| Author |
Message |
Fabio Berutti
Guest
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 Posted:
Wed Jul 27, 2005 5:46 pm Post subject:
Re: The man wants to make a 212 SET amp. |
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That unit is very pretty indeed...in the picture at least. The label is
written in German, but the schematic is surely "made in Japan". From an
electronic-common-sense viewpoint it simply makes no sense. An 845 used as
a driver, lots of transformers with their relevant phase shifts, then a
monster tube used at "only" 1000V with CATHODE BIAS!!!!
OK, I can trust it sounds good, but making all that stuff for perhaps 20W is
like using a Peterbuilt truck to move a box of cigarettes, and I'd bet a
couple of $ that it is not better sounding than any other "sensible"
project, costing like the volume knob of that insanity.
Ciao
Fabio
"Patrick Turner" <info@turneraudio.com.au> ha scritto nel messaggio
news:42E75DF3.DB60839A@turneraudio.com.au...
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Ian Iveson
Guest
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| Posted:
Wed Jul 27, 2005 8:39 pm Post subject:
Re: The man wants to make a 212 SET amp. |
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"Oskari Heinonen" <Oskari.Heinonen@cs.Helsinki.FI> wrote
Hi Oskari
The operating point doesn't quite square with the datasheet posted
by John.
Stepping up from the 845 seems perverse to me. Can the 212 be driven
into A2 with a 10k winding I wonder? Looks to me like grid
resistance drops to 1k pretty quick when it goes +ve.
Also wondering what the cross-hatching on many of the cap symbols
means.
Is it permissible to use a distributed load with these things? Not
much gain to play with but DF could be significantly improved all
the same with perhaps 30% of the winding on the cathode. Need plenty
drive volts though.
All scary stuff to me.
cheers, Ian |
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Patrick Turner
Guest
|
| Posted:
Wed Jul 27, 2005 8:58 pm Post subject:
Re: The man wants to make a 212 SET amp. |
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|
Fabio Berutti wrote:
| Quote: | That unit is very pretty indeed...in the picture at least. The label is
written in German, but the schematic is surely "made in Japan". From an
electronic-common-sense viewpoint it simply makes no sense. An 845 used as
a driver, lots of transformers with their relevant phase shifts, then a
monster tube used at "only" 1000V with CATHODE BIAS!!!!
OK, I can trust it sounds good, but making all that stuff for perhaps 20W is
like using a Peterbuilt truck to move a box of cigarettes, and I'd bet a
couple of $ that it is not better sounding than any other "sensible"
project, costing like the volume knob of that insanity.
|
Yes, well, look, ah, hmm, yeah, I know.
But then 20 watts is a heck of a lotta power to some folks, including
my prospective client who said he only wants 20 watts.
Its the sound they are chasing, and 20 watts is like setting up a
300B and settling for 4 watts only, with a higher load value than "normal".
But at the higher primary loads, the triode is most linear, and the OPT
has the highest ratio, so the damping factor is much better than if we set up
the triode
to screw the most power out of it.
The 845 on it's own could give enough power.
But some folks like to use whatever is out there, and you gotta admit it, the
4212E
does have a certain flair and style that not many other tubes have.
It makes the 845 look like a wussy teensy weenie toobe.
Now they say the Pda of the 212 is 275 watts max, and the
linearity going into the grid current area looks fairly good,
assuming the driver tube has a nice low Ra, so if one set up
such a tube at 275 watts Pda, then an efficiency of 40%
in A2 would give 110 watts, or about what a quad of 211 etc in PSE
would give, or what 14 x 6CA7 in PSE tetrode with CFB would give,
or what 16 x KT88 in triode would give, etc.
But one would never dare have such a tube set up on 275 watts Pda at idle.
It surely wouldn't last long, and not many are around, and
afaik none are being made now.
They are designed to be able to handle 275 watts of Pd, but in practice
the duty cycle in most class AB apps would make the average Pda
far lower; they were designed to make huge amounts of power
in PP circuits with a low idle bias current.
I think about 150 watts of Pd would be about right, so
if Ea was 1,500V at 100mA Ia, it would be fine, and a
class A1 Vswing of about 700Vrms into 14k is possible giving only 35 watts,
as much as my SECFB amp with a quad of 6CA7.
