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Is there something about Fender circuit design that is conducive to cleans?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by xmd5a, Feb 5, 2019.

  1. xmd5a

    xmd5a Member

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    It's common to see people say that Fender amps have the best clean tone, but what I'm wondering is, why? I've tried searching for talk about what makes their circuitry conducive to good clean tones, but I can't find much, just talk about the fact that it "is" the best, not "why". That's why I'm asking this more technically oriented amp forum.

    Could it be that other amp circuits are designed to have a better high gain sound? Could be be that Fender amps are just more often associated with open back cabs, rather than enclosed 4x12s?

    It seems strange to me that a single brand of guitar amp could be known for good clean even though they've gone through countless circuit changes over time, is there really something about them, on a technical level, that remains constant?
     
  2. Steppin' Wolfe

    Steppin' Wolfe Member

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    The BF circuit is the clean machine. The Fender 5F6A tweed Bassman is not that kind of clean...and in fact was the basis for the early Marshall’s and for a large percentage of almost all of the high Gain amps that are far from being known as clean machines. Fender has built a wide variety of amps over the years. I find the general description of ‘Fender cleans’ to be restrictive and inaccurate. It seems you feel the same.
    I suppose that ‘Fender clean’ thing comes from the rock world of the 1960’s when Marshall appropriated that tweed circuit and those hot and nasty Marshall amps took over the ‘dirt’ to a large extent.
     
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  3. ahhlou

    ahhlou Member

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    Early Fender amps are not clean (think tweed amps). This type of cct. is what some say Marshall used to model the their early amps.

    When people think of clean Fender amps, I would say they are talking about the black face era. Higher power amp voltages and a unique tone stack (compared to Marshall) that is placed early in the pre-amp section. Qualities that have carried on since...
     
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  4. HotBluePlates

    HotBluePlates Member

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    Mainly, yes.

    The Fender amps people associate with clean tone have a relatively small margin of amplification available over what it takes to push the output tubes to their maximum clean output power. That extra margin is mostly to accommodate there being stronger & weaker pickups, so that even weak pickups get the amp to make full power.

    When you add gain to get the amp to distort more, a lot of problems arise. The solutions to those problems often involve shaving bass (to keep the distortion clearer) and treble (to keep the sound from being harsh) along with other measures. In a lot of ways, it's the extra EQ to make the distortion sound good that monkeys with the clean sound.
     
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  5. HotBluePlates

    HotBluePlates Member

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    Agreed that most probably mean blackface/silverface when they say "Fender clean". Probably has a lot to do with the 20-22dB mid-scoop, centered on 400-500Hz.

    But tweed Fender amps can be played cleanly. There's also a much wider array of circuits in the tweed family than in the brown/blonde, blackface or silverface family. It is an error to lump all those amps together as a "tweed sound" because an early Princeton is a bit different sound than a narrow-panel Tremolux, is a different sound than the 5E_ Bandmaster/Pro/Super, is a different sound than a 5F6-A Bassman or high-power Twin.
     
  6. Bucksears

    Bucksears Silver Supporting Member

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    IIRC, I read somewhere that the distortion that occurred when tweed amps were turned up was an accident, one that Leo Fender aimed to correct; hence, the later Brownface/Blonde/Blackface Fenders were cleaner then their tweed counterparts.

    But yeah, there's 'tweed clean', Blonde clean, Blackface, etc. Pick a flavor.
     
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  7. DGDGBD

    DGDGBD Member

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    The negative feedback loop from OT secondary to PI also plays a big role in the fender BF cleans.
     
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  8. xtian

    xtian Member

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    This!
     
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  9. HotBluePlates

    HotBluePlates Member

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    Let's just say Fender "designed their amps with clean sound, and clean output power in mind." No Fender up to the mid-60's was designed or marketed based on its distorted voice (which is funny given how good some of the models' mild-moderate distortion sounds are).

    Yes, though there are feedback loops (with similar levels of feedback) to be found in tweed and blonde/brown amps.
     
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  10. DGDGBD

    DGDGBD Member

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    true enough. i always figured the feedback resistor value in the blackface series was much much lower, proving more NFB, making the PI stage cleaner. I guess its like comparing apples and oranges since the PI tubes are different.
     
  11. HotBluePlates

    HotBluePlates Member

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    There are at least 2 components that set the amount of feedback. The resistor you're thinking of is the "series feedback resistor" while the other is the "shunt feedback resistor" and generally runs to ground.

    In the later amps you're think of, the shunt feedback resistor is a smaller value (typically 47Ω or 100Ω) than those found in the earlier amps (maybe 1.5k-5kΩ). If you look at either the theory of voltage dividers or opamp feedback theory, making the shunt feedback resistor smaller means you have to also make the series feedback resistor smaller to get the same amount of feedback.

    So it's not just whether one resistor is larger or smaller, but the relative values of two resistors (as well as the expected voltage at the speaker terminals for the amp's power output).
     
