211 A2 amp

211 A2 amp

Hi All.

I finally finished the A2 211 amp I mentioned in some postings to
Patrick some time ago, so I thought I would post a link to some images.

* w w w .lurcher.org/nick/images/211/

Sorry the quality of the images leave a lot to be desired as they were
from a camera phone, but they give the general idea.

I am very pleased with the sound it makes, Running in A2 seems to have a
powerfull character about it, more so than the previous 211 amps I have
built. I think I might try a similar idea with a 572b output next.

All TX wound by Sowter, as I know Iain rates them very highly I thought
I would give them a try.

--
Nick

Re: 211 A2 amp



"Nick Gorham" <nick@lurcher.org> wrote in message
news:013f5448$0$13384$c3e8da3@news.astraweb . com ...
> Hi All.
>
> I finally finished the A2 211 amp I mentioned in some postings to Patrick
> some time ago, so I thought I would post a link to some images.
>
> * w w w .lurcher.org/nick/images/211/
>
> Sorry the quality of the images leave a lot to be desired as they were
> from a camera phone, but they give the general idea.

Hi Nick. Lovely project:-) Just when RAT seems to
have reached an all-time low - karamba - someone
like you posts details of a project like your 211
and restores our faith in human nature and endevour:-))

Some test results would be of interest, as and when.

> I am very pleased with the sound it makes, Running in A2 seems to have a
> powerfull character about it, more so than the previous 211 amps I have
> built. I think I might try a similar idea with a 572b output next.
>
> All TX wound by Sowter, as I know Iain rates them very highly I thought I
> would give them a try.

And by the sound of it, you were not disappointed:;-)
There transformers are very good indeed.
I have a magnificient set of Sowter potted OPTs for
a KT88 PPP amp, which is looming up.

Meanwhile, today I put the finishing touches to a 30W EL34
mononloc prototype. After discussions here, particularly with
Patrick, I was keen to make an unconditionally stable amp,
even at the risk of a slight deterioration in other parameters.
Rock-steady into 0.22µF is sometimes not easy to obtain:-)

It has THD of 0.05% at 1W, and 0.25% at 25W.
Some square wave pics are shown at:

* w w w .kolumbus.fi/iain.churches/Projects/M30SquareWavePics.html



Regards to all
Iain

Re: 211 A2 amp



Iain Churches wrote:
>
> "Nick Gorham" <nick@lurcher.org> wrote in message
> news:013f5448$0$13384$c3e8da3@news.astraweb . com ...
> > Hi All.
> >
> > I finally finished the A2 211 amp I mentioned in some postings to Patrick
> > some time ago, so I thought I would post a link to some images.
> >
> > * w w w .lurcher.org/nick/images/211/
> >
> > Sorry the quality of the images leave a lot to be desired as they were
> > from a camera phone, but they give the general idea.
>
> Hi Nick. Lovely project:-) Just when RAT seems to
> have reached an all-time low - karamba - someone
> like you posts details of a project like your 211
> and restores our faith in human nature and endevour:-))
>
> Some test results would be of interest, as and when.
>
> > I am very pleased with the sound it makes, Running in A2 seems to have a
> > powerfull character about it, more so than the previous 211 amps I have
> > built. I think I might try a similar idea with a 572b output next.
> >
> > All TX wound by Sowter, as I know Iain rates them very highly I thought I
> > would give them a try.
>
> And by the sound of it, you were not disappointed:;-)
> There transformers are very good indeed.
> I have a magnificient set of Sowter potted OPTs for
> a KT88 PPP amp, which is looming up.
>
> Meanwhile, today I put the finishing touches to a 30W EL34
> mononloc prototype. After discussions here, particularly with
> Patrick, I was keen to make an unconditionally stable amp,
> even at the risk of a slight deterioration in other parameters.
> Rock-steady into 0.22µF is sometimes not easy to obtain:-)


Well, I don't think a tube amp of any kind using global NFB and which
has been unconditionally stabilised
is going to suffer "deterioration of other parameters", which presumably
are imaging and
"openess of sound", whatever that is.
When you have a large amount of class A, wide open loop BW,
and enough NFB to get Rout low enough, and THD below 0.05% at a few
watts,
generally there is better sound than if you removed the NFB,
even with triode output tubes.

A few AB tests I've conducted over the years have been a torture for the
anti NFB fetishistas.

Its preferable for peace that they consider some mild NFB
as a means to allow tubes to sing like a choir of angels
but with a gag tied around the mouths of the few
inevitable choir members who can't sing in tune.

Patrick Turner.


