What about that crazy Brown/Blonde Harmonic Trem?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by vibroverbus, Apr 9, 2008.

  1. vibroverbus

    vibroverbus Member

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    Snarfling through schematics tonight just for fun (note to self: get a life) I noticed how crazy complex the more unique Brown/Blonde era trems are, specifically the Bandmaster/Pro/Showman/Super/Twin/Vibrosonic circuits that have the 'harmonic tremolo'. Not all the brown/blonde trems used this type, Deluxe/Tremolux/Vibrolux/Vibroverb are simple conventional bias-modulating concepts.

    I dig that these are highly regarded and famous in their own little circle, but does anybody have a good tech breakdown on the circuit? I've stared for a few minutes at them and some sections are obvious (start with an oscillator...) but don't get the whole picture yet. Any web pages or previous threads explaining the theory here?

    Also out of curiosity, how many boutiques are using this kind of circuit these days? Bias trem kicks ass on a bug-driven trem, and this is supposed to be even better... Clearly back in the day it was too expensive to justify (as many components in the trem than in the whole rest of the preamp...) but today component variable cost would hardly be any hurdle.
     
  2. Blue Strat

    Blue Strat Member

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    Yeah, that's an awesome sound alright. Can't help with the theory of operation.

    I remember some boutique builder (Victoria?) sold an outboard unit with harmonic trem and (I believe) reverb. I remember it being discontinued too. Not cost effective.
     
  3. VikingAmps

    VikingAmps Member

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    If I remember right its basically a filter that sends the highs to one phase and the lows to opposite phase signals. Ya they're cool. There are some kits that have a similar circuit and a reverb that you can get. Fender had to stop putting it in thier amps by the blackfaces because Magnatone had a patent for it by then.
     
  4. billyguitar

    billyguitar Member

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    The Fender doesn't change the pitch as much as a Magnatone. My brown Super sounds almost like a Univibe.
     
  5. Wakarusa

    Wakarusa Member

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    Kinderspiel :)

    The circuit consists of four basic components. Reading the schematic from left to right there is an oscillator, a phase inverter, a frequency divider, and an amplifier.

    The oscillator is easily identified -- looks, smells, runs the same as the trem oscillators in most every amp you've seen. One triode is the oscillator itself, the other triode is a buffer to provide a high impedance load for the oscillator. This mess feeds through an RC network (and past the intensity control which just dumps oscillator signal to ground) to the grid of the "next" triode.

    That next triode is just a split phase (cathodyne) phase inverter. Same one you've seen just before the output stage on Princtons, tweed deluxes, etc. So on the input side you have the oscillator (a fairly high amplitude low frequency sine wave). On the cathode output (bottom) you have the same thing, on the plate output (top) you've got the low frequency sine wave 180 degrees out of phase (inverted) with respect to what's on the cathode.

    This all feeds through an isolating RC network (all of those 0.1uF, 0.047uF caps and 1 Meg resistors) into a frequency divider. The other feed is from the channel 2 preamp. Note that the signal from the preamp comes from a split plate load. The driving triode uses 100K and 10K plate resistors and picks the signal from between them. This is a simple trick to attenuate the output of the stage. If you just picked off of the 100K (like the other preamp stage) you'd overload the input to the trem circuit.

    The frequency divider is pretty simple as well. The high frequency component (top part) feeds through the 250pF cap, while the low frequency component goes on to the "bottom part" through the 200K, .01uF etc network. After the input signal is divided into hi and lo freq components, they are individually mixed with the low frequency sine waves from the phase inverter.

    From here, we go into a simple amplifier/mix stage to bring the signal level back up and push the hi and lo components back together. Note that the amplifier uses a partially bypassed cathode (4uF? I forget) so tends to emphasize the hi frequency response. This doesn't make things bright -- it just helps recover some of the losses from the earlier signal manipulation.

    Sooo.... you take the musical signal, divide it into hi and lo freq parts, mix those with a pair of out of phase low frequency signals, and then glue the parts back together. The end effect is that the hi and lo frequency parts are being "tremulated" :) out of phase -- which gives you that nifty roto-vibe kind of sound.
     
  6. vibroverbus

    vibroverbus Member

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    YES! That is what I'm talking about. :bow

    Seems so obvious when you put it that way.... something like
    Oscillator -> PI ------------>
    audio signal -> freq divider -> amp/mix

    Cool. no wonder it sounds cool. Now I've really got a jones to build one.
     
  7. Wakarusa

    Wakarusa Member

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    It's also fun to play with the frequency divider. One nifty mod (and big dark secret :)) is to replace the 220K with a fixed resistor and a pot (say 120K and 100K respectively). Now you can tweak the balance between what goes to the hi and low frequency portions of the circuit.
     
  8. vibroverbus

    vibroverbus Member

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    Talk about wanting balanced dual-triode halves though... since the entire audio signal flows through that splitter-amp-mixer arrangement, even with the trem off, the mismatch would be impacting the bass-side treble-side balance, right? I guess there's some self-balancing factor with the shared cathode resistor for bias...
     
  9. Wakarusa

    Wakarusa Member

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    It's a valid point, but I'd argue not worth the trouble of a matched triode. For the matched triode to be useful, you'd have to assume balanced losses in the preceding circuitry and that's just not the case. Further, you could readily compensate for any perceived mismatch with the preamp tone controls.
     
  10. phsyconoodler

    phsyconoodler Member

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    I looked at that circuit for an amp model I built and it has too many tubes to accomplish the goal.I also looked at the Magnatone vibrato and again,too complicated.I finally stumbled on my own system,which is in the process of a patent.I don't know if it's that unique,but it is a bit different.
    There are some sound clips on my website.The amp is what I call a V-Verb,not solely because some of it's vibro-verb circuitry(it also has elements of super,twin and champ in the circuit),but because the guy that worked with me on the project is Johnny V.He's a blues musician,and a damn good one.
    Anyway,it's not the same as either an old brown fender or a magnatone,but it does indeed pitch shift the notes.
    I think vibrato/trem is a very cool feature and the Brown era Fender sounds delicious to me.I think guys who understand oscillators can come up with some cool stuff.I am not one of those guys,having almost totally stumbled on my system by accident.
    It seems too few guys use the feature anymore,and that's a shame.
     
  11. Jahn

    Jahn Listens to Johnny Marr, plays like John Denver Supporting Member

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  12. VikingAmps

    VikingAmps Member

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    That's what I said in less words. A lot less.

    :moon
     

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