V or so (depending on the tube) in the worst case voltage swing. *Alternative is to put a 2nd set of triodes between the splitter and the output tubes, but not sure if that adds potential imbalance or not (you'd have to match more parts again). *At that point I'd do the 'long tail' thing and match the Rp's, and use a constant current source.- Hide quoted text - - Show quoted text - My website shows numerous schematics which speak volumes about my preferences for best-ness. But in the last few years I have re-engineered a few high end amps including ARC VT100, Manley Labs Snappers, and an old pair of Dynaco Mk-VI. In these amps there were 4 x 9 pin sockets for each channel so to make good use of the sockets. With the ARC VT100, I did the following.... I stripped out nearly all of what ARC used, leaving only about 5% of basic heater circuits and PSU. A carpenter's chisel was used remove the tracks and surplus stuff to the bin. There's a lotta stuff which does not give more hi-fi, so its best removed.. New tracks were used using links of wire. 101 other things were don, like soldering copper wire to bases of 6550 to be able to wire them in so they can't fall out of the terrible octal sockets ARC have used which have very poor grip on tube pins. Then, Input = LTP with a one twin triode 6DJ8 on each side. MJE340 CCS cathode tail to a negative rail. Driver = 2 x 12BH7, one on each side of balanced amp with common Rk to negative rail. Balance drive was better than 1%. Output tubes all fitted with individually adjustable fixed bias and bi colour green/red LED used to indicate bias condition of tubes and with full protection against bias failures. The original ARC had a total of 5 TO92 tiny j-fets used as CCS, but they are all unsuitable fragile and unecessary. Just why Big Boys like ARC don't include such things I routinely do is beyond my comprehension. I did a similar thing on Dynaco mk-V1 but with EL84 in triode used on each side of driving balanced amp. Patrick Turner. |
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In RDH4, P344, there is a 30W beam tetrode amp with 807 with SE
pentode input, pentode concertina. Page 344 has the triode Williamson with 807. P353 is interesting though, positive and negative FB loops are used. This is PURE QUIRKINESS for those who must have quirks in their amplifier collection. I have never seen any commercial design with PFB and NFB - too hard to get right. But most amps do have quite a few quarks lurking about. Nothing is ever perfect. Patrick Turner.[/quote] That 807 amp on p344 is simply a barely passable PA type, nothing more. The NFB connexion used makes the OP stage more susceptable to PS ripple. The amp shown on p353 was probably never available commercially, but the text gives a good description of how the circuit works. Several of the Electrovoice Circlotrons used +ve current FB from the secondary of the OPT to accomplish very low to zero internal impedance & go even negative. Another way of looking at it is infinite or -ve DF. If we looked a bit we could probably find others. But not something for an amatuer to attack. Makes up for speaker lead resistance. And something no one seems to talk about, that being the DC resistance of the speaker coil itself. Cheers to all, John |
#11
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NFB windings, was there a US style and UK style?
On May 22, 9:47*am, John L Stewart John.L.Stewart.
wrote: In RDH4, P344, there is a 30W beam tetrode amp with 807 with SE pentode input, pentode concertina. Page 344 has the triode Williamson with 807. P353 is interesting though, positive and negative FB loops are used. This is PURE QUIRKINESS for those who must have quirks in their amplifier collection. I have never seen any commercial design with PFB and NFB - too hard to get right. But most amps do have quite a few quarks lurking about. Nothing is ever perfect. Patrick Turner. That 807 amp on p344 is simply a barely passable PA type, nothing more. The NFB connexion used makes the OP stage more susceptable to PS ripple. Agreed. I like the RDH4 book, its like a bible, but the wisdom needs to be applied in a modern world. The amp shown on p353 was probably never available commercially, but the text gives a good description of how the circuit works. Indeed. Several of the Electrovoice Circlotrons used +ve current FB from the secondary of the OPT to accomplish very low to zero internal impedance & go even negative. Another way of looking at it is infinite or -ve DF. Bogan also did variable damping factor with positive current FB. I've tried it. Dagerous, because Vo rises when RL is reduced, and if the load is a short the damn thing tries to oscillate to death. Not nice. But the PCFB can be made to work at LF only which then vastly improves the bass "tighness", whatever that is. Such things are gimics. The PCFB increases THD/IMD and reduces BW. If we looked a bit we could probably find others. But not something for an amatuer to attack. Depends. 1% of amateurs know more than many professionals. Pros only know so much, and bean counters limit realization of many ideas. Makes up for speaker lead resistance. And something no one seems to talk about, that being the DC resistance of the speaker coil itself. Theil and Small had a lot to say about speakers; A few lines of incomprehensible text, then a formula stretching across the page, then another bit of text, then another formula. They would lecture everyone about speakers et all and at the end of any lecture there remained a stubborn palpable presence of the non diminished mysteries of how speakers worked. Anyway, about 40 years later, a few blokes who understood a little of the math were able to cobble up a program or three so that other blokes just could dial in a speaker driver characteristics, go CLICK, and there was the best kinda box needed without the trial and errors involved with guesswork which always produces expensive samples of firewood. Somehere buried in the input data was the winding resistance. With other books and info stuff a bloke then could learn how to make most dynamic speakers look like a humble resistance as far as eyes of an amplifier were concerned. You liddle beauty, because then the damn crossovers would work as intended. But then a good speaker maker needs to build around the measured and non flat frequency response of the selected drivers, and if you've ever build a 4 way speaker, bass, lower MF, uper MF, and HF, and all with second order filters, boy, what a lot to get right! - many, many, many tings and tings. The world is chokka block full of both amateurs and pros who for 101 reasons never consider enough factoids, and most won't admit it. Patrick Turner. Cheers to all, John -- John L Stewart |
#12
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NFB windings, was there a US style and UK style?
On 05/21/11 22:03, flipper so wittily quipped:
I'm not really a 'fan' of the paraphase but the battery filament tubes I was using in Batman didn't leave much choice. The pentode-triode (and diode I'm not using) share the same filament in the 1D8GT and that ruled out anything under the 'cathodes'. ugh, yeah probably no choice. even a long tail wouldn't work well unless you can somehow CCS both the filament AND Ik at the same time [unlikely]. So definitely limited in what's possible. the basic circuit design is kind of like the way a differential amplifier circuit works, where the sum of the inverting + non-inverting must become zero when the circuit is in balance. The error is then infinitely amplified, and NFB re-balances everything to create a reliably amplified signal (or that's the theory). So when the output of the 'thing' that produces the inverting signal is summed with the original, this creates the 'error' signal. Ideally, you have zero error when the circuit works. Unfortunately, low gain triodes will result in higher error, so you'll probably need something to adjust balance on it. Hence... (your additional comments) I call mine a 'hybrid' paraphase because, as mentioned, the triodes don't have a lot of gain so it starts off looking like a plain paraphase where you tap a 'small' signal off the first phase to drive the second. But rather than just a tap I also link the second phase into it, like a floating paraphase, so the tap 'predisposes' the signal level and the feedback works around that predisposition. I'm guessing this corrects for what I mentioned above. 'Tricky' stuff. If the tubes are identical and exactly as spec'd for bogey then everything gains and sums up to what's expected so there's, essentially, 'nothing for the feedback to do'. But if either are off then the summation doesn't 'come out right' and tends to push the second triode toward a match. and that's another problem with paraphase - you need matching 'things' and component aging isn't compensated for in the least. It isn't very much NFB, maybe 9 dB, but, hey, why not? Actually, I'm not quite sure how to calculate it but there's a 3 to 1 voltage difference with R2 connected there ('closed loop') vs not so I figure that's 9.5dB. http://flipperhome.dyndns.org/Batman.htm pretty cool. FYI something LIKE THIS is ideal for generating "tube amp distortion" without wasting a lot of power. You could use an 8 ohm dummy load and overdrive it into a different amp (which then gets you the volume). /me getting ideas. The amp is still dominated by 2's harmonics but that's from the open ended first stage. a tetrode? Ok, just watch out for the reverse-resistance range at low current. 100k on the plate may suggest that you're already in that slot, hence the 2nd harmonic distortion [which may get even worse]. You might halve that and adjust bias for twice the current. OK gain may be lower, or then again may not. You're probably dropping 40v or so across the 100k which is .4 ma so 47k and .8 ma should get you a bit more linearity. [I'll leave it up to you to tweek the bias on it]. Ep too low is also likely to create problems. My rule is to pick a voltage that's about 1/2 B+ for the 'rest state', using a plate current that's reasonably linear for an easily obtainable G1 bias. Anyway, if you can put in test signals you might want to do that to generate your own curves, maybe 3VDC with a potentiometer and manual readings (or storage scope if you have one). /me reminded of the $89 pocket scope I bought a few months ago - very nice, goes up to 1Mhz. Down side for cathodyne is that you have to make sure that the Rp-Rk never drops below around 50V or so (depending on the tube) in the worst case voltage swing. yes. using high B+ helps. If you've got power tubes that need 400V that's not too difficult. You can add an R+C filter, drop down to maybe 380V with a relatively large R and make that the supply for the cathodyne part. I like to split the voltage so that the G1 bias is less than the voltage drop across Rp or Rk at steady state, and is also SIGNIFICANTLY less than 1/5 of the supply voltage. So then the best case voltages for 60V negative bias (on the power tubes) would be 70V on the cathode and ~310V on the plate (steady state, B+ 380), with 130 on the cathode and 240 on the plate at the 'highest current' point. Unfortunately this will have higher 2nd harmonic distortion due to the wider current swing. Anyway, Rp - Rk is 110, which is more than enough, so this could be tweeked a bit more, maybe 90V on the cathode and 290 on the plate at steady state, then Rp - Rk is 80V, still well within the linear range for a typical triode. Then you just pick resistors that match your desired current, and make sure the G1 volts at steady state is around Ek + bias. But if this is directly driven from a prior stage then you must increase Ek to an even higher amount to prevent Ep on the driving stage from getting too low. So now you tweek it up a bit more, with steady state Ek at 110 (~50V min on driver tube plate), and 280V Ep, with 'worst case' at 170V Ek and 220V Ep (now down to 50V), and you're "there". Current only has to change by a fraction of the quiescent value, you can direct drive it from the prior stage, and if you choose the right parts (and maybe even regulate the B+ for the pre-amps) it self-balances. Anyway, point well taken on min Ep - Ek. At that point I'd do the 'long tail' thing and match the Rp's, and use a constant current source. Neither Williamson nor Baxandall had transistors to conveniently create a CCS. you could do it with tubes. Ground grid, put Rk that sets the current to what you want, plate in series with long tail cathodes, and use a voltage divider (filtered) to bias the grids. But then Ek is much higher than it would be with a transistor, since you could drop down below 2v and still get your CCS (thereby using ground potential for the grid bias). Cheating and using germanium transistors you could go even lower. It also depends on whether it matters enough to bother with. PP gain imbalance manifests predominately as 2'nd harmonic and, remember, that's 'the first to go' with NFB. interesting point. yeah, you can tolerate a bit of imbalance. But the result is likely to be a reduction in output power, or uneven clipping when overdriven. |
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NFB windings, was there a US style and UK style?
