6505+ Bias Mod

Discussion in 'Gear & Equipment' started by maggotspawn, Jan 1, 2017.

  1. maggotspawn

    maggotspawn SS.org Regular

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    Looking to swap the bias resistor in my 6505+ so I can bias the amp a little warmer. I'm looking for the correct value to change the resistor to. Eurotubes says 6.8k, others on the net say 5-7k. Has anybody that's done this mod care to chime in?
     
  2. xero7

    xero7 SS.org Regular

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    I reversed the mod recently in a block letter 5150. Along with resistor change you would want to add the trim pot to adjust the bias for whatever tubes you plan on using. Most people use the resistors you mentioned. Without the trim pot I would think a 10k would be alright cause stock it's a 15. I think the amp sounds better stock.
     
  3. Yodel

    Yodel Banned

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    Peaveys sound better stock, it was designed with that cold setup for a reason.
     
  4. maggotspawn

    maggotspawn SS.org Regular

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    The 6505+ already has a bias pot. Stock it's only letting me bias to 22ma.
    The amp sounds like a can of bees right now. I'm going to change the resistor.
     
  5. xero7

    xero7 SS.org Regular

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    Have you tried the green channel boosted? Tube swap might go along way as well.
     
  6. maggotspawn

    maggotspawn SS.org Regular

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    Swapped out the resistor for a 5.6k one. Biased to 32ma, sounds good but I'm going to bias it 38-40ma. Unfortunately I broke the key on one of the tubes, so I have to hunt down a matched quad. Less bees, less Fizz is a win.
     
  7. KailM

    KailM SS.org Regular

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    FWIW, ever since I started running an MXR 10-band EQ in my 6505+'s effects loop and cutting the 16khz slider by 6db, fizz has been a thing of the past. No fizz, just grinding mids, smooth highs, and guttural lows.
     
  8. maggotspawn

    maggotspawn SS.org Regular

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    I actually have a clone of the MXR coming in. I just wanted to get my base tone as good as I can before I tweak on it.
     
  9. KailM

    KailM SS.org Regular

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    That's a good plan. I also found that running 5751 tubes in V1 and the phase inverter positions reduces fizz and makes the preamp noise quieter, in addition to making the distortion more pleasing in every aspect. After doing that and running my MXR the way I described, I've just been very, very happy with my tone. I wouldn't describe it as "cold" or "lifeless" by any stretch of the imagination. I would call it a warm, thick, yet punchy and grindy tone.

    I'll be staying tuned to your thread if you do the bias mod though. I'm getting a new 6505 head in a couple days; probably going to be very happy with it though.:)
     
  10. Wookieslayer

    Wookieslayer OOOAAARWWWRR

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    Have you seen the Amptweaker De-fizzerator?
     
  11. maggotspawn

    maggotspawn SS.org Regular

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    Saw it. Don't need it now. I sorted out my tube problem, biased it up to 38ma.
    No more bees or fizz, still grinds.
     

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