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DIY Tube Amp - 5F6A "Bassman" with 6550's

Discussion in 'Amps and Cabs [BG]' started by priesty, Oct 25, 2009.


  1. Here's one for you all, my 1st attempt at building a bass amplifier, a Bassman 5F6A copy with Dual GZ34 Rectifiers and twin 6550 output tubes:

    I've seen plenty of guff on the internet that a bassman with twin 6550's isn't worth it, I beg to differ! 60 to 70 Watts of bass valve power!!!
     

    Attached Files:

  2. rbonner

    rbonner

    Sep 25, 2008
    That looks pretty, lets see the inside.
     
  3. Coelho

    Coelho

    May 10, 2006
    Astoria, NY
    Looks very good!
     
  4. I actually like the idea of a somewhat low wattage tube bass amp, as it would be small enough to crank it to get some grindyness without being unfathomably loud.

    What controls are there?
     
  5. James Hart

    James Hart

    Feb 1, 2002
    toms_river.nj.us
    Endorsing Artist: see profile
    Impressive! I'm about to jump from design to prototyping phase for my first build... I too think a pair of 6550 (or KT88) is plenty.

    Why tube rectified?

    and lets see some gut shots!!!!!
     
  6. scotch

    scotch It's not rocket science! Supporting Member

    Nov 12, 2006
    Albany, NY USA
    Please see Profile for Endorsement disclosures
    Whoa! Nice! More details: where, how pricey, sound...

    I'm guessing trad Bassman tone, but w/ more headroom?
     
  7. Chef

    Chef Moderator Staff Member Supporting Member

    May 23, 2004
    Columbia MO
    Staff Reviewer; Bass Gear Magazine
    If that's a first effort, it's a dang nice job there chief. Hat's off!
     
  8. What is rectified, anyway?
     
  9. scotch

    scotch It's not rocket science! Supporting Member

    Nov 12, 2006
    Albany, NY USA
    Please see Profile for Endorsement disclosures
    The Rectifier converts AC power to DC power to be utilized in different portions of the amps various circuits. Rectifiers are more commonly solid state in modern all-tube bass amps. But, tube rectifiers are decidedly 'old-school' and inherent to the classic Bassman design.
     
  10. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Very nice.
     
  11. Interceptor

    Interceptor Supporting Member

    Mar 29, 2005
    Madison, WI
    That's pretty!

    I happen to like the Fender tone stack. And 6550's. And, it's getting to be the time of the year that us living in the North get busy in the shop.

    Inspirational.
     
  12. Thanks for your comments Gents, I have attached two "internals" photos for your viewing pleasure. Note the salvaged lightning conductor used as an earth bus in photo 1!

    On your questions:
    - I went for Tube rectification as I wanted all components in the signal path to be as genuine as possible to the valve amp ethos. To me putting silicon diodes in would be like putting fuel injection on a scratch built shelby cobra!!
    - The dual GZ34's are required for running the two 6550's, its a design that SUNN amps used a fair bit.
    - The amp has the Bassman "Normal" and "Bright" inputs (which can be bridged with a patch lead to great effect!)
    - Controls are as per Bassman 5F6A - Volume, Volume Bright, Treble, Middle, Bass, Presence.
    - Sounds - Tubey! Iv'e only played through an SVT once, this is similar to my ears, but Bassman like in that it does distort at very high volume (Pre amp distorsion though, might fool around with a 1st stage cathode bypas cap lift switch)
    - Price - Components £500ish, Materials, £80ish, Paint Job (pro powder coat) £40, Tubes, £120ish, total around £750 (approx $1,230 for those in US).
    - Building time circa. 200 manhours.
     

    Attached Files:

  13. scotch

    scotch It's not rocket science! Supporting Member

    Nov 12, 2006
    Albany, NY USA
    Please see Profile for Endorsement disclosures
    Awesome, priesty, just awesome!

    Next time I play in the UK, can I look you up & rent this thing for a show? I'm so intrigued!
     
  14. Beautiful amp, that looks really cool!

    Just out of curiosity, where did you find the wiring diagram for that?
     
  15. See post above, did it myself.
     
  16. scotch

    scotch It's not rocket science! Supporting Member

    Nov 12, 2006
    Albany, NY USA
    Please see Profile for Endorsement disclosures
    Uh-oh! That'll be off the track! :)
     
  17. pgk

    pgk

    Aug 19, 2007
    my god, a man after my own heart! and here i thought i was all by my lonesome in extolling the virtues of an (essentially) bassman with 6550's. it is the BOMB for a two output tube design, and i've been persuing this direction for over 20 years. let people have their mega-watt amps, if one knows what they're doing design wise this is all one needs, with the right cabs to match. looks bee you tee ful. can you explain the cathode cap bypass mod further, and where did you find that chassis? also, since i don't read schematics what percentages of your design are fender sunn and ampeg?
     
  18. Thanks pgk, my thoughts exactly! Who needs 500W never mind 1000W!!!

    With PA’s as good as they are now, just needed something on 60 to 70 watt range to keep up with drums, Ampeg SVTDI and PA does the rest!!

    As for the circuit make up, the input jacks to phase inverter is pure 5F6A Bassman, the power section is Ampeg (of sorts) as it is inspired by 6L6 loaded B-15, but with 6550’s (why did ampeg never do this?). The rectifier section is as per SUNN Sonaro or I-40 amps. My Input is on the Bias Circuit design and the DC heaters, regulated with a LM1084 low drop out regulator.
     
  19. James Hart

    James Hart

    Feb 1, 2002
    toms_river.nj.us
    Endorsing Artist: see profile
    very cool! Mine is turning out to be 20% Hiwatt, 30% Ampeg, 50% Me (as compiled from old known circuits from ancient textbooks and my continuing discovery of the science behind tubes/audio).

    What did you use to create your schematic? I've got a bunch of hand sketched drawings and such... I need to go CAD (or at least photoshop) with the stuff. I worked as an Electrical Draftsman in the late 80s so they aren't too bad, but near clean enough.
     
  20. Just drew it up on MS Word, the draw function is pretty good! What tubes are you using James?
     

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