On 05/21/11 04:41, Patrick Turner so wittily quipped:
Williamson fell under the spell of the bean counters who had declared war upon anyone who ever used one more resistor than necessary acording to the rules laid down by the Society Of Bean Counters. that can happen. accountants driving design. sometimes good, sometimes not. 'cost reduction' phases are typical. So he used that stupid arrangement with his KT66 cathode circuit involving pots to balance ****. Balance it did, until a tube over heated, and in EVERY single Williamson sample I have had to repair, guess what - fried pots where thy just should *never ever* be used. Leak's idea of cathode bias with two R&C bias networks was best, and **** the rest. Its because the dual R gave the best bias regulation. self-correcting (inherently stable) circuit designs. who'd a thunk it. That as opposed to potentially UNstable ones. Like that one. Assuming nothing ever overloads/overheats, nothing ever ages, nothing ever works differently than it does in the design lab, etc. is just a recipe for RMA's and costly design revs. And unhappy customers. it's my opinion that adjustments are bad, anyway. It's an extra step during the assembly that requires a technician to perform. Adding a few extra parts to PREVENT that step is preferable, my $.10 worth anyway. |
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NFB windings, was there a US style and UK style?
In article ,
Patrick Turner wrote: In RDH4, P344, there is a 30W beam tetrode amp with 807 with SE pentode input, pentode concertina. Page 344 has the triode Williamson with 807. P353 is interesting though, positive and negative FB loops are used. This is PURE QUIRKINESS for those who must have quirks in their amplifier collection. I have never seen any commercial design with PFB and NFB - too hard to get right. But most amps do have quite a few quarks lurking about. Look here for a common combined PFB & NFB circuit. http://www.triodeel.com/dynast35.gif -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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NFB windings, was there a US style and UK style?
In article ,
Patrick Turner wrote: Quad-II used a kind of floating paraphase. Nobody much understands exactly how it works but fact is that the output of one EF86 is reduced by resistance divider and fed to the grid of another EF86 and both thus have a stage which effectively has 6dB of local positive FB. Both EF86 have to make up to about 40Vrms for each KT66 grid, ( depending on the load ). It's not really that hard to figure out how it works, a lot of the quirkiness is due to the acrobatics needed to apply negative feedback to the common cathode of the voltage amplifier and phase splitter. This was probably a bean counter thing to eliminate a couple of electrolytic capacitors. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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NFB windings, was there a US style and UK style?
Somebody said...
Neither Williamson nor Baxandall had transistors to conveniently create a CCS. you could do it with tubes. *Ground grid, put Rk that sets the current to what you want, plate in series with long tail cathodes, and use a voltage divider (filtered) to bias the grids. *But then Ek is much higher than it would be with a transistor, since you could drop down below 2v and still get your CCS (thereby using ground potential for the grid bias). *Cheating and using germanium transistors you could go even lower. Easiest 10mA CCS for an LTP is with a 6AU6 pentode. Anode to the common LTP cathodes, screen bypassed to cathode, fixed Eg1 bias at -100V, fixed cathode supply at -150V with Rk at about 5k2, so that if the 6AU6 µ is say 1,600, effective Ra = 8.7Meg. But with an MJE340, you get much more than that easily. Tube CCS really need two tubes in series to get really high finite Ra values, and bean counters in 1955 had firmly forbiden any such extravagances in consumer grade audio gear. "Cracked carbon" resistors with accurate and stable R values were also expensive and forbidden. But making do with sub-optimal solutions was very popular. Patrick Turner. |
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NFB windings, was there a US style and UK style?
"John Byrns" wrote in message ... In article , Patrick Turner wrote: In RDH4, P344, there is a 30W beam tetrode amp with 807 with SE pentode input, pentode concertina. Page 344 has the triode Williamson with 807. P353 is interesting though, positive and negative FB loops are used. This is PURE QUIRKINESS for those who must have quirks in their amplifier collection. I have never seen any commercial design with PFB and NFB - too hard to get right. But most amps do have quite a few quarks lurking about. Look here for a common combined PFB & NFB circuit. http://www.triodeel.com/dynast35.gif 6.3VAC on the 6BQ5 (common) cathodes???? Gio -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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NFB windings, was there a US style and UK style?
On 05/22/11 04:40, flipper so wittily quipped:
On Sun, 22 May 2011 03:49:29 -0700, Big Bad Bob wrote: On 05/21/11 22:03, flipper so wittily quipped: I'm not really a 'fan' of the paraphase but the battery filament tubes I was using in Batman didn't leave much choice. The pentode-triode (and diode I'm not using) share the same filament in the 1D8GT and that ruled out anything under the 'cathodes'. ugh, yeah probably no choice. even a long tail wouldn't work well unless you can somehow CCS both the filament AND Ik at the same time [unlikely]. So definitely limited in what's possible. Filament IS Ik. And when more than one tube in the bottle it's the combined Ik from all of them. No kind of 'tail' will work. yeah, ok. technically Ik would be the sum of Ip + Ig (the latter irrelevant for triodes) but ok. Filament current is really a different path and that's what I meant. So in your case "Ik" = Ip + filament current (whatever Ix that would be). That's just a distraction, anyway. However, if you _did_ have a way of isolating the actual cathode EMISSION current from the filament current you might be able to get it to work. /me can't think of any way to do that, though. I could have used single triodes but it still isn't so easy to power the filaments since they're the 'cathode' and would be floating on the 'tail'. Gets to be a real mess. and that's the point. |
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Before this thread went off the rails did anyone mention that McIntosh uses a seperate winding on the OPT for overall NFB?
Cheers, John |
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NFB windings, was there a US style and UK style?
I mentioned......
I have never seen any commercial design with PFB and NFB - Of course you have and I gave you examples the last time you said the same thing. I've been saying the same thing because I've never seen PFB used in commercial amps like the way its done in RDH4 at that amp. Sometimes bootstrapping is PFB, but usually the gain increase is mild, from gain with a load to gain which approaches µ. However, come to think about it, Dynaco bootstrapped the pentode input tube anode RL ahead of the triode concertina to boost the gain of the pentode. The gain with bootstrapping a pentode often rises much more than with a triode tube because the pentode has its anode feedback screened off from the electron stream. Pentode µ is gm x Ra, and as pentode Ra is so high then µ is high. BTW, I have to repair the design results of acountants and bean counters all too often. Patrick Turner. |
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NFB windings, was there a US style and UK style?
