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5E7 build please help

Discussion in 'Shock Brother's DIY Amps' started by Doctormo4, Mar 12, 2018.

  1. Inventour

    Inventour Tele-Holic

    740
    Feb 10, 2011
    USA
    I would replace the 1.5k screen grid resistors usingvthe 1/2 watt rating. Those are too big.

    Here is my 59 Pro, its the same as your build except a few resistors.

    Make sure you install the outside foil just like my original. If your tone/ coupling caps ate not marked for outside foil, you need to test and mark them.

    Some solder joints are not shiny, not enough heat, may cause a cold joint. You have massive 2 watt resistors in places that should be 1/2 watt.

    Use my amp as a guide.

    [​IMG]


    [​IMG]

    [​IMG]

    [​IMG]
     
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  2. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Good ideas, gentlemen, but is it likely that either the choke location or resistor size are causing the OP's weak sound output?

    5e7 choke size and location are complex, but unless (maybe??) we have the 200mA choke wired *after* the first plate node (where a 90mA is appropriate) I'm not sure it would produce the weak sound the OP describes. There's another 5e7 thread right now where folks talk about choke size and location, and it made me go back to school on the topic. I found this old TGP thread with ideas from Jeff Gehring and Mark Norwine (long-time amp gurus) who discuss the issue pretty thoroughly.

    I'm not sure... do 1W or even 2W resistors actually hurt an amp circuit? I seem to recall Merlin even suggesting that if you use CC resistors you want to upsize in hiss-prone slots.

    Cold solder joints are of course a potential suspect.

    Doctormo4, I’ll be interested to see what folks think about your voltages and wiring photos. Disclosure: I’ve built a couple amps that had weak, odd tone at startup, so I’ve read a lot of troubleshooting threads about that.

    Usual suspects include input jack wiring, pot wiring, preamp grounding, overall ground scheme, resistor values (470k instead of 470, etc.), output jack wiring, and (sadly) about a million other issues further down the list.

    If this was built from a Weber kit, their layouts can be quirky. Going back to the Fender schematic and layout like you did is generally helpful, but they contain quite a few areas (heater wiring, as you know, bias circuit, AC power, etc.) that you have to modernize.

    The 'pop test' is helpful, done right, but what really helped me was to install a free signal generator app on my phone, sacrifice a cheap mini-stereo output cable to connect to an alligator clip, and then use another amp as a 'signal probe' at different points down the signal chain. Maybe someone here can supply more specific instructions.
     
    Last edited: Mar 14, 2018
    D'tar likes this.
  3. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    With all due respect to that 59 Pro (which is gorgeous BTW), I don't think the OP's problems are caused by resistor power rating or coupling cap foil orientation. At most those would be noise floor issues, but probably would make no difference at all, and if anything the higher power rating for a screen grid resistor is preferred for a modern build.

    A cold solder joint is a good point though. Based on the OP's pictures either a bad joint or an overheated component seems like a good possibility.
     
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  4. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    the 500 volts on pin 7 of V3 is the voltage that stands out as wrong.

    looks to be the grid of the cathodyne phase inverter.

    if that is highly positive, a large signal would go to the power tubes and could explain the red plating.



    its hard to understand how it (500) is higher than the B+. I would track that down.
     
  5. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    doctormo4, I appreciate your efforts on the voltage chart. It would be easier to read and work with if it were rotated 90 degrees counterclockwise.
    IF that voltage on V3, P7 is really 500vdc, then yes that is a problem. Check your cathode bias resistor and connections on V3/P3. follow the voltage supply back to the power rail through that 100K power resistor. Check the power rail. There should only be 332vdc per the schematic on the power rail side of that 100K power resistor....and 213vdc after the drop from that 100K power resistor. IF the first triode in V3 is not conducting current, then that plate voltage coming off of V#,P1 enroute to that V3,P7 will be high.....but how it gets to be 500vdc I don't know.
    +1 on solder connections. There are a few other things that bother me about what I see...in particular I don't like the frayed cloth on the wiring. Cloth wire should not be frayed back. The wire should be cut to length and the cloth pulled back for soldering at the joint. When released, the cloth should then move toward the solder joint and provide some insulation rather that leaving a bare wire.
    The pictures of the tube socket areas are out of focus and not conducive to analysis. IN particular, I would like to see a better picture of V2.
    upload_2018-3-14_10-55-44.jpeg

