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5E3 Negative Feedback Issue

Discussion in 'Shock Brother's DIY Amps' started by Baron13, Jun 20, 2020.

  1. Baron13

    Baron13 TDPRI Member

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    I recently built a 5E3 and added the three way negative feedback mod. Initially I did have to swap the OT wires. After that all positions worked as expected. Last night I swapped power tubes and everything was fine until I switched over to the negative feedback position. I get a loud hum through the speakers even with volume down. I poked around with a chopstick but everything seems fine. Any ideas? Bad switch? Thanks
     
  2. wabashslim

    wabashslim Friend of Leo's

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    What happens when you put the tubes back where they were?
     
  3. Baron13

    Baron13 TDPRI Member

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    I tried that this morning but the hum is still there. Tested continuity in the switch and that checked out.
     
  4. wabashslim

    wabashslim Friend of Leo's

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    But its still OK with the NFB off?
     
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  5. The Ballzz

    The Ballzz Tele-Afflicted

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    I'm gonna spitball a guess here. Changing the power tubes jostled an already iffy/cold solder joint on a power tube socket, and now.....?
    Just Guessin'?
    Gene
     
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  6. Baron13

    Baron13 TDPRI Member

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    Yes. Stock setting and cap bypass work fine and sound great.
     
  7. ocduff

    ocduff Tele-Meister

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    Not to be glib, but did you find a sound you liked in the switch other than stock?

    If not, this mod may not be worth your trouble here.

    A dialed in 5E3 is a thing of beauty.
     
    Last edited: Jun 20, 2020
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  8. wabashslim

    wabashslim Friend of Leo's

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    Swap the 12AX7? Check the above suggestions? Check your wiring for the umpteenth time? PM Rob?
     
  9. Baron13

    Baron13 TDPRI Member

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    I do appreciate simplicity and the stock circuit is great. I have an “esquired” tele and a junior and can’t really beat the 5F1 for responsive tone. The 5E3 mods may not be worth the trouble as it is nice and brash. I went back and forth and thought you won’t know unless you hear it for yourself. Wanted to hear the settings at band volume before deciding. Taking the time to dial in the amp and working the interactive controls is well worth it and may prove that it’s best left alone.

    The cap bypass tone is cool, but it is less of what you are there for. Maybe it holds on a bit longer with a boost, but then I guess you could slightly dial back the stock setting.
     
  10. ocduff

    ocduff Tele-Meister

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    The 5E3, in stock form, running 360V and tubes at 100% dissipation into a true low power vintage type speaker, and that’s as good as it’s going to get - and that’s a really great sound.

    But, there’s just warts in the design and limitations to what it ever was meant to be - and attempts to dial them out always take away from what makes it good.

    I liken the mid 50’s tweeds to the early Corvettes - Drum brakes, solid axle rear, 6 cylinder, chassis construction.

    Then by the 5F6 Bassman, 5F8 Twin and 5G9 Tremolux, suddenly its like a late 50’s Corvette - disc brakes, fuel injection, V8, independent suspension - Things are starting to cook, and those designs stuck.

    Have fun with the 5E3. It’s a great amp. I miss the ones I built but really, when you do these “mods” imho, they make you realize that Leo Fender considered them - tried them himself, slept on it - and then said “it sounded better with no NFB” and made that amp for 5 years - probably his longest running design.

    And that amp has become famous. Had he put in NFB perhaps no one ever would’ve thought much of them.

    I mean, even taking out the .1uf coupling caps takes away that gooey goodness.

    People have been trying to change the 5E3 into something it’s not - really, when Leo made the 6G3 he perfected the design of a small single tone control 6V6 amp. Period.

    However, the 5E3 sounds better at first blush - but all those warts reveal themselves through time to reveal an amp that is just pretty archaic.

    Step into a 6G3 and all those problems are solved and then some, but that loosely gooey woody bloom (with the 5E3’s way too much low end and inverter fizz when driven) is gone.

    So you kind of have to live with the warts and enjoy it for what it is and realize that its limitations can’t be bypassed without killing what makes it magic.

    I didn’t mean to rant and suck the fun out of the journey - Just sharing my .02, and that’s all it’s worth.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    Baton, you might do a voltage and bias number check on the power tubes. The all’s is on the track with his suspicion, imho.
     
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  12. D'tar

    D'tar Friend of Leo's

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    I like pictures and as built schematics/drawings. Sometimes we look at things and guarantee we are seeing something other than reality.
     
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  13. Baron13

    Baron13 TDPRI Member

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    Fixed! Thanks. I was getting some photos to upload and took new voltage readings. Sometimes I clip the ground to the speaker out ground lug while probing for readings. I wasn’t getting any accurate readings. Clipped to the ground bus and readings looked good. Checked continuity to ground at output jack...nothing. I had pinched a wire when I last tightened the jack. Cleaned up the wire, tightened it up and continuity! Checked NFB and everything works as it should. Thanks for the help. Thanks for your time. Thanks for making me get back in there for voltages.
     
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  14. Baron13

    Baron13 TDPRI Member

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    These are the readings. Let me know if you think they look ok. Amp sounds good.

    B+1 375
    B+2 334
    B+3 248

    Heater 6.4
    Wall 115

    Rectifier 4 266
    8 376

    V4 2 and 7 6.4
    3 368
    4 335
    8 21.1
    V3 2 and 7 6.4
    3 368
    4 335
    8 21.1
    V2 1 167
    3 1.2
    4 5 and 9 6.4
    6 200
    8 47
    V1 1 132
    3 1.9
    4 5 and 9 6.4
    6 130
    8 1.9
     
  15. The Ballzz

    The Ballzz Tele-Afflicted

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    Couldn't get much closer to perfect! :cool: I'd only add that I'd prefer those EXCACT voltages at 123 volts or so at the wall! ;) Still a right on the money ratio of readings! :D Finding just the right NOS or ANOS rectumfryer might get those same voltages at 120+ wall volts. On the other hand, I think I'd usually prefer the voltages of a 5E3 to be a couple high than the neutering of a few low!

    Just curious as to whose transformers/model numbers did you use for the build?

    Congrats On The Successful Troubleshoot/Repair!
    Gene
     
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  16. Baron13

    Baron13 TDPRI Member

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    Thanks! I used the Classic Tone 40-18021 PT and 40-18090 OT.
     
  17. The Ballzz

    The Ballzz Tele-Afflicted

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    NOICE! :D Let me guess, rectumfryer is a SOVTEK or JJ? Is your typical wall voltage around SoCal in that 115 range or did you dial it in with a variac, of some sort? If a variac, what's you're "actual" wall voltage?

    Just Guessin'?
    Gene
     
  18. Baron13

    Baron13 TDPRI Member

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    I had a JJ in first but switched to an RCA. Voltages are really close between the two.

    The 115 was read at the fuse terminal. I don’t know if it’s this old duplex or typical SoCal. Does it tend to be above 120?
     
  19. The Ballzz

    The Ballzz Tele-Afflicted

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    Typical here in Las Vegas had been 121-124 for a long time, but the last couple years has tamed down to like 119-123, but with more frequent and wider fluctuations than in the past. If you play in different spots, it doesn't hurt to have a volt meter handy. Not so much to do anything (a few volts high won't likely damage anything) but to at least know why your amp may sound a bit harsh or wimpy in a given situation. The simpler the design of an amp, the more sonically susceptible it may be to such variations.

    All indications are that you have a good handle on things! :cool:

    Just Sayin'
    Gene
     
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