fatten SE EL84 (6bQ5) amp

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by CodfishLabs, Mar 15, 2015.

  1. CodfishLabs

    CodfishLabs Member

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    anyone have any ideas on how to add some growl to a Harmony 410A?

    have one here at near no cost.

    When I got it, I swapped a vintage 7025 for its newer 12AX7 and I swapped a vintage Sylvania 6BQ5 for an unmarked 9-pin in the power tube slot.

    I replaced 2 wax/paper caps that were clearly oozing - one btw the 7025 and the tone control and one btw the volume and tone.

    Speaker is a new 8" Weber so I don't really care for that.

    Was thinking of R/R the Weber for a 10" Jensen and also remember Fender ran a bypass cap cathode to ground on their 12AX7's and this amp has no bypass cap on that tube.

    Any and all ideas would be much appreciated.

    Joe
     
  2. Blue Strat

    Blue Strat Member

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    Different tubes won't help unless you've got weak ones in there now. There's very little to work with, circuit-wise, but the only hope would be to completely redesign the preamp...probably with no tone control. Your best bet is a boost pedal in front of the amp.
     
  3. pdf64

    pdf64 Member

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  4. Avatar Tech

    Avatar Tech Member

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    You could try some larger values for R7 and R8. That should increase output voltage from those stages and maybe help overdrive the power tube.

    R19 could be decreased in value to provide higher voltage to the plates of V1. I'm thinking of possibly helping overdrive the power tube again here. I'm not sure if increased plate voltage here will change things much though.
    Make sure you safely discharge the caps before you start tinkering :)
    Colin
     
    Last edited: Mar 15, 2015
  5. teemuk

    teemuk Member

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    Back in the days the answer was to mutilate the speaker cone. Vintage amp, vintage mods....
     
  6. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    I'd try a EP Boost in front - that should fatten it just fine.

    - T
     
  7. Prattacaster

    Prattacaster Member

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    I agree with others, bypassing the first and/or second stage is a great way to beef things up. You can throw a whole cow at it but if you're playing into an 8in speaker its still going to be a thin steak.

    The values for the bypass caps might have to be tailored to your speaker. 20uF caps for both might be a little to much. Might want a smaller value for the second stage bypass(1uf-5uF)

    Before doing the bypass caps you could raise R10 to 510k or 1meg. This will give an increase in gain and low end.
     
  8. telenut62

    telenut62 Member

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    +1
    220K's would be the go, see how it sounds...if you want more gain....a 10-25uf bypass on V1 2.2K resistor....hey make it switchable!
     
  9. zenas

    zenas Member

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    So other than the two wax and paper caps the rest are original ? Bet there's a cap can that's junk and maybe a couple more electrolytics.

    Maybe it's just me but I like to get an amp working right as it was built, add a grounded cord (because shocks suck) then start modding it if need be.
     
  10. teemuk

    teemuk Member

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    Good suggestions.

    With low end, however, it's worthwhile to bear in mind the restrictions introduced by the low output power rating and the small-diameter cone of that loudspeaker. There's simply not much "headroom" for clear low end and once those low frequencies distort they intermodulate everything into "buzzy/farty" mess. So there's a point when you simply have "too much" low end, and with low power amps and small cones that point is usually reached rather quickly.
     
  11. CodfishLabs

    CodfishLabs Member

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    hey - thanks to everyone for the quick response and all great ideas.

    here's what i'm thinking, test between each step and stop when happy:
    1. go pedals first because i can use them with other amps
    2. do the resistors, caps around the 12AX7 cathode
    3. R/R resistor in power supply to get more B+ on the 12AX7 plate
    4. R/R Weber speaker last - $$$

    I hope to post results in a few weeks when the temperatures allow me to get in my garage workshop.

    Thanks again to all. Joe
     
  12. CodfishLabs

    CodfishLabs Member

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    forgot to mention - the amp came with a 3 prong cord and the correct fuse in the fuse holder. Joe
     

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