On May 23, 5:33*am, John Byrns wrote:
In article , *Patrick Turner wrote: In RDH4, P344, there is a 30W beam tetrode amp with 807 with SE pentode input, pentode concertina. Page 344 has the triode Williamson with 807. P353 is interesting though, positive and negative FB loops are used. This is PURE QUIRKINESS for those who must have quirks in their amplifier collection. I have never seen any commercial design with PFB and NFB - too hard to get right. But most amps do have quite a few quarks lurking about. Look here for a common combined PFB & NFB circuit. http://www.triodeel.com/dynast35.gif -- Regards, John Byrns Surf my web pages at, *http://fmamradios.com/ Indeed, PFB cathode to cathode of the input stages and in the presence of NFB. But how much PFB is there? What effect would it have on stability margins and reactive ( capacitance) load tolerance? I've always seen PFB as one step forward, 2 steps backward - a cheap nasty fix if ever there was one! Patrick Turner. |
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NFB windings, was there a US style and UK style?
On May 23, 5:38*am, John Byrns wrote:
In article , *Patrick Turner wrote: Quad-II used a kind of floating paraphase. Nobody much understands exactly how it works but fact is that the output of one EF86 is reduced by resistance divider and fed to the grid of another EF86 and both thus have a stage which effectively has 6dB of local positive FB. Both EF86 have to make up to about 40Vrms for each KT66 grid, ( depending on the load ). It's not really that hard to figure out how it works, a lot of the quirkiness is due to the acrobatics needed to apply negative feedback to the common cathode of the voltage amplifier and phase splitter. *This was probably a bean counter thing to eliminate a couple of electrolytic capacitors. The solution to the problem of low open loop gain suited bean counters at Quad. But if you make the two EF86 behave as a real LTP with common cathode R to -400Vdc, or a CCS, and without the mild 6dB PFB boost of the paraphase inverter principle used, then the differential gain is halved and if the same amount of NFB was applied globally, you'd need 2.8Vrms for clipping power instead of only 1.4Vrms. One solution is to use 6BX6 as input tubes with Ia in each at nearly 2mA and then the gm rises well above what it is in EF86 and by carefully juggling the load values and biasing R to OP tubes which would better be EL34 with more gain than KT66, then you get increased open loop gain without the paraphase connection and better stability etc, and the input pair may be set up as a true LTP. I've also used KT88 and KT90 in Quad-II. Sure anyone may work out ther Quad circuit workings. But I've rarely ever met that anyone. Patrick Turner. -- Regards, John Byrns Surf my web pages at, *http://fmamradios.com/ |
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NFB windings, was there a US style and UK style?
On May 26, 7:35*pm, flipper wrote:
On Thu, 26 May 2011 00:37:17 -0700 (PDT), Patrick Turner wrote: I mentioned...... I have never seen any commercial design with PFB and NFB - Of course you have and I gave you examples the last time you said the same thing. I've been saying the same thing because I've never seen PFB used in commercial amps like the way its done in RDH4 at that amp. Now you've changed it again by adding "like the way." Sometimes bootstrapping is PFB, but usually the gain increase is mild, from gain with a load to gain which approaches µ. However, come to think about it, Dynaco bootstrapped the pentode input tube anode RL ahead of the triode concertina to boost the gain of the pentode. Told ya so. The Harmon Kardon 'Trio' also uses PFB and is the basic topology I've used in at least three of mine. The gain with bootstrapping a pentode often rises much more than with a triode tube because the pentode has its anode feedback screened off from the electron stream. Pentode µ is gm x Ra, and as pentode Ra is so high then µ is high. BTW, I have to repair the design results of acountants and bean counters all too often. You've never had to do it even once because bean counters don't do design. Never have, don't now, and never will. IMHO, you are not quite correct because bean counters and accountants do considerable design work during development of any product. You may laugh, but bean counters and accountants are employed to "just say no" to engineers' first, second and third attempts to design a prototype. This is the standard method of Bean Counter Design Process, BCDP. Often engineers are not even employed at all lest they cost money and crerate arguments and waste everyone's time around the office. But if engineers are employed, design might be complete after attempt No 4, when finally the parts count and complexity, size, and weight and strength have been reduced to a horrible minimum to get the product to barely make it past the warranty and its defects ensure it will never be collectable or a classic design or any kind of sample of goodness. The manufacture of many many products is put through the same old dumb down mill run by the bean counters. Bean counters also ensure that minimum or even zero product development ever takes place; what is simulated once during 2 hours on a PC becomes the product sold to the public, and R&D really becomes "Run and Duck" the irate buyers of a product which fails before the warranty period expires. The Quad-II Forty made in China is a very good example of Applied Crapology By Bean Counters, or ACBBC, where every effort has been avoided to make a good product while disguising what has been made with a chic paint job which does not improve the sound by even +0.002 dB. The Product Promoters ensure the quality removal design work by bean counters will yield nice fat company profits for shareholders who have a nasty habit of selling their shares in a panic if the fatness isn't fat enough, because they are into fatness maximums. But then the company in China might be "owned" by a relative of a high ranking member of China's elitist Communist Party and profits are REALLY FAT because the Quad amps are sold high while workers maybe only earn 64c per hour. People can be persuaded to eat a **** sandwich very easily, and big profits have been made throughout history selling **** which has been diluted before being applied between very thin slices of bread made from nutrition free white flour with chemical additives cheaper than providing decent bread. So you can't even get a good **** sandwich if you wanted one; even this product has been Designed Down to maximise profits. We are seeing Chinese made cars in shops here for 10 grand drive away price. They fail crash tests dismally, and maybe you need to buy two to ensure you have a spare during next years's breakdowns, but hey, its better than a bicycle. I hate shopping and travel. If I shop, its mostly junk I have to buy, and if I travel, the people at arrival line up to empty my wallet, and there's nothing to show for it except an empty wallet. Lots of things in life are shambolic, devoid of any authenticity or wonderment. I like bicycles. I don't like Big Tobbacco, Big Hudrocarbon, or Big Brother and a whole big list of other Mr Bigs in manufacturing who perpetuate products I don't have any desire for. They all want me to be a debt ridden but addicted and compliant little consumer. But I say no to most of them. They deal in negativity, so from me they get negativity. Patrick Turner. Patrick Turner.- Hide quoted text - - Show quoted text - |
#24
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NFB windings, was there a US style and UK style?
In article ,
Patrick Turner wrote: On May 23, 5:38*am, John Byrns wrote: In article , *Patrick Turner wrote: Quad-II used a kind of floating paraphase. Nobody much understands exactly how it works but fact is that the output of one EF86 is reduced by resistance divider and fed to the grid of another EF86 and both thus have a stage which effectively has 6dB of local positive FB. Both EF86 have to make up to about 40Vrms for each KT66 grid, ( depending on the load ). It's not really that hard to figure out how it works, a lot of the quirkiness is due to the acrobatics needed to apply negative feedback to the common cathode of the voltage amplifier and phase splitter. *This was probably a bean counter thing to eliminate a couple of electrolytic capacitors. The solution to the problem of low open loop gain suited bean counters at Quad. But if you make the two EF86 behave as a real LTP with common cathode R to -400Vdc, or a CCS, and without the mild 6dB PFB boost of the paraphase inverter principle used, then the differential gain is halved and if the same amount of NFB was applied globally, you'd need 2.8Vrms for clipping power instead of only 1.4Vrms. You keep talking about "the mild 6dB PFB boost of the paraphase inverter principle". I don't understand where you are getting this 6 dB boost? The floating paraphase has the same gain as the concertina phase inverter, how does the concertina equal the gain of the floating paraphse without PFB, or does the concertina also use PFB? In any case only one out of the many many possible variations of the floating paraphse could be said to incorporate PFB. I'm not an expert on the LTP, and am not a big fan of it, I suspect that the truth here is that the LTP throws away 6 dB of gain relative to the concertina and floating paraphase. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#25
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NFB windings, was there a US style and UK style?
