JFC....2203x mod failure...hmmmm

Discussion in 'The Workbench' started by shredless, Jan 17, 2021.

  1. shredless

    shredless Well-Known Member

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    Welp, l settled on the mods, cleaned up my mess on the board, loved the amp for a week and now its acting up.

    I should mention this is not really a new problem. When the amp is in my kitchen, upside down chassis out it always sounds great. I put it together, move to my room and it usually sounds turdified within a couple days.

    It sounds tubby, farty, bloated, crappy distortion and the attack of the notes have a fizz on them.

    I realize l have to learn to fix issues if lm gonna mod...lm trying.
    This weird seemingly room specific madness has happened numerous times over the years. Sometimes l think its a outlet issue or ghost. Ive checked wall voltage and its the same as the kitchen.

    Im going to clean power tube sockets and the impedance switch with deoxit. Does deoxit gold leave a residue?

    If that doest fix it l will open it up and do a poke test and maybe change power tubes.

    Any ideas,? Thanks for your time and thoughts
     
  2. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Maybe when you move it right side up something shifts. Definitely do the poke test to check for loose components while it’s running on the bench.
     
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  3. thetragichero

    thetragichero Well-Known Member

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    could also potentially be heat related. air convection cools a lot better than inside an insulated box
     
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  4. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    In regards to wall voltage is it the same as the kitchen when the amp sounds shit? Wall voltage can be affected by numerous thins in the house cycling on and off.
     
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  5. shredless

    shredless Well-Known Member

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    Thanks, l think the wall voltage is not the issue.

    Moved it to kitchen, still poo
    Though its more like uh oh l need to go but can make it home first. The other day it was no way must go now
    Cleaned tube sockets, still poo
    Dint open it up cuz l was drinkin jack n coke, will open it after l eat some white pizza here soon.

    Stay tuned...bummer it was purrrfect last week
     
  6. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Solder joint?
     
  7. RickyLee

    RickyLee Well-Known Member VIP Member

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    Dam that River. It sounds like you and myself are still in the same school of hard mods knocks. My last three projects were very underwhelming and then I undertook this last one to give it another go.

    Does it feel as if there could be some emissions inside the chassis somewhere that are getting injected into the signal path? And I am sure you ruled out different 12AX7's.

    I noticed the other day on mine, that my 5V tap wires can inject noise/hum or they can actually be used to cancel out noise by where they are placed. It does seem to also slightly color the tone depending on location.

    I think some of my past issues on projects was my stubbornness to give up on my old stash of carbon comp/film resistors. And I am still not sure which type some actually are lol. This time I gave in and got some metal film and used them in a few key areas. But I am still wondering about the old shielded cable method I was using everywhere. This time, I actually only used shielded runs from input to the first stage. And I did not ground the shield at first. When I went back and grounded the shield, it seemed to make no difference in hiss/hum/noise.
     
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  8. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Ricky's post reminded me of high frequency oscillation. This you can't hear but will make an amp sound ass.
     
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  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    This is my current project but this was before being close to finished. It is a test bed at this point and will need transformer leads trimmed and such. I will probably get slammed for this rats nest, but I tried to do a complicated 2 channel build in a Deluxe Reverb platform. Surprisingly, it is very quiet. So I might get afraid to actually change anything if it is still quiet and sounds good. Might be like a jinx if I try to go back and trim and re-route wires lol.

    That happened on another project many years ago, as it sounded good and I was afraid to change anything.

    :D

     
  10. shredless

    shredless Well-Known Member

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    Interesting...any chance theres a cap l could place in a specific spot to knock this out, if it may be occuring?

    I just retensioned the sockets and found a loose black insert, it pops back and forth where the others dont. I did break a tube locator in that socket recently. Im only running two so l swapped to the outers.

    I cant get any change by tapping anything or even pounding on chassis with fist.

    It sounds better but not the same as a week ago. Ive also suspected the fx loop....seems maybe it throws fits after being used...maybe.

    I didnt swap pres today, any chance l could be damaging a 12ax7, like hittin it too hard electrically? Like burning them out...l dont think l did anything severe mod wise.

    Thats next, try another pre
     
  11. thetragichero

    thetragichero Well-Known Member

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    no to most/all of that
    can't deal with an oscillation until you know where/what it is, and besides running the amp while moving leads around with a chopstick i wouldn't know how to narrow it down without a scope
     
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  12. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    The HFO can be observed on a scope quite easily. Without a scope you can not rule this out. It can be cause by a capacitor but also other things. Have you messed around with the grid stoppers on the power tubes?
     
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  13. anitoli

    anitoli Well-Known Member Gold Supporting Member

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  14. thetragichero

    thetragichero Well-Known Member

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    one of the things I've run into making my own versions of older fender amps (silverface vibrochamp but with better power filtering and 6l6g output among other brown/blackface-inspired builds) is that they were SUPER stingy with grid stoppers. after trying all sort of plate bypass capacitors, etc adding a 33k resistor here or there (1k5 or so on power tubes) solved all sorts of yuck
    but blindly adding stuff without isolating where the problem starts is frustrating at best, if not making things worse/messier
     
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  15. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    I bought a JCM800 Super Bass and that thing was oscillating big time if i had 4 tubes in but wouldn't with two tubes. The curious thing was they only called for TWO 5.6k stoppers on the schem and that's all that was in there. Two more resistors solved that problem.
     
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  16. shredless

    shredless Well-Known Member

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    Seems it was the phase inverter tube. This is not the first time in the 8 years lve had this amp that it got weak, l just tend to forget...swapped it out and got quite a volume, gain boost and brought back last weeks mojo
     
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  17. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Awesome you got it. :thumbs:
     
  18. RickyLee

    RickyLee Well-Known Member VIP Member

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    Check and write down all your voltages within the phase inverter circuit. And your B+ feeding the plates as well.

    Is this just the standard 2203/4 power amp circuit?

    Are you running any snubber caps anywhere? How about the fizz cap across the phase inverter?

    I had forgot to ground the end of my PI and left out the fizz cap across PI plates when I fired mine up for the first time lol.
     
  19. thetragichero

    thetragichero Well-Known Member

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    quick ebay search for tektronix oscilloscope shows you should be able to get one for about a hundred bucks (although check craigslist/facebook marketplace/electronics surplus stores)
    i use a tek453 that spent 20+ years in my father's attic, so i went in and replaced all of the electro caps. pain in the ass but for my purposes it's perfect and the price (free) was right. I've mentioned elsewhere but it was life-changing
     
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  20. shredless

    shredless Well-Known Member

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    Ill keep this in mind...
    Its a 2203x ...PI n power is all stock circuitry. Dont know snubbers or fizzers...
     

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