>
> It has THD of 0.05% at 1W, and 0.25% at 25W.
> Some square wave pics are shown at:
>
> * w w w .kolumbus.fi/iain.churches/Projects/M30SquareWavePics.html
>
> Regards to all
> Iain

Re: 211 A2 amp



"Patrick Turner" <info@turneraudio . com .au> wrote in message
news:47ED92B2.B370A44F@turneraudio . com .au...
>
>
> Iain Churches wrote:
>>
>> "Nick Gorham" <nick@lurcher.org> wrote in message
>> news:013f5448$0$13384$c3e8da3@news.astraweb . com ...
>> > Hi All.
>> >
>> > I finally finished the A2 211 amp I mentioned in some postings to
>> > Patrick
>> > some time ago, so I thought I would post a link to some images.
>> >
>> > * w w w .lurcher.org/nick/images/211/
>> >
>> > Sorry the quality of the images leave a lot to be desired as they were
>> > from a camera phone, but they give the general idea.
>>
>> Hi Nick. Lovely project:-) Just when RAT seems to
>> have reached an all-time low - karamba - someone
>> like you posts details of a project like your 211
>> and restores our faith in human nature and endevour:-))
>>
>> Some test results would be of interest, as and when.
>>
>> > I am very pleased with the sound it makes, Running in A2 seems to have
>> > a
>> > powerfull character about it, more so than the previous 211 amps I have
>> > built. I think I might try a similar idea with a 572b output next.
>> >
>> > All TX wound by Sowter, as I know Iain rates them very highly I thought
>> > I
>> > would give them a try.
>>
>> And by the sound of it, you were not disappointed:;-)
>> There transformers are very good indeed.
>> I have a magnificient set of Sowter potted OPTs for
>> a KT88 PPP amp, which is looming up.
>>
>> Meanwhile, today I put the finishing touches to a 30W EL34
>> mononloc prototype. After discussions here, particularly with
>> Patrick, I was keen to make an unconditionally stable amp,
>> even at the risk of a slight deterioration in other parameters.
>> Rock-steady into 0.22µF is sometimes not easy to obtain:-)
>
>
> Well, I don't think a tube amp of any kind using global NFB and which
> has been unconditionally stabilised
> is going to suffer "deterioration of other parameters", which presumably
> are imaging and
> "openess of sound", whatever that is.

I increased the stability margin by several dB, and so with a little
less NFB the THD was marginally higher.

Regards to all
Iain

Re: 211 A2 amp


I said...
> > Well, I don't think a tube amp of any kind using global NFB and which
> > has been unconditionally stabilised
> > is going to suffer "deterioration of other parameters", which presumably
> > are imaging and
> > "openess of sound", whatever that is.
>
> I increased the stability margin by several dB, and so with a little
> less NFB the THD was marginally higher.

The best stability margin you can have is with no global NFB.

Then as you add NFB, you can draw a plot of the margin of stability if
you really must,
and by the time you have say 30dB applied, and the test with 0.22uF
across the
output causes inevitable HF oscillations no matter what you do to
the compensation networks, or, you get LF oscillations that cannot be
cured with
gain tweaker networks, then you have reached the point of maximal
applied FB for that
amp to get the wanted bandwidth say of 10Hz to 50kHz.
Its possible to tweak open loop phase shift even further if you settle
for less closed loop bandwidth
but you shouldn't, because you'll end up with an amp with
maybe 35dB NFB applied at say between 500Hz and 5kHz, and each side of
the that
the efectively applied NFB will reduce because of the reduction of OLG
with networks. With such reductions of OLG, the reduction of distortion
products
at above 5kHz becomes less, and sound goes hard, most probably.

30dB of NFB was applied around a large number of amps back in the 1960s
to get typical
THD < 0.05% at a dB below clip at 1kHz, R load only.
So at 3 watts typical was <0.02%. Companies liked to take part in wars
of Watts,
and battles of THD to get sales.


In my mind there is no need for such low THD/IMD figures simply
because the products formed in class A working of tube amps is much more
bearable
than say the same amount but higher numbered H in a class B SS amp that
has not been designed very well.
In fact 0.1% THD in any SET amp at 3 watts is fine, because its all 2H,
and IMD results are not as appalling as one might suspect.

But I get samples of SET amps made by DIYers and some companies like
Silk Amps from Thailand, and they have maybe 5% THD at 2 watts from a
very poorly
conceived circuit, and this is way too high.

The use of a pair of KT88, KT90 or 6550 of 300B for the output stage
means
anode idle dissipation is naturally higher than lesser capable tubes
such as
EL34, 6L6, KT66, 6V6, EL84 etc, so therefore the class A PO before class
AB action commences
is higher.