On May 27, 10:47*am, John Byrns wrote:
In article , *Patrick Turner wrote: On May 23, 5:38*am, John Byrns wrote: In article , *Patrick Turner wrote: Quad-II used a kind of floating paraphase. Nobody much understands exactly how it works but fact is that the output of one EF86 is reduced by resistance divider and fed to the grid of another EF86 and both thus have a stage which effectively has 6dB of local positive FB. Both EF86 have to make up to about 40Vrms for each KT66 grid, ( depending on the load ). It's not really that hard to figure out how it works, a lot of the quirkiness is due to the acrobatics needed to apply negative feedback to the common cathode of the voltage amplifier and phase splitter. *This was probably a bean counter thing to eliminate a couple of electrolytic capacitors. The solution to the problem of low open loop gain suited bean counters at Quad. But if you make the two EF86 behave as a real LTP with common cathode R to -400Vdc, or a CCS, and without the mild 6dB PFB boost of the paraphase inverter principle used, then the differential gain is halved and if the same amount of NFB was applied globally, you'd need 2.8Vrms for clipping power instead of only 1.4Vrms. You keep talking about "the mild 6dB PFB boost of the paraphase inverter principle". *I don't understand where you are getting this 6 dB boost? *The floating paraphase has the same gain as the concertina phase inverter, how does the concertina equal the gain of the floating paraphse without PFB, or does the concertina also use PFB? *In any case only one out of the many many possible variations of the floating paraphse could be said to incorporate PFB. *I'm not an expert on the LTP, and am not a big fan of it, I suspect that the truth here is that the LTP throws away 6 dB of gain relative to the concertina and floating paraphase. With any pair if paraphase input tubes with or without the "floating" drive for V2 grid, there is 6dB PFB because you are using a fraction of the output from V1 to drive V2. So say tou have a V1 and V2 input parphase pair = 6SN7. Each may havea gain = 17, so that from V1 you get -17Vo for a +1V input Vgk. The -17Vo V1 output signal is divided down to -1Vgk and applied to V2 grid, and the % distortion created by V1 is also created in V2, so that distortion is inceased in its percentage V2 duely creates its expected +17Vo, and so for only 1Vin at V1, you get a total of 34Vo from the two tubes at higher THD %. This is certainly true of Quad-II and its feeble use of EF86. I've measured it all. Now if the pair of 6SN7 triodes are set up as an LTP with one grid taken to 0V, then to get 34Vo you need to apply 2V in, and you might say gain is halved because you need 2Vi to get 17Vo from each anode. Such an LTP is a form of one common cathode gain tube driving a grounded grid tube, but there is no added gain by means of PFB, and if the tail is a high R value, there is excellent even order H reduction from balanced operation. Try doing some measurements. The concertina "throws away" most of its gain so that +19Vin to a 6SN7 produces -17Va and +17Vk, with the +2Vgk needed to make the total 34Vak output, so open loop gain = 17, but closed loop gain is 34 / 19 = 1.79, and this has beautiful low THD because the 19dB current NFB present. The paraphase works OK if the input pair have only to make a low voltage output less than 2Vrms to drive a balanced amp gain stage, say a second 6SN7 LTP, which works best with a high resistance tail R, or CCS. If the balanced driver amp has balanced drive to each of its grids, the the cathode signal should only have distortion signal present, while each triode of the pair produces gain of 17. Input topologies used by many makers and designers "who didn't design for mass production" as someone else ruefully explained, are determined by need to gain fair input sensitivity while also being able to sustain 20dB reduction of OLG by applied GNFB. Hence Williamson's triode amp needed 2Vrms input for 16 Watts in triode from KT66. Of course UL connection gave 32 Watts and sensitivity was doubled if expressed as Watts per input volt. If instead of Williamson's SET input stage and concertina with 19dB NCFB, you use a paraphase with no NFB but with PFB, then input sensitivity is doubled. It so happens that a pair of cascaded 6SN7 triodes might have gain of 17, but typically its 15 in old amps because makers were loathe to run driver tubes with more than 2mA. So gm is low and Ra is high and gain = 15. Two stages got OLG = 225. But the use of EF86 became irresistable to makers because accountants and bean counters said you only needed 2 little compact EF86 to do what 4 triodes in 2 bulky octal 6SN7 could do. Just how well the devices did things was never of much concern. But it was amoung those few who liked the best technical performance in terms of OL THD and bandwidth. The Williamson input/ driver stage ****s all over a pair of EF86 trying to make +/- 30Vrms drive to a pair of OP tubes. But EF86 and the accountants prevailed. Mullard went to EF86 plus a 12AX7 LTP, and yes, only two nine pin sockets, and yes, loads more OLG, so Vin with 20dB global NFB was 0.2Vrms, so therefore the preamp could have -20dB of gain and maybe one whole tube could be removed out of production. Preamps typically could have one EF86 for a magnetic MM phono cart, followed by another EF86 which wore "all the hats" possible including unity gain Baxandall tone control, and ande follower buffereing of any input signals before the power amp. A typical 1960 UL Mullard concoction producing 1/2 a watt to a sensitive 16 ohm speaker of the day would fill a house with sound, and produce bugger all THD. I would say the Williamson triode amp with all triode input tubes and more of them might sound better, and it to would have "bugger all THD", ie, less than 0.02% at normal civilised listening levels. The Willy amp gave the best SNR. I have used 6BX6 in Quad-II amps set up as a true LTP with CCS cathode R and THD is much less than if one tries the same tubes in paraphase. In Quad-II, the GNFB is applied in an unusual way to maintain paraphase operation while being able to apply the GNFB. But if ever there is a bodge in circuits, here it is! If the input pair is set up as a true LTP, sensitivity is halved, and if the same *amount* of GNFB is used, ie, about 10dB, then Vin needed for clipping is doubled, using the same EF86 operating conditions as for the original Quad-II amps. With 6BX6 / EF80, or even 6EJ7 / EF184, higher gm and higher OLG is possible at at the same or lower OLG as EF86 and with higher current to counter Miller C. Anyway, the last Quad-II upgrade I did used 6BX6 plus KT88 which cope better with appalling inability of Quad- II to get a good load match to any RL below 8 ohms. In about 1999, I used SET tube = 12AT7 paralleled and driving 12AU7 LTP for trioded KT88 with fixed bias for a very nice 20W class AB1 in triode - to me it sounded better than original Quad-II. See the schematic at the bottom of the page at http://www.turneraudio.com.au/quad2powerampmods.htm So Quad always could have used 4 triodes for its input -driver stage. So why didn't they do it? Probably because some stinking rotten small minded parsimonious accountant or bean counter said "Peter, if we use an extra 2 resistors in the Quad-II, we won't be able to buy the latest Morris Major this year..." While Mullard might have been saying "Peter, we have terrific deals on EF86..." Meanwhile, trying to manufacture anything in the UK after WW2 was difficult. All the best guys had been killed in WW2, and copper and iron was scarce. Unions wanted bucket fulls of cash for very little work. OK, make miniature OPTs. Smaller things cost less to make . The missus was always pregnant with baby boomer kids. So how to get ppl to buy Quad? get the price down, get the hype up, and don't sell 'em very much at all - just a fashionable bare minimum. See, design by accountants. The curmudgeonly spending habits of an average John Smith of 1960 also determined the design. We have lived for a considerable time now as an integrated society where product design is affected by everyone. Patrick Turner Regards, John Byrns Surf my web pages at, *http://fmamradios.com/- Hide quoted text - - Show quoted text - |
#26
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NFB windings, was there a US style and UK style?
