Mojotone 5E3 Voltage Question - V2B Pin 8

Discussion in 'Amp Tech Center' started by peterferguson9, Nov 4, 2019.

  1. peterferguson9

    peterferguson9 TDPRI Member

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    First time poster on this forum. First time amp build as well! Calling amp techs/5E3 kit experts:

    Started putting together my first amp, a Mojotone 5E3, about a month and a half ago. It has been an awesome experience, and I can't wait to do another. I've got a Celestion Gold for this rig and I think it is going to be rad.

    The amp is fully together, and everything seems to be testing within tolerance except for one thing - V2B Pin 6 and 8. Pin 6/V2B Plate/Phase-inverted output is way too high, and Pin 8/V2B Cathode/non-inverted output is way too low.

    When I plug a guitar in, I have..sound. No hum, no buzz, but it kinda sounds like a low-biased gated fuzz pedal. Super distorted, not too loud, and very glitchy. In other words, it sounds REALLY cool...but REALLY wrong.

    Things I've tried, with no changes to the problems:
    - Confirmed continuity from the .022uF coupling cap to the PI inverter grid leak
    - All resistors in the cathodyne (1M, 1.5k, 56k) all test to spec.
    - Confirmed that the export lines form the PT are properly insulated and tucked away
    - Confirmed wall outlet is 120VAC.
    - Swapped 12AY7 and 12AX7 (V1/V2).
    - Tried another working 12AX7 in V2.

    Here are the tested voltages across the amp:

    V1:
    Pin 1: 136VDC
    Pins 2/7: 0VDC
    Pins 3/8: 2.1VDC
    Pins 4/5/9: 6.1VAC (2.9 to ground)
    Pin 6: 129VDC

    V2:
    Pin 1: 162VDC
    Pin 2: 0VDC
    Pin 3: 1.4VDC
    Pins 4/5/9: 6.1VAC (2.9 to ground)
    Pin 6: 242VDC
    Pin 7: 3.7VDC
    Pin 8: .02VDC

    V3:
    Pins 2/7: 6.1VAC (2.9 to ground)
    Pin 3: 356VDC
    Pin 4: 332VDC
    Pins 5/6: 0VDC
    Pin 8: 24VDC

    V4:
    Pins 2/7: 6.1VAC (2.9 to ground)
    Pin 3: 344VDC
    Pin 4: 332VDC
    Pins 5/6: 0VDC
    Pin 8: 24VDC

    5Y3:
    Pin 2: 361VDC
    Pin 4: 355VAC
    Pin 6: 355VAC
    Pin 8: 362VDC

    Photos of the build: 5EA69187-243F-4F8C-AF14-D222F5DA41CD.jpg 16C3D7A9-76ED-4C6F-9F0B-1CA79DEF1179.jpg 20A658D1-51DC-4F0F-A9C9-5093B6445236.jpg B6DA363C-69CC-497A-BDD9-B59AF7234F27.jpg E13FC8F3-193D-49FB-A4C3-FD1749DC0D7A.jpg FB826903-EDA7-485E-A23B-6653A5EA123E.jpg
     
  2. corliss1

    corliss1 Friend of Leo's

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    Measure resistance from that pin 8 to ground. What do you get?
     
    King Fan likes this.
  3. muchxs

    muchxs Doctor of Teleocity

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    Recently repaired one with symptoms almost exactly like that. Blown output transformer.
     
  4. King Fan

    King Fan Friend of Leo's

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    I know this isn't about the rectifier, and it may be a typo, but shouldn't pin 2 be 5VAC? Is it odd your B+ at pin 8 is only 362VDC if your PT secondaries are at 355VAC? Is the low 6.1 heater voltage on modern wall voltage a sign of the same thing?

    I'm guessing (and hoping) @corliss1 is onto something with the grounding question.
     
  5. peterferguson9

    peterferguson9 TDPRI Member

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    @corliss1 @King Fan @muchxs
    Thanks for your responses. I found the issue and am almost embarrassed to explain! I quadruple checked my wiring during the build, and thought everything was fine. Turned out my lines for pin 7 and pin 8 were switched. Such a simple mistake. Switched them back and the amp roars! Already using it on overdubs and live.

    Looking to redeem myself on another build - either a Princeton non-verb or brownface deluxe. Really addicting stuff!
     
    D'tar and King Fan like this.
  6. corliss1

    corliss1 Friend of Leo's

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    You think you're the first guy that's switched the wire on a pin? :D

    My point for measuring that was "is pin 8 grounded." If you measured something other than the correct value there, that's a clue, and after you've been looking at something for a while a good measurement is often more helpful than a quadruple visual check.
     
  7. peterferguson9

    peterferguson9 TDPRI Member

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    Should have mentioned - I checked that value, which led me to follow that wiring again, which led me to finding the problem!
     
    corliss1 likes this.
  8. Preacher

    Preacher Friend of Leo's

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    Been there, done that!!
     
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