Boogie Bias Mod - Help!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by EFrost, Feb 21, 2006.


  1. EFrost

    EFrost Member

    Messages:
    12
    Joined:
    Nov 9, 2005
    Hey all. I decided to open up my Boogie Rectoverb the other day (head version) and suddenly felt the urge to add a pot and resistor in place of the single bias resistor. I've looked high and low, but haven't been able to find any schematics or websites that detail this procedure. So I beseech ye brilliant members of TGP to assist me in my quest - to find the Holy Resistor!

    I've got a couple of snapshots Here of the guts, hopefully they are large and clear enough! I can take more pictures if necessary. Thanks!


    Eric
     
  2. Blue Strat

    Blue Strat Member

    Messages:
    30,013
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Thanks for reminding me why I don't work on Mesas ;)
     
  3. EFrost

    EFrost Member

    Messages:
    12
    Joined:
    Nov 9, 2005
    Hahaha damn! That's pretty much why I was afriad to ask... Why can't Randall Smith just put a bias adjust knob in :horse
     
  4. Blue Strat

    Blue Strat Member

    Messages:
    30,013
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    So he can sell more tubes:)

    In reality, any tube dealer familiar with the Mesa bias range can sell you power tubes that will bias up fine.
     
  5. EFrost

    EFrost Member

    Messages:
    12
    Joined:
    Nov 9, 2005
    I have seen that offered from almost all the major tube vendors, but it's just not the same as biasing it to taste or on the fly or whatever. I have a whole bunch of tubes, but I don't think any are Mesa spec'd, and I'd hate to have them all go to waste.:BITCH
     
  6. Blue Strat

    Blue Strat Member

    Messages:
    30,013
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Since you'll have to measure bias current anyway, get a bias probe (or use whichever method you prefer) and check the tubes you have with the amp "as is". Some may work out for you.
     
  7. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    What Mike said...

    In addition I find there is often not enough negative voltage available to really make an adjustment- especially EL84 "designs". I usually get these right by adding a cathode resistor, thus making them "mixed-bias". (A phrase I coined for a combination of Cathode-Bias and Fixed-Bias.)

    In EL34 and 6L6 examples you can usually find a pair or quartet that work if you spec it all out, assuming you have a large tube inventory with high plate currents. Fortunately I have the stock, knowledge, equipment to do this. A Mesa Consumer is usually nor so lucky.

    For the Consumer, it's just a way to sell Mesa tubes. They are usually Sovtek but testing on the Mesa-marked examples has shown very close matching.

    Don't get me started on Mesa! Everyone eventually buys at least one Mesa, but likely not another! Perhaps too much said...

    -Jon
     
  8. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    I've never even bought one... I happen to think most of them (newer ones anyhow) don't sound so hot...

    definitely too much said!
     
  9. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    Brad- good decision.Sir!

    The last good Mesa was'nt a Mesa at all but a Boogie MkII. (Fortunately I have an early Boogie Princeton Reverb.)
    From this point onward it's a long and tangled tale of Lawyers and I won't go any further without first-hand knowledge.
    I have discussed a couple of design problems with Mesa Engineering and I can't report anything that gives me hope...

    I had a Mesa-EL84-something-or-other that I converted to Mixed-Bias and put up for sale. It sold over a year ago and I haven'y heard any problem.
    since. It was OK sounding for a Mesa, but not a keeper for me.

    Latest Mesa production follows Bessent's 3rd Law which states that "The amount of trouble you will have with an amp is directly related to the number of knobs on it. The more knobs, the more trouble and less satisfaction you will have with it."

    -Jon Bessent
     
  10. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    Jon, it may sound silly, but I judge an amp mostly by

    a) looks

    and

    b) number of knobs.


    The cooler looking the amp is and the fewer knobs it has the more likely i am to be happy with it. Seriously.
     
  11. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    Brad,
    You are follwing Bessent's 3rd Law!

    My favorites have between one and three knobs.

    May The Tone Be WIth You,
    Jon
     
  12. scottywompas

    scottywompas Member

    Messages:
    1,595
    Joined:
    Feb 9, 2006
    Location:
    Sunny California
    I'm a MarkIV owner and a fifteen year boogie user. I've never had a problem with the mesa tubes or the amps. I realize everyone's need for customization and the quest for the perfect tone but mesas are designed without a bias pot for a reason. If that's what you need move on to something else
     
  13. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    reason being: so they can sell more tubes! :D
     
  14. EFrost

    EFrost Member

    Messages:
    12
    Joined:
    Nov 9, 2005
    Ka-Ching!:BEER
     
  15. EFrost

    EFrost Member

    Messages:
    12
    Joined:
    Nov 9, 2005
    If anyone is still interested, I found the bias resistors. They are the 220k ohmers in the red circle: http://i34.photobucket.com/albums/d148/boxedlight/untitled.jpg

    Sorry for the cruddy quality but it's the best I can do. Anyways, I verified that those are in fact the bias resistors. Randall Smith does a neat trick with 4 bars of color on the resistors. I don't know if what he has is common, but the third and fourth bars multiply together to give you the actual value.

    If anybody else owns a Recto and is interested in this mod, it's fairly simple (more simple than I imagined) with a decent soldering gun (25 watt Weller from Home Depot will work). If one doesn't like the mod, and doesn't snip the original resistors, it can be returned stock in no time.

    Eric
     
  16. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    EForst- THANKS for send though I'll have to check as 220K pair are likely isolation/voltage-divider rather than the bias-limit resistor...

    Mike- looking at pics I wonder why I mess with it too! It looks just like I recall, yet I go forth as some poor fool, with big sad puppy-eyes and a sad story, has bought it and I wanna save his ass if I can!
    One day I'm gonna come to my senses and stop going into the money-hole...

    May The Tone Be With Ya'll,
    Jon
     
  17. trdlasvegas

    trdlasvegas Member

    Messages:
    167
    Joined:
    Aug 30, 2005
    Location:
    Las Vegas
    Randall doesn't do any trick with the colors. Those are standard Vishay 1% Metal Film resistors available from Mouser. You read the color code just any other 1 or 2 % resistor. The first three bands translate into numbers, the 4 band is the multiplier, and the fifth band is the tolerance.

    The ones you are refering to are:

    red - 2
    red - 2
    brown - 1
    orange - 1000
    brown - 1%

    221000 ohms or 221K ohms

    -Tony
     
  18. EFrost

    EFrost Member

    Messages:
    12
    Joined:
    Nov 9, 2005

    Blast! Foiled again... So after all that I haven't found the bias resistors?
     
  19. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    As I said earlier- it's hell to find and likely won't be enough bias voltage to do you any real good anyway. My "mixed-bias" mod is still recommended...

    -Jon
     
  20. Beam Tetrode

    Beam Tetrode Member

    Messages:
    1,439
    Joined:
    Nov 16, 2005
    Location:
    The Moon
    I have a couple of questions relevant to this topic...

    1) What is the Mesa spec bias current for a 6L6 in a Dual Recto head?
    2) How does the Bold/Spongy switch affect the bias voltage and current?
     

Share This Page