    .....but all of the tube sockets are of interest.
    The connections to the power rail shown here are less than desirable, imho.
    upload_2018-3-14_10-55-0.jpeg
    I suppose being a doctor you had easy access to that bandaging tape...the beige colored stuff???
    (;^)
    upload_2018-3-14_10-57-21.jpeg
     
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  6. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Age:
    34
    Sep 10, 2013
    Georgia
    Ironically, I've had the same type issue on a Super I built. The bad thing about amps is they aren't go/no-go. They can still "work" when something is wrong, just not well, and that can almost make things harder to diagnose. The one I built made sound, the volumes & tone knobs worked like they should, the overdrive even kicked in like it should on the dial. But the output was less than a Champ, maybe 50% of a Champ. Definitely not a 6L6 2x10 Fender.

    After nearly 2 weeks of trouble shooting, I finally found it with the continuity tester on my meter. It's been a couple years, but I had accidentally mixed up the leads on pins 2 & 3 (I think) on V2. Anyway, it took checking the continuity against every connection to that point (I wish I had started from the input side instead of output...) against the layout until I finally found a point that didn't check out. Swapped them around and it worked like a charm.
     
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  7. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    So you spend months reading and learning about tube amp circuits. Then you wire up your first cathode follower. Wuuut!!! Back to the drawing board!:)

    Thats a beauty there!

    Dr....

    That's a lot to chew on and a bunch of good info/suggestions..

    BTW what's on the other end of the OT? The original schematic has three speakers. I think you are using the 4ohm tap if I remember correctly.

    On your first set of voltages your phase inverter readings looked OK. Hopefully your 500v is an error.

    And the red and white striped wires from the p/t that are cut real close to the housing, you may try to add some heat shrink to close those up.
     
  8. jmp81sc

    jmp81sc Tele-Meister

    221
    Feb 6, 2015
    Orange County
    Maybe not relevant to your problems, but your grounding solder joints look cold to me. You may consider grounding to a lug bolted to the chassis at your PT side, and running a ground bus wire for your preamp side. Fender did not do it this way, but it seems to be the preferred method on this board.

    Good luck
    John
     
    Nickfl likes this.
  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I see one grounding issue just from the chart. V1 pin 7 should read very close to zero, not 6. That would explain your weird volume control interactions. That points to an issue in the bright channel input jack wiring, especially around the 1M grid leak resistor. Now, if there's one there's probably more, especially since there's a few noted dull looking joints. Getting solid solder joints to ground via pot casings takes a strong iron and patience, which is part of why several of us have switched to the ground bus method - it's simple to implement and reliable.

    You'll need your amp unplugged with the filter caps drained to zero - in other words, safe to reach into. You can use a specially built jumper lead w/ resistor (333k/3W works well) or simply monitor voltage until the caps go to zero. I prefer the jumper. Use the resistance function of your meter, clip one probe to chassis, and verify every ground called out on the layout reads zero ohms to chassis, or as close as your meter can show.
     
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  10. Doctormo4

    Doctormo4 TDPRI Member

    Age:
    61
    11
    Mar 12, 2018
    Pittsburgh Pa.
    Thanks for the critique and advise. I am going to re-explore this weekend when I have a little time
     
  11. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Let us know. We're rooting for you.

    BTW, if that pink stuff is Coban, I'll ignore the whole 'hospital duct tape' vibe and just suggest the adhesive on that stuff gets real old real fast. Your local hardware store will sell you a bag of nylon cable ties in useful sizes for next to nothing... :)
     
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