Page 2 of 2 FirstFirst 12
Results 36 to 60 of 60

Thread: Output tube impedance

  1. #36
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,393
    Thumbs Up/Down
    Received: 1,410/21
    Given: 1,196/0
    Rep Power
    14
    Quote Originally Posted by Chuck H View Post
    I imagine it like telling someone how great a bottle of wine is before they try it.
    Roger THAT ^^^^^ !!!

    Also veddy smaht of Ken to keep a copy to compare his other builds. McIntosh did the same with their hi fi gear, at least in the tube days.

    Quote Originally Posted by fretts
    with a strong focus on "purity" - few components, short wires, few features, no frills.
    Matches my principle: "Simplicity yields clarity."

    And you needn't worry about the OP, he's been missing for 9 years more or less. Lots of hijacking and necrothread revivals around here, that's the way we roll. Kind of a pirates of the Caribbean for guitar and amp guys. I'm gonna have a tot of rum now, shiver me timbers!

    0 Not allowed! Not allowed!

  2. #37
    Member
    Join Date
    Sep 2006
    Location
    California
    Posts
    59
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    14
    Quote Originally Posted by Chuck H View Post
    And yet, not so much. It's actually hard to find (or filter for) actual Trainwreck circuits on line now. .......
    Ken even kept his own personal Express that he called "Reality Check" so that he wouldn't get lost in the effort of building a new Express. Age, ear fatigue, how you ate that week, the rain, etc. can all play games on your perception. But the point is that he used the same reference point for each new amp. Smart, but it hardly implies custom for customer. But if you're Ken Fischer you can tell anyone that their amp is special weshial and they'll just swoon and love it.
    I do have some gut shots of Francesca and of Ingrid from when it was possible just search for them. Chris Merren prototyped transformers for Ken after the Stancors disappeared - 12 revisions he says, Ken was looking for something a bit brighter, says he - and sent the specs to Pacific Transformer to buy 50 at a time. Chris told me the story of how bad those Pacific transformers were. The QC was terrible, most had specs all over the place and they didn't follow the winding sheet. Bob Gjika says that all that hand tweaking wasn't custom-tuning for a customer so much as trying to wrangle an amp with an out-of-spec Pacific transformer back to sounding like the benchmark, the Reality Check Amp. If he couldn't get it after altering three or four parts, the tranny came out and went into a box of duds, and another one was swapped in. John Mark - "JM" - corroborates this. He was a close friend for a long time and in recent years, the Fischer family and JM came to an agreement whereby he would build new Trainwrecks, in their house, on Ken's workbench, to Ken's specs, with their blessing. All Ken's "lab notes" are there and the Reality Check amp is still there for reference. So you could buy a sanctioned "real" Express now if you wanted to - they're $5K.

    0 Not allowed! Not allowed!

  3. #38
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,308
    Thumbs Up/Down
    Received: 2,307/4
    Given: 2,665/0
    Rep Power
    30
    Quote Originally Posted by Fretts View Post
    So you could buy a sanctioned "real" Express now if you wanted to - they're $5K.
    Which is the only happy ending to any of this. Provided the orders are there and the people who need the income are ok. Ken worked hard when he could with what he had and died undiagnosed. He actually couldn't work much for more than a decade prior and if the family can use what he built to generate an income and provide a wanted product to some happy buyers then everybody wins. If you're hankerin' for an Express then there's no better way I can imagine to get one than from one of Kens actual confidants that is working with his notes, on his bench and comparing to his existing benchmark. It's not built by Ken, but you can't possibly get closer and you can get it for 20% or less than you'd pay for an original. Not to mention that since there are only 100-"ish" original Trainwreck amps of any make, model, prototype or custom incarnation, continuing manufacture of the line with some sense of integrity to the original intent satisfies a demand and perpetuates the only classic amp ever made in such low numbers. Sort of like the amp version of saving an endangered species and setting a family table at the same time

    0 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  4. #39
    Senior Member Pedro Vecino's Avatar
    Join Date
    Jul 2006
    Location
    Spain
    Posts
    666
    Thumbs Up/Down
    Received: 199/2
    Given: 99/0
    Rep Power
    14
    I do not know that amplifier. I imagine that if they are so exclusive, few people will have had them.
    Can someone put a video of the most representative example of that amplifier to understand what is being pursued?
    Thanks!