And if you ever draw a graph of the distortion against output voltage
you'll see that THD is very low while the amp is in class A, and then it
increases
more rapidly above the A to AB threshold, but which really only need to
handle transients
such as drum beats and cannon shots.

The maximum amount of class A available depends on the RLa-a load,
anode dissipation, ie, Ea x Ia at idle, and maximum anode efficiency for
class A
which is about 45% for multigrids and up to about 30% for the same
trioded
or for 300B.
More PO is available in A2.


If you carefully read all my pages on load matching to triodes and beam
tetrodes
you will see what load values required for pure class A and how the load
affects the maximum PO and balance between A and AB PO.
And if anyone is allergic to NFB, then to get anywhere decent ENOUGH
measurements that
the best sound from tubes depends, they must use triodes and a high
value RLa-a,
say 12k with KT88 and Ea = 500V, Ia = 60mA per output tube, ( only with
multi grids,
not 300B )

This means they'll select a Hammond 1650P which has 6k6 : 4, 8, 16 ohms
and
they will never use the 8 and 16 ohm outlet config,
and with 8 ohm speakers the tube RLa-a is 13.2k, and PO ALL class A.
OPT winding losses become lower, as you increase speaker Z
with a give outlet value.


Then it will be found that no matter what tricks are played, the output
stage
has around 1% THD mainly 3H at a dB below clipping, with about 20 watts
available.
If more PO is wanted, halve RLa-a and use 4 output tubes.
The 40 watts of pure class A will be sublime, but nobody bothers
to waste so much electricity, and they back off the bias to 35mA even
with only
2 output tubes.
Then class A PO drops but 90% of what they listen to is covered by pure
class A
and the distortions rise only slightly for the class A region although
its not unusual to get 2% THD or more with class AB at full PO.
Suitable input/driver stages with low THD are shown at my website.
At 3 watts in a 20 watt PP pure class A amp, expect 0.3%,
and 0.15% in the 40 watter.
Pure class A has THD dropping linearly with VO, not PO.

The least THD should be where there is no RL connected, and the load
line is a
nearly horizontal line, and maximal Va occurs with almost no Ia change.
But you will still see up to 1% THD just under clipping.

The internally applied local NFB action within the triodes is maximal
because gain is maximal.

Things are a lot worse with tetrodes or pentodes used as such.
The UL connection is a great help, but the CFB connection is better,
and can give lower THD than triodes at the same PO,
and maximal A1 PO is also greater.

I have not tried the 211 in a PP circuit.
Suppose Pda = 60 watts per tube.
At max class efficiency of say 25%, expect 30 watts of pure class A,
which is 948Vrms across the 30k OPT primary.

The OPT is a devil to wind compared to making one for 6k,
and to get the same wide BW from 10Hz to at least 50kHz.

845 under similar dissipation amounts would make 30% efficiency,
and give 36 watts, but one can run them with Pda = 75 watts each
and go into a little A2 and get 60 watts class A from a pair in either
SE or PP.
The PP load needs to be 24k, and the OPT is a difficult thing to wind
to get the bandwidth expected today.
Its possible that where the bandwidth is difficult to get,
you have two OPT, one for below 1kHz, and the other cap coupled to the
anodes for the above 1 kHz F.
global NFB arrangements if used become fiddly.

A circlotron with 845 could be done, but is entirely unecessary
because Rout and THD/IMD is all low enough with the anode loaded tubes
and no
loop NFB at all.

The 805 of some other high µ large triode may be suited to a circlotron
because you need a lotta FB with 805.
And you'd need a shirt and trouser load of driver voltage, which is
bleedin difficult,
wheras global NFB applied back to an input triode cathode is so much
easier.



Patrick Turner.















>
> Regards to all
> Iain

Re: 211 A2 amp



"Patrick Turner" <info@turneraudio . com .au> wrote in message
news:47EE43F0.3C805E18@turneraudio . com .au...
>
>> > Well, I don't think a tube amp of any kind using global NFB and which
>> > has been unconditionally stabilised
>> > is going to suffer "deterioration of other parameters", which
>> > presumably
>> > are imaging and
>> > "openess of sound", whatever that is.
>>
Iain said
>> I increased the stability margin by several dB, and so with a little
>> less NFB the THD was marginally higher.
>
> The best stability margin you can have is with no global NFB.

Indeed but a little NFB does wonders for a PP amp I find.
>
> Then as you add NFB, you can draw a plot of the margin of stability if
> you really must,
> and by the time you have say 30dB applied,

Never managed to get that far 26dB is my best:-)
I usually check the response of the amp open loop, and then
add a 10k pot for Rfb. I decrease the value until the amp starts
to oscillate and then back off ten (or more) dB

That's the empirical method I was shown as a lad.
Is there a better way?

