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Thread: sunn 200S choke theory question

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    sunn 200S choke theory question

    So I had one of those "wake up in the middle of the night with an exclamation mark above my head" moments.

    Looking at the schema (and the actual wiring), the choke filtered supply is the direct B+ to the OT. Which I guess might make sense b/c the OT is ultralinear, and doesn't' need a screen supply.

    But usual practice is to run the screens and preamp off the choke, which is not much current. The 200S B+ is sucking >100mA just idling. I didn't think you could do that to a choke without saturation, maybe an air gap in the iron, but once you do that, you kill the inductance, so whats the point?

    Is just a design FU? (Like Leo exceeding max plate V in the Deluxe?)

    I'm going to disconnect the choke and measure it, just for curiosity...

    Thanks!

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    Lifetime Member Enzo's Avatar
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    What is a "car"? Is it one of those tiny things that holds one guy? Or is it a monster tired thing that drives over VWs at shows?

    What is a choke? A choke is an electrical concept, you can make chokes for any level of current you want. It can be a tiny thing or a monster car crushing choke.


    I hate when people wave "design flaw" in the air. Leo didn't fuck up, he just realized the tubes would run just fine 100v over the book spec. 50-60 years later and those same amps are still running just fine, thanks, at those overly large voltages.

    So until I start seeing old Sunn amps coming in with blown chokes on a regular basis, I won;t be waving the DF flag

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    Thanks for the information-free snarky reply. I was looking for the theoretical underpinnings of the design, you just wanted to throw things.

    For the tech-interested, the choke measured about 1H and 58 ohms at 100hz. I don't know what the L at actual current is, it will be less. My best info on this is from Merlin Blencowe's "Designing Power Supplies for Tube Amplifiers" which suggests minimum L for this ap at about 2.6H. I did scope the filter performance (it is a 20/40 pi) and it did well at reducing 120hz ripple. The unanswered question (if I get bored I'll put it in Spice) is whether a 15 cent 58 ohm resistor can do as well as a $15 choke in this app.

    That, I would call a design FU.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by marku52 View Post
    ...The unanswered question (if I get bored I'll put it in Spice) is whether a 15 cent 58 ohm resistor can do as well as a $15 choke in this app...
    The power supply with the choke vice a resistor will have lower ripple.

    The early Sunn amps used Dynaco transformers. The Sunn 200 power amp was based on the Dynakit Mark III design which Dynaco published in their transformer catalog. The Dynaco circuits did show a power supply with a pi filter using a choke before the B+ feed to the OT. Of course, there is no separate screen power supply node because the power amp is an ultra linear design. Since Dynaco was primarily marketing to the Hi Fi crowd it all makes sense to me. Sunn used the Dynaco suggested circuit with very few changes. The choke that Dynaco sold for use in that circuit was their model C-354. The description in the Dynaco catalog of the day simply says "C-354 choke (200ma) Net price $1.50" No inductance value is listed. However, another schematic in the catalog titled "Improved Mullard 30 Watt Amplifier using Dynaco A-420 Transformer" shows a choke marked "8.5 HY 200 mA"

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    Lifetime Member Enzo's Avatar
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    I apologize for any offense.

    I didn't think you could do that to a choke without saturation,
    When you said that, it seemed to me you were thinking of "choke" as something generic. I tried to make a real-world example of something that is not well specified by its name - "car". I tried to point out that chokes come in all sizes, just as cars do. The implication would be that for any given current, there would be chokes too small and chokes too large, but we can't rule one out just because it is a choke.

    The higher current the choke is designed for, the more expensive it will be, it is larger. The reason the guitar amps skip the output plates before running B+ through a choke, is cost. The cap filters work well enough. But look at early Fender choke circuits like the 5E2 Princeton or Bassman 5E6. The entire B+ flows through a choke. They quickly found they could move the choke down stream a step and use a smaller part. They found they could get the same improvements in other ways.

    If you consider that a design flaw, that's OK. In my own mind a design flaw would be a quirk in the nature that caused a problem. Having cheaper ways to do things is not a "problem". A design flaw would be the sort of thing one tries to correct when one sees the amp. But that is only an opinion.