    0 Not allowed! Not allowed!

  5. #40
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,308
    Thumbs Up/Down
    Received: 2,307/4
    Given: 2,665/0
    Rep Power
    30
    There are a couple of session players that used them on many commercials for TV and radio, but it's obscure finding out exactly which ones. There are also rumors about a couple of bands whose guitar players used them on this or that track. Some denied by guitarists though. I've read there one parked in a famous Nashville studio, so who knows how many tracks and from whom it may be on. These amps have propagated behind the scenes in the industry among those in the know. Probably the best known proponent of these amps is a very good player named Glen Kuykendall and he has several clips on *outube that are easy to find. There are also a great many clones and straight up copies on *outube. So if you want to hear the real deal, not a clone or copy, stick with Glen's videos.

    This is a clip I always liked because it's NOT a PAF guitar or a strat like all the other TW clips. It's P-90's and it really shows of the amps particular character. Sort of crunchy but saturated with nice swirl and bloom.

    http://www.kometamps.com/user/uploads/60_09.mp3

    0 Not allowed! Not allowed!
    Last edited by Chuck H; 02-25-2018 at 02:14 PM.
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  6. #41
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,753
    Thumbs Up/Down
    Received: 1,880/23
    Given: 1,463/35
    Rep Power
    27
    That ^^^^^
    To see what it means electronically, hereīs one of the back engineered circuits.
    Which differ all over the place, in great part because the *originals* were all over the place too:
    http://www.blueguitar.org/new/schem/...k/wreckxpr.pdf

    it starts with (why am I not surprised?) .... "a Fender"
    Basically same preamp and power amp

    To which 6 tweaks are added

    1) an extra tube gain stage (so far, what Randall Smith started with, 40 years ago, with Mesa Boogie Mark 1 , go figure), so of course it has much more sustain and grind than, say, a Twin or Super Reverb on 10.

    2) BUT, itīs not a plain "Fender" gain stage (which would make it same as the example mentioned above) , instead itīs the very particular sounding "Marshall cold biased" gain/clipper stage of JCM800 fame (and most High Gain amps following it, from Soldano to MBDR to ....) so it has a more grinding, buzzier and biting sound than any Fender, yet it does not fully cross into Marshall territory.

    3) to make it cleaner, even when clipping, he added a highpass/low cut á la VOX, which makes for a janglier sound.
    2000pF into 150k is quite close to VOX 500pF into 470k .

    A plain gain boosted Fender might be farty, because they have lots of Bass; cutting them sharply improves sound while keeping gain high at mid/high frequencies which improves harmonics big way.

    So in a way, for me itīs a "Fender meets Marshall meets VOX" combination, which happens to be very tasty.

    I was almost forgetting a *very* important point: it has NO repeat NO master Volume of any kind (not even PPIMV) so what you hear is either very clean jangly sound or pure creamy power tube distortion, with some added bite added by the cold clipper stage, enough to make it interesting.

    4) he replaced Fender NFB with a Tweed/Marshall type, including Presence.

    5) tone controls are "halfway" between Blackface and Tweed/Marshall, just check values, play with them in TSC and compare to Classic ones.
    In general it has slightly more mids and less range than classic Fender (Treble is shifted lower, Bass is shifted higher, so Mid dip is shallower) which is not bad at all.

    6) there is an undocumented tweak which is NOT in the schematic (might be in the layout though, pity thereīs none "officially approved" available)

    If you listen to YT videos or MP3s youīll notice there is a very bright ring present, very tasty ... which is not reflected in what the schematic shows.
    No bright cap in the volume control, no small value caps in parallel with cathode resistors ... yet that brightness is there.

    Myth says that Designer had put certain wires in close contact which should beter kept well away, under penalty of uncontrolled oscillation.

    Now oscillations happen at certain frequencies, usually very low (motorboating) of which there is no chance here, because of strong low cut, *or* at quite high frequencies , basically because the small parasite capacitances coupling "what you do not want to couple" let high and very high frequencies through easier than lower ones.

