The MarPegDer (Clean + Dirty Tube Amp Design & Build Log)

Discussion in 'Amps and Cabs [BG]' started by christw, Sep 16, 2016.

  1. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    Howdy folks, I am still as crazy as ever and this project aims to prove it!

    The concept behind this design is pure self indulgence. It is based upon some of my favorite amplifiers and several designs I've been kicking around that never made it to fruition. It will feature two very different preamplifiers for the explicit purpose of blending some of my favorite clean and dirty tones together. They will then be mated to a sweet spot 100-130w tube power amp.

    The design will evolve and be refined over the next few days. The following is a rough outline of what I plan to do.

    Preamp 1: Ampeg 6SL7 Baxandall V/B/T

    -This will be a typical Ampeg B15N preamp
    -Feeds into a cathode follower and a master volume

    Preamp 2: FMV 2x 12AX7 stack V/G/B/M/T

    -This will be a modified FMV style preamp (some specifics still TBD)
    -A second gain stage will be added before the tone stack
    -Again, this will feed into a cathode follower and a master volume

    Channel "Switching"
    -I am considering using a simple two button channel on/off relay setup
    -Powered from from the filament tap
    -More TBD

    Output section
    -V4 based
    -4x 6550
    -More TBD

    Power Supply
    -V4 based

    I have at my disposal:

    -A large power transformer capable of supporting well over 100 watts of output with tube rectification(!!!!)
    -A large output transformer capable of handling 120w RMS, maybe more.
    -Plenty of miscellaneous supplies (sockets, jacks, capacitors, switches, etc)
    -A large amp box and chassis from a crappy solid state amp given to me by the shop I work for part time

    Next up?
    -draw up preamplifier schematics
    -draw up power supply schematics
    -draw up output section schematics
    Last edited: Sep 17, 2016
    grisezd, wyrtti, TC.65 and 3 others like this.
  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Woo hoo! Sounds like a party!
    christw likes this.
  3. Martin Beer

    Martin Beer

    Dec 4, 2004
    Sounds interesting, I'll be watching this thread. The valve rectifiers are an unusual choice in an amp this size, but if you prefer that then why not?
    christw likes this.
  4. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I had wanted to do tube rectifiers because this is my last big transformer with the 5V taps. Realistically, I should probably just avoid them on a bass amp of this size. I don't think I'll have enough real estate to fit them in anyways.
  5. BassmanPaul

    BassmanPaul Inactive

    I think you’d be better served by running a quad of 6550 in place of the 6L6GCs.
    spankdaplank and christw like this.
  6. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I'd have to agree. :D

    I made some updates above. First draft of schematics were drawn up this afternoon but there are revisions to be made. I drew up several variations of the overdrive channel. I can't decide if I want to do a typical cathode follower into the tone stack or a parallel triode like my Univox had. I really liked the tones I got out of that particular amp.

    I think my output and power supply will be based on the V4 series Ampegs.
    TC.65 likes this.
  7. Passinwind

    Passinwind I know nothing.

    How about using both, SS for the mains and a small tOOb recto for the preamp?
    spankdaplank and deathness like this.
  8. rufus.K


    Oct 18, 2015
  9. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I would have never thought of that! Neat idea... I like it.
    Passinwind likes this.
  10. Passinwind

    Passinwind I know nothing.

    Not sure how much work it'd take to get enough sag to be useful, but I've been kicking around the idea for a while.

    Sounds like a fun project in any case Chris.
    Last edited: Sep 17, 2016
  11. CatSquare