If class A2 is used with Pd = 150watts and Ea = 1,500V, then
you get 1,000 Vrms into 15k, and about 65 watts.
That's an efficiency of 43%, and not too bad, since any other class A tube amp
won't
achieve much higher efficiency, bearing in mind the max theoretical
possible class A efficiency is 50%.
With a 15k load, and Ra = about 2,500 ohms at Iaq = 100 mA,
the DF without any NFB is quite good at 6
and as good as a 300B loaded with 6k.
The linearity at 15k looks good, I reckoned about 2% thd at 50 watts
from the load lines.
Expect about 0.63% at 5 watts, and at 0.5 watts about 0.2%,
all of which would be mainly 2H.
12 dB of NFB would make the amp measure
about 0.5 % at 50 watts, 0.15% at 5 watts, and 0.05% at 0.5 watts.
These figures would be actually drastically reduced in practice if a tube such
as a 300B
2A3, or trioded KT66 etc was was used as a driver and loaded appropriately
since the thd of the driver having to produce 63vrms, which is not much
for a power tube, will be about 2%, and it will cancel the 2H in the output
stage
to a large extent, and since nothing is badly loaded, the amount of second
order products due to distortion cancelling would have to be tiny indeed.
So without any loop FB, a substantially linear 60 watt SET
amp can be made using only 3 tubes, perhaps 4 if we includ
a cathode follower buffer between the driver and 212 grids.
People can argue that when linearity is dialled into the design of an SET
amp it won't sound any good, but I've never heard any evidence to support this
idea.
I figured the 15k OPT would need to be about 14Kgs, and a
400VA PT would also be about 10Kgs, and with a choke in the CLC supply,
the amp won't be light.
I don't see any need for mercury vapour rectifiers or oiler caps
when standard practice for HV supplies in recent years is to series electros,
so small chokes may be used, and to series SS diodes.
But a mono will be around 30 Kgs.
My prospective client says he has tonnes of gear, including
30,000 tubes. He says he has 6,000 x 12AT7, all NOS.
Maybe he has a few 866, and many chokes and oil caps.
Patrick Turner.
| Quote: |
Ciao
Fabio
"Patrick Turner" <info@turneraudio.com.au> ha scritto nel messaggio
news:42E75DF3.DB60839A@turneraudio.com.au...
Oskari Heinonen wrote:
Patrick Turner wrote:
Fabio Berutti wrote:
There's a schematic at http://www10.big.or.jp/~dh/work/0402.html
but I'm sure I saw the project of a guy who made a monster SE
using some STC 4212 tubes
The url you gave has a PP amp with 1,100V applied to the CT and
cathode biasing, so it must be a largely class A1 PP amp.
Did you notice this 845/4212 SE amp?
http://www10.big.or.jp/~dh/gallery/pic/4212.html
http://www10.big.or.jp/~dh/gallery/pic/4212c.html
--
Hi Oskari, your eye for detail is as fine as ever, and no I
didn't see the above, but one has to say its a spectacular amp.
It uses a little iddy biddy 845 as the driver.......
Patrick Turner.
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Bret Ludwig
Guest
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| Posted:
Thu Jul 28, 2005 3:56 am Post subject:
Re: The man wants to make a 212 SET amp. |
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| Quote: |
I don't know if this is much use, but at http://audiobizarro.com/ you
can see a couple of photographs of amps created with the 212E, the
first is on three chassis, whilst the second, is on more reasonable
one.
|
By definition, there is nothing reasonable about building an amplifier
with a 212 today. Might as well go the full monty! |
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Ian Iveson
Guest
|
| Posted:
Thu Jul 28, 2005 4:45 am Post subject:
Re: The man wants to make a 212 SET amp. |
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"Bret Ludwig" <bretldwig@yahoo.com> wrote
| Quote: | ...there is nothing reasonable about building an amplifier
with a 212 today...
|
Exactly the point. The key element of the design process is getting
the styling right. Ludicrously over-engineered build quality then
has to complete the impression of a thoroughgoing but civilised
unreasonableness.
They will need totally unreasonable speakers to complete the
statement.
