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Thread: Output tube impedance

  1. #1
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    Output tube impedance

    Is there a chart somewhere showing the output impedance for various power tubes in push-pull. I've seen various differing opinions in other forums, just would like to have a chart for reference on future projects

    Thanks

    D

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    Common values are:

    6L6/KT66/6550/KT88/KT90 - 4K/tube nominal for Fender (though the 3x10 bandmaster ran 2.67K), 3.6K to 4.2K are typical, sometimes as low as 2.5K, maybe up to 5K+ for an amp that has a compromise primary Z & can run 6V6 as well. Some old PA amps ran 7.5K. I'd normally go with the Fender value as this gives some leeway with ext cab jacks. Halve the speaker load with a 2.5K primary and you can wave goodbye to your power tubes!

    EL34 - typically a shade lower than 6L6 (3.5K to 3.6K) but to all intents and purposes interchangeable with 6L6.

    6V6/EL84 - Centre value might be considered around 8K/tube, some old PA amps ran 4.7K, early low voltage SE amps might be as low as 5K-5.5K, BF Deluxe is around 6.6K, up to 8.5K at the higher end for Princeton/Tweed Deluxe.

    In short, stick to 3.6-4.2K for the bigger tubes & 6.6-8.5K for the smaller tubes if you intend to wire the amp Fender style, with parallel ext. jack, fed by the same OT secondary - an amp set up like this will normaly tolerate halving or doubling of the matched speaker load. If you swing to more extreme hi/lo primary Z, make sure you use a multitap OT & always match speaker loads.

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    Senior Member Old Tele man's Avatar
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    ...footnote at the bottom of page 188 in F. E. Terman's ELECTRONIC & RADIO ENGINEERING, 4th ed., 1955, McGraw-Hill:

    "To the extent mu (µ) is constant and that Eq. (6-10) is true, the plate resistance depends only on the plate current Ib, and is inversely proportional to the 'cube-root of Ib'."

    Eq. (6-15) rp = (2/3)*mu / (Ib*K^2)^(1/3)

    where:
    rp = tube dynamic plate resistance, ohms
    mu = tube triode amplification factor, dimensionless
    Ib = plate current, amps
    K = tube constant Perveance, amps-per-volt^(3/2)
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    The short answer is that yes, there is a chart somewhere that does this. I don't have it, and it may well not be accurate, or not all accurate.

    First of all, if I don't misunderstand you, you're interested in what plate load to apply to the plates of a pair (or two, or three pairs) of output tubes. There was a large amount of work done on what was the "best" load for a pair of output tubes back in the Golden Age of Tubes. They came up with answers, but the answers weren't simple.

    If you had a magic output transformer which could dial in a load on a pair of output tubes while you drove it, and you looked at things like maximum output power before distortion and the curve of distortion versus output power level, you could make charts of output versus loading and distortion versus loading, so you could pick off the "best" loading points. That was done back in the Golden Age, and I've seen a few of these charts.

    For power pentodes (like the EL34) and power beam tubes (like the 6L6) there is a broad maximum of power output at a specific loading level. For 6L6s this is about 4K to 4.4K plate to plate. For EL34s this is at a higher loading. However, the "peak" in power output is not really a peak - it's a broad hump. Missing the magic maximum only loses you a little power output, and your ears do not hear all that much change in output level. So it's entirely possible to use the same loading for 6L6 and EL34 and not miss much in loudness.

    But then there's that other curve of distortion versus loading. It turns out that the distortion before clipping starts is minimized at some point of loading. This loading is not the same point as maximum power. For the 6L6, it's up at about 6.6K plate to plate.

    The result is that hifi tube amps tend to (or tended to, back when people knew how to design amps with tubes, not easter-egg in parts) use the lowest-distortion loading, and guitar amps tend to use the highest-power loading.

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    Supporting Member tubeswell's Avatar
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    Quote Originally Posted by Old Tele man View Post
    ...footnote at the bottom of page 188 in F. E. Terman's ELECTRONIC & RADIO ENGINEERING, 4th ed., 1955, McGraw-Hill:

    "To the extent mu (µ) is constant and that Eq. (6-10) is true, the plate resistance depends only on the plate current Ib, and is inversely proportional to the 'cube-root of Ib'."

