[mike s]

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-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.


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 -

[Patrick Turner]

I happened to mention......
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.

The original Williamson was a 16W class AB1 amp with lots of class A
from two hot running KT66. It didn't need to run hot if biased for for
less Ia, because the OPT was very wide BW ( for 1947 ) and the
crossover THD was mostly 3H and reduced by NFB OK.
The Williamson could easily be made to run with input and driver being
two 9 pin mini twin triodes, 12AT7 for input and concertina phase
inverter and 12AU7 for balanced amp. If the OP stage was configured in
UL, then you have a truly superlative tube amp for 30W class AB with
KT66, and the tubes need not take up any more space than a Quad-II,
many of which were parked under benches out of sight, out of mind
until their terrible biasing method made tubes run with unbalanced Ia
thus slowly ruining the sound. The main benefit of a genuine
Williamson was avoided like the plague by most makers because none
took Williamson's ideas seriously about OPTs, including Quad. Only a
few old Leaks ever had OPTs made to the Willy specs. After awhile,
makers just used crap in all things they foisted upon the public.
After Williamson, there was only lip service to quality.

There were a few companies which evolved to have a "no compromise"
approach to tube amp engineering such as ARC and others, but you paid
through the nose to buy such things, and more recently even these
companies make things without many refinements such as "simplicity
with circuit integrity", ie, over engieneered stuff, and it all too
often ends up at my repair bench because of bad smoking habits.

AFAIK, the OPTs by ARC, Conrad Johnson, Manley Labs were pretty damn
good, and may still be, and the beat anything the Pomms pumped out in
1955.

Patrick Turner.

[Bret Ludwig[_2_]]

On May 21, 4:53*am, Patrick Turner wrote:
I happened to mention......

*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.

The original Williamson was a 16W class AB1 amp with lots of class A
from two hot running KT66. It didn't need to run hot if biased for for
less Ia, because the OPT was very wide BW ( for 1947 ) and the
crossover THD was mostly 3H and reduced by NFB OK.
The Williamson could easily be made to run with input and driver being
two 9 pin mini twin triodes, 12AT7 for input and concertina phase
inverter and 12AU7 for balanced amp. If the OP stage was configured in
UL, then you have a truly superlative tube amp for 30W class AB with
KT66, and the tubes need not take up any more space than a Quad-II,
many of which were parked under benches out of sight, out of mind
until their terrible biasing method made tubes run with unbalanced Ia
thus slowly ruining the sound. The main benefit of a genuine
Williamson was avoided like the plague by most makers because none
took Williamson's ideas seriously about OPTs, including Quad. Only a
few old Leaks ever had OPTs made to the Willy specs. After awhile,
makers just used crap in all things they foisted upon the public.
After Williamson, there was only lip service to quality.

There were a few companies which evolved to have a "no compromise"
approach to tube amp engineering such as ARC and others, but you paid
through the nose to buy such things, and more recently even these
companies make things without many refinements such as "simplicity
with circuit integrity", ie, over engieneered stuff, and it all too
often ends up at my repair bench because of bad smoking habits.

The original Willy OPT is WAY too complicated and the amplifier
needlessly complicated. Commercial opt winders in the UK (Partridge),
USA (several) and Japan (Hashimoto, Tango, Lux) all excelled the Willy
design with much simpler interleaving.

[Patrick Turner]

On May 30, 9:42*am, Bret Ludwig wrote:
On May 21, 4:53*am, Patrick Turner wrote:





I happened to mention......

*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.

The original Williamson was a 16W class AB1 amp with lots of class A
from two hot running KT66. It didn't need to run hot if biased for for
less Ia, because the OPT was very wide BW ( for 1947 ) and the
crossover THD was mostly 3H and reduced by NFB OK.
The Williamson could easily be made to run with input and driver being
two 9 pin mini twin triodes, 12AT7 for input and concertina phase
inverter and 12AU7 for balanced amp. If the OP stage was configured in
UL, then you have a truly superlative tube amp for 30W class AB with
KT66, and the tubes need not take up any more space than a Quad-II,
many of which were parked under benches out of sight, out of mind
until their terrible biasing method made tubes run with unbalanced Ia
thus slowly ruining the sound. The main benefit of a genuine
Williamson was avoided like the plague by most makers because none
took Williamson's ideas seriously about OPTs, including Quad. Only a
few old Leaks ever had OPTs made to the Willy specs. After awhile,
makers just used crap in all things they foisted upon the public.
After Williamson, there was only lip service to quality.

There were a few companies which evolved to have a "no compromise"
approach to tube amp engineering such as ARC and others, but you paid
through the nose to buy such things, and more recently even these
companies make things without many refinements such as "simplicity
with circuit integrity", ie, over engieneered stuff, and it all too
often ends up at my repair bench because of bad smoking habits.

*The original Willy OPT is WAY too complicated and the amplifier
needlessly complicated. Commercial opt winders in the UK (Partridge),
USA (several) and Japan (Hashimoto, Tango, Lux) all excelled the Willy
design with much simpler interleaving.- Hide quoted text -

- Show quoted text -

Yes, mainly because the twin bobbins are not needed because perfect
DCR and symetrical LL and capacitances are not required.

But all the good makers of OPTs use plenty of interleaving, 5 P
sections with 4 S sections, just concentrically wound. But few makers
ever went that far and you got 3P + 2S and 60mH of LL, ****ing
attrocious, but it was promoted as wonderful, when it wasn't.

Not all the OPT makers you mention did all god stuff. I've seen crap
from a lot them.

OPT makers got their repuations from the most expensive models they
promoted, but what sold the most was the cheap range of rubbish. Its
the same now with audio gear, out comes something high end and
suddenly, for example, Arcam is Cool. Well, I had an Arcam unit here
for a fix and it was anything but cool - total rubbish, but the person
bought it because they heard Arcam was a great product somewhere from
someone.

OPTS made in Oz by A&R of Melbourne and Fergeson of Sydney in 1960
varied between cheap open fram types with P-S- P interleaving with
lousy iron and too few turns to real swish with GOSS, plenty turns,
good interleaving and potted in diecast case nice paint with screen
printed lable, and 100kHz BW. These companies sold very few swish
trannies at twice the price of the crap. Crap paid the bills.

Patrick Turner.