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    Thanks to you both. I knew there was a dynaco connection, both the trannys and the can cap say it on them. Using a choke before the B+ feed makes sense in a Hi Fi app. Physically, the choke doesn't look any larger than the ones Fender used in a screen application. Maybe it is one EI size bigger. Making a choke with enough iron to take the full B+ current would be expensive, or you could put an air gap in the magnetic circuit to reduce the flux density, at the expense of the inductance. And the only tech ref I had suggested that 1H would be too small to be effective. That said, the scope says it is working pretty well. Does the reduced inductance work as well as the 15 cent resistor? Ah, questions....

    How did Sunn end up with a bunch of Dynaco parts? did Dynaco go out of business? I know they sold these as kits, my dad built one back in '62 or so....Maybe wiki knows

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    Senior Member potatofarmer's Avatar
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    Quote Originally Posted by marku52 View Post
    The unanswered question (if I get bored I'll put it in Spice) is whether a 15 cent 58 ohm resistor can do as well as a $15 choke in this app.
    I'm willing to bet it wouldn't. You only have the one CLC stage to minimize the ripple going to the plates AND the screens. When Fender dipped their toe into distributed load operation, they used a bias balance control so you could null out the ripple buzz reaching the speaker, without either a CLC or CRC before the plate supply. The best solution would probably be the combination of a CLC stage and a bias adjustment for each tube, but a CRC stage and individual bias controls would probably work nicely.

    Quote Originally Posted by marku52 View Post
    How did Sunn end up with a bunch of Dynaco parts?
    That's how Sunn got started, adding preamps to Dynakits and putting them in a nice big chassis.

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    Old Timer Leo_Gnardo's Avatar
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    Markus, when a power supply type choke like Sunn/Dynaco has much less than its ideal current passing through, it doesn't work as well as it should and the Henry reading is derated. They like to be run at near-saturation. And if the current is way too much, the wire acts like a fuse and that's the end of the story. Seen it happen rarely in Fenders. Only once in a Marshall but that Marshall choke developed an internal short so it was useless for filtering but the amp still "worked" - however there was a gargling sort of interference especially on single notes as the power supply ripple modulated the note frequency. If you jumper across your Sunn choke, you might hear a similar effect.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by marku52 View Post
    ...Making a choke with enough iron to take the full B+ current would be expensive, or you could put an air gap in the magnetic circuit to reduce the flux density, at the expense of the inductance...
    I expect that the choke in the Sunn does have an air gap. There is some interesting choke info at http://www.vias.org/crowhurstba/crow..._vol3_025.html. The article discusses the "Swinging choke" and how it behaves as the current increases. They use a choke input filter as an example but the theory may also apply to a C-L-C Pi filter performance.

    Quote Originally Posted by marku52 View Post
    ...How did Sunn end up with a bunch of Dynaco parts?...
    Part of Dynaco's business was transformer sales. They were going strong at the time and were probably very happy to sell parts to Sunn.

    Quote Originally Posted by marku52 View Post
    ...Does the reduced inductance work as well as the 15 cent resistor?...
    No! The choke works better. The resistor substitutions come about as cost saving measures forced on the designers by bean counters.

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    Last edited by Tom Phillips; 05-15-2014 at 09:59 PM. Reason: Swinging choke info added

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    well this is annoying, I write a reply, a hit post and it says token expired.

    to rewrite:

    Here another question: It is common knowledge (even in Merlin's book) that if you saturate the choke, it will burn up. Now this definitely true of a transformer, as the magnetizing current will head to the moon. But in a choke? This choke is 1H +58 ohms resistive. When you saturate it, the permeability of the iron goes to that of air, and the inductance drops to that of a air core coil. But the heat is driven by the 58 ohms resistive, which has been there all along. I jsut see the amp getting hummy, because 500 ohms of inductive filter reactance went to about zero, but the 58 ohms remains as it was before. I don't see some runaway current to burn it up.

    I think I'm calling BS on Commons knowledge in this particular case, but welcome any correction. I'll check out that choke link

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    Very interesting stuff at that link. As I understand it, as the current drawn increases, the current pulses start to saturate the choke, and now the filter caps get charged through Rcoil ohms resistive, which is probably 7X smaller than Zl ohms reactive. The more current you draw, the more current is allowed to charge the cap. So it regulates. Drop the current, and it is reactive all the time, fed by a higher reactance. Increase the current, and it starts acting resistive in pulses.

    Clever.