In article ,
Patrick Turner wrote: On May 27, 10:47*am, John Byrns wrote: In article , *Patrick Turner wrote: On May 23, 5:38*am, John Byrns wrote: In article , *Patrick Turner wrote: Quad-II used a kind of floating paraphase. Nobody much understands exactly how it works but fact is that the output of one EF86 is reduced by resistance divider and fed to the grid of another EF86 and both thus have a stage which effectively has 6dB of local positive FB. Both EF86 have to make up to about 40Vrms for each KT66 grid, ( depending on the load ). It's not really that hard to figure out how it works, a lot of the quirkiness is due to the acrobatics needed to apply negative feedback to the common cathode of the voltage amplifier and phase splitter. *This was probably a bean counter thing to eliminate a couple of electrolytic capacitors. The solution to the problem of low open loop gain suited bean counters at Quad. But if you make the two EF86 behave as a real LTP with common cathode R to -400Vdc, or a CCS, and without the mild 6dB PFB boost of the paraphase inverter principle used, then the differential gain is halved and if the same amount of NFB was applied globally, you'd need 2.8Vrms for clipping power instead of only 1.4Vrms. You keep talking about "the mild 6dB PFB boost of the paraphase inverter principle". *I don't understand where you are getting this 6 dB boost? *The floating paraphase has the same gain as the concertina phase inverter, how does the concertina equal the gain of the floating paraphse without PFB, or does the concertina also use PFB? *In any case only one out of the many many possible variations of the floating paraphse could be said to incorporate PFB. *I'm not an expert on the LTP, and am not a big fan of it, I suspect that the truth here is that the LTP throws away 6 dB of gain relative to the concertina and floating paraphase. With any pair if paraphase input tubes with or without the "floating" drive for V2 grid, there is 6dB PFB because you are using a fraction of the output from V1 to drive V2. So say tou have a V1 and V2 input parphase pair = 6SN7. Each may havea gain = 17, so that from V1 you get -17Vo for a +1V input Vgk. The -17Vo V1 output signal is divided down to -1Vgk and applied to V2 grid, and the % distortion created by V1 is also created in V2, so that distortion is inceased in its percentage V2 duely creates its expected +17Vo, and so for only 1Vin at V1, you get a total of 34Vo from the two tubes at higher THD %. This is certainly true of Quad-II and its feeble use of EF86. I've measured it all. Now if the pair of 6SN7 triodes are set up as an LTP with one grid taken to 0V, then to get 34Vo you need to apply 2V in, and you might say gain is halved because you need 2Vi to get 17Vo from each anode. Such an LTP is a form of one common cathode gain tube driving a grounded grid tube, but there is no added gain by means of PFB, and if the tail is a high R value, there is excellent even order H reduction from balanced operation. Try doing some measurements. I fail to see how doing some measurements would help to answer my question? You mentioned "PFB" twice again in the above paragraph. My question was, where is the "PFB in the Paraphase? Most implementations of the paraphase don't have any "PFB" at all. "PFB" is extremely rare in American paraphase designs, either in old radios or in tube Hi-Fi. The only version of the paraphase that could be said to incorporate "PFB" is where V1 & V2 share a common unbypassed cathode resistor, this scheme is more commonly seen in British designs. The concertina "throws away" most of its gain so that +19Vin to a 6SN7 produces -17Va and +17Vk, with the +2Vgk needed to make the total 34Vak output, so open loop gain = 17, but closed loop gain is 34 / 19 = 1.79, and this has beautiful low THD because the 19dB current NFB present. So the concertina requires 1.12 Volts in for the specified 34V grid to grid output drive vs. 1.0 Volts in for the paraphase and 2.0 Volts in for the LTP. 1.12 Volts is close enough to 1.0 Volts for Government work, at least it is a lot closer to 1.0 Volts than it is to 2.0 Volts. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#27
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NFB windings, was there a US style and UK style?
In article ,
John L Stewart wrote: Before this thread went off the rails did anyone mention that McIntosh uses a seperate winding on the OPT for overall NFB? A separate winding, a tertiary winding, provides better high frequency stability than taking the NFB from the secondary winding. The down side of using a tertiary winding for NFB is that the NFB doesn't include the secondary so that the damping factor suffers, this probably has no practical consequence, but doesn't make for good ad copy. Marantz used an interesting NFB scheme that took the NFB from the secondary at low frequencies to provide a good damping factor and then used a crossover filter to take the high frequency NFB from tertiary windings. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#28
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NFB windings, was there a US style and UK style?
Quirky old stuff is "my thing", so I stash away old transformers and the like. It does seem however that UK designers took a different tack when designing amps/transformers for this style of feedback. *So the vintage British transformers I have won't simply "drop in". *I'm OK with that. Yes, you have to respect a thing for what it is and not want it isn't. I bought an old Plymouth with four doors, a Slant Six and three on the tree a couple of years ago and sold it. The new owner gutted it as it didn't have what he wanted, lost interest, and the next guy pulled off the chrome trim and scrapped it. A perfectly good car scrapped. * There have been an amazing number of different amps designed by guys who yearned to be unique. "Yearning For Uniqueness" would have to an extremely vain pre-occupation of anyone's mind when we consider the **** of one man smells almost identically to the **** of the next man along. We all like redheads, brunnettes, and blondes, and what we do with them is remarkably similar, ie, the divorce rate in the US is about the same as in Britain. I grew up in a house with two parents, a grandmther, and two older brothers and a sometimes visiting older half sister. If I went into the bathroom within a few minutes of any of the above having taken a **** there I could without fail identify which had done so. I knew everyone's **** smell, all different. But the implementation of any one amplifier idea is often just as important as the idea itself. Ultralinear was the favourite in the US, and so was global NFB around 3 gain stages with FB froma speaker winding to cathode of V1. The Pomms showed how in the Williamson which was a PP triode thinge which quickly became UL if you wanted it to be for more power. The americans were fond of power, and their cars were bigger, waistlines longer, houses bigger and government more influential after WW2. Everything in the USA had to be bigger brighter, louder, et all while in Deare Olde England which was savaged by WW2 things remained GRIM with housing, rates of pay, and standard of living for far longer than in the US after WW2. With smaller houses and smaller budgets, only the rich in the UK could buy Quad-II amps with the luxury of powerful octal based tubes such as KT66. KT88 had to wait until 1957, 12 years after WW2. It wasa time when interest in hi-fi was seen by many as vain, vapid, absurd, and eccentric. In my observations of audiophiles, I can see that not much has changed. Socialism ruined Britain postwar. The country has never recovered as it went from being poor but at least British to being overrun with immigrants, nonwhite immigrants, none of which have any interest in being British. The BNP is doing as best it can but too little too late. All these old circuits have serious shortcomings which are only made worse when someone tries to use an unsuitable OPT at home and they do not understand how NOT to build an oscillator while trying to build an amplifier. Hopefully whatever I pick won't be unsuitable, *but is going to need some changes. *Perhaps in the end the amount of NFB will need to be reduced. The use of NFB from a dedicated NFB winding which uses fine wire because there is little current was something major makers never adopted. Baxandal and Williamson were the most ignored blokes of 1950. People wanted SIMPLE CHEAP solutions, and these two boffins offered neither cheap nor simple, and insisted on going the extra country mile to fetch a bucket full of real hi-fi. Baxandall was brilliant, and his analysis of the Quad ESL57 requirements of the step up trannies shows how damn dumb most consequent people were about basic audio LCR ideas. And many online DIY ppl who have tried to build ESL show how dumb they continue to be. Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. The Williamson is *FAR BETTER amp design than anything with two pentode input tubes. But in 1950, many designers had a real bad case of pentoditis, the affliction where blokes chose a pentode instead of two low µ triodes which give much less THD and IMD. Not necessarily. The resulting amp used fewer tubes and therefore was more reliable. It used less heater power. The Quad worked fine for decades with the original tube set in many instances, and was much simpler than the American Mcintosh and it did the job for its intended market. The Quad buyer was an affluent sort that just wanted it to play the music with little fuss. That ti did. I would advise having a new transformer wound to the original spec if the amp is worth keeping. |
#29
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NFB windings, was there a US style and UK style?
GE published a book giving 17 amps from 5W to 1,100W and in it they recommended both Williamson and paraphase circuits. The best paraphase circuits are done using low µ triodes with a following balanced amp so that the paraphasing is done at low signal levels where the positive FB doesn't increase HD very much because it is so low to begin with. GEC, not GE. BIG difference. http://www.tubebooks.org/Books/GEC_approach.pdf |
#30
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NFB windings, was there a US style and UK style?
there's a general move to create something 'patentable' so that you have a portfolio of intellectual property, regardless of whether or not it is actually better. *But in cases where another guy's patent steps on your design, it might be necessary to go a slightly inferior route to avoid paying royalties. *By now all of those patents are expired. None the less no one makes a true McIntosh circuit or transformer, despite the superiority of the basic circuit and the transformer design. |
#31
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NFB windings, was there a US style and UK style?
On May 24, 6:03*am, John L Stewart John.L.Stewart.
wrote: Before this thread went off the rails did anyone mention that McIntosh uses a seperate winding on the OPT for overall NFB? Cheers, John -- John L Stewart They use several windings for NFB if you look carefully. Lots of amplifiers had a tertiary for NFB so the secondary could float, and that is a very good idea. |
#32
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NFB windings, was there a US style and UK style?