    CatSquare Guest

    Mar 7, 2014
    Sounds like an amp I would want to play, eager to hear this puppy when you've finished!
    TC.65 likes this.
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    What is the primary impedance of the output transformer. This might indicate what the output tube compliment should be (type and number)
  13. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    the classic Soldano Slo 100 preamp is a very good classic high gain implementation of the fender style tone stack and extra gain stages for distortion/overdrive. the classic cathode followers of course have the benefit of a low output impedance but it's a well known factor for overdrive circuits that a
    gain recovery stage or pregain stage with cathode followers produces a different/ better overdrive waveform. and no it's not the mythical asymmetric distortion that's claimed to be tube magic. it's actually more symmetrical distortion. anyways the slo preamp used a lot of highpass filtering for smoother distortion. and the values should lowered for bass use. but highpass filtering is still essential for bass distortion. Likewise opinions vary on using extremely high voltage on phase spliters to reduce them from being overdriven or using lower voltages and allow them to be overdriven. and the classic " blackface" phase splinters actually had pretty poorly matched gain stages so many find them more pleasing for distortion cause they cause the output stage to distort more asymmetrical. since the actual crunch or better sounding output stage distortion on a class
    a/b amplifier is symmetrical as well. and kinda puts a end to the mythical tube magic asymmetrical distortion. but of course using the horrible blackface style phase spliter will change the symmetrical distortion to more asymmetrical which I personally find buttery n boring. but when fender was bought out by cbs and they updated the bassman the bassman 135 has a much more accurate and textbook perfect phase spliter and If I remember correctly runs the splitter at very high voltages above what the data sheet gives as max. which makes it much better for overhead. likewise aged horse is correct and curious what
    ratio your output tranny is. maybe you have eenough to do 6x6L6 instead of a quad 6550 to do something with balls wattage wise. but still breakup like 6l6 100 watt bassman and to just say I did it for no real reason. Likewise the rectifier heater tap will run slightly higher voltage if it isn't loaded down to
    3 amps and could run two 1 amp heaters for the extra 2 6l6 output tubes
    Last edited: Sep 18, 2016
  14. BassmanPaul

    BassmanPaul Inactive

    Agreed but if the OPT has not yet been purchased 6550s are a better option over the 6L6GCs.
  15. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    I thought he already had one, maybe I didn't read the OP correctly though?
  16. BassmanPaul

    BassmanPaul Inactive

    You are indeed correct Andy, the OP has an OPT at his disposal.
  17. charlie monroe

    charlie monroe Gold Supporting Member

    Feb 14, 2011
    Buffalo, NY
    No hair pic?!?!
  18. cfsporn


    Aug 20, 2011
    New York City
  19. beans-on-toast


    Aug 7, 2008
    Specifying the design is important.

    Although budget is always an issue, try avoid using old recycled power supply caps. Use modern caps, they are way better than the old cap cans that so many use. I like Nichicon snap in or equivalent.

    If you want a tube rectifier for the pre, consider a switch between tube and solid state rectifier. An alternative is to build it for a tube rectifier, then you'll have the option of using a solid state rectifier module in the tube socket. But a 100w amp or larger as you are proposing will require solid state.

    Any chance you'll want to include a separate high impedance input to handle piezo pickups.

    Do you want a mid boost or cut in the tone stage. Cut is easier than boost. Consider a bypassable tone stage (relays or switches).

    Make sure that the power transformer meets the design goals. Nothing worse having to drop voltage that is too high for the first stage.

    Layout and grounding is very important. Don't tackle that as an afterthought, it should be part of the initial design. Think about tube proximity to the transformers.

    Lots of power tube bias options. Do you want adjustable bias? Do you want dual bias for push and pull sides, individual bias for each power tube, or a single bias? Build in a way of measuring cathode current.

    Are ferrite beads instead of plate resistors a good idea? Ampeg does this in all their modern SVT's (digikey 490-10999-1-ND for example). Consider if this is good or bad. Is high frequency oscillation supression good/bad/necessary to build into the design.

    Consider a DC heater design for the preamp to minimize noise.

    Is this a bass or guitar amp, or intended for both. If both, are different tone stage voicings worth considering.

    Will there be a standby switch? There are different ways of doing this. Will the power tube bias be energized when the amp is in standby. Is this necessary? A separate bias supply power transformer can be used if you want this feature.

    There are so many options and tradeoffs to consider. Things can get complicated fast.
    Last edited: Sep 20, 2016
    BassmanPaul likes this.
  20. rufus.K


    Oct 18, 2015
    :speechless: some people plan before they build?! :whistle: