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Exploring Fender's Harmonic Tremolo

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

  1. Snfoilhat

    Snfoilhat Tele-Afflicted

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    ^Rad. Thank you, Bendyha!


    IMG_1965.JPG
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    My harmonic trem (6G*-A version) amp in a new box -- this week I pulled the guts out of a friend's failed Fender Deluxe 90 amp. Sounding pretty good, impedance mismatch and Chinese "Ipswich England" Celestion and all :rolleyes:
    Gotta cut a plug of black acrylic for that open centimeter where the chassis is offset. Following this thread closely for any cool circuit tweaks since I've got the amp closer to the front of my mind.
     
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  2. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    How did you deal with the left-side chassis strap in this ^ ? And did you glue a wider wood chassis support block in there?

    Asking because even though it might be cheaper mdf or plywood, there are and will for a long time be a healthy supply of unfixable pcb and ic amps out there, with good condition cabinets and chassis and some other parts. It'd be a shame to toss those in the dumpster.

    I like the red faceplate!

    //end hijack
     
  3. Snfoilhat

    Snfoilhat Tele-Afflicted

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    I haven't felt the loss of the two narrow strips of wood to which the back panels are screwed in on a proper brown/black/silver-era Fender cab. I just locate the new chassis flush against the power transformer side and drill up to 4 new holes for whichever chassis straps fit my chassis. I usually cut a rectangle of black acrylic to fill whatever sized gap is on the input side of the chassis. Thanks! And apologies for the tangent!
     
  4. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    R.H. Dorf has a subtle arch-post-Helmholtz humorous writing style. Liked it, not just the CAPS LOCK parts.

    Thank you Mr B for posting those articles.

    Also:

    Sorry to ask. Where did you see that?
     
    Last edited: Apr 28, 2018
  5. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    "Universal" - Line 1 - page 3. -

    "The first tube of the vibrato circuit itself is V2a, the phase splitter. It looks like an ordinary "long-tailed" phase splitter, with equal loads in cathode and plate circuits to produce two outputs 180 degrees apart. And this is just what it is."

    And "Wurlitzer" - Line 6 - page 1 -

    "V2 is a phase splitter of the “long-tailed” type, with one signal taken from the cathode circuit, across R7, and the
    other from the plate, the two signals 180° apart in phase."


    Wheras the two schematics clearly show the V2 tube to be a cathodyne.
     
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  6. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    In the planning phase of my 5G4 (6G4 circuit) build and there are a couple of things that are bugging me. It's the 4 triode version. Those amps (5G4/5/7/12/13) all had a 500k linear volume control, and a split plate load in the second gain stage (100k/6.8k). Is split plate load there because of extra gain from the output triodes of the tremolo? Is there even extra gain from the tremolo or a loss? Or is the reduced gain from the second gain stage to prevent overloading the tremolo?
     
  7. Gibsonsmu

    Gibsonsmu Tele-Holic

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  8. clintj

    clintj Friend of Leo's

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    Referring to the previous two posts:

    My impression, from looking at the 6G4 and 6G4-A, is they tweaked the mixer stage and second signal gain stage to alter the tremolo effect. Basically, it looks like the changes altered the input network to the mixer, which could have affected the input impedance, making a change to the signal strength sent to it necessary. That's one explanation for the different split plate loads between the two circuits.

    Remember, these circuits are all a delicate balancing act of guitar, LFO, and mixer bias. Any change to one component may make multiple other changes necessary to rebalance the system.
     
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  9. Gibsonsmu

    Gibsonsmu Tele-Holic

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    In my thread what is perplexing me is the difference between even these two seemingly identical 6G4 circutis - is what you are describing the change there referred to in my prior thread and they just made an even bigger change in the 6g4-A? What would be the difference in signal strength you described? Less resistance, so less signal? Just odd that they just took those two resistors out of later 6G4s in circuits that are otherwise identical. Guessing it wast enough since they completely redid it in the "A"
     
  10. Gibsonsmu

    Gibsonsmu Tele-Holic

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    6G4 Board EB.png 1959 With 220K.png 6g4 Layout Pointed.png
     
  11. clintj

    clintj Friend of Leo's

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    I agree it's odd, but Fender was definitely prone to making tweaks on the factory floor, often undocumented.