    Eq. (6-15) rp = (2/3)*mu / (Ib*K^2)^(1/3)

    where:
    rp = tube dynamic plate resistance, ohms
    mu = tube triode amplification factor, dimensionless
    Ib = plate current, amps
    K = tube constant Perveance, amps-per-volt^(3/2)
    Yeah but what is "Eq. (6-10)"?

    And how does that differ from "Eq. (6-15)"? - (Apart from "5"??)
    Building a better world (one tube amp at a time)

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    Senior Member Old Tele man's Avatar
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    Quote Originally Posted by tubeswell View Post
    Yeah but what is "Eq. (6-10)"?

    And how does that differ from "Eq. (6-15)"? - (Apart from "5"??)
    Eq. (6-10) is the "Child-Langmuir 3/2's Law" for triodes:

    Ip = K*(Vg + Vp/mu)^(3/2)

    ...which, with proper messaging also works for tetrodes and pentodes:

    Ik = (Ip+Is) = K*(Vg + Vs/mu.triode + Vp/mu.tetrode)^(3/2)

    where:
    Ik = cathode current
    Ip = plate current
    Is = screen current
    K = Perveance
    Vg = control grid voltage
    Vs = screen grid voltage
    Vp = plate voltage
    mu.triode = tube triode amplification factor (mu1)
    mu.tetrode = tube tetrode or pentode amplification factor (mu2)
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    A couple more points/questions

    1) Contrary to what's been posted, the Weber site apparantly figures the impedance of KT66's at about 8k, as the shift the OT taps in their kits when a KT66 is used. Are they wrong?

    2) When doing calcs, is it better to use 3.2 for 4 ohm speakers and 6.8 for 8 ohm speakers which seems normal when reading the speakers?

    thanks

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by MWJB View Post
    Common values are:...

    6L6/KT66/6550/KT88/KT90 - 4K/tube nominal for Fender (though the 3x10 bandmaster ran 2.67K), ...
    With respect to the 3x10 Bandmaster, your # of 2.67 is right but that would be secondary load in ohms not primary zed in Kilo-ohms.
    Actually the tweed Bandmaster and tweed Super ran the same impedance OT... which was not 4K but between 6200 and 6500 ohms (depending on the year and all that rot) so the Bandmaster, at 2.67 ohms was around <4K35, very close to the 4K you mentioned above.
    The tweed Pro also used a 6200/6500 Ohm OT but with an 8 ohm secondary.
    To be honest, all those amps sound really good even with the wrong 4K output in them but they do sound a bit different and more classic with the right impedance.
    What's really cool is you can use a softer rectifier tube and sub 6V6s right in those old tweed amps with no change to the OT or speaker loads.
    Bruce

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    Senior Member Old Tele man's Avatar
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    ...Bruce, you're referring to the original Triad/Stancor transformers and not the later Woodward-Schumacher transformers, correct?
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    If you check out the KT66 data sheet, it calls for a 8K load, but this is for minimum distortion, so no it's not incorrect but max power calls for a load between 6 and 8K.
    See here; http://www.mif.pg.gda.pl/homepages/f...086/k/KT66.pdf
    Speaker impedance includes inductance, so it is higher than the measured DC resistance.

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    It would be nice if there was a sort of tutorial out there in basic terms so everyone could get a handle on it that said something to the effect of

    "If you use an OT with a primary impedance of 4k with a 6L6GC pair, and you change to a 6k6 load, THIS is what will happen to the sound. Or if you use an EL34 on those same transformers, THIS is what happens to the sound."
    But of course everything changes with applied voltage and current and what not, so I know its not that simple......

    Greg

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    "1) Contrary to what's been posted, the Weber site apparantly figures the impedance of KT66's at about 8k, as the shift the OT taps in their kits when a KT66 is used. Are they wrong?" Marshall used 4K, to all intents & purposes KT66 & 6L6 are interchangeable in most aspects, except heater current draw (some current production KT66 even match that - check with mfrs notes).

    2) Transformers work on AC impedance not dc resistance, so go with the nominal impedance rating. However ratings are just that "nominal" & float with freq response, also the discrepancy between DCR and AC-Z (20%?) won't be enough to cause damage in a middle of the road design.

  13. #13
    Senior Member Old Tele man's Avatar
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    ...the published speaker impedance values are "measured" at 1,000 Hz, which is just one point on their spectrum, so obviously actual values will be different, but "...at..." 1KHz they will roughly (within tolerances) produce their rated impedance. As the frequency varies, so does the XL reactance and thus the speakers impedance. 1KHz is just the standardized point of measurement.