On May 28, 7:59*pm, flipper wrote:
On Thu, 26 May 2011 16:32:44 -0700 (PDT), Patrick Turner wrote: On May 26, 7:35*pm, flipper wrote: On Thu, 26 May 2011 00:37:17 -0700 (PDT), Patrick Turner wrote: I mentioned...... I have never seen any commercial design with PFB and NFB - Of course you have and I gave you examples the last time you said the same thing. I've been saying the same thing because I've never seen PFB used in commercial amps like the way its done in RDH4 at that amp. Now you've changed it again by adding "like the way." Sometimes bootstrapping is PFB, but usually the gain increase is mild, from gain with a load to gain which approaches µ. However, come to think about it, Dynaco bootstrapped the pentode input tube anode RL ahead of the triode concertina to boost the gain of the pentode. Told ya so. The Harmon Kardon 'Trio' also uses PFB and is the basic topology I've used in at least three of mine. The gain with bootstrapping a pentode often rises much more than with a triode tube because the pentode has its anode feedback screened off from the electron stream. Pentode µ is gm x Ra, and as pentode Ra is so high then µ is high. BTW, I have to repair the design results of acountants and bean counters all too often. You've never had to do it even once because bean counters don't do design. Never have, don't now, and never will. IMHO, Your 'opinion', and following gibberish, is twaddle. It may fool fellow ignorants who, like you, haven't a clue but it's drivel to those of us who've worked with your so called 'bean counters' and done professional product design. I don't speak from 'opinion' and cartoons. I speak from fact, having been there and done that. Its OK, every man's facts are another man's fictions, and one man's trash is another man's treasure. I just see lots of what I don't want to buy or approve of or want to be involved in and if everyone was like me the world would collapse. But fear not and be grateful for the human diversity around you now. Patrick Turner. |
#33
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NFB windings, was there a US style and UK style?
On May 29, 3:47*am, John Byrns wrote:
In article , *Patrick Turner wrote: On May 27, 10:47*am, John Byrns wrote: In article , *Patrick Turner wrote: On May 23, 5:38*am, John Byrns wrote: In article , *Patrick Turner wrote: Quad-II used a kind of floating paraphase. Nobody much understands exactly how it works but fact is that the output of one EF86 is reduced by resistance divider and fed to the grid of another EF86 and both thus have a stage which effectively has 6dB of local positive FB. |
#34
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NFB windings, was there a US style and UK style?
On May 29, 11:23*am, John Byrns wrote:
In article , *John L Stewart wrote: Before this thread went off the rails did anyone mention that McIntosh uses a seperate winding on the OPT for overall NFB? A separate winding, a tertiary winding, provides better high frequency stability than taking the NFB from the secondary winding. *The down side of using a tertiary winding for NFB is that the NFB doesn't include the secondary so that the damping factor suffers, this probably has no practical consequence, but doesn't make for good ad copy. Yes but the NFB winding position relative to anode turns or secondary turns will have an effect on what is fed back. I'd say that if a fine wire NFB winding is wound right over the top of a thick wire speaker secondary then its signal is near equal to the speaker's signal and the "error" made by the secondary Rw and LL and phase shift et all is fed back for correction. But if the FB tertiary is next to the anode primary then you have a good hum free FB source with anode signal without LL and Sec Rw effects and the amp is MUCH easier to stabilise unconditionally. Such a low voltage and grounded winding gives a conveniently small FB signal with low source resistance which can be applied to an earlier stage very effectively. Such a signal picks up a sample of the whole Va-a, and is a better NFB than using a cap plus R divider from one of the PP anodes back to an input tube cathode. But bean counters mostly hated paying for a dedicated NFB tertiary. Why have one when the speaker secondary exists? Indeed why have one? if you make the OPT with sufficient interleaving then the Williamson way of NFB is fine, but bean counters hated Williamson too, and laughed at what he said makers should do, so we all mainly ended up with OPT with very high LL and appalling stability problems. Marantz used an interesting NFB scheme that took the NFB from the secondary at low frequencies to provide a good damping factor and then used a crossover filter to take the high frequency NFB from tertiary windings. I'm not sure about that one. Some makers just used the speaker sec for normal global NFB with R divider to feed V1 cathode but then the cap that is normally strapped across the FB R to advance the phase of the fed back signal at HF is not used, but a cap from V1 cathode is taken to one of the OP tube anodes. I've always found no real joy in such measures and I prefer the normal conventional Williamson idea where one makes the OPT have wide BW which then pushes the regions where instabilty occurs further below and above the AF band and thus to where its easy to instal gain reducing and phase tailoring networks while maintaining 10Hz to 65kHz full power bandwidth with complete stability and 20dB global NFB is desired. I'll follow the Williamson principles before I'll follow any others. I happen to like using tertiary windings for NFB myself, but in the form of CFB windings which are a part of the OPT primary and which carry Ia. Its the Acoustical. Thus the OP stage is substantially linearised and given the character of triode Ra and low THD while retaining the tertrode AB1 or A1 power. Then only 10dB of global NFB is needed, if any at all. Patrick Turner. -- Regards, John Byrns Surf my web pages at, *http://fmamradios.com/ |
#35
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 29, 10:25*pm, flipper wrote:
On Fri, 27 May 2011 17:42:35 -0700 (PDT), Patrick Turner wrote: On May 27, 10:47*am, John Byrns wrote: In article , *Patrick Turner wrote: On May 23, 5:38*am, John Byrns wrote: In article , *Patrick Turner wrote: Quad-II used a kind of floating paraphase. Nobody much understands exactly how it works but fact is that the output of one EF86 is reduced by resistance divider and fed to the grid of another EF86 and both thus have a stage which effectively has 6dB of local positive FB.. Both EF86 have to make up to about 40Vrms for each KT66 grid, ( depending on the load ). It's not really that hard to figure out how it works, a lot of the quirkiness is due to the acrobatics needed to apply negative feedback to the common cathode of the voltage amplifier and phase splitter. *This was probably a bean counter thing to eliminate a couple of electrolytic capacitors. The solution to the problem of low open loop gain suited bean counters at Quad. But if you make the two EF86 behave as a real LTP with common cathode R to -400Vdc, or a CCS, and without the mild 6dB PFB boost of the paraphase inverter principle used, then the differential gain is halved and if the same amount of NFB was applied globally, you'd need 2.8Vrms for clipping power instead of only 1.4Vrms. You keep talking about "the mild 6dB PFB boost of the paraphase inverter principle". *I don't understand where you are getting this 6 dB boost? *The floating paraphase has the same gain as the concertina phase inverter, how does the concertina equal the gain of the floating paraphse without PFB, or does the concertina also use PFB? *In any case only one out of the many many possible variations of the floating paraphse could be said to incorporate PFB. *I'm not an expert on the LTP, and am not a big fan of it, I suspect that the truth here is that the LTP throws away 6 dB of gain relative to the concertina and floating paraphase. With any pair if paraphase input tubes with or without the "floating" drive for V2 grid, It makes a difference which. there is 6dB PFB because you are using a fraction of the output from V1 to drive V2. So say tou have a V1 and V2 input parphase pair = 6SN7. Each may havea gain = 17, so that from V1 you get -17Vo for a +1V input Vgk. The -17Vo V1 output signal is divided down to -1Vgk and applied to V2 grid, and the % distortion created by V1 is also created in V2, so that distortion is inceased in its percentage V2 duely creates its expected +17Vo, and so for only 1Vin at V1, you get a total of 34Vo from the two tubes at higher THD %. The 'plain' paraphase you describe is simply two gain stages with an intermediate attenuator and, in that regard, is no different than, say, a volume control stage with the knob turned to a 1/17 ratio. That, in the paraphase, the ratio is fixed and one observes the two signals are of opposite phase doesn't magically turn it into positive feedback. It's still simply two gain stages with the attenuation 'specifically selected' for the purpose. The floating paraphase is a different beast and uses negative feedback. In that regard it's similar to the concertina except it uses two summing resistors to the grid for unity gain rather than cathode follower feedback, but they both use NFB to reduce distortion in the second tube and have similar gain. The concertina's topology makes it inherently balanced but also limits it's use to triodes while the floating paraphase can use pentodes for greater levels of NFB, with the balance of both types being limited by resistor tolerances. The floating paraphase is also capable of larger output voltage swing than the concertina. None of them, however, use PFB in the basic circuit. This is certainly true of Quad-II and its feeble use of EF86. I've measured it all. Now if the pair of 6SN7 triodes are set up as an LTP with one grid taken to 0V, then to get 34Vo you need to apply 2V in, and you might say gain is halved because you need 2Vi to get 17Vo from each anode. Such an LTP is a form of one common cathode gain tube driving a grounded grid tube, but there is no added gain by means of PFB, and if the tail is a high R value, there is excellent even order H reduction from balanced operation. Try doing some measurements. The "even order H reduction" comes from OPT summing of the two phases, not the LTP itself. However, even if one had a 'perfect' CCS you do not end up with 'perfectly balanced' LTP outputs because of the tubes. Current may be constant but the voltages in each tube to produce those currents are not identical and it's the voltages which are propagated as signals. Plus, of course, the ever present resistor tolerances. The concertina "throws away" most of its gain so that +19Vin to a 6SN7 produces -17Va and +17Vk, with the +2Vgk needed to make the total 34Vak output, so open loop gain = 17, but closed loop gain is 34 / 19 = 1.79, and this has beautiful low THD because the 19dB current NFB present. The floating paraphase, if using the same tubes, has the same NFB. Your reasoning seems so flawed to me that I'd waste too much of my precious time to refute everything you've said which doesn't add up. I humbly suggest you do a pile of basic measurements like I did and you may conclude that the paraphase is a impure way and a poor man's way to boost gain by mild PFB. I've never seen fit to ever use any kind of paraphase connection of input or driver tubes. I have my reasons which remain obscure to you. Don't let that stop you using a paraphase though; usually the defects of paraphase won't cripple the music. Patrick Turner. |
