2W High-Gain build into an old SS combo

Discussion in 'Shock Brother's DIY Amps' started by Badside, Feb 28, 2020.

  1. Badside

    Badside Tele-Meister

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    Location:
    Montreal, Qc, Canada
    So, there's this old Beckemer combo in my basement. It's... not doing well. But it's got a nicely sized (15x9x2.5") thick chassis that would have no trouble holding good iron, and a surprisingly good 10" speaker.

    Intention is to keep the front panel unchanced, meaning working with the existing control layout (still replacing the pots though since they are PCB mounted). It's a 2-channel amp.

    Controls are:
    - Level 1 = Clean volume basically
    - Channel Switch (between Clean and Distortion)
    - Drive = Gain for the Distorted side
    - Level 2 = Volume for the Distorted Side
    - "Contour" = Not sure what it does, but I'll reuse it as a backward Mid control
    - "Bite" switch = makes it more aggressive, so I'll just make it a NFB defeat switch
    - Bass and Treble = Self explanatory

    So therefore I started researching, and I found my inspiration in the Marshall JCM1 (see here: https://guitar-gear.ru/forum/index....attach&attach_rel_module=post&attach_id=40985)

    Except I want one more gain stage than this, because I'm replicating a preamp topology I built last year and that I really like: Marshallesque input stage into non-bypassed warm-biased stage into cold clipper into DC-coupled cathode follower feeding a tone stack with some tweaked values (not as mid strong as Marshall, but not as scooped as Fender).

    So I'm adding a 4th tube which gives me my extra gain stage, and with the extra triode to work with I'm going back to a tube cathode follower instead of the MOSFET. Then a Cathodyne splitter into an ECC99 power amp.

    The NFB is interesting here, it's obtained by wiring the output transformer in reverse polarity, so that you can feed the output back into the input of the Cathodyne. Good old swap the blue and brown wires. This is exactly as Marshall did it.

    The Clean mode will be very early Plexi like. The Distorted mode will be like a JCM800 with a Tubescreamer in front.

    Transformers are chosen based on... I already have them. The OT is a 18W transformer (Hammond 1750PA), so running the 8 ohm speaker into the 4 ohm tap gives me a 16.8K primary impedance, which is right on track.

    Anyway, here's where I'm at so far
     

    Attached Files:

    BB and Ditchgator like this.
  2. Badside

    Badside Tele-Meister

    Posts:
    379
    Joined:
    Jul 5, 2011
    Location:
    Montreal, Qc, Canada
    Working on the layout. Since I'm reusing a preamp topology I've already built, I figured I'd reuse the layout since that amp is dead quiet despite having SLO levels of distortion. It uses a Hoffmanesque approach with B+ and Ground rails at the top. I save space (and money) by using radial filter caps.

    Past the Cathode Follower though, it's a brand new design since that other amp then has a MOSFET FX Loop, a tube recovery stage and LTPI. Instead here, I go straight to a Cathodyne PI.

    So I decided to split this into 3 boards: Preamp, PI and Power amp.
    Cause I'm always changing my mind! This way, I can insert an FX Loop if I so choose, I can also move to EL84 or 6V6 output without ending up with a butchered board like I often do.

    I've added a load resistor and a Slave output (repurposing the Headphone out, and using the Reverb control as a Send Level). My plan for this amp is to not mic it but instead slave it out into my HX Stomp for "post amp" effects and IR cab sim into the PA. This way I can run the power amp as hard as I want and still get clean delays and reverb and a lead boost.

    The layout looks more complicated than it really is because of the channel switch. It's a 3PDT, has to be. First pole (from the right) selects what feeds into the 4th stage: the Level 1 output (taken after the 1st stage), or the output of the 3rd stage (aka, the "Clean" mode bypasses stages 2 and 3). 2nd pole (middle), grounds the input of the 3rd stage when in clean mode (else you'll hear some distorted sound behind your clean, not sure if it's because a 12AX7 envelope is being shared, either way it's quieter when doing this). And 3rd pole bypasses "Level 2" in clean mode (allows you to balance the volume of both channels).

    Again, already built this preamp in a 20W version and it's super quiet so I'm confident in this layout. HOWEVER, that version had relays for channel switching, which allowed me to distribute the switching closer to the affected circuits. That said... if I like this amp, I'll probably end up adding relays anyway (in which case the front panel switch controls the relays in absence of a footswitch). Just want to keep things simple at first.

    This is a work in progress of course. Not a build ready layout!
     

    Attached Files:

    Last edited: Mar 2, 2020
    FRANKENJAG and FenderLover like this.
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