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Exploring Fender's Harmonic Tremolo

Discussion in 'Amp Tech Center' started by moosie, Dec 23, 2017.

  1. moosie

    moosie Doctor of Teleocity Silver Supporter

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    During the process of building a ReVibe, a discussion has sprung up where a few of us either want to know, or are helping to explain, how the variants of the Fender Harmonic tremolo work. @keithb7 asked about the 6G8-A, and here's @clintj's great explanation:

    I'm just hoping to learn more, and in that spirit, I'm posting the 6G8-A schematic, with the bare bones markup of what I think Clint said. I'd appreciate it if others would flesh it out a bit further, along the lines of @robrob's fine signal trace / markups of the Deluxe and the 6G15.

    To demonstrate the limits of my knowledge, I had to look up "cathodyne". And I think I just learned about cathode followers yesterday (you tell me).


    fender_twin-6g8a.pdf_1.png

    For instance, and I know this is standard Fender, but how exactly do those three caps create the LFO? Basically - and I hope I'm not the only one - I'm interested to know what each component does, beyond standard coupling caps, and tube current and signal flow.
     
    Last edited: Dec 23, 2017
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  2. clintj

    clintj Friend of Leo's

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    You need a few of Merlin's chapters to piece these bad boys together.

    The tremolo oscillator is here.

    http://www.valvewizard.co.uk/trem1.html

    Basically, the three caps together cause a 180 degree phase shift, which coupled with an inverting gain stage, creates a positive feedback loop.

    Now, the two different ways Fender built this:

    The four triode version is built around a paraphase inverter.

    http://www.valvewizard.co.uk/paraphase.html

    A tap is taken from the output of the LFO loop after the Depth control, and sent to the inverter stage. Then the two signals are balanced via voltage dividers and sent to each subsequent triode out of phase with each other.

    The five triode version uses the same oscillator, but is then amplified by one stage, like you see in a Princeton Reverb, brown Deluxe, and others. Now we have a large LFO signal. This is sent to a cathodyne phase inverter.

    http://www.valvewizard.co.uk/cathodyne.html

    That produces equal and opposite signals that drive the following stages.
     
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  3. theprofessor

    theprofessor Poster Extraordinaire

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    Thanks very much for starting this thread, @moosie! About a year ago, I started a thread on the history of amp tremolo ("vibrato") that some knowledgeable folks contributed to (not me!) : http://www.tdpri.com/threads/teach-me-about-the-history-of-amp-vibrato.698270/ I'm sure your thread will be a great resource for others who want to know the ins and outs of harmonic tremolo in the future. Hey, maybe you could start a thread on how every single one of the several types of amp tremolo works! :) Or not... 'cause then you might have a bad Christmas!
     
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  4. theprofessor

    theprofessor Poster Extraordinaire

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    P.S. @moosie - I see that you're now a Doctor of Teleocity! I have a Ph.D., but I think I might rather have a Tel.D. 'cause that's awesome! Cheers!
     
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  5. keithb7

    keithb7 Friend of Leo's

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    Thanks for starting this thread @moosie . I look forward to also learning about this harmonic vibrato circuit.

    I need some time to print out the schematic and study it. Re-read what Clint said and try to comprehend. I’ll read the suggested links to Merlin’s info too.

    I just need some down time to digest all this. Today our family all started showing up for a Chevy Chase style Christmas here. I’ll try and sneak away when I can between family interventions. Lol.
     
    Last edited: Dec 23, 2017
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  6. clintj

    clintj Friend of Leo's

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    Bus ride time, so let's add some more to this.

    What's the point of having opposite phase oscillator signals?

    If you've studied the later bias wiggle tremolo, you know that it works by driving the bias of the tube up and down, causing the tube to cut off sound by shifting its bias point away from its normal point to an extreme. Same thing happens in the following stages, but at opposite times.

    If you look at the schematic above, you see that the signal goes to both triodes. On its way there, it gets split by a high pass and a low pass filter network. High frequency content gets sent to one triode, low to the other. If both triodes followed a single oacillator signal, they would both cut off at the same time and it would sound like plain old bias tremolo like you hear in a PR. Cool sound, but not the goal.

    Since the two signal triodes are being driven into cutoff at opposite times, you get something really cool happening. The high frequency triode gets cut off, leaving only low frequency content. As it starts to recover and pass signal again, the other triode (LF content) goes into cutoff leaving only high frequency content. These signals get recombine into one, and sent on through to the power amp.

    That's the magic. You get some perceived volume shift as the signals change, and that high/low dance as the two halves of the signal are silenced at opposite times. Very cool sound.

    Now look at how much real estate in that schematic is eaten up by that circuit. It's what, a quarter of the page? There's a LOT of parts, and either 2 or 3 vacuum tubes involved. The brown Concert (6G12-A), for example, has six preamp tubes. That's the same number as a later blackface amp, Deluxe Reverb or larger. If you want to add reverb like all the cool surf guitar guys use, something's got to go to make room.
     
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  7. moosie

    moosie Doctor of Teleocity Silver Supporter

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    I'm a who now?

    What was I before? I suppose it changed at 10k, which really just tells me I have too much time on my hands. :lol:
     
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  8. theprofessor

    theprofessor Poster Extraordinaire

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    No idea. I think all these names are pretty hilarious. But you've got to admit that your avatar makes you look super-important: you've got a banner that says "silver supporter," and you're called a "Doctor"... of a web site!
     
  9. moosie

    moosie Doctor of Teleocity Silver Supporter

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    One new member thought Silver Supporter was my name! :lol:

    I just like that I can easily find my avatar, with the red Tele, so I can find my own posts in long threads.
     
  10. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Let's add the other harmaonic trem schematic into the ring of discussion....I'm off for a long walk now, but am looking forward to this developing discusion.
     