    Last edited: Feb 5, 2019
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  12. pdf64

    pdf64 Member

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    To understand how it works, measure the open loop gain AOL, note the feedback ratio β and read https://en.wikipedia.org/wiki/Negative-feedback_amplifier
     
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  13. 79Stone1

    79Stone1 Member

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    Here's a quote from this page:
    https://www.audioxpress.com/article/selecting-a-power-tube
    "Some musicians prefer power pentodes as output tubes in electric guitar amplifiers because of their higher distortion characteristics, which are partially the result of higher transconductance, which leads to higher gain in the amplifier circuit. For example, a 6L6GC operating with 250 V on the plate has 4,700-microsiemens (μS) transconductance, compared to 11,000 μS for an EL34 power pentode under similar conditions."
    Could this be partly responsible for British amps being associated with higher gain relative to American circuits? Among other factors, of course
     
  14. PushedGlass

    PushedGlass Member

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    My take (as in, take it for what it's worth) has been that Fender took a big ride on surf and country (especially the Bakersfield Sound) and became especially associated with those genres' clean tones. Yet there were still people like Steve Howe and his Twin Reverbs and Dual Showmans who were winding them out past clean, and Pete Townshend and his Bandmasters who went past Howe in that regard. Then there's the Princeton Reverb, which does not have clean and loud in its envelope (loud, that being said, is glorious). Remember that "Layla" is a wound-out Champ, and a lot of recent Jeff Beck is Fender-amped.
     
  15. HotBluePlates

    HotBluePlates Member

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    Not the way you're thinking, no.

    To get power output, the output tubes receive a voltage signal and conduct a varying current through the output transformer. That varying current ("current swing") creates a voltage drop across the transformer's primary ("voltage swing"), and the (voltage swing) * (current swing) = power output.

    Transconductance (Gm) is expressed in "micromhos" or "microsiemens" but the old European way of describing it is more informative. 1,000 micromhos = 1 milliampere / volt, or 1mA/V. That is, a tube with a Gm of 1,000 micromhos has a plate current change of 1mA for a signal voltage change of 1 volt.

    Now look at those 6L6 and EL34 numbers again. 4,700 micromhos = 4.7mA/volt for 6L6 (usually closer to 6mA/v, and 11,000 micromhos = 11mA/volt for EL34. If we wanted the same plate current change for the same output power across the same output transformer primary impedance, then the EL34 will only need about half as much input signal voltage as the 6L6 to get there. If we start with the same pickup signal, there can be less amplification between the amp's input jacks and the output tubes if we use the EL34 to get our power output.

    In one sense the EL34 "has more gain" than the 6L6, and in another sense we don't need as much gain to drive it fully. This has little (directly) to do with guitarists getting distortion. It is the reason why EL34s use a very different bias voltage than 6L6s (because the EL34 grid is more effective at controlling plate current, so bias voltage is smaller).
     
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  16. aynirar27

    aynirar27 All You Need Is Rock and Roll Gold Supporting Member

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  17. aiq

    aiq Supporting Member

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    I don’t know why but I give thanks to the old gods and the new.
     
  18. HotBluePlates

    HotBluePlates Member

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    Just re-read the whole article. There is much that is implied in the article, and it uses terms in a way that will be unfamiliar to non-technical guitarists. The article also assumes the reader has a similar level of amplifier design experience as the author. It appears to be geared to a designer that is passingly-familiar with power tube types, and is looking to select a tube type for a new amplifier design.

    A key point is the quote, "Since voltage gain is directly proportional to transconductance, the EL34 would have about 2.34 times the gain of a 6L6GC in the same circuit." Guitarists habitually think of "gain" as equal to distortion (pedal & amp knob labels help create this impression) and might take the article statement to be universally true (instead of applying specifically to output tubes).

    But look at the Gm of common preamp triodes:
    Type Amp. Factor Gm
    12AX7 100 1250
    12AT7 60 4000
    12AY7 44 1750
    12AU7 20 3100

    Looking at the table, lower gain tubes have higher Gm (except that 12AY7), so something else is at play. Part of that "something else" is internal plate resistance of a tube and how that works against load resistance. Designing with pentodes (and beam tubes) one assumes the internal resistance is so high compared to the load that it can be ignored, and Gain = Gm * Load Resistance. This isn't the case for preamp triode tubes.

    There's a minefield waiting for those trying to understand tube electronics without first getting the proper fundamentals down pat (said as someone who went through that minefield without those fundamentals, blew off a foot or two, and now walks smartly with prosthetics... :D).
     
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  19. 79Stone1

    79Stone1 Member

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    Lol I've been bumbling through that mine field for years myself.
    I also noticed that article classified EL84 as a beam tetrode. I always thought they were a true pentode. Anyhow....
    Thanks for the explanation. Informative as always.
     
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  20. HotBluePlates

    HotBluePlates Member

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    Exactly right! The American type number 6BQ5 is a true pentode without aligned grids or beam-forming plates (the article also omitted the importance of the aligned grid structure for beam power tubes).
     
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