    ...just remember, the DC-resistance (R) measurement of a speaker will typically be about 80% of it's rated AC-impedance (Z) value...give or take a little.

    ...and +100% on tube rp variance vs. (a) minimum distortion and (b) maximum power...commonly 10% or more apart in value, with a lower value yielding lower distortion and a higher value yielding higher power.
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    "...the published speaker impedance values are "measured" at 1,000 Hz, which is just one point on their spectrum," - Not so. Just around the speaker's resonant frequency there is a spike in impedance, nominal impedance is usually taken at the foot of the trough following this spike, for many guitar speakers this will be somewhere around the 100Hz mark. Often by 1KHz an "8ohm speaker" is nearer 10ohms.

    Not that I have idled away far too many hours gawping at speaker charts, or anything like that...:-). (Still no substitute for hearing them though).

    There was a thread recently where the OP was querying AC voltages & primary Z on current Fender schems, it transpires that when you work out turns ratios that the 8ohm amps seem to be operating at 10ohms...suspiciously like running the test into a speaker rather than a 8ohm dummy load?

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    Senior Member hasserl's Avatar
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    On the old forum Speedracer (Joe from Obsolete Electronics) posted this as "Common Output Tranny Primary Z ratings"

    Amplifier Primary Impedance

    Marshall, 50W 2xEL34 3,400 Ohms

    Marshall 100W 4xEL34 1,750 Ohms

    Marshall JTM45 2xKT66 8,000 Ohms

    Vox 30W 4xEL84 4,000 Ohms

    Vox 15W 2xEL84 8,000 Ohms

    Fender 50W 2x6L6 4,100 Ohms

    Fender 100W 4x6L6 2,000 Ohms

    Tweed-Spec Cathode Bias 6L6 p-p 6,600 Ohms

    Matchless 15W* 2xEL84 4,000 Ohm

    Matchless 30W 4xEL84 4,000 Ohms

    Park 50W 2xEL34 5,000 Ohms

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    I heard that the JTM30 had an output tranny that matched 8K:16ohms, but by using 2x16ohms speakers in parallel, cut it back to 4K:8ohms.

    6.6K is more typical of the fixed bias BF/SF Deluxe. P-P Princeton (also fixed bias) and cathode biased Deluxe more like 8K-8.5k.

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Old Tele man View Post
    ...Bruce, you're referring to the original Triad/Stancor transformers and not the later Woodward-Schumacher transformers, correct?
    Yes... the classic, later tweed Bassman amps must have all been 4K but the lower powered 6L6 amps were higher.
    It seems that after that very late 50's push, (moving into early brown face amps and beyond), most of the amps (not counting the SE amps) became fixed bias amps and the 6L6 OTs changed to the lower zed of +4K.
    The Deluxe 6V6 amps also went lower into the 6K6 range but the Princeton and Vibrolux remained the same zed as the tweed Deluxe. I'm not sure about the 6V6 Tremolux, I can't remember off the top of my head.
    I don't know if that zed change was just because of the higher plate voltages, power tube load line curves, how the power tubes were biased or quest for power vs cleanliness, but it seems plausible doesn't it?
    Bruce

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    "With respect to the 3x10 Bandmaster, your # of 2.67 is right but that would be secondary load in ohms not primary zed in Kilo-ohms." I stand corrected re. the tweeds, thanks Bruce. Though I didn't say so, I was thinking about the brown tolex 3x10"...

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    Quote Originally Posted by hasserl View Post
    On the old forum Speedracer (Joe from Obsolete Electronics) posted this as "Common Output Tranny Primary Z ratings"

    Amplifier Primary Impedance

    Marshall, 50W 2xEL34 3,400 Ohms

    Marshall 100W 4xEL34 1,750 Ohms

    Marshall JTM45 2xKT66 8,000 Ohms

    Vox 30W 4xEL84 4,000 Ohms

    Vox 15W 2xEL84 8,000 Ohms

    Fender 50W 2x6L6 4,100 Ohms

    Fender 100W 4x6L6 2,000 Ohms

    Tweed-Spec Cathode Bias 6L6 p-p 6,600 Ohms

    Matchless 15W* 2xEL84 4,000 Ohm

    Matchless 30W 4xEL84 4,000 Ohms

    Park 50W 2xEL34 5,000 Ohms
    I thought I'd seen that somewhere before.
    Building a better world (one tube amp at a time)

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    RG, On this topic, let's say you were maybe from Colonia, New Jersey and let's say you wanted to use primary output impedance to finesse the onset of distortion in the power amp. What deviations from "cleanest recommended impedance" might you consider when choosing the primary impedance load for a guitar amp? What might you expect from choosing the lower end of the "hump", the upper end, or the middle? Or to the extreme, what consequences might you suffer from running a pair of 6L6s much lower and much higher than the middle ground?