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  11. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Thanks for that, @Bendyha!

    That's the same 4 triode design that I'm having trouble with at the moment in my Weber ReVibe. Here's the relevant schematic, which is essentially identical to the post above, but with the benefit of some component numbering for easier discussion, plus it's not so grainy.

    The markup is my interpretation of @clintj's great explanations, and Merlin Blencowe's articles.

    The dry signal comes in from the top left, and the wet (pulsating) exits via C33 on the far right, into a mixing circuit with the reverb, in this case.

    The LFO, once started, pumps out the oscillation endlessly. The in-phase pulsating current is sent to both V4b (in this case), and the Paraphase Inverter.

    The inverter is a simple gain stage, pushing out 180-degree out-of-phase pulsating current. This is sent to V4a.

    I've tried to identify the high and low pass filters, but am not sure it's correct. Anyone?

    I haven't tried to identify the voltage dividers that match the amplified OOP level with the unamplified in-phase level. Again, anyone? Bueller?


    Screen Shot 2017-12-24 at 5.57.19 AM.png


    Does anyone know the lineage of these two circuits? Which came first? Which models received them? Keith, didn't your HPTT have something like this in it?
     
    Last edited: Dec 24, 2017
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  12. keithb7

    keithb7 Friend of Leo's

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    @moosie, The High Power Tweed Twin quite a simple circuit with no vibrato. Just a push-pull phase inverter of some type for the power tube pairs.

    Until just reading these recent posts, I had no idea that there was a 4 and a 5 tube version of Harmonic Vibrato from Fender. My understanding is, in my 6G8A blonde Twin half a pre-amp tube, or 1 triode is left abandoned. Half of a 12Ax7, is not used as it was not needed for the circuit. It has 6 pre-amp tubes. As seen here. This would be the 5 tube system Clint is talking about. (which is actually using 4 1/2 tubes)

    Looking at the Blonde Concert Schematic 6G12A, it too seems to be set up with the same vibrato system as my Blonde Twin. A 5 tube trem.

    It seems some amps used 4 tubes. Looking at schematics now I see that the Blonde Bandmaster for example used 4 in the harmonic trem system. The 6G7 for example. Interesting. I had no idea.

    [​IMG]
     
    Last edited: Dec 24, 2017
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  13. keithb7

    keithb7 Friend of Leo's

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    Looking at V4 here with only 3 scant wires coming off it, I'd say that's the one running on 1 lung.

    [​IMG]
     
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  14. FenderLover

    FenderLover Poster Extraordinaire

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    I just stumbled on to this post, but had just written a few of my impressions on the circuit here:
    http://www.tdpri.com/threads/harmonic-tremolo-amp-2.790875/

    Fender didn't come up with this circuit. The 2-tube circuit came from an RF circuit than had been published a few years before Fender got a hold of it, and their 2-1/2 tube re-design (the "-A" Fender models) actually made the circuit more similar to what was originally published. There are a lot of components and component values that I think we can be sure were arrived at empirically. Leo's boys in the lab didn't have their slide rules out for that one.

    Review my ideas in the link above, and I think you'll agree that the circuit can be greatly simplified, and that Leo could have made a rush to judgement.
     
  15. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Vibrato was a big thing in the fifties for electric organs, and so it was clear that the effect would be wanted by guitar players. There are many differing versions, some with only slight variations, others with very different approaches and results, such as that used by Magnatone -which did real pitch shifting, and the concept used by Vox on the AC30's.
    Here is Leo Fenders own patent filed in june 1959, and released on the day of my birth (part of the reason I feel a close conection to this version with 4 triodes)
    If you can get your mind around the patent attorney stile of writing, it does explain how the thing is ment to work.
    If you look at the bottom of the patent, there is a list of all the other patents that he sites relating to vibrato circuits. I have copies of all of them (I think), and many others, if you want to look into the subject more.
     

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  16. FenderLover

    FenderLover Poster Extraordinaire

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    Looks like the schematic labelled A-FJ were 4-tube and the K-FJ were the 5-tube. I generalized the change as the "-A", but that's not correct.
     
  17. FenderLover

    FenderLover Poster Extraordinaire

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    Notice the output of the top triode has 1M+1M to the lower plate, and not ground. The center connection to the grid is essentially virtual ground, and the circuit is called a self-balancing phase inverter in the patent that Bendyha posted.
     
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  18. FenderLover

    FenderLover Poster Extraordinaire

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    From The Soul of Tone, page 269:

    Hartley Peavey speculates that an article written by Richard H. Dorf in the April 1954 edition of Radio Television News might have inspired the Fender circuit....The first edition of Fender's variant came out in the Vibrasonic and was extended to the first Concert. Fender later added another tube and made the circuit even more like Drof's article..."

    Dorf's article is in fact referenced in Fender's patent.
     
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  19. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Here is an article and patent by Dorf ;
     

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  20. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Y'all are the best!
    I don't know if this will help Moosie, since we are working on two different versions of the trem, but I solved my issue!
    I had remarkably deep trem for a later, 5-triode, version of the circuit, but with not enough of the vibey modulation. I have been following Moosie's Revibe build thread and this one, hoping to glean a little better understanding of the circuit. Today I rechecked the intensity-controlled DC voltage changes on the grids, found both sides of the triode to be functioning correctly, but noticed I had omitted a cap in the low-pass/high-pass filters!

    Here is an unmodified Super 6G4-a schematic for reference, with the missing cap highlighted. My amp incorporates the trem pretty much whole:
    super_6g4a.png
    I think putting this cap on a switch might make the circuits of this family more flexible. It really is a good-sounding trem, when the harmonic is too much. Happy accident.
    Cheers!
     
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