    Now when you separate wires to correct that, there is one point when continuous oscillation is stopped, but there is still some "ring" at that frequency ... which if within the audio band will sound like a narrow but strong boost at a certain frequency, think a single band in a Graphic Equalizer or what you can achieve with a Parametric filter.

    Myth says that Designer moved wires here and there until he got the right sound

    Of course, there is no way to show that in the schematic, and I guess is part of the Mystery surrounding this amplifier and makes it hard to clone exactly.

    Where is that "ring"? ... 2 kHz? ... 3 kHz? .... higher?
    All I know is itīs very distinctive.

    Maybe the old shop hand mentioned above, who has built them before and apparently has all of Kenīs Lab notes can reproduce them the right way.

    I think Ceriatone also cloned these (like so many others) but I fear they are missing this important point.

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  7. #42
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,308
    Thumbs Up/Down
    Received: 2,307/4
    Given: 2,665/0
    Rep Power
    30
    Continued discussion of other aspects of the circuit could probably be directed to another existing thread, but hey, NBD really. So... The TW does look in some ways like a kludge of ideas from existing amps. The Fender BF topography, the added gain stage (Boogie), the 10k cathode clipper (Marshall) and the HP filter close to VOX frequencies. But you can say this or that amp is like this or that amp pretty much across the board. And we all know that you can't just piecemeal circuits from different amps together and expect performance. In the end I think it's just the circuit he ended up with. One unique feature Juan didn't mention was the .1uf coupling cap to the PI. Since the PI is doing most of the duty cycle shift (swirl and bloom) this cap value is important to the effect because it charges and discharges slowly (time constant) by comparison the the typically lower values used here. I've experimented with this and watched it on a scope and the results are very interesting. I did try it out on a model I build that's sort of like a Trainwreck design. I didn't like it (for that amp). It compressed the attack in a way that felt strange to me. But in the Express design I think it's a major part of the amps tonal character.

    0 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  8. #43
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,393
    Thumbs Up/Down
    Received: 1,410/21
    Given: 1,196/0
    Rep Power
    14
    Quote Originally Posted by J M Fahey View Post
    I think Ceriatone also cloned these (like so many others) but I fear they are missing this important point.
    Good analysis Juan! I got to work on one of those Ceria "clones", hmmm, maybe 12 years ago? It was actually pretty darn good, except the guy who bought it had only one complaint, that it rang like a bell! A very small bell at that, "pinggggg..." Turned out to be a stiff wire that ran from the circus board to the grid of the EQ drive tube. I could pluck that wire and generate a "sproing" noise that quickly settled down to a high frequency whistle, maybe 4-5 KHz or so, then settled down. Simply replaced the wire with one that wasn't so stiff, with soft plastic insulation. Still had plenty of zing, but no more "ping" accompanying every note. Goes to show, not only wire dress, but selection of just one wire's stiffness / insulation physical qualities can make the difference between unacceptable and terrific.

    0 Not allowed! Not allowed!

  9. #44
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,308
    Thumbs Up/Down
    Received: 2,307/4
    Given: 2,665/0
    Rep Power
    30
    Hmmmm..? Does make me think about Ken reportedly discussing wire selection. That's one that was harder for me to buy into before now. Since he used predominantly NOS preamp tubes and at high gain levels most go "PING" if you tap them you actually ARE influencing the tone of the amp via tube microphonics and physical vibration (feedback). Golden ears? I'm starting to believe it. Of course that means mine are more like, I dunno, aluminum maybe?

    0 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  10. #45
    Member
    Join Date
    Sep 2006
    Location
    California
    Posts
    59
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    14
    There was a book I once had about "lateral thinking" or something like that regarding creativity in business. There's a story of a guy in the 70's who made an uncopyable amplifying module that had a gain of 12,000 or something crazy like that that was theoretically impossible given the parts being used. Turns out that in order to make it super compact, he took what would have been a long, narrow circuit board, halved it and folded one on top of the other, parts facing each other. Then some final trim tweak was performed until it "went into mode" (quote from the book) and then it was potted in a small enclosure. Reverse engineers that cut open examples and managed to identify the parts never got more than a gain of 400 or so, they could not figure out what he was doing. The original designer eventually divulged decades later, when the whole thing was obsolete, the secret - there was positive coupling of some kind that serendipitously occurred when the two circuit halves were placed too close together like that. It was an unintended bonus. He harnessed it and made it controllable and had great success with it that was never duplicated by the competition.
    So anyway, it's not unprecedented for somebody with a quick and open mind to believe it when they see it, and then go ahead and run with it.