If I had a Peterbuilt I'd go to the shop for cigs in it, even though
I don't smoke.
cheers, Ian |
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Patrick Turner
Guest
|
| Posted:
Thu Jul 28, 2005 3:06 pm Post subject:
Re: The man wants to make a 212 SET amp. |
|
|
Ian Iveson wrote:
Part of the grid swing can be +ve to drive into 10k as the
above schematic proposes, but if A2 isn't addopted, then po in A1 is
rather low.
The lower the Ea, higher the Ia, and lower the RL,
the worse things become, +ve grid drive is increased,
thd is increased, even in the A1 part of the op,
and Ro increases, all of which would spoil the sonic and
measured technical outcome.
Rgin of 1k is easily overcome by using a CF tube with Ro = 100 ohms.
Even when gI isn't used, a CF is supposed to improve the sound
as borne out with the Ongaku amp, which has a CF drive to the 211 grid
anyway.
| Quote: |
Also wondering what the cross-hatching on many of the cap symbols
means.
Is it permissible to use a distributed load with these things? Not
much gain to play with but DF could be significantly improved all
the same with perhaps 30% of the winding on the cathode. Need plenty
drive volts though.
|
As I explained in several posts, the last of which was last night, the
use of a high Ea,
RL of 15k, and almost all A1 op will give a DF of 6 without CFB.
6dB of CFB will double the drive voltage from around 70vrms to 140vrms,
and make matters worse, with very little reduction of Ro and thd if any.
If FB is to be used, it should be global, and around a tube drive stage
with no
ISTs, and with an OPT with about 50kHz open loop bw to start with.
Patrick Turner.
| Quote: |
All scary stuff to me.
|
1,600 volts isn't something you take lightly, you need to be frightened
of such voltages at all times.
Patrick Turner.
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Oskari Heinonen
Guest
|
| Posted:
Thu Jul 28, 2005 3:58 pm Post subject:
Re: The man wants to make a 212 SET amp. |
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|
Ian Iveson wrote:
| Quote: |
"Bret Ludwig" <bretldwig@yahoo.com> wrote
...there is nothing reasonable about building an amplifier
with a 212 today...
Exactly the point. The key element of the design process is getting
the styling right. Ludicrously over-engineered build quality then
has to complete the impression of a thoroughgoing but civilised
unreasonableness.
They will need totally unreasonable speakers to complete the
statement.
|
Ian, may I suggest the Acapella Sphäron?
http://www.zulum.co.kr/bbs/data/ACAPELLA/spharon.jpg
Ion tweeters...
| Quote: | If I had a Peterbuilt I'd go to the shop for cigs in it, even though
I don't smoke.
|
:-)
O.
--
Oskari Heinonen * University of Helsinki * Department of Computer Science
Oskari.Heinonen@cs.Helsinki.FI * http://www.cs.Helsinki.FI/Oskari.Heinonen/ |
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moby@kcbbs.gen.nz
Guest
|
| Posted:
Thu Jul 28, 2005 4:34 pm Post subject:
Re: The man wants to make a 212 SET amp. |
|
|
bill ramsay wrote:
| Quote: | On Mon, 25 Jul 2005 16:28:16 GMT, Patrick Turner
info@turneraudio.com.au> wrote:
Anyone have any experience in using the 212 triode
for an SET amp?
I have been asked for some input to a project a man
is about to embark on; he has the tubes, but
needs a little help.
Patrick Turner
Patrick,
I don't know if this is much use, but at http://audiobizarro.com/ you
can see a couple of photographs of amps created with the 212E, the
first is on three chassis, whilst the second, is on more reasonable
one.
I seem to remembe seeing diagrams of these, but the site has changed
somewhat and they have dissapeared/
watch your fingers.
bill
|
Here's a fine example of agricultural functionality over aesthetics. It
was built in the late '60s for the Seventh Day Adventists for their big
ralleys and was the "prime mover" for a stadium full of Vitavox
horns/GP1 drivers. It uses a pair of 212s driven by a pair of 845s
driven by a pair of KT66s. The clock on the mixer works, haven't got
around to finding out if the rest works (chicken!!!).
http://homepages.kcbbs.gen.nz/moby/bigamp.JPG
Caution - 1.7MB
M |
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Ian Iveson
Guest
|
| Posted:
Thu Jul 28, 2005 5:57 pm Post subject:
Re: The man wants to make a 212 SET amp. |
|
|
"Patrick Turner" <info@turneraudio.com.au> wrote
| Quote: | ...Rgin of 1k is easily overcome by using a CF tube with Ro = 100
ohms.
|
Of course, I was commenting on the design posted by Oskari. Have you
discussed this CF idea with your customer?
| Quote: | Even when gI isn't used, a CF is supposed to improve the sound
|
Some say, others disagree.
| Quote: | as borne out with the Ongaku amp, which has a CF drive to the 211
grid
anyway.