> and the test with 0.22uF
> across the
> output causes inevitable HF oscillations no matter what you do to
> the compensation networks, or, you get LF oscillations that cannot be
> cured with
> gain tweaker networks, then you have reached the point of maximal
> applied FB for that
> amp to get the wanted bandwidth say of 10Hz to 50kHz.

Once I have got the Rfb sorted out, and replaced
with a fixed R then I start to look at the stability into
O/C and a cap. Once again, I use empirical methods,
but the more you do it, the better you get at it:-)

> Its possible to tweak open loop phase shift even further if you settle
> for less closed loop bandwidth
> but you shouldn't, because you'll end up with an amp with
> maybe 35dB NFB applied at say between 500Hz and 5kHz, and each side of
> the that
> the efectively applied NFB will reduce because of the reduction of OLG
> with networks. With such reductions of OLG, the reduction of distortion
> products
> at above 5kHz becomes less, and sound goes hard, most probably.

How does one go about tweaking the open loop phase shift?

>
> 30dB of NFB was applied around a large number of amps back in the 1960s
> to get typical
> THD < 0.05% at a dB below clip at 1kHz, R load only.
> So at 3 watts typical was <0.02%. Companies liked to take part in wars
> of Watts,
> and battles of THD to get sales.

These days many tube amps seem to have power quoted at
1% THD. In the 60s+70s 0.1% was more common.
>
>
> And if you ever draw a graph of the distortion against output voltage
> you'll see that THD is very low while the amp is in class A, and then it
> increases
> more rapidly above the A to AB threshold, but which really only need to
> handle transients
> such as drum beats and cannon shots.

Yes I have done that, and also compared a KT88 Class A with
a KT88 AB1 at various levels.

I also plotted distortion against frequency at various levels.
Now that's an eyeopener!

Iain

Re: 211 A2 amp



Iain Churches wrote:
>
> "Patrick Turner" <info@turneraudio . com .au> wrote in message
> news:47EE43F0.3C805E18@turneraudio . com .au...
> >
> >> > Well, I don't think a tube amp of any kind using global NFB and which
> >> > has been unconditionally stabilised
> >> > is going to suffer "deterioration of other parameters", which
> >> > presumably
> >> > are imaging and
> >> > "openess of sound", whatever that is.
> >>
> Iain said
> >> I increased the stability margin by several dB, and so with a little
> >> less NFB the THD was marginally higher.
> >
> > The best stability margin you can have is with no global NFB.
>
> Indeed but a little NFB does wonders for a PP amp I find.
> >
> > Then as you add NFB, you can draw a plot of the margin of stability if
> > you really must,
> > and by the time you have say 30dB applied,
>
> Never managed to get that far 26dB is my best:-)

I got 35dB max global NFB with the early version of 8585 which has about
12dB NFB
in the output stage from the CFB connection while in class A where OP
tube gain is maximal.

> I usually check the response of the amp open loop, and then
> add a 10k pot for Rfb. I decrease the value until the amp starts
> to oscillate and then back off ten (or more) dB


>
> That's the empirical method I was shown as a lad.
> Is there a better way?

Its OK, but you are not changeing the OLG phase character to
suit the wanted NFB, or the compo cap across the Rfb,
or the gain step networks to reduce gain at extremes of F.

There are actually about 5 things to be juggled, not just 1.

After about 20 amp projects, and trying to see just what has to be done
to get
stablity good and bandwidth wide with global NFB, you'll get the hang,
while appreciating the old fashioned empircals.


>
> > and the test with 0.22uF
> > across the
> > output causes inevitable HF oscillations no matter what you do to
> > the compensation networks, or, you get LF oscillations that cannot be
> > cured with
> > gain tweaker networks, then you have reached the point of maximal
> > applied FB for that
> > amp to get the wanted bandwidth say of 10Hz to 50kHz.
>
> Once I have got the Rfb sorted out, and replaced
> with a fixed R then I start to look at the stability into
> O/C and a cap. Once again, I use empirical methods,
> but the more you do it, the better you get at it:-)

Exactly. And if you are not doing it very often, you forget how to.


> > Its possible to tweak open loop phase shift even further if you settle
> > for less closed loop bandwidth
> > but you shouldn't, because you'll end up with an amp with
> > maybe 35dB NFB applied at say between 500Hz and 5kHz, and each side of
> > the that
> > the efectively applied NFB will reduce because of the reduction of OLG
> > with networks. With such reductions of OLG, the reduction of distortion
> > products
> > at above 5kHz becomes less, and sound goes hard, most probably.
>
> How does one go about tweaking the open loop phase shift?

