Another Parts-On-Hand build... 6V6 Spitfire Reverb

Discussion in 'Shock Brother's DIY Amps' started by RollingBender, Aug 3, 2019.

  1. RollingBender

    RollingBender Tele-Afflicted

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    Since I’m trying to do a parts-on-hand build, I’m using a rotary switch I pulled out of something. It’s a 4 position switch that I will be using for the speaker impedance selector.
    2E1A3E52-EC63-4C81-B690-678A41D46FC6.jpeg

    My OT has 3 taps, the switch has 4 positions. Not a good idea to flip it to #4 and have nothing connected there. What could I hook up to that 4th connection to keep the OT safe should someone flip it to that position?
     
  2. FRANKENJAG

    FRANKENJAG TDPRI Member

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    Dummy load?
     
  3. RollingBender

    RollingBender Tele-Afflicted

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    That’s what I was thinking. It would be great if it did something cool but a dummy load would serve a purpose.

    Got the turrets installed and most of the parts loaded and soldered. Missing two 22uf caps that I don’t have. How close is close enough? I have some “nearby” values.
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    Start putting on the jumper leads next.
     
  4. RollingBender

    RollingBender Tele-Afflicted

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    Got the jumpers soldered to the board, heaters and power section wired, board installed and have begun work on connecting the jumpers.

    C4C4A0DE-72FA-497A-A5C7-9199E41D186B.jpeg
     
  5. RollingBender

    RollingBender Tele-Afflicted

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    Most of the wiring done but still waiting for those 2 missing caps.

    I’ll be tracing the schematic against what I have done a couple more times to make sure I have it all wired correctly and then I will start going through the startup procedure.
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  6. RollingBender

    RollingBender Tele-Afflicted

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    It’s alive!!!

    After some confusion of pins on the power tubes (converting an EL84 design to 6V6), I got it sorted out and fired up and most everything works. The volume control (Master) is acting a bit wonky but I think I can figure it out.

    I played it for about an hour tonight at pretty good volume. It’s definitely not the same amp as the Spitfire clone I built earlier even though the circuit is almost identical. This one is way more dirty yet it stays pretty tight. As I overdrive it towards max, it starts getting a little boomy but it doesn’t fart out. The reverb is a pretty weak but I want to do a thorough test of voltages before I start tearing into that. I need to run a bias check next.
     
  7. RollingBender

    RollingBender Tele-Afflicted

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    As first assembled, the bias was running WAY hot. I changed to a higher value cathode resistor and got that under control. Doing that also helped the reverb a bit.

    The main issue with the volume (post PI Master) was the pot wired wrong...I have the pots in a different order than my Spitfire clone and was looking at the wrong pot on my layout diagram.

    Since this is a cathode biased 6V6 push/pull, I went looking at RobRob’s 5E3 mid page and clipped in a negative feedback resistor. That helped tremendously with clean headroom and also helped with the reverb.

    A/B comparison with my Spitfire clone reveals very similar tones though the 6V6 version is a bit brighter and has smoother breakup. Gain cranked and master set to desired volume produces a tone reminiscent of a JTM45!

    Cabinet build has begun...
     
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  8. RollingBender

    RollingBender Tele-Afflicted

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    I guess the theme of this build has become doing everything twice. What I thought was going to be a perfectly dimensioned cabinet is not meant to be so. The speaker interferes with the OT.
    436424AD-D76D-4761-9639-511F77F468FB.jpeg

    I’ll be building a new box 1-1/4” taller and maybe just a bit wider too so I can maintain some proportion like what I already have.

    All is not in vain, however. This box is the same width and depth as a 5E3 cabinet and only 3/8” taller. I need to make another Deluxe cab anyway so I’m already part way there.
     
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  9. RollingBender

    RollingBender Tele-Afflicted

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    Been putting in a fair amount of time shaking this thing down and doing a few tweaks to get things working at optimal level.

    I have a bit of hum in the reverb that is completely reverb level dependent (zero at zero, worst at 100%). The ground for the reverb circuit runs to the power section ground and the pre-amp ground is separate. Suppose splitting the reverb ground off the power ground and tying it directly to the star ground would help this?
     
  10. RollingBender

    RollingBender Tele-Afflicted

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    I found a way to salvage the cabinet. A few more tweaks to the box design and then I can glue it together and finish it somehow.

    Here’s a cellphone video of what it sounds like so far...

     
  11. zook

    zook Friend of Leo's

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    Connect pin 3 and pin 4 so both will reflect the 16 ohm tap.
     
  12. dunner84

    dunner84 Tele-Meister

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    I'm curious if you modified the PI to be more 6V6 friendly, or did you keep it stock?

    I recently built a spitfire/lightning, and I will be converting it to 6v6 with switchable cathode/fixed bias, so I'm curious what you did there.
     
  13. RollingBender

    RollingBender Tele-Afflicted

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    I left the architecture from the original Spitfire schematic as it was but changed some values of components.

    Here's what I have right now as-built...

    6V6 Spitfire Reverb.png
     
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