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    Last edited by marku52; 05-15-2014 at 09:41 PM. Reason: missing word
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    Sunn started because Norm Sundholm played bass in the Kingsmen and wanted a better amp. His brother Conrad was into hifi and built him a dynakit mk3 and a cabinet of his own design. Norm toured the country and a lot of people liked the sound. Norm started finding dealers for the amps and told them to call his brother. Conrad bought lots of Dynakits and put them together in a new box and called them Sunns. Eventually they put them in a single chassis and made some changes to make it sound better for musical instruments, used just the transformers, and sold more. Later they changed to Schumacher transformers instead of Dynaco. These amps have higher voltages and a less sweet but more powerful sound to them. That is a very simplified version but essentially the gist of it. All of the amps are loud and clean and when they distort, the distortion is very gritty and hard. They are all quite reliable amps too. The original PT for the 200S was a little undersized/overstressed so they sometimes blow. Triode Electronics sells an upgraded replacement.

    Greg

    Greg

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    I grew up in Gainesville, FLA, and I remember seeing my first SUNN amp. We were all befuddled totally as to how to pronounce "Tualitin"

    "Where the heck is that?"
    "2 Ah Litten?"

    Strange that later on I ended up in Portland...

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    Member patlaw's Avatar
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    Quote Originally Posted by soundmasterg View Post
    Sunn started because Norm Sundholm played bass in the Kingsmen and wanted a better amp. His brother Conrad was into hifi and built him a dynakit mk3 and a cabinet of his own design. Norm toured the country and a lot of people liked the sound. Norm started finding dealers for the amps and told them to call his brother. Conrad bought lots of Dynakits and put them together in a new box and called them Sunns.
    The first time I ever saw a Sunn amp was when I saw The Kingsmen with Norm playing bass. He and I talked for a while because my family owned a guitar store at the time. We became a Sunn dealer, which is how I got my Sunn Sentura II in 1969.

    Not only did they have the cool looking Sunn amps, but as I recall, they had Barry Curtis' chopped-up Hammond organ with Sunn tolex. I don't remember which Hammond it was. Barry told me, but I have since forgotten.

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    Quote Originally Posted by marku52 View Post
    This choke is 1H +58 ohms resistive. When you saturate it, the permeability of the iron goes to that of air, and the inductance drops to that of a air core coil. But the heat is driven by the 58 ohms resistive, which has been there all along. I jsut see the amp getting hummy, because 500 ohms of inductive filter reactance went to about zero, but the 58 ohms remains as it was before. I don't see some runaway current to burn it up.

    I think I'm calling BS on Commons knowledge in this particular case, but welcome any correction. I'll check out that choke link
    Hi, I know I'm replying years after this thread was started but I just joined the forum a few days ago. The Dynaco choke in question is rated 1.5 Henries @ 200ma. These were common in Dynaco tube amps like the ST-70 and others.
    It is my understanding that as the current goes up in a choke, the inductance decreases. As current goes down, inductance goes up. They used to make "swinging chokes" which had two ratings: One at a low current and another rating at a much higher current.
    See:
    diyAudio - Swinging Choke or Smoothing Choke
    Now, in push-pull amps, chokes aren't really required for the output stage because of common mode rejection. Any noise created in the output section cancels out in the output transformer as the two sides are dynamically opposed in currents. A choke in the power supply before the output transformer may help, but like stated above, isn't that cost effective vs benefit. IE. A really large and expensive choke would be needed to make a noticeable difference in sound. If the output transformer is Ultra-Linear, then the output transformer acts as a "choke filter" to the screens besides making them function as "pseudo-triode".
    Downwind of the output stage, everything works in class "A" so power supply noise rejection isn't super important as each stage pulls constant current from the PS. A choke will tighten up the sound by smoothing the supply ripple, so there is a benefit. Everyone has to remember that 50+ years ago, Electrolytic capacitor technology wasn't what it is today; power supply caps were pretty dismal and expensive. This is where chokes really came in handy in power supplies. Look at old Western Electric amplifiers from the 1920's and they had very few filter caps in them, but a lot of chokes. W.E. actually used a lot of neat tricks to get the job done. Worth taking a look at.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by danfrank View Post
    Hi, I know I'm replying years after this thread was started but I just joined the forum a few days ago. The Dynaco choke in question is rated 1.5 Henries @ 200ma. These were common in Dynaco tube amps like the ST-70 and others.
    It is my understanding that as the current goes up in a choke, the inductance decreases. As current goes down, inductance goes up. They used to make "swinging chokes" which had two ratings: One at a low current and another rating at a much higher current.
    See:
    diyAudio - Swinging Choke or Smoothing Choke
    Now, in push-pull amps, chokes aren't really required for the output stage because of common mode rejection. Any noise created in the output section cancels out in the output transformer as the two sides are dynamically opposed in currents. A choke in the power supply before the output transformer may help, but like stated above, isn't that cost effective vs benefit. IE. A really large and expensive choke would be needed to make a noticeable difference in sound. If the output transformer is Ultra-Linear, then the output transformer acts as a "choke filter" to the screens besides making them function as "pseudo-triode".
    Downwind of the output stage, everything works in class "A" so power supply noise rejection isn't super important as each stage pulls constant current from the PS. A choke will tighten up the sound by smoothing the supply ripple, so there is a benefit. Everyone has to remember that 50+ years ago, Electrolytic capacitor technology wasn't what it is today; power supply caps were pretty dismal and expensive. This is where chokes really came in handy in power supplies. Look at old Western Electric amplifiers from the 1920's and they had very few filter caps in them, but a lot of chokes. W.E. actually used a lot of neat tricks to get the job done. Worth taking a look at.
    Thanks for posting. There's some truth to a choke's change in value as small to larger currents are drawn but since current stays nearly even in a class A or near-A amp, you can expect the choke's value to be what it's expected to be. There were/are some swinging choke designs where inductance changes more radically. These aren't much found in audio amps, they're more suited to RF transmitters.