    I wish I had time to analyze what exactly dropping those two resistors did *exactly*, but I'm pretty booked up this week. Stack of repair work waiting and some household chores. Such is life.
     
  12. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I was thinking about using a 500k audio volume pot in my 5G4(6G4). While I was talking to some else about this, they were suggesting that I stick with the linear pot. They basically said that with the audio pot, there would be nothing through the first portion of the control. If that's true, I would assume that's because of the low gain from the second gain stage.
     
  13. clintj

    clintj Friend of Leo's

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    There's at least one person here with an original Brownie, I wonder what they find on theirs? The idea and reasoning are certainly sound. If you find it to be touchy, they do make 30% audio pots as an option. They're harder to find, but they're out there.
     
  14. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I'd need the correct bushing size and length, as well as the correct shaft length. Most of those 30% 'J' taper pots are low friction for guitar. I can't even find CTS 500k linear, so I'd have to use Alpha. Although, I've seen claims from experienced builders that the Alpha are better. I've been using CTS so far.
     
    Last edited: Jul 26, 2018
  15. clintj

    clintj Friend of Leo's

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    I used Alphas from Hoffman Amps in my last homebrew build over a year ago, and they've been excellent so far.
     
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  16. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I'll also have to use the 10M reverse audio from Revisit/Weber. Not sure about the quality of those. I know Moosie used it in his Revibe build. It looks like the tapped treble pots are available again, so I may order a few of those also.
     
    Last edited: Jul 26, 2018
  17. Snfoilhat

    Snfoilhat Tele-Afflicted

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    My amp combined the trem from the 6G*-A with a cathodyne PI and a pair of 6V6s (a lot like a 5E3 Deluxe) for the power amp. The change from LTP to cathodyne left the whole amp's gain staging wrong, and this took me a while to figure out :oops:

    The upshot is that I did a bunch of experiments with the gain, including removing the split plate load and inserting a pre-trem volume control to test the headroom of the trem stage. I also tried to make the tremolo switchable between proper 'harmonic' trem and a simple preamp bias vary sound like you'd see in a Vibrochamp -- with limited success.

    What I learned in this messing around is that the trem circuit block did seem to have extra headroom, and so the voltage divider at the split plate load could be tweaked to taste for more gain. I have read that despite the rarity and specialness of the 6G* tremolo, some owners of original 6G* amps use the normal channel exclusively, because the trem channel has noticeably less gain. This seems like a conservative design choice probably driven by that balancing act that clintj refers to.

    I also learned that the trem stage itself seemingly provides lots of gain when the high and low pass filter system is broken (intentionally or unintentionally) and doesn't split the signal evenly, but when it is set up according to the schematic the gain of the trem overall sounds to my ears much less. This makes sense becausethe amplitude of half the signal is descending just as the other half is ascending, by design. My hack to turn the trem to simple preamp bias vary boosted the signal in a way that messed up the gain structure even more, so I abandoned it. Maybe revisit it another time.
     
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  18. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I said 5M, but I meant 10M for the insensitivity pot. The original spec for the 4 triode version was for a linear intensity pot. Also, I assume that since this is still basically the same oscillator as the later blackface amps that the 3M-RA speed is an acceptable sub for the 4M-RA.
     
  19. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Another interesting is that all of the early browns used a 100k/6.8k split plate load, while the later ones are different between amps. The Vibrosonic, Concert, and Bandmaster had 100k/10k, while the Pro and Super used 100k/22k.

    Edit: The 6G8 had 100k/4.7k and the 6G8-A had 100k/22k
     
  20. gtomax

    gtomax TDPRI Member

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    Reviving this from the dead, would upping the .01's to .02 in the LFO feedback section slow the minimum speed similar to the BF photo trem circuit?
     
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