Also wondering what the cross-hatching on many of the cap symbols
means.
Is it permissible to use a distributed load with these things?
Not
much gain to play with but DF could be significantly improved all
the same with perhaps 30% of the winding on the cathode. Need
plenty
drive volts though.
As I explained in several posts, the last of which was last night,
|
I don't generally read your posts.
| Quote: | the use of a high Ea,
RL of 15k, and almost all A1 op
|
I was talking about the circuit posted by Oskari
| Quote: | will give a DF of 6 without CFB.
|
Par for a SET.
| Quote: | 6dB of CFB will double the drive voltage from around 70vrms to
140vrms,
|
Correct. And halve the effective input impedance. You have 1000V HT,
what's the problem?
| Quote: | and make matters worse,
|
What matters? How?
| Quote: | with very little reduction of Ro and thd if any.
|
Rubbish. In proportion to the amount of feedback. As I said, not
much gain to play with, but the improvement in DF without the use of
global feedback could be worthwhile. Do you know what speakers your
target final user has?
My question was whether a distributed load was allowable in the
tradition of the genre.
As for thd, much would depend on how well the voltage amp and output
stages suit each other. Feedback round the output stage will reduce
the distortion produced by that stage in proportion. The problem of
designing a voltage amp for best fit, distortion-wise, would be the
same.
| Quote: | If FB is to be used, it should be global
|
Says who?
| Quote: | and around a tube drive stage
with no
ISTs, and with an OPT with about 50kHz open loop bw to start with.
|
Have you discussed this global fb idea with your customer?
And why ask a question if you already know all the answers?
Still wondering what the crosshatching means...
cheers, Ian |
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Patrick Turner
Guest
|
| Posted:
Thu Jul 28, 2005 9:07 pm Post subject:
Re: The man wants to make a 212 SET amp. |
|
|
"moby@kcbbs.gen.nz" wrote:
| Quote: | bill ramsay wrote:
On Mon, 25 Jul 2005 16:28:16 GMT, Patrick Turner
info@turneraudio.com.au> wrote:
Anyone have any experience in using the 212 triode
for an SET amp?
I have been asked for some input to a project a man
is about to embark on; he has the tubes, but
needs a little help.
Patrick Turner
Patrick,
I don't know if this is much use, but at http://audiobizarro.com/ you
can see a couple of photographs of amps created with the 212E, the
first is on three chassis, whilst the second, is on more reasonable
one.
I seem to remembe seeing diagrams of these, but the site has changed
somewhat and they have dissapeared/
watch your fingers.
bill
Here's a fine example of agricultural functionality over aesthetics. It
was built in the late '60s for the Seventh Day Adventists for their big
ralleys and was the "prime mover" for a stadium full of Vitavox
horns/GP1 drivers. It uses a pair of 212s driven by a pair of 845s
driven by a pair of KT66s. The clock on the mixer works, haven't got
around to finding out if the rest works (chicken!!!).
http://homepages.kcbbs.gen.nz/moby/bigamp.JPG
Caution - 1.7MB
M
|
Well now Bill, that's quite some lotta gear you got there.
But yeah, quite some complexity, and obviously tailor made for the
prosletizing
of the Faith, and I notice that on the bottom left hand corner a meter with
control knob
for Demonic Control Effect, and top right hand corner God's Grace Effect,
and in the middle there's a knob labelled Converted Soul Level,
my my, what a fine old time they musta had back in those glory to be days.
Sure it was the late sixties? more like late fifties maybe, but hey
maybe the gear was purchased from Billy Graham, he was big back then,
and they used to broadcast live in Oz via special heavenly transmit link
from the US that could only have been arranged between Billy and
Our Redeemer, after a few serious prayers.
