StewMac/MojoTone 5E3 Volume Fades to Nothing After Five Minutes

Discussion in 'Amp Tech Center' started by eponymous, Jun 14, 2021.

  1. eponymous

    eponymous TDPRI Member

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    Hi everyone. I've read a lot of posts here, but this is my first post. I built one of the Stew Mac '59 Tweed Deluxe kits late last year, and it was working very well, but I let it sit unused for a few months while recovering from a wrist injury.

    Now I find that it works happily for about five minutes after powering it up, then the output volume fades to silence over the course of around 15-30 seconds, and doesn't recover until the amp has been turned off and allowed to cool down.

    I took a bunch of voltage measurements before and after to see what had changed (all measured at the eyelets on the tube socket/speaker output side of the eyelet board, relative to chassis ground):

    B+1: 392V (no change)
    B+2: 348V (no change)
    B+3: 255V before, 302V after
    V2B Cathode: 52V before, 62V after
    V2B Plate: 201V before, 240V after
    V2A Plate: 164V before, 196V after
    V2A Cathode: 1.4V before, 1.7V after
    V1B Plate: 132V before, 300V after
    V1A Plate: 134V before, 300V after
    V1 Cathode (pins 3 and 8 jumpered): 2.1V before, 0V after

    Obviously the biggest changes are around V1, with significant but smaller increases around V2. Chopsticking around the components and solder joints on the eyelet board didn't produce any audible results, but I still get the crunchy noise when touching the multimeter probe to the B+ eyelets.

    Do you have any tips on how to narrow down the problem? I would hardly be surprised if I have one or more solder joints that could do with some attention. I have spare 12AX7/ECC83S tubes on hand, but not a 12AY7, if I should be trying switching out the preamp tubes to see if the behaviour changes.

    Thanks very much.
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    Check you have the 1M grid leak resistors from the two 68k to ground are in the right place.
    From your description, I would say that either pin 2, 7 or both are not connected to ground through the 1M resistors and float more and more negative allowing the 12AY7 to gradually turn off.
     
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  3. eponymous

    eponymous TDPRI Member

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    Thank you, that's very helpful. I think I have the issue sorted out - at least the resistance to ground from pins 2 and 7 of the V1 socket now measure > 1M.

    Some time ago I'd added a pre-phase inverter master volume, with the 1M pot in the place of the Normal Low input. The Normal Hi jack is re-wired as per Rob Robinette's diagram:

    [​IMG]

    I found that if I have that connection between the Ground and Switch tabs on the Normal Hi jack, I only have about 33k resistance between V1 pin 2 and ground, so I've left them disconnected for now. Is that indicative of another problem elsewhere, or is that as it should be? I'm probably only going to use the Normal Hi jack if it's to jumper to the Bright Lo jack.
     
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    About 33K is what would normally be on pin2 when nothing is plugged in.
    I don't understand what is disconnected and/or connected. Was it disconnected before or after the fade issue?

    Another possible for the fade issue would be the heater of V1. If the heater is not heating the amp could react this way. Try a different tube, check solder connections and clean the tube pins.

    Edit: Some heater wires break inside the insulation, so you may want to move those around on your inspection.
     
    Last edited: Jun 14, 2021
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  5. eponymous

    eponymous TDPRI Member

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    That's also really helpful. It looks like the AC voltage between pins 1 and either 4 or 5 will drop from 3.6V to around 0.8V when I move the heater wires, so it looks like I have either a cold solder or a broken wire.

    (By the way, I meant the connection between the Ground and Switch terminals on the Normal Hi input - I've since re-soldered that connection. It was only while re-connecting the 1M resistor on the jack that I'd left that one connection incomplete temporarily.)
     
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  6. Dacious

    Dacious Poster Extraordinaire

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    What tube do you have in V2? That's a cathodyne phase inverter. You need a tough NOS tube or JJ or Shuguang 12AX7 there. Otherwise the grid to plate current overheats and and - bingo. Sound cuts out
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I am sure you meant *between pins 9 and either 4 or 5*.
    The heater wiring could very well be the problem then.
     
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  8. eponymous

    eponymous TDPRI Member

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    Correct. Brain fade on my part. I've reflowed the solder on those pins and the problem persists, so I expect that I have a break somewhere - I'll replace the heater wiring and see how I go. Thanks again.

    Update: I've been able to narrow the problem down to the heater wire between pins 4 and 5 on V1 and pin 9 on V2, as the resistance goes from under half an ohm to open circuit when I move those wires around...
     
    Last edited: Jun 14, 2021
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  9. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good deduction here by @Lowerleftcoast , and good testing by you. If it’s the wiring, which sounds likely, remember you can easily use 22ga heater wires, especially over in the preamp; it bends more easily so may be less prone to break from being tortured into place.
     
    Last edited: Jun 15, 2021
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  10. Paul G.

    Paul G. Friend of Leo's

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    The fashion nowadays is to use a drill and UBER! TIGHTLY! twist heater wiring. It looks cool in the photos you post of your build and all but...the purpose of twisting is to keep the wires parallel and presenting more or less a similar profile to external interference. A looser twist is just as effective, uses less wire, and in the case of solid core, minimizes the chance of breaking the wire inside the insulation.

    The problem is, due to the low resistance of the heater winding, a break is not always easy to pinpoint.

    My 2¢.
     
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  11. eponymous

    eponymous TDPRI Member

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    Well, the heater wiring was definitely a problem (although I suspect it was actually poor soldering on the V2 socket) - I'm now seeing 7V AC between pin 9 and 4/5 on both the V1 and V2 sockets.

    Now to track down a problem I seem to have introduced while working on the one I already had - a high pitched squeal when I turn up the Bright volume that varies with the position of the volume control. I expect that I've messed something up in the input jack wiring on that channel while trying to ensure that I had the 1M resistor wired correctly.

    Thank you again for all the help, and for not making me feel like an idiot with my beginner's questions :) (I've been a software guy for 25 years, so I can usually diagnose a software problem from the far side of the room... It's good to remember what being a beginner feels like when I have to help others learn, and I'm really grateful for not just the advice here, but how it has been provided generously and without judgement.)
     
  12. eponymous

    eponymous TDPRI Member

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    Just to wrap it all up:
    1. I had a problem with the heater wire from pin 9 on V2 to pins 4/5 on V1, which had obviously been there since I originally built the amp (based on the voltage measurements in my build notes), as I was just lucky for a while that the wire hadn't moved enough to cause a noticeable problem.
    2. The squeal on the bright channel was because I forgot that I got interrupted part way through re-soldering the connections there, and one of the leads was just floating in the breeze :oops:
    Thanks again for all the help, including the tips that I'll be taking into any future builds (like not having to twist the heater wiring so tightly that it looks like green fusilli :D). I now have an amp that sounds better than it has ever sounded.
     
    Last edited: Jun 16, 2021
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  13. Dave Skowron

    Dave Skowron TDPRI Member

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    Here's a tip for the heater wire that I picked up somewhere, and it works fine. Use 22 gauge zip cord. Being in close proximity to one another the magnetic fields will mostly cancel out. I believe Soldano uses this method. You could also go one step further and twist the zip cord but that's probably overkill.
     
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