Filament voltages, MojoTone 5e3

Discussion in 'Shock Brother's DIY Amps' started by Tele Slacker, Sep 24, 2019.

  1. Tele Slacker

    Tele Slacker TDPRI Member Silver Supporter

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    I wired the referenced kit in July 2019, and it has worked *nearly* flawlessly from the beginning. However, occasionally it will fire up (rectifier, pilot light), but makes no sound. Doesn’t appear the power tubes and preamp tubes are heating up at the filament. This is an intermittent issue - amp has worked today, then stopped. Will likely work again tonight. Rewired filament runs, and tested for continuity - no identifiable issues.

    Took voltages (when not operating properly) and all are 12-15 volts high at many of the referenced locations on the MojoTone wiring diagram. When operating properly, voltages are nearly exact.

    Took filament voltages from power xformer, and it appears in the 3-4v range (with respect to ground). My limited understanding is that filament voltages should be 6+. Heaters in 6v6 and preamp tubes never appear to heat up.

    Not sure if this is related, but I am measuring continuity between the filament connections to the pilot light (pic attached). Is that an issue?

    Any ideas?
    92EF184A-1D6E-40C7-A9E4-9C82404B6802.jpeg
    Thanks all!
     
  2. Wyatt

    Wyatt Tele-Afflicted

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    Don't measure the heater wires to Ground. Measure the voltage across them.

    • One probe on each terminal at the pilot light (exact where you have arrows in the pic) ... 6.3V AC? (don't measure DC!)
    • One probe on pin 2 and one on pin 7 at each 6V6 ... 6.3V AC?
    • One probe on pins 4/5 (they should be connected) and one probe on pin 9 of each 12AX7 ... 6.3V AC?
    • For the rectifier, one probe on pin 2 and one on pin 8 ... 5V AC? (don't measure DC!)

    Yes, there should be continuity across the winding, it's one, long, continuous wire and there should be continuity to Ground (Ground reference). Does your amp use a center tap (usually green and yellow wire) for the heater winding? Or are you using a faux CT (two 100-ohm resistors)? And you are only using one or the other?

    More pics would help A LOT.
     
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  3. Paul G.

    Paul G. Friend of Leo's

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    It's 6.3Vac across the terminals. 3.15Vac to ground. Are you measuring DC? I see the connection to the transformer, and the connection to the tubes, I don't see resistors to ground making the artificial center tap.

    Can you take more photos?
     
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  4. Tele Slacker

    Tele Slacker TDPRI Member Silver Supporter

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    I have the two 100 ohm resistors-to-ground. You both helped me a lot. I was uncertain if the 6.3v ac was relative to ground, or across terminals. Thank you both very much. All relevant voltages were high side of 6v ac.

    As I was checking voltage across pilot light terminals, the pressure applied caused filaments to come to life. Apparently a bad solder joint (I couldn’t tell visually). I have resoldered, it works (!) and will test it out for a while.

    For sake of posterity, I’ll attach a pic of my first build.

    Thanks Paul & Wyatt!

    Ted
     

    Attached Files:

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  5. King Fan

    King Fan Friend of Leo's

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    Good for you!!! I like the way you described the problem and what you didn’t know and then quickly (and cheerfully) responded to these good suggestions. Nice fix.

    Looking at the solder joints in your pic, I noticed there didn’t seem to be much if any wire sticking out beyond the lug, and none wrapped around it. With all respect, are you trimming your tails that short, stripping too little wire, or ??? You want the wires to stay mechanically joined through at least mild tugs before soldering.
     
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  6. Tele Slacker

    Tele Slacker TDPRI Member Silver Supporter

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    Yeah, I think you’re right. May have cut that one a little shorter than ideal. If the problem persists, I may reroute the Xformer filament lead to use what little slack I left, and more securely attached that connection.

    Appreciate the feedback!
     
  7. Tele Slacker

    Tele Slacker TDPRI Member Silver Supporter

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    More issues tonight. After hours of playing at moderate to low volume, I noticed some noise (described as low level motor boating, although I’ve never heard real motorboating before... so take my description with a grain of salt). Unplugged guitar and sound remained with nothing plugged in. Looked at tubes and noticed some dim blue flashes in the output tubes.

    turned it off, came back later... all power and preamp tubes heater are lit, but rectifier tube appears dead. No sound. So looks like I’m ordering a new 5Y3.

    now my question... poking around, rectifier filament voltage was fine (5+ vac). However when checking 2ndary hv ac from power xformer, I’m getting 800v ac across pins 4 & 6 of the rectifier tube. Is this ok? Seems really high. Don’t want to buy another rectifier tube until I know I don’t have a xformer issue.

    thanks all!
    Ted
     
  8. Wally

    Wally Telefied Ad Free Member

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    Okay...so..here is what the current Mojo kit calls for...384VAC on each leg....768VAC across pins 4&6. So your PT seems to be what Mojo uses. ????? What were your voltages in the working circuit? The last vintage 5E3 I worked on had 365VDC on the plates.
    Imho, that PT yields higher voltage than I would want. This PT is closer, imho, as it has a 660VAC HT.

    https://www.mojotone.com/amp-parts/Power-Transformers/Tweed-Deluxe-Power-Transformer_2

    t
    hat said, that HT voltage doesn’t seem to be improper for the PT the kit uses, and your PT does not seem to be the problem??

    did you make a voltage chart for the finished amp? Such a chart is a good baseline reference.
     
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  9. Wally

    Wally Telefied Ad Free Member

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    One might suspect a filter cap, too, since you hear a motorboat??
     
  10. Tele Slacker

    Tele Slacker TDPRI Member Silver Supporter

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    I have voltages on several test points recorded, but none on the secondary HV - I may need to look a little deeper… probably some opportunity to extrapolate. Excellent suggestion on formally creating a voltage chart. Thank you for your feedback!
     
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