#36
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 30, 9:38*am, Bret Ludwig wrote:
Quirky old stuff is "my thing", so I stash away old transformers and the like. It does seem however that UK designers took a different tack when designing amps/transformers for this style of feedback. *So the vintage British transformers I have won't simply "drop in". *I'm OK with that. *Yes, you have to respect a thing for what it is and not want it isn't. I bought an old Plymouth with four doors, a Slant Six and three on the tree a couple of years ago and sold it. The new owner gutted it as it didn't have what he wanted, lost interest, and the next guy pulled off the chrome trim and scrapped it. A perfectly good car scrapped. * There have been an amazing number of different amps designed by guys who yearned to be unique. "Yearning For Uniqueness" would have to an extremely vain pre-occupation of anyone's mind when we consider the **** of one man smells almost identically to the **** of the next man along. We all like redheads, brunnettes, and blondes, and what we do with them is remarkably similar, ie, the divorce rate in the US is about the same as in Britain. *I grew up in a house with two parents, a grandmther, and two older brothers and a sometimes visiting older half sister. If I went into the bathroom within a few minutes of any of the above having taken a **** there I could without fail identify which had done so. I knew everyone's **** smell, all different. Ah, well, your powers of perception are remarkable, bearing in mind your fascinations, and, but, unlike yourself, it must have been my almost complete dis-interest in the smell of the other 7 ppl in the house I grew up i that i completely missed out on knowing whose crap was whose, or else perhaps the toilet in our house was better ventilated than in yours. :-) But the implementation of any one amplifier idea is often just as important as the idea itself. Ultralinear was the favourite in the US, and so was global NFB around 3 gain stages with FB froma speaker winding to cathode of V1. The Pomms showed how in the Williamson which was a PP triode thinge which quickly became UL if you wanted it to be for more power. The americans were fond of power, and their cars were bigger, waistlines longer, houses bigger and government more influential after WW2. Everything in the USA had to be bigger brighter, louder, et all while in Deare Olde England which was savaged by WW2 things remained GRIM with housing, rates of pay, and standard of living for far longer than in the US after WW2. With smaller houses and smaller budgets, only the rich in the UK could buy Quad-II amps with the luxury of powerful octal based tubes such as KT66. KT88 had to wait until 1957, 12 years after WW2. It was a time when interest in hi-fi was seen by many as vain, vapid, absurd, and eccentric. In my observations of audiophiles, I can see that not much has changed. *Socialism ruined Britain postwar. The country has never recovered as it went from being poor but at least British to being overrun with immigrants, nonwhite immigrants, none of which have any interest in being British. The BNP is doing as best it can but too little too late. So where were you when ya could have lent them your machine gun to make Britain a better place? Oh, I guess you really hate all those Mexicans crossing the border too, yet they do all the dirty work so nice and cheap now don't they? Probably you hate a browned skin man being President Of the USA, and giving speaches in Ireland last week which would have been totally beyond the capability of Dubbya, or of Sarah Palin. All these old circuits have serious shortcomings which are only made worse when someone tries to use an unsuitable OPT at home and they do not understand how NOT to build an oscillator while trying to build an amplifier. Hopefully whatever I pick won't be unsuitable, *but is going to need some changes. *Perhaps in the end the amount of NFB will need to be reduced. The use of NFB from a dedicated NFB winding which uses fine wire because there is little current was something major makers never adopted. Baxandal and Williamson were the most ignored blokes of 1950. People wanted SIMPLE CHEAP solutions, and these two boffins offered neither cheap nor simple, and insisted on going the extra country mile to fetch a bucket full of real hi-fi. Baxandall was brilliant, and his analysis of the Quad ESL57 requirements of the step up trannies shows how damn dumb most consequent people were about basic audio LCR ideas. And many online DIY ppl who have tried to build ESL show how dumb they continue to be. Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. The Williamson is *FAR BETTER amp design than anything with two pentode input tubes. But in 1950, many designers had a real bad case of pentoditis, the affliction where blokes chose a pentode instead of two low µ triodes which give much less THD and IMD. *Not necessarily. The resulting amp used fewer tubes and therefore was more reliable. It used less heater power. The Quad worked fine for decades with the original tube set in many instances, and was much simpler than the American Mcintosh and it did the job for its intended market. *The Quad buyer was an affluent sort that just wanted it to play the music with little fuss. That ti did. The Quad-II was a lot better than much other worse garbage being flogged to a gullible public in 1960. But its easy to make a better sounding amp than Quad-II with the same 2 x KT66 and same number of input tube sockets where a EF86 in triode would make a nice SE input tube and then you have LTP driver/phase inverter with 6CG7. As long as 3 pairs of mint quads are in a museum someplace, I'm happy. But all the rest are worth complete re-wiring. I have a pair underway here where I have wound much better OPT than the terrible originals. Unlike ppl who ruin nice old cars, I create something far better while making use of the basics like chassis and PT. I finish what I start. Patrick Turner. *I would advise having a new transformer wound to the original spec if the amp is worth keeping.- Yeah, very nice idea, but who can ever do EXACTLY what was done 50 years ago? To me the past is gone, there is only the future in which lessons of the past may be incorperated without the shortcomings of the past. Patrick Turner. |
#37
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 30, 8:43*pm, Patrick Turner wrote:
On May 30, 9:38*am, Bret Ludwig wrote: Quirky old stuff is "my thing", so I stash away old transformers and the like. It does seem however that UK designers took a different tack when designing amps/transformers for this style of feedback. *So the vintage British transformers I have won't simply "drop in". *I'm OK with that. *Yes, you have to respect a thing for what it is and not want it isn't. I bought an old Plymouth with four doors, a Slant Six and three on the tree a couple of years ago and sold it. The new owner gutted it as it didn't have what he wanted, lost interest, and the next guy pulled off the chrome trim and scrapped it. A perfectly good car scrapped. * There have been an amazing number of different amps designed by guys who yearned to be unique. "Yearning For Uniqueness" would have to an extremely vain pre-occupation of anyone's mind when we consider the **** of one man smells almost identically to the **** of the next man along. We all like redheads, brunnettes, and blondes, and what we do with them is remarkably similar, ie, the divorce rate in the US is about the same as in Britain. *I grew up in a house with two parents, a grandmther, and two older brothers and a sometimes visiting older half sister. If I went into the bathroom within a few minutes of any of the above having taken a **** there I could without fail identify which had done so. I knew everyone's **** smell, all different. Ah, well, your powers of perception are remarkable, bearing in mind your fascinations, and, but, unlike yourself, it must have been my almost complete dis-interest in the smell of the other 7 ppl in the house I grew up i that i completely missed out on knowing whose crap was whose, or else perhaps the toilet in our house was better ventilated than in yours. :-) But the implementation of any one amplifier idea is often just as important as the idea itself. Ultralinear was the favourite in the US, and so was global NFB around 3 gain stages with FB froma speaker winding to cathode of V1. The Pomms showed how in the Williamson which was a PP triode thinge which quickly became UL if you wanted it to be for more power. The americans were fond of power, and their cars were bigger, waistlines longer, houses bigger and government more influential after WW2. Everything in the USA had to be bigger brighter, louder, et all while in Deare Olde England which was savaged by WW2 things remained GRIM with housing, rates of pay, and standard of living for far longer than in the US after WW2. With smaller houses and smaller budgets, only the rich in the UK could buy Quad-II amps with the luxury of powerful octal based tubes such as KT66. KT88 had to wait until 1957, 12 years after WW2. It was a time when interest in hi-fi was seen by many as vain, vapid, absurd, and eccentric. In my observations of audiophiles, I can see that not much has changed. *Socialism ruined Britain postwar. The country has never recovered as it went from being poor but at least British to being