    0 Not allowed! Not allowed!

  11. #46
    Member
    Join Date
    Sep 2006
    Location
    California
    Posts
    59
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    14
    ... it has already been said that "Kenny" was not a heavy duty electrical engineer. You'd expect a proper, disciplined engineer to correct the problem of spurious coupling like that and keep those wires well apart. Being free from electronic dogma probably is what allowed him to think, 'wait a minute, that sounds great' and tinker with it rather than instinctively engineer the "problem" out.
    I never knew him outside some postcards and a few phone calls, but those who did know him well and have been interviewed, paint pretty much that kind of picture. He would just move things around and see what they did, and then obsessively refine it to the nth degree, conventional wisdom be damned.
    Besides the actual sound that came out of his amp designs, that's why I admire him so much.

    0 Not allowed! Not allowed!

  12. #47
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,393
    Thumbs Up/Down
    Received: 1,410/21
    Given: 1,196/0
    Rep Power
    14
    Quote Originally Posted by Fretts View Post
    ... it has already been said that "Kenny" was not a heavy duty electrical engineer.
    Perhaps not "heavy duty" as an MIT Electrical Engineer, but he was trained by US Army Signal Corps leading to practical knowledge that served him well. In his Ampeg years he distinguished himself by getting off the production line and becoming "the guy who could fix anything."

    There are certain basic truths in electronics, they are inescapable, call it "dogma" if you will but that's what the universe hands us and we all have to work with it. Maxwell's equations, Ohms law, power law, etc. Thevenin and Kirchoff gave us techniques to engineer circuitry so that we could reasonably expect plans modeled on paper to work the way we expect when we build them. Ken Fisher cannot have been ignorant of them, and nobody can escape them even if they think they're such a genius they can color outside the lines. You can complain about conventional wisdom but you can't run away from it. Stand on the shoulders of giants and get used to it.

    0 Not allowed! Not allowed!

  13. #48
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    31,994
    Thumbs Up/Down
    Received: 1,803/7
    Given: 0/0
    Rep Power
    54
    Seems like the wire type wasn't at fault so much as that it could move. I bet if the sproingy wire had been solidly anchored to something, it wouldn't have happened.


    And the super high gain folded up circuit? Sounds a lot like a regenerative radio set. An amplifier about to break into oscillation goes through an area of high gain before instability. In the old days of regen sets, you turned up the RF gain until it almost oscillated, then the radio was super sensitive, and you could receive weak signals. The regenerative feedback gave you much more gain than just the stage gain.

    And Leo is right. Whether we understand what is going on or not, it is all explainable by the laws of physics, the laws of electronics.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  14. #49
    Member
    Join Date
    Sep 2006
    Location
    California
    Posts
    59
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    14
    Ken's bio does show his military electronics training, sure. Not discounting the value of the tried and true either.
    He had a track record of being kind of an iconoclast. The only point I wanted to make is that I doubt he went into the design of the Express with thoughts of exploiting spurious coupling or stray capacitance as a game plan. Only that he was of a mindset that wouldn't immediately try to engineer it out if/when he encountered it, without tinkering with it first and evaluating its... value.

    0 Not allowed! Not allowed!

  15. #50
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,393
    Thumbs Up/Down
    Received: 1,410/21
    Given: 1,196/0
    Rep Power
    14
    Quote Originally Posted by Enzo View Post
    Seems like the wire type wasn't at fault so much as that it could move. I bet if the sproingy wire had been solidly anchored to something, it wouldn't have happened.
    I think you're right about that Enzo, or I could have placed a layer or 2 of heat shrink tubing over it to damp its movement.