    Quote Originally Posted by R.G. View Post
    The short answer is that yes, there is a chart somewhere that does this. I don't have it, and it may well not be accurate, or not all accurate.

    First of all, if I don't misunderstand you, you're interested in what plate load to apply to the plates of a pair (or two, or three pairs) of output tubes. There was a large amount of work done on what was the "best" load for a pair of output tubes back in the Golden Age of Tubes. They came up with answers, but the answers weren't simple.

    If you had a magic output transformer which could dial in a load on a pair of output tubes while you drove it, and you looked at things like maximum output power before distortion and the curve of distortion versus output power level, you could make charts of output versus loading and distortion versus loading, so you could pick off the "best" loading points. That was done back in the Golden Age, and I've seen a few of these charts.

    For power pentodes (like the EL34) and power beam tubes (like the 6L6) there is a broad maximum of power output at a specific loading level. For 6L6s this is about 4K to 4.4K plate to plate. For EL34s this is at a higher loading. However, the "peak" in power output is not really a peak - it's a broad hump. Missing the magic maximum only loses you a little power output, and your ears do not hear all that much change in output level. So it's entirely possible to use the same loading for 6L6 and EL34 and not miss much in loudness.

    But then there's that other curve of distortion versus loading. It turns out that the distortion before clipping starts is minimized at some point of loading. This loading is not the same point as maximum power. For the 6L6, it's up at about 6.6K plate to plate.

    The result is that hifi tube amps tend to (or tended to, back when people knew how to design amps with tubes, not easter-egg in parts) use the lowest-distortion loading, and guitar amps tend to use the highest-power loading.

  21. #21
    Old Timer J M Fahey's Avatar
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    Hope you notice you are asking to a 2009 thread, forgotten and sleeping for 9 years.
    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Hope you notice you are asking to a 2009 thread, forgotten and sleeping for 9 years.
    --No, I didn't really notice, but apparently it gets read... by you and me at least!
    J M Fahey and eschertron like this.

  23. #23
    Old Timer J M Fahey's Avatar
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    Oh, you digged it from the grave , once you or anybody adds a post, it gets a current date and everybody sees it again.
    No problem with that, I was just warning you that usually people writing there has not visited the Forum for years, so don´t hold your breath waiting for quick answers.
    As of your questions:
    let's say you were maybe from Colonia, New Jersey
    ??????????????????????????????????????
    let's say you wanted to use primary output impedance to finesse the onset of distortion in the power amp.
    Amps are not designed that way.

    What deviations from "cleanest recommended impedance" might you consider when choosing the primary impedance load for a guitar amp?
    See above answer.
    In any case, differences will be subtle, and practically inaudible in the Guitar World.
    A Hi Fi designer may agonize trying to turn 0.27% into 0.22% and try literally *everything* , in a Guitar amp that is absolutely undetectable.

    You need a gross 2:1 or 1:2 mismatch to start hearing something, that´s why it´s not a useful tool here.
    Guitar amp designers quote output power at 5% or 10% distortion, well into visible clipping, subtle variations are lost in the noise floor.
    Juan Manuel Fahey

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    Supporting Member Chuck H's Avatar
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    Where Fretts is going is trying to determine Ken Fischer's process when he designed the Train Wreck amps. Because they don't use the ideal primary impedance and no one could believe that Ken did anything without a specific intent on tone I'm sure it's commonly believed in some circles that the specific distortion effect caused by the non ideal output transformer primary impedance is critical. Further, any other criteria for OT primary impedance is unacceptable.



