mike s
May 20th 11, 12:11 PM
On Friday, May 20, 2011 11:22:13 AM UTC+1, Patrick Turner wrote:
> 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-geekery/valve-amplifiers/other-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.
>
During WWII PX25 and KT66 valves were unobtainable by most, requiring special authority because they were deemed transmitter valves. After the war purchase tax on high-end radiograms was 66% but export was very much supported so lots of Leak, etc. went to the wealthy of other countries. (rather like today).
> >
> > > 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.
>
I'll post the whole Baxandall article soon. The design was a direct response to Williamson, attempting to show how equivalent performance could be achieved with fewer components and greater efficiency. This is probably why broadcasters favoured pentodes and tetrodes over triodes, because if you've got a lot of amplifiers in a small space all triode amps would be very hot.
> >
> > > 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 -
> 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-geekery/valve-amplifiers/other-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.
>
During WWII PX25 and KT66 valves were unobtainable by most, requiring special authority because they were deemed transmitter valves. After the war purchase tax on high-end radiograms was 66% but export was very much supported so lots of Leak, etc. went to the wealthy of other countries. (rather like today).
> >
> > > 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.
>
I'll post the whole Baxandall article soon. The design was a direct response to Williamson, attempting to show how equivalent performance could be achieved with fewer components and greater efficiency. This is probably why broadcasters favoured pentodes and tetrodes over triodes, because if you've got a lot of amplifiers in a small space all triode amps would be very hot.
> >
> > > 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 -