Use the networks detailed and shown in all my many amp schematics at my
website.

If you cannot understand the networks, build samples and measure them
until you do know what to expect.
Its all just basic applied and practical LCR theory.

>
> >
> > 30dB of NFB was applied around a large number of amps back in the 1960s
> > to get typical
> > THD < 0.05% at a dB below clip at 1kHz, R load only.
> > So at 3 watts typical was <0.02%. Companies liked to take part in wars
> > of Watts,
> > and battles of THD to get sales.
>
> These days many tube amps seem to have power quoted at
> 1% THD. In the 60s+70s 0.1% was more common.

By the 60s, tubes were still mainstream, and many articles and reviews
on amps
mainly considered the technical aspects. A radio or sound system once
was the most
complex gear in anyone's house, and reading audiophiles had little to do
in evenings
except read audio mags while listening to their latest audio gear
acquisitions.
So readers all knew basic aspects about THD and its legacy of related
IMD etc.
TV started a lot of rot.

Later, when tubes got mainly dumped, and THD went down below 0.01%,
the technicals became unimportant, and looks and features and utter
fucking bull shit
about sound became the norm. Readers went dumb, and houshold gear
couldn't
be understood any longer; it either worked or it didn't, and any
co-relations between sound quality and where the transistors were made
was all mainly
boring BS.
By mid 70s, most young folks were into hi-fi because it was a backround
noise
to accompany early family life and/or seduction of the liberated woman
who was by then taking the Pill and who could be seduced 4 times week
with the Beatles or Fleetwood Mac playing along and life was grand.

The same ppl now rarely play a tune, and goodness me, rootin the missus
with candle light and
fine wine and fine music simply isn't on, and ears ain't what they used
to be, the missus sure ain't,
and the Picture comes first now, so most ppl have huge screen size for
HT, and shithouse sound quality,
and its a much more distracting balm against the realization that the
older they get the better they was.

Many tube amps are made so badly now that they dare not mention the
technical specs.
> >
> >
> > And if you ever draw a graph of the distortion against output voltage
> > you'll see that THD is very low while the amp is in class A, and then it
> > increases
> > more rapidly above the A to AB threshold, but which really only need to
> > handle transients
> > such as drum beats and cannon shots.
>
> Yes I have done that, and also compared a KT88 Class A with
> a KT88 AB1 at various levels.
>
> I also plotted distortion against frequency at various levels.
> Now that's an eyeopener!

Yes, you get more THD at 10Hz and 40kHz than at 1kHz.
Its a function of OLG and amount of applied NFB.

Best not to worry that a typical 50W PP tube amp
will double its 1kHz THD at say 30Hz and 20kHz.

It does not matter.

Patrick Turner.


>
> Iain

Re: 211 A2 amp



"Patrick Turner" <info@turneraudio . com .au> wrote in message
news:47F23482.B39B6A89@turneraudio . com .au...

>> > Then as you add NFB, you can draw a plot of the margin of stability if
>> > you really must,
>> > and by the time you have say 30dB applied,
>>
> Iain wrote:

>> Never managed to get that far 26dB is my best:-)
>
> I got 35dB max global NFB with the early version of 8585 which has about
> 12dB NFB
> in the output stage from the CFB connection while in class A where OP
> tube gain is maximal.

The Radford STA 35 has 24dB of NFB and is completely stable.
John Widgery, who was Radford's chief engineer told me that
Arthur Radford was one of the few people who understood
trannsformer design, and thought it important enough to make
them properly.

>> I usually check the response of the amp open loop, and then
>> add a 10k pot for Rfb. I decrease the value until the amp starts
>> to oscillate and then back off ten (or more) dB

> Its OK, but you are not changeing the OLG phase character to
> suit the wanted NFB, or the compo cap across the Rfb,
> or the gain step networks to reduce gain at extremes of F.
>
> There are actually about 5 things to be juggled, not just 1.

Yes. I see. I am just looking at the output power, the input
sensitivity and the THD.
>
> After about 20 amp projects, and trying to see just what has to be done
> to get
> stablity good and bandwidth wide with global NFB, you'll get the hang,
> while appreciating the old fashioned empircals.

Each amp is a separate challenge in itself it seems to me.
I am never entirely satisfied. Some people build tube amps
without taking any test measurements, they just check the
wiring and the DC conditions, and start top play music.
They always seem happy:-)

I have found the use of Sowter transformers make
the task of building a stable amplifier easier than Hammond.