    I've run across a similar problem in lots of classic 60-70's Fender reverb amps. The wire that connects the reverb/dry mix node to the next pre tube's grid has a tendency to "talk" a lot. Tap it with your handy chopstick and you'll hear it clack. Not as much of a problem as the Ceria but in some amps it really is annoying to have this electro-mechanical noise. I tried tacking the wire down with RTV, no dice. Change of wire, mmm, not that either. In those amps where it's really buggin' me, I attach the mix resistors directly to the tube socket, then there's NO wire to cause a racket. One fly lead each to the eyelets on the board that source dry, reverb, and ground, no more problems. Mind you I don't do this to every amp, just the ones that exhibit an objectionable problem. Why some do, and others not so much, there remains a mystery.

    0 Not allowed! Not allowed!

  16. #51
    Supporting Member loudthud's Avatar
    Join Date
    May 2006
    Location
    Near Dallas Texas
    Posts
    3,537
    Thumbs Up/Down
    Received: 220/0
    Given: 113/1
    Rep Power
    18
    Quote Originally Posted by Chuck H View Post
    One unique feature Juan didn't mention was the .1uf coupling cap to the PI. Since the PI is doing most of the duty cycle shift (swirl and bloom) this cap value is important to the effect because it charges and discharges slowly (time constant) by comparison the the typically lower values used here. I've experimented with this and watched it on a scope and the results are very interesting. I did try it out on a model I build that's sort of like a Trainwreck design. I didn't like it (for that amp). It compressed the attack in a way that felt strange to me. But in the Express design I think it's a major part of the amps tonal character.
    The thing you didn't mention is the effect of the PI being driven by the Cold Clipper stage. Bias on the Cold Clipper means it doesn't overdrive the PI too hard on positive peaks so bias shift is limited. It wouldn't react the same way if the PI was driven by a normally biased stage.

    On the clone I built, the magic seems to be in the power supply. There is a sweet spot with B+ around 400V. With an 80uF reservoir cap, B+ is stiff, hardly any ripple or signal on the main B+ node but the 1K "choke" resistor gives plenty of sag to the preamp. Despite this, measured compression is low. The plate Voltage on the EL34's can swing very low due to the high OT impedance and that causes high screen current.

    0 Not allowed! Not allowed!
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

  17. #52
    Supporting Member
    Join Date
    Sep 2007
    Posts
    3,137
    Thumbs Up/Down
    Received: 182/0
    Given: 0/0
    Rep Power
    17
    Remember regenerative radio receivers? They were invented in about 1912 and obsolete for most purposes in 1918 when superhet was invented. They used a tickler coil to get positive feedback, reasonably controlled, but used a gain control just for the purpose of adjusting the gain for current conditions. A regenerative guitar amp, sitting on the edge of oscillation under all conditions at a useful audio frequency with the positive feedback feedback resulting from stray capacitive coupling would be an unusual design accomplishment.

    0 Not allowed! Not allowed!

  18. #53
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,308
    Thumbs Up/Down
    Received: 2,307/4
    Given: 2,665/0
    Rep Power
    30
    Quote Originally Posted by Mike Sulzer View Post
    They used a tickler coil to get positive feedback,..
    I like that for a component name. Tickler coil. Says it all technically, but still maintains a mad scientist feel Who wouldn't want to try one of those into a guitar amp

    0 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  19. #54
    Supporting Member
    Join Date
    Sep 2007
    Posts
    3,137
    Thumbs Up/Down
    Received: 182/0
    Given: 0/0
    Rep Power
    17
    Quote Originally Posted by Chuck H View Post
    I like that for a component name. Tickler coil. Says it all technically, but still maintains a mad scientist feel Who wouldn't want to try one of those into a guitar amp
    But first you need the main coil of a resonant circuit to tickle, or other inductor. Otherwise, you are stuck with doing it capacitively.

    One inductive approach would be to use an interstage transformer with a special extra winding. Positive feedback is usually rejected as a good approach in audio circuit design because it increases distortion. Oh, but wait; that is exactly what you want for a guitar amp.

    I suppose you can dream about a design in which you have a push pull output stage and one high gain pentode driving it through an interstage transformer that would provide the phase splitting as well as a tickler winding, allowing positive feedback around just the output stage so it would not be too hard to control. Well, maybe.