    And let me say THIS on THAT... The few percent of specific order distortions and their ratios at differing impedance are so incredibly tiny compared to the distortion generated by those amps at their intended drive level that if Ken were alive today even HE would be rolling his eyes.
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    Quote Originally Posted by Chuck H View Post
    And let me say THIS on THAT... The few percent of specific order distortions and their ratios at differing impedance are so incredibly tiny compared to the distortion generated by those amps at their intended drive level that if Ken were alive today even HE would be rolling his eyes.
    Then he'd casually stroll over and turn the volume knob UP. There's 90% of the mojo right there. It's a GUITAR amp - it's SUPPOSED to make some noise! Oh, wait... I see that every time I post.

    Justin
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    Quote Originally Posted by J M Fahey View Post
    Oh, you digged it from the grave , once you or anybody adds a post, it gets a current date and everybody sees it again.
    No problem with that, I was just warning you that usually people writing there has not visited the Forum for years, so don´t hold your breath waiting for quick answers.
    As of your questions:
    ??????????????????????????????????????

    Amps are not designed that way.


    See above answer.
    In any case, differences will be subtle, and practically inaudible in the Guitar World.
    A Hi Fi designer may agonize trying to turn 0.27% into 0.22% and try literally *everything* , in a Guitar amp that is absolutely undetectable.

    You need a gross 2:1 or 1:2 mismatch to start hearing something, that´s why it´s not a useful tool here.
    Guitar amp designers quote output power at 5% or 10% distortion, well into visible clipping, subtle variations are lost in the noise floor.
    Yup, the Zombie lives again, sorry.
    So why would a designer deliberately choose a primary impedance that is not a dead match for the output tubes? It seems Fender chose assorted primary impedances, at least on some models that don't exactly line up. Ken Fischer of Trainwreck fame (Colonia, NJ) was very particular about his output transformers and much lore exists about him choosing a nonstandard primary impedance on his OTs that had to be custom made. Can you postulate why he might have done so?

  27. #27
    Supporting Member Chuck H's Avatar
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    My understanding was that the Express transformers Ken commissioned from Heyboer had a primary impedance 1100 ohms lower than the original Stancor A-3801. Well why would he do THAT!?! Was there something wrong with the originals? People are practically knocking each other over to get to them. I think Ken was more concerned about other aspects like inductance, DCR, core material and other aspects of transformer design that, pooled together, still don't get as much attention on the guru forums than the magic figure of primary impedance. In other words, if Ken thought that there was only one best design for the Express OT, why would he have changed something about the OT from what he designed the amp with!?! He's known to have used two off the shelf transformers and one proprietary model in different Express amps of different era's. He's said to have used at least two different OT's for the Liverpool model and they were of different primary impedances as well. So, to get to the question at hand..

    Why would someone choose a non ideal OT primary impedance AND what would be their criteria? I don't know and it probably doesn't matter as much as some people might think it does. At least Ken didn't think so.
    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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  28. #28
    Senior Member Old Tele man's Avatar
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    Ask Ken!
    ...and the Devil said: "...yes, but it's a DRY heat!"

  29. #29
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Old Tele man View Post
    Ask Ken!
    A lot of people would love to except they can't because he's dead. I suppose one could ask any of his supposed confidants. There have been several claiming the position.
    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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    I expect that all the elements of a KF design were contributing toward the same end. A subtle thing here, a threshold setting there... it seemed to work because the amps were enthusiastically embraced at a time when there was no legend and no reputation to inflate their status. Looks like he commissioned transformers because he had to, Stancor quit making his go-to, and while he was at it, spec'ed an additional tweak. We'll never know now, unless current new-Trainwreck-builder, "JM" chooses to divulge what he sees in the original notes left by Ken.
    Well, I certainly took a detour here, whew!

