Bogen E30 Conversion

Discussion in 'Shock Brother's DIY Amps' started by Jesco, May 23, 2020.

  1. Jesco

    Jesco TDPRI Member

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    Hello again, all!

    I bought a corroded, old Bogen E30 with the hopes of giving it a glorious second life. The Bogen E30 is a 6L6, cathode biased, paraphase, dual volume PA amp that has long since ceased to shush the churlish children of some long forgotten, godforsaken middle school.

    The original preamp tubes are two 7B4s (6SF5=½ of a 12AX7), one 7F7 (6SL7=12AX7), and one 7N7 (6NL7=12AU7).

    Because of the sever corrosion, I gutted the amp and will only be using the transformers, chassis, tube sockets, and tubes (possibly). The transformers both checked out good, thankfully. I don't know if I will use the choke, it is so heavy already.

    My question for the amazing TDPRI community:

    I was thinking about turning this into a 5E5a circuit. However, I'm don't know if I can expect pleasant results if I build it using octals tubes. If I swap the tubes to Octal equivalents to the 5E5a's two 12AY7, one 12AX7 compliment, can I just build it as is? Or would I have to draw out the load lines for each tube and essentially redesign this?

    More fun: If you were given the option to build a 6L6 amp with octal equivalents of 2 12AX7s and a 12AU7, what would you build?

    Or should I forget octals and convert to 9 pin?

    Here are some pics:
    Original Flavor. I plan to learn to screen print on metal for this. I will try to recreate the front panel's art deco style. Something close. Next up is painting the chassis and planning the layout.
    Bogen E30 Chassis 1.jpg Bogen E30 Chassis 2.JPG Bogen E30 Chassis 3.jpg Bogen E30 Chassis Gut OG 1 .JPG Bogen E30 Chassis Gut OG 2.JPG Bogen E30 Chassis Gut Clean 1.JPG Bogen E30 Chassis Gut Clean 2.JPG
     
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  2. tubegeek

    tubegeek Tele-Afflicted

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    "Bueller? Bueller?"

    Nice wire brush work - that thing didn't look so good at first.

    Loktals +sockets=octals

    Octals > novals

    Metal octals > chocolate ice cream*

    *top end of the scale
     
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  3. Jesco

    Jesco TDPRI Member

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    So your vote is for octals!

    I'm still concerned about just plugging them into a Fender circuit designed for novals.
     
  4. tubegeek

    tubegeek Tele-Afflicted

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    You may need to adjust the plate & cathode R's a bit depending on what you use where. There are folks you can ask.
    Q: the 7xxx tubes are not octals, they are Loktals. Can you put octals directly into a Loktal socket? I know it doesn't go the other way but I'm not sure about this one.
     
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  5. The Ballzz

    The Ballzz Tele-Afflicted

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    Last edited: May 24, 2020
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  6. Jesco

    Jesco TDPRI Member

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    Yes, you are correct: Loktals. I plan on probably using the Loktals that came with the amp.
     
  7. Jesco

    Jesco TDPRI Member

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    The planning stage is done, for now.

    I have decided on a 5E5A circuit using three 7F7 (70 mu: rough equivalent 6SL7/12AX7) tubes. I plan on implementing the following mods:

    1. Implement 3 way NFB instead of presence pot
    2. Only 2 inputs, no low input on either channel
    3. Jumper Switch to jumper 1 and 2
    4. Cascade switch to cascade Channel 1 into 2, probably on a on/off/on with the jumper switch
    5. Fixed and Cathode Bias switch
    6. Phase Inverter Grid Stop Resistor
    7. PI Cathode heater elevation
    8. No Standby

    I weathered Mouser and now must go back to Blencowe's chapter on channel switching. I want to figure out how to make the switching quiet.
     
  8. Jesco

    Jesco TDPRI Member

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    So a new skill: DIYLC. It took me about 6-8 to get it downloaded and working on my High Sierra Macbook. I was determined.

    The tubes are set up in the chassis in a way that made using a normal layout impossible without super long runs from the board to the controls, which are mounted outside the chassis. I came up with a pretty interesting new layout using the Mojotone Turret 100 1 3/4" boards. Essentially, all the signal wires are routed from the tubes to the control on one board while the power and ground wires are routed on another board placed at the opposite side of the tubes. The layout of the Loktal socket made this rather convenient. There are almost signal wires crossing power wires in the preamp circuit. Maybe I'm on to something, or maybe it will be a disaster.

    Anywho, if you're interested, have a look. Feel free to harshly criticize or kindly point out any obvious errors I made. I'm not expecting anyone to comb through the design, obviously.

    Bogen E30 to 5E5A Layout.png
     

    Attached Files:

    RollingBender likes this.
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