    0 Not allowed! Not allowed!

  20. #55
    Supporting Member tubeswell's Avatar
    Join Date
    Feb 2007
    Location
    Wellington NZ
    Posts
    4,201
    Blog Entries
    1
    Thumbs Up/Down
    Received: 132/0
    Given: 43/0
    Rep Power
    18
    Hmmm - this has got me thinking about Tickler Coils

    Dave's Homemade Radios Tube Radio Schematic Selector

    0 Not allowed! Not allowed!
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

  21. #56
    Member
    Join Date
    Sep 2006
    Location
    California
    Posts
    59
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    14
    Quote Originally Posted by Chuck H View Post
    ...One unique feature Juan didn't mention was the .1uf coupling cap to the PI. Since the PI is doing most of the duty cycle shift (swirl and bloom) this cap value is important to the effect because it charges and discharges slowly (time constant) by comparison the the typically lower values used here. I've experimented with this and watched it on a scope and the results are very interesting. I did try it out on a model I build that's sort of like a Trainwreck design. I didn't like it (for that amp). It compressed the attack in a way that felt strange to me. But in the Express design I think it's a major part of the amps tonal character.
    On the Komet 60, there is a fast/slow switch on the back of the amp which changes something in the PI. I didn't have access to the opened chassis long enough to dig into details, but it sounds like that may be what he was doing in there -- changing the 'tightness' of the response. The little cylindrical switch body was wrapped in wire. I was in there long enough to determine that it was a grounded wire, wrapped around the switch body as a shield. Shield?, gimmick capacitor? antenna?... with Ken Fischer I'd believe almost anything. The Komet is a tremendous sounding amp - that I can vouch for.

    0 Not allowed! Not allowed!

  22. #57
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,920
    Thumbs Up/Down
    Received: 520/3
    Given: 274/0
    Rep Power
    26
    A little info that I found on "The Gear Page":

    "The Fast gradual switch is just a plain old Split plate load switch. It takes the standard 100K plate load resistor and splits it into 82K and 22K in this case. Fast mode is the full 100K load resistor. Gradual mode takes it's signal from the junction of the 82K and 22K to soften the tone by reducing gain. "

    0 Not allowed! Not allowed!

  23. #58
    Member
    Join Date
    Sep 2006
    Location
    California
    Posts
    59
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    14
    Ah, great! Thank you for that. The handmade spiral wire shield looked tricky though.

    0 Not allowed! Not allowed!

  24. #59
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,431
    Thumbs Up/Down
    Received: 162/0
    Given: 2,444/0
    Rep Power
    4
    Quote Originally Posted by Enzo View Post
    because there is no "dead match" for the tubes. All this happens in a spectrum. There is a wide range of values that are perfectly fine for the circuit. The output impedance at the load is only a nominal value, if you look at any speaker impedance curve you see a huge range of values with respect to frequency. All those impedances are reflected back to the tubes in equally varying amount.


    The designer also has to consider how it sounds. We are not making hifi amps, here, we are making the opposite: amps that do indeed color the sound and distort. If a "wrong" impedance pushes the tube operation down its curves a bit and we like the result, well, it isn't wrong then.
    Evidence for the "magic" 1 in 1000 amps.

    0 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  25. #60
    New Member
    Join Date
    Jun 2019
    Posts
    1
    Thumbs Up/Down
    Received: 0/0
    Given: 1/0
    Rep Power
    0
    Reading all of this thread is great. Now, my next amp will go to Eleven.

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. output trans impedance
    By stingray_65 in forum Theory & Design
    Replies: 3
    Last Post: 08-23-2009, 02:38 AM
  2. NFB Lowers Output Impedance?
    By tbryanh in forum Theory & Design
    Replies: 5
    Last Post: 05-02-2009, 09:27 PM
  3. Calculating Output Impedance for a 6G15
    By tubeswell in forum Theory & Design
    Replies: 28
    Last Post: 12-15-2008, 04:55 PM
  4. Magnatone 460 output impedance
    By imaradiostar in forum Maintenance, Troubleshooting & Repair
    Replies: 1
    Last Post: 04-29-2008, 02:30 AM
  5. How to change an amp's output impedance
    By jondanger in forum Mods & Tweaks
    Replies: 25
    Last Post: 03-09-2007, 06:57 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •