Mesa Buster: Power Amp Schematic, Adjustable Bias Mod, and 7581A

Discussion in 'Amps and Cabs [BG]' started by BassDuex, Jun 11, 2019.

  1. BassDuex

    BassDuex Supporting Member

    Jul 15, 2006
    Placerville, CA
    I would like to add a bias mod to my Buster head, but I cannot find a schematic for the power amp.
    While I’m no amp tech, I can safely work on high voltage equipment.
    I haven’t opened up the amp itself yet, as it gets played nearly every day.

    As far as the bias mod, I know in theory how to add a pot for each tube, but it seems like it might be easier and as effective (since I would be using a matched set) to use 1 pot for each triplet? Or for pairs?
    If pairs or by trio, would you mind detailing where to insert?
    Do you remove the resistors, tie the grounds together, replace with a resistor/pot of 1/3 range (for a trio) - due to the current from 3 tubes?

    Would anyone mind helping me understand how to best approach this?

    Also, has anyone used the Tung-Sol 7581A tubes in these?

    Thank you!
     
  2. Bertr

    Bertr

    May 6, 2013
    Apparently you can ask Mesa directly:
    Schematic for Mesa Buster?

    I would not bother with individual bias (I can not say I have ever heard the difference between amps that have it and amps that do not, especially wih matched valves). Adjustable bias is likely to be only 1 resistor mod.

    I would measure the bias and the pwr valves dissipation first. If it is fine, why modify?
     
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  3. BassmanPaul

    BassmanPaul Inactive

    There has been a huge amount of music made using tube amps with fixed bias. Very few of them had a bias pot. If the amp is working as it should leave the poor thing alone. Adding a pot will just add one more thing to go wrong.
     
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  4. pfschim

    pfschim Just a Skeleton with a Jazz bass

    Apr 26, 2006
    SF Bay Area
    well, I'm no tech, but if the OP was considering using 7581A power tubes in the Buster (I use them in a Fender Studio Bass amp in place of the original 6L6's) .. he might want the bias mod as the 7581A's have a hotter (bias?) than the 6L6's and it would make using them easier.
     
    Wasnex likes this.
  5. BassDuex

    BassDuex Supporting Member

    Jul 15, 2006
    Placerville, CA
    Point taken. She stays as is.
     
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  6. Red Planet

    Red Planet Inactive

    May 29, 2005
    Atlanta
    I'm cranky in my old age.
    A Bias Mod for a Mesa is a great idea. Mesa Biases their tube amps really cold. Not having a Bias Mechanism limits your choices.

    By the way the Buster Amp would be considered fixed bias whether it has an adjusting pot or not.
    Non Fixed Bias is Cathode Bias which self adjusts. For example a Fender Bassman is fixed Bias but it has an adjusting pot to set Bias.

    The Buster amp will function well without adjustable Bias providing the Bias is at an acceptable level/the tubes used are at the right ma draw.
    Having a bias pot greatly increases the amount of tubes one can use with ones amp though it does require a higher skill level and some math.
     
    Wasnex likes this.
  7. chaosMK

    chaosMK

    May 26, 2005
    Albuquerque, NM
    Too much hip thrust
    Buster has a great power section, quick boomy and clean. I'd add the bias mod and put some KT88's in it. That could be pretty legit (but expensive!).
     
  8. BassmanPaul

    BassmanPaul Inactive

    The output stage including the Output Transformer was configured for
    And what would be gained? Any increase in available power wouldn’t even be noticeable and the future failure modes would increase. In my book don’t try to second guess the very knowledgable engineers who designed the thing in the first place.
     
    agedhorse likes this.
  9. chaosMK

    chaosMK

    May 26, 2005
    Albuquerque, NM
    Too much hip thrust
    You can put 6550/88's in most amps which run 6L6, if the tubes are correct spec. In the case of Mesa amps it would be the spec matching Mesa's tested stock 6L6 tubes... which fall in the median range for 6L6 (their process of selection is to discard the tubes which fall too far outside of median range). The fixed bias on Mesa amps is fixed for use with the most common range for 6L6. I think it is around 35-40mA. Basically you want to use 6550/88's that test like 6L6's, which is very low.

    I use 6550s in my Bass 400 (granted, they say it was designed for it) and also use 6L6/6550 combo in my Dual Rectifier. The benefits... it's not so much about dramatic increases of power output although 6550/88's should make the amp sound bigger (they will not make it sound smaller)... mainly more pushiness to the tone and deeper depth. 6550s are sometimes a flatter-response tube and can add more mids that you wont hear out of 6L6 as much, KT88's (JJ's anyway) seem to have less of a flat response and add low end. Breakup and power section saturation is also different than 6L6 if you go there. Overall the 6550's add a bulldozer element to my Recto, which is nice because of how low I tune (F#).

