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Thread: Rebuilding Dad's amp

  1. #1
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    Rebuilding Dad's amp

    This is my father’s amp. I’m not sure if he built it in the 30s or 40s. I haven’t been able to find a data sheet for the power tubes (6B5). What I know is they are dual triodes with the cathode of the first direct coupled to the grid of the second. This makes them self biasing, although he used a bias resistor.

    The preamp and PI tubes (6SC7) have a unipotential cathode. That is a new term to me I don’t know if it’s one cathode with a plate and grid on either side or two cathodes connected internally.

    First I replaced the filter caps in the PS and brought the B+ voltage up from 238 to 480.

    I traced the schematic and started with a plan to just replace some caps, but found a lot of resistors out of tolerance. The 50K resistor in the PS, measures 15K. That might explain the 282V on one of the plates in the PI, which exceeds the 250V max. The 105V on the other I assume is due to a leaky coupling cap. I was about to replace the coupling caps when I came to my senses. With this point to point (rats nest) wiring changing one part at a time was pointless and I would have to gut the whole thing and start from scratch.

    The primary on the OT is 8600ohms. The possibility of using the chassis and transformers for a Princeton reverb clone is a possibility for the future. Although the voltage on the PT is a little high for a PR. For now I want to work with the original tubes and transformers.

    My questions are,

    Has anyone seen a tone circuit like this? It works well, but has the limitation of only 4 positions. I’m thinking of perhaps a tone stack between the first and second preamp stages when I’m done.

    The choke seems to be in an odd place. Could it be that the choke along with the 16uf caps and 52K resistors form a 120HZ filter? Or should I move the choke to a conventional location in the circuit.

    The amp sounded pretty good with guitar before and after the caps were replaced. A lot cleaner than I remember it when I last used it in the 90s. The last thing I did was plug in the old Turner mic. That didn’t work but it might be the mic. It hasn’t been used since the 70s.

    I’m open to any suggestions, comments, or smart ass remarks. And thank you in advance for your help.
    Attached Thumbnails Attached Thumbnails schmetic-da.gif   front-view.jpg   top-view.jpg   bottom-view.jpg  

  2. #2
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    Dunno about the other questions, but I found the data sheets for the 6B5 http://frank.pocnet.net/sheets/127/6/6B5.pdf No curves though.

  3. #3
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    Thanks for that, it looks like my voltages are too high. I wonder how it survived this long.

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  5. #5
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    Thanks thats a great resource.

  6. #6
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    Well, that's what I'd call a work of Love.
    Congratulations
    Feel free to ask for any help.

  7. #7
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    Interesting amp. I sure don't see the choke you're talking about.

    I'd add a cap bypassing the 15K R at the bottom of the page. Anything like 10 - 47uF will do fine.

    That tone control is a bit crazy but if it works I'd keep it.

    6SC7 has two triodes with the cathodes connected internally

    I would restore this and then build something else. This thing deserves preservation.

  8. #8
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    I made a few mistakes on that schematic. I corrected them as I took it apart. Right now my files are on dvds waiting for the parts to arrive for my new computer. When I get it assembled I'll post the right schematic, and what I changed when it went back together.

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    I have the corrected schematic now. I kept the power amp and tone circuit as close to original as possible. Replacing everything but the tubes and transformers. I used orange drops in the tone circuit and coupling caps.

    Since I started this project some one else posted a schematic with the choke wired like this one. And my questions were answered there.

    For the preamp I simply made two gain stages with a gain control after each stage. It works good except when I turn the first stage up past 6 it starts to oscillate. At first it makes a nice tremolo effect, but gets out of control as the knob gets to 8 or 9. Putting a scope on the + rail you can see the voltage change with the oscillation. I'm thinking if I replace the 8uf cap in the power supply with a 22uf cap it might eliminate the problem. If you see something I did that makes no sense, or could be improved let me know.

    It sounds good to me, and that is the goal. It even makes a faint crackling sound for the first few minutes that it is on. No problem there it made the same noise when I played 78s through it as a child. Reminders of my youth are not bad.

    As for my ignorance in terminology? When I started reading what madkatb sent me, I wanted an eraser so badly it hurt. We're here to learn, right?
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    You have the same bypass cap/ resistor connected to two cascaded triodes. Use separate caps/resistors for each triode. Oops, just realised that the cathodes are connected internally. You can't make two gain stages with that tube, have to use one with two separate cathodes (6SN7, 6SL7, etc).

  11. #11
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    You actually can do two gain stages with common cathode, either by cathode coupling the second stage or using enough bypass cap on the cathode. OR, you get NFB from the second stage that reduces gain of the first. Which still works, just not the way we're used to.

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    Could you show a quick schematic of that? I know that I inadvertantly connected the ground end of the caps on both sides of a 12ax7 (to make them switchable) and it didn't work out well. Same problem that olive2sing had, oscillation and squealing pig demons.

  13. #13
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    Thanks for your replies. I've been looking for info on cathode coupling, haven't found it yet but will keep looking.

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