>> Once I have got the Rfb sorted out, and replaced
>> with a fixed R then I start to look at the stability into
>> O/C and a cap. Once again, I use empirical methods,
>> but the more you do it, the better you get at it:-)
>
> Exactly. And if you are not doing it very often, you forget how to.

I make careful sketches and keep notes.
>
>
>> How does one go about tweaking the open loop phase shift?
>
> Use the networks detailed and shown in all my many amp schematics at my
> website.

I bought a dedicated phase shift meter (working it all out
from elipses on the CRO was a tad too difficult) My
25Wamp is +5 degrees at 10Hz and -30degrees at 100Hz.
>
> If you cannot understand the networks, build samples and measure them
> until you do know what to expect.
> Its all just basic applied and practical LCR theory.

But there are so many variables - so much going on at
once:-)
>

Regards
Iain

Re: 211 A2 amp



Iain Churches wrote:
>
> "Patrick Turner" <info@turneraudio . com .au> wrote in message
> news:47F23482.B39B6A89@turneraudio . com .au...
>
> >> > Then as you add NFB, you can draw a plot of the margin of stability if
> >> > you really must,
> >> > and by the time you have say 30dB applied,
> >>
> > Iain wrote:
>
> >> Never managed to get that far 26dB is my best:-)
> >
> > I got 35dB max global NFB with the early version of 8585 which has about
> > 12dB NFB
> > in the output stage from the CFB connection while in class A where OP
> > tube gain is maximal.
>
> The Radford STA 35 has 24dB of NFB and is completely stable.
> John Widgery, who was Radford's chief engineer told me that
> Arthur Radford was one of the few people who understood
> trannsformer design, and thought it important enough to make
> them properly.

Apart from Arthur, I am the other guy.


>
> >> I usually check the response of the amp open loop, and then
> >> add a 10k pot for Rfb. I decrease the value until the amp starts
> >> to oscillate and then back off ten (or more) dB
>
> > Its OK, but you are not changeing the OLG phase character to
> > suit the wanted NFB, or the compo cap across the Rfb,
> > or the gain step networks to reduce gain at extremes of F.
> >
> > There are actually about 5 things to be juggled, not just 1.
>
> Yes. I see. I am just looking at the output power, the input
> sensitivity and the THD.
> >
> > After about 20 amp projects, and trying to see just what has to be done
> > to get
> > stablity good and bandwidth wide with global NFB, you'll get the hang,
> > while appreciating the old fashioned empircals.
>
> Each amp is a separate challenge in itself it seems to me.
> I am never entirely satisfied. Some people build tube amps
> without taking any test measurements, they just check the
> wiring and the DC conditions, and start top play music.
> They always seem happy:-)
>
> I have found the use of Sowter transformers make
> the task of building a stable amplifier easier than Hammond.
>
> >> Once I have got the Rfb sorted out, and replaced
> >> with a fixed R then I start to look at the stability into
> >> O/C and a cap. Once again, I use empirical methods,
> >> but the more you do it, the better you get at it:-)
> >
> > Exactly. And if you are not doing it very often, you forget how to.
>
> I make careful sketches and keep notes.
> >
> >
> >> How does one go about tweaking the open loop phase shift?
> >
> > Use the networks detailed and shown in all my many amp schematics at my
> > website.
>
> I bought a dedicated phase shift meter (working it all out
> from elipses on the CRO was a tad too difficult) My
> 25Wamp is +5 degrees at 10Hz and -30degrees at 100Hz.

Numbers shumbers.

Just do an XY on the CRO and with 0 degrees phase shift at say 1kHz, you
see a slanting line.

Phase shift either way shows up as an elipse at first,
and then at 90 degrees it becomes a circle, and it SO EASY.

You actually don't need to get exact numbers using my methods.
There are no critical calculations required.
Where needed, Zobel networks across the 1/2 primaries of the OPT
are based on making the load to the output tubes resistive somewhere
above 20kHz,
so where RLa-a is 5k, you'd choose about 2k7 plus 0.022 as a Zobel.
Why?, because the pole for the R&C is at 27kHz, easily calculated by a
dumb school boy.
The R damps the otherwise resonant behaviour of the OPT
somewhere above 20kHz.

But the CRO is your eye upon the wave, and the means by which a response
is plotted,
because the CRO makes a great volt meter. Again you do not need exact
numbers,
Seeing the amplitude drop by half its amplitude means -6dB, two lines
down from 0dB in an
exercize book, 1 line = -3dB, 2 = -6, 3 = -9, 4 = -12 and so on,
and an amp response can be drawn, hard copy, in 8 minutes, 4 for HF, 4
for LF.

Square wave testing also tells you a lot you need to know about critical
damping and stability.