Patrick Turner. |
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Patrick Turner
Guest
|
| Posted:
Thu Jul 28, 2005 10:09 pm Post subject:
Re: The man wants to make a 212 SET amp. |
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|
Ian Iveson wrote:
| Quote: | "Patrick Turner" <info@turneraudio.com.au> wrote
...Rgin of 1k is easily overcome by using a CF tube with Ro = 100
ohms.
Of course, I was commenting on the design posted by Oskari. Have you
discussed this CF idea with your customer?
|
I doubt he'd have the foggiest idea if i did.
But no, not yet.
| Quote: |
Even when gI isn't used, a CF is supposed to improve the sound
Some say, others disagree.
as borne out with the Ongaku amp, which has a CF drive to the 211
grid
anyway.
Also wondering what the cross-hatching on many of the cap symbols
means.
Is it permissible to use a distributed load with these things?
Not
much gain to play with but DF could be significantly improved all
the same with perhaps 30% of the winding on the cathode. Need
plenty
drive volts though.
As I explained in several posts, the last of which was last night,
I don't generally read your posts.
|
Yes, I am aware of that, and I have no comment.
| Quote: |
the use of a high Ea,
RL of 15k, and almost all A1 op
I was talking about the circuit posted by Oskari
will give a DF of 6 without CFB.
Par for a SET.
|
Its not, actually. Eg, many ppl use about a 3.5k RL for 300B,
and go for maybe 9 watts, and since Ra is 800 ohms, and the OPT winding
R
is equivalent to say 350 ohms, then the DF = 3,500 / ( 800 + 350 ) =
approx 3,
not 6, if you are lucky.
| Quote: |
6dB of CFB will double the drive voltage from around 70vrms to
140vrms,
Correct. And halve the effective input impedance. You have 1000V HT,
what's the problem?
|
The problem is that in producing twice the drive voltage,
you have twice the thd in the drive amp, which
may/may not cancel the output stage's thd, maybe the thd outcome
is worse with a the higher drive voltage.
| Quote: | and make matters worse,
What matters? How?
with very little reduction of Ro and thd if any.
Rubbish. In proportion to the amount of feedback.
|
Its not quite that simple as you suggest.
Using only 6 dB of CFB in the output stage means that
there will be some of the 2H converted to 3H and higher products, and
the
thd of the drive stage will add to this.
The output stage without the transformer complexities of a CFB winding
is a fine output stage in its own right; there is just no need for the
local CFB.
Other tubes, 805, 833, would benefit from CFB because the gains
of these other triodes is high, and the Ra also high, which needs a lot
of reduction.
Using CFB with the 212 is like operating a Quad II in triode, and of
course one
must retain the CFB since one cannot afford to waste the windings, but
all that does is apply a feeble 3dB of NFB to the already lowish Ro
output stage.
I have done this though, and used KT88 as well, and the CFB merely
compensates for the high winding R of the Quad OPT.
I then applied 12 dB of global FB around the revised Quad amp, and the
sound
became better.
We'd tried 6 dB and triode, and that wasn't so good.
The spectral content in the thd is slightly better with triodes in such
amps,
compared to a tetrode output stage.
But I digress.
A big fat triode like the 212 is a fairly fine thing without local NFB.
Global FB requires careful design, but its very effective if used right.
| Quote: | As I said, not
much gain to play with, but the improvement in DF without the use of
global feedback could be worthwhile. Do you know what speakers your
target final user has?
|
He wants a switch to change from "with FB" to "without FB".
The only time the DF will be really healthy with no global FB
is where the ratio of RL to Ra is especially high, and I place that at
around
15k for 212, and the plate voltage is high, and the load line is thus as
flat
as possible, with as wide a max swing voltage as possible.
Its a challenge to get wide BW with a 15k load though;
at full power voltage, to get the onset of oveload distortion
at 20Hz would require ZLp = 15kohms at around 14Hz,
so Lp should be 170H, an almost impossible figure to attain.
| Quote: | My question was whether a distributed load was allowable in the
tradition of the genre.
|
By all means be my guest and use CFB if you want to with a 212.
I would decline to.
CFB is just series voltage FB, and as such it is frowned upon by
those involved in the "tradition of the genre", presumably
you mean the cult of the SET amp.