overrun with immigrants, nonwhite immigrants, none of which have any interest in being British. The BNP is doing as best it can but too little too late. So where were you when ya could have lent them your machine gun to make Britain a better place? Oh, I guess you really hate all those Mexicans crossing the border too, yet they do all the dirty work so nice and cheap now don't they? Probably you hate a browned skin man being President Of the USA, and giving speaches in Ireland last week which would have been totally beyond the capability of Dubbya, or of Sarah Palin. All these old circuits have serious shortcomings which are only made worse when someone tries to use an unsuitable OPT at home and they do not understand how NOT to build an oscillator while trying to build an amplifier. Hopefully whatever I pick won't be unsuitable, *but is going to need some changes. *Perhaps in the end the amount of NFB will need to be reduced. The use of NFB from a dedicated NFB winding which uses fine wire because there is little current was something major makers never adopted. Baxandal and Williamson were the most ignored blokes of 1950.. People wanted SIMPLE CHEAP solutions, and these two boffins offered neither cheap nor simple, and insisted on going the extra country mile to fetch a bucket full of real hi-fi. Baxandall was brilliant, and his analysis of the Quad ESL57 requirements of the step up trannies shows how damn dumb most consequent people were about basic audio LCR ideas.. And many online DIY ppl who have tried to build ESL show how dumb they continue to be. Baxandall's amp isn't real bad, but the first input pentode must produce a voltage large enough to drive one 6L6 grid in beam tetrode mode with fixed Eg2. The other input pentode operates as an inverting anode follower with unity gain, but any THD generated in V1 is passed on to be reproduced in V2. In effect, driver amp distortion is double that of a single tube, and this defect also occurs in any amp using a paraphase driver. The Williamson is *FAR BETTER amp design than anything with two pentode input tubes. But in 1950, many designers had a real bad case of pentoditis, the affliction where blokes chose a pentode instead of two low µ triodes which give much less THD and IMD. *Not necessarily. The resulting amp used fewer tubes and therefore was more reliable. It used less heater power. The Quad worked fine for decades with the original tube set in many instances, and was much simpler than the American Mcintosh and it did the job for its intended market. *The Quad buyer was an affluent sort that just wanted it to play the music with little fuss. That ti did. The Quad-II was a lot better than much other worse garbage being flogged to a gullible public in 1960. But its easy to make a better sounding amp than Quad-II with the same 2 x KT66 and same number of input tube sockets where a EF86 in triode would make a nice SE input tube and then you have LTP driver/phase inverter with 6CG7. As long as 3 pairs of mint quads are in a museum someplace, I'm happy. But all the rest are worth complete re-wiring. I have a pair underway here where I have wound much better OPT than the terrible originals. Unlike ppl who ruin nice old cars, I create something far better while making use of the basics like chassis and PT. I finish what I start. Patrick Turner. *I would advise having a new transformer wound to the original spec if the amp is worth keeping.- Yeah, very nice idea, but who can ever do EXACTLY what was done 50 years ago? To me the past is gone, there is only the future in which lessons of the past may be incorperated without the shortcomings of the past. Patrick Turner.- Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - |
#38
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 30, 9:44*am, Bret Ludwig wrote:
GE published a book giving 17 amps from 5W to 1,100W and in it they recommended both Williamson and paraphase circuits. The best paraphase circuits are done using low µ triodes with a following balanced amp so that the paraphasing is done at low signal levels where the positive FB doesn't increase HD very much because it is so low to begin with. *GEC, not GE. BIG difference. http://www.tubebooks.org/Books/GEC_approach.pdf Indeed. Everyone should have a copy of the GEC book. Patrick Turner. |
#39
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
On May 30, 7:51*pm, flipper wrote:
On Mon, 30 May 2011 02:23:45 -0700 (PDT), Patrick Turner wrote: On May 28, 7:59*pm, flipper wrote: On Thu, 26 May 2011 16:32:44 -0700 (PDT), Patrick Turner wrote: On May 26, 7:35*pm, flipper wrote: On Thu, 26 May 2011 00:37:17 -0700 (PDT), Patrick Turner wrote: I mentioned...... I have never seen any commercial design with PFB and NFB - Of course you have and I gave you examples the last time you said the same thing. I've been saying the same thing because I've never seen PFB used in commercial amps like the way its done in RDH4 at that amp. Now you've changed it again by adding "like the way." Sometimes bootstrapping is PFB, but usually the gain increase is mild, from gain with a load to gain which approaches µ. However, come to think about it, Dynaco bootstrapped the pentode input tube anode RL ahead of the triode concertina to boost the gain of the pentode. Told ya so. The Harmon Kardon 'Trio' also uses PFB and is the basic topology I've used in at least three of mine. The gain with bootstrapping a pentode often rises much more than with a triode tube because the pentode has its anode feedback screened off from the electron stream. Pentode µ is gm x Ra, and as pentode Ra is so high then µ is high. BTW, I have to repair the design results of acountants and bean counters all too often. You've never had to do it even once because bean counters don't do design. Never have, don't now, and never will. IMHO, Your 'opinion', and following gibberish, is twaddle. It may fool fellow ignorants who, like you, haven't a clue but it's drivel to those of us who've worked with your so called 'bean counters' and done professional product design. I don't speak from 'opinion' and cartoons. I speak from fact, having been there and done that. Its OK, every man's facts are another man's fictions, and one man's trash is another man's treasure. More gibberish. Go bang your head on a wall and see how far you get wishing it were 'fiction' rather than solid fact. I just see lots of what I don't want to buy or approve of or want to be involved in and if everyone was like me the world would collapse. But fear not and be grateful for the human diversity around you now. It should be blindingly obvious by now that you are not representative of the market. Of course, it's also blindingly obvious that accountants are trained in accounting, which is pretty much why they're called accountants, and not design but, then, it being obvious to everyone else has never stopped your delusions yet. I agree almost entirely. Acountants are acountants. No ****in doubt about it. They count beans, so to speak. And so often one will say to a CEO, "Jesus Fred, doncha reckon if the OPT was smaller and lighter you'd make more profit?" CEO replies, "Yeah, your'e right again Bill, let's talk to the winder guy...", and its a little more quality demolished by a bean counter. Similar crap conversations happen in BP which polluted the gulf so badly......the all mighty DOOLAH must be worshpipped and the true price which should be paid is avoided. Human failings have few limitations. People can resist all sorts of things, except temptations. Patrick Turner. |
#40
Posted to rec.audio.tubes
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NFB windings, was there a US style and UK style?
In article ,
Patrick Turner wrote: On May 29, 3:47*am, John Byrns wrote: In article , *Patrick Turner wrote: Well, I can't explain it any better than I have. If circuit gain is increased by feeding back a fraction of an output signal of one stage to an earlier stage and the gain is increased, THD increased, bandwidth reduced, then I'd say PFB exists. We see feathers, a ducks bill, wings and webbed feet, a ducks tail that wags and we say there's a duck. Unfortunately this is a completely incorrect description of how a generic paraphase inverter operates, so the duck analogy doesn't apply. The concertina "throws away" most of its gain so that +19Vin to a 6SN7 produces -17Va and +17Vk, with the +2Vgk needed to make the total 34Vak output, so open loop gain = 17, but closed loop gain is 34 / 19 = 1.79, and this has beautiful low THD because the 19dB current NFB present. So the concertina requires 1.12 Volts in for the specified 34V grid to grid output drive vs. 1.0 Volts in for the paraphase and 2.0 Volts in for the LTP. * 1.12 Volts is close enough to 1.0 Volts for Government work, at least it is a lot closer to 1.0 Volts than it is to 2.0 Volts. The paraphase tends to add gain "artificially" for a given number of tubes, moreso than merely cascading them. "artificial" gain is the best kind, it can be made so pure that its sound is simply sublime. And in the case of Quad-II, the KT66 grids are biased via 680k Rgs, and one has a 2k7 at its bottom from which the signal to V2 pentode grid is wrought. Now that 2k7 is just what Walker chose, but it could be a range of values; if larger, there's more PFB and more gain and weird things happen when you increase that 2k7 too much because there is a definite limit to the amount of PFB which can be applied without terrible instability. I don't know if 2k7 is the correct value for this resistor, I assume that Quad choose correctly here, there is only one correct value for this resistor where the circuit will function properly, other values are not an option! Changing the value of this resistor will change the amount of PFB as you say, thereby changing the gain of V1. The problem with changing the value of this resistor away from the correct value, to some other value, is that in addition to changing the amount of PFB, it also unbalances the phase inverter so that the two output tubes don't receive the same amount of drive voltage. Next time you play around with a paraphase stage, try tinkering with R diver values and drive from V1 to V2. Interesting stuff happens, with not much of it of real value. What exactly are "R diver values"? There is only one correct value for the "drive from V1 to V2", you may find other amounts of drive interesting, I'm not sure why though? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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