For zobels across V1 RL, I use a radio tuning cap in series with a
linear pot of 25k.

DC is kept out with a 0.1uF cap.
The effect of changing R&C values for the Zobel MUST be monitored with a
CRO and square wave with
0.22uF load. "tune" the R&C for least ring at 1VO, using a 5 kHz square
wave.



> >
> > If you cannot understand the networks, build samples and measure them
> > until you do know what to expect.
> > Its all just basic applied and practical LCR theory.
>
> But there are so many variables - so much going on at
> once:-)

There is nothing that capable person with average intelligence cannot
figure out.
Engineers of the past like Arthur at Radford were pretty ordinary
people.
Amps ain't like sending a guy to the moon.

And BTW, the waves are all going through the amp at the same time, just
their phase
relationships vary.

Most ppl get confused about phase lead, or advance, and can't understand
that
it would appear the electricty arrived at its destination before
starting out on the journey.
They understand phase lag, oh yeah, they all know about a train or bus
that was late.


There is rarely any amp I cannot stabilize for unconditional stability.
I have not met one yet where I cannot apply 15dB global NFB and still
get
10Hz to 30kHz, -3dB into the rated load.

Sometimes I have to use more than global 'series voltage' NFB, and I
have to apply
some global 'series current' NFB, as in the case of the Leak amp I
worked on, see my page at

* w w w .turneraudio . com .au/leakampmods.html

Patrick Turner.


>
> Regards
> Iain

Re: 211 A2 amp



"Patrick Turner" <info@turneraudio . com .au> wrote in message
news:47F35B27.F4295@turneraudio . com .au...
>
>
> Iain Churches wrote:
>> The Radford STA 25 has 24dB of NFB and is completely stable.
>> John Widgery, who was Radford's chief engineer told me that
>> Arthur Radford was one of the few people who understood
>> trannsformer design, and thought it important enough to make
>> them properly.
>
> Apart from Arthur, I am the other guy.

Actually there are two other guys. You and Brian Sowter
(We won't mention Per Lundahl, as he doesn't read this NG)

Iain

Re: 211 A2 amp



Iain Churches wrote:
>
> "Patrick Turner" <info@turneraudio . com .au> wrote in message
> news:47F35B27.F4295@turneraudio . com .au...
> >
> >
> > Iain Churches wrote:
> >> The Radford STA 25 has 24dB of NFB and is completely stable.
> >> John Widgery, who was Radford's chief engineer told me that
> >> Arthur Radford was one of the few people who understood
> >> trannsformer design, and thought it important enough to make
> >> them properly.
> >
> > Apart from Arthur, I am the other guy.
>
> Actually there are two other guys. You and Brian Sowter
> (We won't mention Per Lundahl, as he doesn't read this NG)

Well of course.

Arthur Radford is maybe dead, and never seen here, nor is Brian Sowter.

It would appear Brian wouldn't like to part with his expertise and
detailed sectional drawings of bobbin winding details because
he's the usual secretive guy who runs a business so busy he'd never have
time for us.

Per Lundahl is nowhere to be seen.

A hundred other capable designers are nowhere to be seen.

But that might be all the OPT designers in the world.......

I don't have a clue really.

Never fear, I am 'ere!

Patrick Turner.




>
> Iain

Re: 211 A2 amp



"Patrick Turner" <info@turneraudio . com .au> wrote in message
news:47F766D5.331B4260@turneraudio . com .au...
>
>
> Iain Churches wrote:
>>
>> "Patrick Turner" <info@turneraudio . com .au> wrote in message
>> news:47F35B27.F4295@turneraudio . com .au...
>> >
>> >
>> > Iain Churches wrote:
>> >> The Radford STA 25 has 24dB of NFB and is completely stable.
>> >> John Widgery, who was Radford's chief engineer told me that
>> >> Arthur Radford was one of the few people who understood
>> >> trannsformer design, and thought it important enough to make
>> >> them properly.
>> >
>> > Apart from Arthur, I am the other guy.
>>
>> Actually there are two other guys. You and Brian Sowter
>> (We won't mention Per Lundahl, as he doesn't read this NG)
>
> Well of course.
>
> Arthur Radford is maybe dead, and never seen here, nor is Brian Sowter.

Yes AR is probably now in the great transformer winding
shop in the sky. He retired to due ill-health in the early 80's.
Now *that* would have been a firm worth investing in to
ensure its continuity, but it was Mr Radford's wish that the
firm should close. I see there is a new company now with
the name Radford Audio.Their website says they plan to
continue the Radford tradition. They are probably trying
to cash in on a legendary name, and have no product at
the moment.