Not a bad cult, they don't have a strange messiah who tries to get them
all to
suicide at the right moment.
| Quote: |
As for thd, much would depend on how well the voltage amp and output
stages suit each other. Feedback round the output stage will reduce
the distortion produced by that stage in proportion.
|
Yes, but as i said, the drive stage then increases its distortion, and
ther's no global FB to
correct that. Quad II didn't adopt CFB without also using some global,
since the pentode drive stage
isn't the world's cleanest driver stage.
The role of CFB with power tetrodes is altogether different to use with
triodes.
The CFB with tetrodes also means the FB is applied around the screen
circuit, and the effect
is somewhat unique, since the 3H of the tube is reduced more than one
would think it would.
based on gain-feedback equations very much like it is with UL.
But the reduction in Ra with CFB around a tetrode is about twice as a
effective
as plain UL, which does have a big linearizing effect, but the Ra is
still high with plain UL.
The beauty of the CFB around tetrodes is that the lion's share of thd
reduction Ra reduction,
and phase shift reduction is done in the output stage without
paying much of a price in raising the required drive voltage above what
a triode output stage
would need.
Of course ppl forget they could have class A2, or AB2 PP triode amps and
get oddles more power,
but not have to use CFB or UL with tubes such as 6L6, 807, 6550, KT66 et
all.
The GE book I have with a pair of DA1505 in PP which are the same as 212
afaik
simply had a normal output tranny with a 2.5kV supply.
The power output was about 1,100 watts, and no need for CFB because the
drive voltage was already
large.
A pair of KT66 in CF were used to drive a 1:1 coupling tranny to the
1505 grids.
The primary was cathode to cathode, and the two 1/2 secondaries were
taken to the low impedance fixed grid bias supply.
Caps shunt the ends of P and S windings at the same signal to extend the
HF coupling.
the drive voltage to the CF buffer stage is simply a pair of 6BQ5 in PP
triode
and tranny coupled to the CF grids with yet another 1:1 coupling tranny.
considerable signal voltage needed to be applied to the 'Q5 grids.
These lovely old bangers were PA amps, they were
only required to go from 75Hz to 8 kHz, any more might be a problem.
Anyway, there is simply no need whatever to use an 845 to drive a 212.
But since the tubes exist, there is always someone who will use
them in ways that seem unecessary.
The PA stadium amp didn't have global FB; that would have been quite
impossible around so many stages and trannies, but thd was 7% at clip,
which is a lot for a PP amp, since its all mainly odd order, but
3% at 250 watts.
| Quote: | The problem of
designing a voltage amp for best fit, distortion-wise, would be the
same.
If FB is to be used, it should be global
Says who?
|
I am, with regard to this particular tube and the intended use.
Feel free to disagree.
| Quote: |
and around a tube drive stage
with no
ISTs, and with an OPT with about 50kHz open loop bw to start with.
Have you discussed this global fb idea with your customer?
|
As I said, he wants switchable FB.
| Quote: |
And why ask a question if you already know all the answers?
|
I designed a colleagues GM70 amp with a CF driver buffer, CR coupled
gain tube, normal input and global FB, and the amp ran very well and
sounded good.
I never assume I know all the answers.
Patrick Turner.
| Quote: |
Still wondering what the crosshatching means...
cheers, Ian |
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Ian Iveson
Guest
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| Posted:
Thu Jul 28, 2005 10:19 pm Post subject:
Re: The man wants to make a 212 SET amp. |
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"Oskari Heinonen" <Oskari.Heinonen@cs.Helsinki.FI> wrote
Perfect!
Er...odd place to put the wardrobe :-)
cheers, Ian |
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Paul Barker
Joined: 30 Dec 2006
Posts: 1
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| Posted:
Sun Dec 31, 2006 6:41 am Post subject:
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Stranbge place to ask these questions, I just found them.
212 is nice sounding valve, better than the ge vt4c available today and better than the modern production 845.
The Western Electric Oxide coated 211d is probably the best sound I have heard though.
The 212 has a depth and power though that the aformentioned cannot achieve.
Amps for the 212 require a great deal of size and weight, have to be made in modules which even those are only just liftable by one man and then he#d have a bad back for weeks.
It isn't really worth it. A really well executed 211 amp is sufficient for most people. |
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