Mercury Magnetics Output Transformers - Stock Replacement vs Oversized Replacment

Discussion in 'Amp Tech Center' started by AVMfromDenver, Oct 29, 2019.

  1. AVMfromDenver

    AVMfromDenver TDPRI Member

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    I've been working on putting together my spin on a 5E3 deluxe and am opting for the oversized FTDO-59X vs the FTDO-59 output transformer in hopes of creating a little more headroom and to firm up the bottom end. I was wondering if anyone has any experience with either/both.

    FYI, the stock replacement FTDO-59 weighed in at 688 grams and the FTDO-59X was a whopping 1010 grams. Should be an interesting end result (with either a P12Q or P12N)
     

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  2. uriah1

    uriah1 Telefied Ad Free Member

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    My old guitar tech always used mercury for all trannies and chokes.
    I think they have a few lines.
    imho
     
  3. bblumentritt

    bblumentritt Tele-Afflicted Platinum Supporter

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    Early cutoff distortion and uneven frequency response are characteristics of the 5E3. Hint - it's not the trannies. You'll spend high dollar for something that won't resolve your issues.

    The early cutoff at the second preamp stage is caused by too much overall amplification in the first two preamp stages, and the 5E3 amplifies the lower frequencies by almost twice as much as the higher frequencies.


    If I were to create more headroom and firm up the bottom end of a 5E3, I would do the following:
    1. To reduce the amplification, take the simple but effective step of removing the bypass capacitor on the cathode of the second preamp triode. Removing the cathode bypass capacitor allows the voltage at the cathode of the 2nd preamp gain stage to swing in conjunction with the voltage at its grid. This is a form of local negative feedback and thus it reduces the gain and allows more variation in the volume control before cutoff distortion occurs.
    2. Replace the huge .1µF bypass caps coming off the V1 plates with .047 or even .022µF caps.
    Less that $5 cost; dramatic improvement.
     
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  4. AVMfromDenver

    AVMfromDenver TDPRI Member

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    I sincerely appreciate the tips and will experiment the the variations. The bypass capacitor as described in the first part is the one that is in parallel with the 1.5k resistor, right?
     
  5. D'tar

    D'tar Friend of Leo's

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  6. muchxs

    muchxs Doctor of Teleocity

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    Money saving tip:

    Classic Tone builds working man's transformers at a working man's price.

    An old Pro Reverb or Bandmaster Reverb OT generally considered "undersized" in those amps is amazing in a tweed Deluxe.
     
  7. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    And the Pro/ Bandmaster Classictone is like forty bucks new!

    I have three 18w Marshall clones, one with MM OT and the others with Classictone.
    Can't hear any better sound from the MM equipped amp.
    MM has more models now but 15 years ago I called them up to ask which Plexi 100 their Plexi 100 OT was blueprinted from.
    This because I wanted to reproduce a '67 Plexi, which had a different OT from the '66 and '68 100, as well as a different sound. Hard to say how much the OT influenced the sounds of those three different Plexi 100s, but I wanted to know...

    At MM I talked with the phone person who connected me to a sales rep who then hunted down an engineer.
    Nobody there knew what year plexi their Plexi 100 OT was based on, and they chose to make only one version and claim it was a correct replacement for the huge '66, the small '67 and the medium size '68 that was the same as the later metal panel 100 OTs.

    So they essentially blueprinted a metal panel OT and sold it as a Plexi, seemingly unaware of the three different Plexi 100 OT versions.
    For this reason I find them annoying.
    Sorry MM!
     
  8. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    WRT bigger I always think I want bigger and it makes some sense, but in an amp where the OT saturates there is something lost when you replace it with a bigger OT that will never saturate at the amps full power.
    You need to know what parts of the circuit influencing the sound are the parts you like.
     
  9. King Fan

    King Fan Friend of Leo's

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    Sounds like you already spent like, what, $127 on what MM calls their 'oversized upgrade' 8K 8ohm Deluxe OT, so I won't get into the chronic MM "$$ vs. value" argument.

    What @bblumentritt says, as usual, is exactly right. A variant on omitting the V2a bypass cap: I just downsized it to 10uF. Rob's '3-way switched NFB' mod has the omitted bypass as the milder of two 'tamed' settings. It's a noticeable change even from 10uF. It'd be easy to try both -- the idea is the same.

    The .047 or .022 coupling caps are an almost universal approach for folks who want to tame 5e3 bass flub. Neither value gets close to trimming useful, 'valid' bass signal.

    One other simple step to clean up 5e3 bass flub without hurting the sonics is to upsize the first filter cap to 30 or even 40uF -- an idea I got from our super-wizard friend @Wally.
     
    Last edited: Nov 5, 2019
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