    IMO the Buster is loud enough as is but I have a thing for KT88's. If I still had Buster I'd drop some of my spec'd 6550 tubes in and see what happens/if it starts to smell.
     
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Respectfully, I'm going to use this post as an example of why EVERYBODY needs to be aware of the potential for bad information on the internet, even if it's well intentioned.

    I can't think of a potentially more destructive suggestion than what's presented here. The Buster was designed for 6L6 class output tubes which have a rated heater current of 900mA (that's 0.9 amps for the math challenged). Both the 6550 and the KT-88 have a rated heater current of 1.6 amps. Doing the math:
    6 x 6L6, would be 6 x 0.9A = 5.4 amps heater current
    6 x 6550/KT88 would be 6 x 1.6A = 9.6 amps

    The 9.6 amps exceeds the safe heater winding capacity by a wide margin and will result in decreased power transformer life or simply transformer failure. Since this is an expensive part, it is prudent to understand what's behind the technology before making such suggestions or recommendations. This one is simply wrong.
     
  11. BassDuex

    BassDuex Supporting Member

    Jul 15, 2006
    Placerville, CA
    The main reason I was looking at at tube swap, etc was I just can’t get enough clean headroom.
    I’ve got a standard jj 12ax7 in V1, and haven’t tried substations yet.
    Had a rehearsal yesterday and tried running my Avalon U5 straight into the power amp - sounded amazing, didn’t even have to turn the master volume past 45%.
     
  12. Red Planet

    Red Planet Inactive

    May 29, 2005
    Atlanta
    I'm cranky in my old age.

    I think you need to make the Buster your bedroom amp and find you a good used GK 700RB the shear power alone (plus the really good tone) will turn you into a cookie monster. :)
     
  13. Wasnex

    Wasnex

    Dec 25, 2011
    I have seen a few post that claim Mesa Tech Support says says the Buster is safe with 6550s and KT88s.

    I was under the impression that the Buster used the same PT as the 400+. If this is the case the amp will have plenty of capacity on the heater tap. Can't say there will necessarily be a jump in power from running 6550s or KT88s as I thought the OT was from the Triple Rectifier, which I believe is a 150W PT. So maybe it's not a good idea to bias the tubes to a 35W or 42W dissipation spec regardless of what the PT can handle.

    I would be curious to try the 7581A the OP proposed, which is a 35W 6L6 sub. But I would continue biasing the amp as if it were still running 30W tubes.
     
  14. BassmanPaul

    BassmanPaul Inactive

    In my book this comes under the “If it ain’t broke - don’t fix it” heading. :D
     
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  15. Wasnex

    Wasnex

    Dec 25, 2011
    Agreed, but if the amp needs new tubes I consider it broke. In this case I don't think the amp needs new tubes though.
     
  16. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    If the head was eating tubes
    Then 7581 could slightly help.

    But in reality a good testing set of Mesa Factory 6L6 tubes usually can handle alot of abuse. Give old school 7581 run for the money.
     
  17. Red Planet

    Red Planet Inactive

    May 29, 2005
    Atlanta
    I'm cranky in my old age.

    So just what is a set of Mesa Factory 6L6's tubes? Made by Mesa?
     
    Last edited: Jun 22, 2019
  18. Wasnex

    Wasnex

    Dec 25, 2011
    Mesa buys tubes and then tests, grades, and relabels them.
     
  19. Wasnex

    Wasnex

    Dec 25, 2011
    I think the existing tubes are fine...here's the current thread Best way to send Avalon U5 output to 2 separate bass amps
     
  20. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    They are actually 6L6 (we don't short-change customers by selling them 6L5's ;) )

    They are tubes that are manufactured, tested (different tests for different tube types), aged and screened then matched so that the tube grading number (color) is guaranteed to work in the amp with the proper factory bias current if you match the grade number (color).

    Unlike most tube vendors, all Mesa tubes come with a 6 month warranty rather than 90 days, and if you buy a new amp you get a voucher for your first replacement tube set at (I believe) 50% off. Using factory guaranteed tubes really cuts down on the failures that we see in the service dept. We don't cover the cost of repair if the failure is due to non-factory tube issues.

    An amazing number of tubes don't pass all the tests and fall out, first by the vendor's initial testing and then with follow-up testing. These tubes get returned to the various manufacturers and end up getting resold to those with less demanding applications or criteria. Many of them likely end up on the tube re-seller's market.
     
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