6V6 JCM800 ?’s

Discussion in 'Shock Brother's DIY Amps' started by ZMurph, Jan 17, 2020.

  1. ZMurph

    ZMurph TDPRI Member

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    Ok. So I redid a lot of the wires and their routing and touched up several solder joints. I also moved the master volume pot over next to the presence pot so everything has a shorter, more direct route. I’m still getting the oscillation. Now that I’ve turned things up a bit, I can tell it is happening on both inputs. It’s just much more noticeable on the high side and worse when the treble is turned up. Here’s some updated pics. Any ideas?

    F3E76356-484D-4EBB-AE19-811139ECDE18.jpeg 037112CA-DE96-4D20-9CCD-9865C941B2F7.jpeg 44CA7A05-6CFD-440E-962E-1E121B9CC9A0.jpeg 38787B6D-3CEF-435E-A4B5-4433C4C0D44E.jpeg
     
  2. ZMurph

    ZMurph TDPRI Member

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    Is it possible that since my voltages were a bit higher to start with, that I need to increase the size of my grid leak resistors?
     
  3. Mongo Park

    Mongo Park Tele-Holic

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    This is on a different topic, I noticed you wound your heater wires nice and tight and have used large gauge wire. Some discussion a while back on the size of heater wires and the bigger they are the harder they are to manage. What I noticed here is your wires are not twisted where they come close to the tubes and close to the tubes is where it really counts. It is also where it it hard to get them twisted right up to the socket pins. This could be something to work on for your next build.
    I hope you sort things out, I am in the process of collecting parts for the same build so I am watching your progress.
    Cheers Ron.
     
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  4. ZMurph

    ZMurph TDPRI Member

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    Actually, I’m going to redo the heater wires too, but I ran out of wire and the stuff I ordered won’t be here until tomorrow. I’ll try to tighten them up around the sockets. Thanks for the advice!
    Ps.: at this point I’ll say I’m not a fan of the cloth covered wire. ;)
     
  5. ZMurph

    ZMurph TDPRI Member

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    So I redid the heater wires (and one or two others) and the oscillating is somewhat better. It’s, I think, almost exclusively associated with the treble pot, or that part of the tone circuit. It’s pretty much not there unless I turn up the treble pot a little. The middle, bass, and presence don’t seem to effect it.

    I tried to do a better job on twisting the heater wires closer to the tube sockets. I also twisted the wires going to the tone controls. Should I not have done this?

    4AA76C7B-0D24-451F-9879-AD0E9961D7F1.jpeg 354F9AAA-DA29-4BA1-ACE9-D5CAD9B3CC8B.jpeg
     
  6. ZMurph

    ZMurph TDPRI Member

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    Ok. I’ve redone some more wiring and lead dress. No improvement. I’m still getting the trebly ghost notes especially with the treble and/or mid controls are up even a tiny bit. When using the neck pickup, the gain is very muddy and woofy almost like a fuzz pedal. Here’s all my measurements at this point:

    I’m using JJ ECC83S for all preamp tubes and some old Marshall branded winged “C” EL34’s from a DSL 100 I used to own.
    Amp warmed up and all controls full down.
    Line voltage in: 122VAC
    B+1: 424VDC
    B+2: 422
    B+3: 362
    B+4: 320
    B+5: 309
    Heater filaments: 6.38VAC
    65V heater elevation: 65.4VDC
    50V bias supply: 52VAC
    after 2.7K resistor and diode: -46.4VDC
    (With the 470 ohm resistor I couldn’t get the bias levels up to a proper amount. Now the bias voltage sweep after the bias trim and balance pot is -52.6 to -31.85VDC)

    ———V1.———V2.———V3.—————V4.————V5.
    1)(239)—–(175.4)—-(233.6)——-1)—(.3mV)——(.3mV)
    2)(.1mV)——( 0 )——-(25.9)———2)—-( Fil )—-( Fil )
    3)(1.863)—-(1.233)—(43.3)———3)—-(421)——(422)
    4)( Fil )———( Fil )——-( Fil )——-4)—(417.5)—-(418.5)
    5)( Fil )———( Fil )——-( Fil )——-5)—(-36.85)—(-36.88)
    6)(268)——-(320)——-(232)———6)—(419.7)——(419.5)
    7)( 0 )——-(175.9)——(27.8)——-7)——( Fil )——-( Fil)
    8)(2.627)—( 177 )——-(43.3)——-8)—-(.3mV )——(.3mV)
    9)( Fil )——( Fil )——( Fil )
     
    Last edited: Jan 30, 2020
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