  31. #31
    Supporting Member Chuck H's Avatar
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    Well, keep it in the back of your mind so your ears pick up anything in the woods. But I honestly think that Ken simply listened to the amps and decided that a little high was better sounding. So when he ordered the Heyboers he stayed high, though not quite as high as the Stancor. He likely paid more attention to other aspects of the design. Ken could occasionally surprise with some tech. He was careful not to speak out of his depth, but generally came across as a well seasoned, but not technically educated amp guy. I know he had an actual job related to amps with a famous maker before he did his own thing. But IIRC he was just an electronics enthusiast when he took that job and his skill at repairs moved him up the ladder. So he was good, yes. But hardly an engineer. My point is just that I don't think Ken ever made any finite decision about specific harmonic structures and OT primary impedance. He was more likely to notice the tonal difference between impedances in general and consider it in his designs. Not nearly as finite as people would think. He also did things like move OT's around, holding them in his hand while the amp operated so that he could choose the quietest place. He also experimented with component location inside the chassis because it actually can make a difference. But I strongly doubt he was ever the genius that could look at a hand wired board and tell you what would happen if you reoriented "that capacitor" 180*. Through experience (building ham systems) he may have had more odd clues about layout anomalies than most hobbyists, but the stuff engineers know about transformer design was probably not his to own. If people would stop idolizing him and try harder to get into his real head/perspective they might have an easier time unlocking his secrets.
    Justin Thomas and Fretts like this.
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  32. #32
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    Why would someone choose a non ideal OT primary impedance AND what would be their criteria? I don't know and it probably doesn't matter as much as some people might think it does. At least Ken didn't think so.
    When scratch-building, most folks use off the shelf OT's, and few of those have primary impedances that are an exact match to the tube-guide charts. Besides, who knows how close today's tubes - or NOS for that matter - are to their claimed specs. We have to deal with what's available. More advanced builders could special order iron from outfits like Heyboer or maybe Edcor. Another piece of Trainwreck lore that fell into my ear (OW!) for some of them Ken used OT's he pried from Dynaco MkIII power amps, intended to be driven by KT88. Maybe he liked the tone? Could have something to do with it?

    IF there's an answer, Ken took it with him. Another bit of 'Wreck lore, which you may believe or not as you wish: Ken tweaked each amp for its intended owner, so you'll doubtless find variations in each model type. If he was around today he'd probably advise: what's the point in beating your brains out trying to ape the 'Wreck? (And which one???) What's important is that you build an amp that suits what you're looking for, your ears, your speakers, your guitars. The Express is a good format for builders, it's not awfully difficult to make one if you're so inclined. Then set to work dialing it in for yourself. That would be a proper continuation of the Ken Fisher legacy.

    BTW I caught the Colonia reference early on. I grew up about an hour's drive north, and my Aunt Betty lived right down the road a hop & a skip in Old Bridge / East Brunswick. In spite of all that I never got to meet the guy, though I have met some of his amps and their owners.
    Last edited by Leo_Gnardo; 02-23-2018 at 10:01 PM.
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  33. #33
    Lifetime Member Enzo's Avatar
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    So why would a designer deliberately choose a primary impedance that is not a dead match for the output tubes?
    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.
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    Thanks for all the views - very enlightening.
    Bob Gjika has videos on YouTube about his own amps and his experiences with Ken Fischer come up in conversation. He adds detail to a lot of the snippets of gossipy lore that are always around about Ken and his work. The transformers, the problems, their conversations. It's interesting. I had a postcard correspondence going with Ken for a little while in the 80's and called him on the phone a couple times. I thought he was a fast-talking, fidgety character, albeit very forthcoming and generous with his time, with a strong focus on "purity" - few components, short wires, few features, no frills. I don't know personally, but I would not be surprised if his motorcycles were stripped-down like that, just the pure essence.
    I have to apologize to DaBreeze - I seem to have totally highjacked this conversation! Sorry about that. Don't ever start one about Tesla, we'll be here for weeks.
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  35. #35
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    Ken tweaked each amp for its intended owner, so you'll doubtless find variations in each model type.
    And yet, not so much. It's actually hard to find (or filter for) actual Trainwreck circuits on line now. But once upon a time there were a few chassis shots and some circuit blueprinting going on for a few others and the Express amps are pretty much all the same. The only exceptions I've noted have been a 1.5k grid stop on the second triode in one amp, shielded or unshielded input, where that shield was tied and the slightly different OT's used. In the Liverpool's there were some differences in the load to the third triode with some examples having 68k and some as high as the Express value of 150k. but really, not much deviation to the circuit in the majority of amps. If he was tuning these amps for individual owners he was doing it within the component specs because the marked values are the same. He did talk a lot about selecting the right tubes. So that could be another customer tweak I suppose. But since he's dead and tubes all differ a little, well... Glen Kuykendall has a few videos where he compares his and someone else's Express or his two Express's side by side and talks about the differences, but the differences are so subtle as to be almost ignorable. I imagine it like telling someone how great a bottle of wine is before they try it. 90% will love it immediately based only on what they heard about it with their taste buds fast asleep. 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'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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