    I doubt David Hafler would have included the "pre filter" if he didn't think it necessary for the MkIII, MkIV & Stereo70 Dynaco amps which were copied into Sunn tube guitar/bass/PA amps. One fine day some 25 years ago, for a lark I tried running MkIII's from the raw B+ supply. It didn't work out well, sure I got another couple watts, barely more than stock. But I also got audible distortion. Running sine waves thru the MkIII's and watching the scope I easily saw the "blip" added = distortion. A bridge too far, retreated one step to stock and all's OK with the world & the Dyna MkIII's. Dam' I had 'em sold too, and in the time I wasted faffing around, the buyer found himself a pair of McIntosh C40 instead. Well no worries, he was happy with them and the C20 preamp he also scored, and had me work them over, so I ended up making a couple bucks & learning a lesson: my "improvement" to Dynacos was no improvement.

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    Senior Member ThermionicScott's Avatar
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    Having a CRC filter before the plates is still a good thing for Class AB amps, as the ripple-cancelling benefit of push-pull goes away as soon as one tube in the pair goes into cutoff.

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    Quote Originally Posted by danfrank View Post

    ...

    Downwind of the output stage, everything works in class "A" so power supply noise rejection isn't super important as each stage pulls constant current from the PS.
    ...
    Thanks for the info on Dynaco choke specs.

    In reality, the earlier stages are extremely sensitive to power supply noise. The average current is constant but it's signal current that matters. A typical 12AX7 common cathode stage has only about 14dB of power supply noise rejection i.e 1V of ripple on the power rail will produce 200mV at the plate. Also, the signal level is smaller so meaning you really have to pay attention to ripple in these stages. That is why there is often a RC filter ladder to successively attenuate it down the the lowest level input stage.

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    Quote Originally Posted by marku52 View Post
    I jsut see the amp getting hummy, because 500 ohms of inductive filter reactance went to about zero, but the 58 ohms remains as it was before. I don't see some runaway current to burn it up.

    I think I'm calling BS on Commons knowledge in this particular case, but welcome any correction.
    Just hummy? Try that on the bench and let us know how that works out for you...

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    Don't forget the screens are also sensitive to power supply noise. Not a problem with pentode connection, but it can be pretty noisy in UL - hence Fender's bias balance trick.

    I wonder how important the ripple might be to the overdriven sound. I do remember one experiment done where they used rock-solid supplies throughout a tweed bassman and supposedly it sounded awful, but I can't find the link.

    Nevermind, here it is: https://www.ampbooks.com/mobile/clas...ass-AB-ripple/

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    Old Timer J M Fahey's Avatar
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    As soon as the power stage clips, situation changes a lot: hum itself was cancelled at lower powers because of the circuit symmetry, still does if it clips symmetrically, but ripple still modulates the clipped sinewave and that can be heard.

    Compare it to a double sideband transmitter: carrier (in this case 100/120 hz) is cancelled, but the sidebands (audio modulated by it) are not.

    Imagine audio subject to low intensity Tremolo, with sawtooth oscillator set to 100/120Hz, and you'll be describing it exactly.

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