> It would appear Brian wouldn't like to part with his expertise and
> detailed sectional drawings of bobbin winding details because
> he's the usual secretive guy who runs a business so busy he'd never have
> time for us.

Groups like this one are too much of a chimp fight for people
like Brian Sowter, Morgan Jones, Per Lundahl, and a number
of very capable people from the BBC, Swedish and German
Broadcast They do contribute and discuss in closed groups to
which the semi-literate and socially challenged (many of who
inflict their presence on us here) have no access.

Regards
Iain

Re: 211 A2 amp



Iain Churches wrote:
>
> "Patrick Turner" <info@turneraudio . com .au> wrote in message
> news:47F766D5.331B4260@turneraudio . com .au...
> >
> >
> > Iain Churches wrote:
> >>
> >> "Patrick Turner" <info@turneraudio . com .au> wrote in message
> >> news:47F35B27.F4295@turneraudio . com .au...
> >> >
> >> >
> >> > Iain Churches wrote:
> >> >> The Radford STA 25 has 24dB of NFB and is completely stable.
> >> >> John Widgery, who was Radford's chief engineer told me that
> >> >> Arthur Radford was one of the few people who understood
> >> >> trannsformer design, and thought it important enough to make
> >> >> them properly.
> >> >
> >> > Apart from Arthur, I am the other guy.
> >>
> >> Actually there are two other guys. You and Brian Sowter
> >> (We won't mention Per Lundahl, as he doesn't read this NG)
> >
> > Well of course.
> >
> > Arthur Radford is maybe dead, and never seen here, nor is Brian Sowter.
>
> Yes AR is probably now in the great transformer winding
> shop in the sky. He retired to due ill-health in the early 80's.
> Now *that* would have been a firm worth investing in to
> ensure its continuity, but it was Mr Radford's wish that the
> firm should close. I see there is a new company now with
> the name Radford Audio.Their website says they plan to
> continue the Radford tradition. They are probably trying
> to cash in on a legendary name, and have no product at
> the moment.
>
> > It would appear Brian wouldn't like to part with his expertise and
> > detailed sectional drawings of bobbin winding details because
> > he's the usual secretive guy who runs a business so busy he'd never have
> > time for us.
>
> Groups like this one are too much of a chimp fight for people
> like Brian Sowter, Morgan Jones, Per Lundahl, and a number
> of very capable people from the BBC, Swedish and German
> Broadcast They do contribute and discuss in closed groups to
> which the semi-literate and socially challenged (many of who
> inflict their presence on us here) have no access.

The tall poppies you mention couldn't stand the flak,
and wouldn't like their names dragged into the sewers
by the anti tube brigade.

Unlike them I feel I have nothing to loose, no matter how
submerged I am in shite and other trashy arguments.

Patrick Turner
>
> Regards
> Iain

Re: 211 A2 amp

Iain Churches wrote:


>
> Yes AR is probably now in the great transformer winding
> shop in the sky. He retired to due ill-health in the early 80's.
> Now *that* would have been a firm worth investing in to
> ensure its continuity, but it was Mr Radford's wish that the
> firm should close. I see there is a new company now with
> the name Radford Audio.Their website says they plan to
> continue the Radford tradition. They are probably trying
> to cash in on a legendary name, and have no product at
> the moment.
>

Iain, just FYI,

I don't know what their plans are, but the group that has the Radford
name now, are the same people who produce the World Designs kits.

* wduk.worldomain . net /

I have spoken with Peter who runs the group several times now, and his
area of design is loudspeakers (he was tech director at Mission), he
would be the first to admit that he is not specialist when it comes to
valve amps, so normally uses Andy Grove for design work. I suspect he
will do the same with the Radford name.

--
Nick

Re: 211 A2 amp

On Mar 28, 3:10 pm, Nick Gorham <n...@lurcher.org> wrote:
> Hi All.
>
> I finally finished the A2 211 amp I mentioned in some postings to
> Patrick some time ago, so I thought I would post a link to some images.
>
> * w w w .lurcher.org/nick/images/211/
>
> Sorry the quality of the images leave a lot to be desired as they were
> from a camera phone, but they give the general idea.
>
> I am very pleased with the sound it makes, Running in A2 seems to have a
> powerfull character about it, more so than the previous 211 amps I have
> built. I think I might try a similar idea with a 572b output next.
>
> All TX wound by Sowter, as I know Iain rates them very highly I thought
> I would give them a try.
>
> --
> Nick

Nice work, Nick. You may come make the case for my next amp. Are you
publishing a schematic?

Andre Jute
Visit Jute on Amps at * members.lycos.co.uk/fiultra/
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