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Thread: Blues Jr (Cream Board) Overdriven Issue

  1. #1
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    Blues Jr (Cream Board) Overdriven Issue

    Greetings!
    im new here to this forum but not new to looking into amps. I’m set up pretty good to troubleshoot and understand the safety of working inside. I have run into a Blues Jr (cream board) that I can not figure out. At any volume it is sounding like it’s being overdriven/distorted. My test point voltages look good. Installed the following: tamed the bias, tone mod, replaced all plate resistors, replaced IC electrolytics with F&T (kept it at 47uF), switchcraft input jack, new tubes, new reverb tank, tried connecting a new speaker (no change), replaced OT with known working tranny on a whim. Still, the amp sounds overdriven at low volume especially with bass notes but is present with the higher notes as well. Don’t know where else to look. Please help.

    Thanks!!

    reconbrad

  2. #2
    Forum Member Michael Smith's Avatar
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    Re: Blues Jr (Cream Board) Overdriven Issue

    Check for a bad solder joint somewhere, although with the amount of components you've already changed, there probably aren't that many more to check. With an old school amp with an eyelet board, you can tap around on the board with a wood stick while strumming a guitar (or a signal generator if you have one), but I don't know how effective this would be on an amp like yours with a PCB board.

    Here's a link to a short video I made recently.

    https://www.youtube.com/watch?v=4E8Hs9uXq2Q
    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

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    Re: Blues Jr (Cream Board) Overdriven Issue

    Quote Originally Posted by Michael Smith View Post
    Check for a bad solder joint somewhere, although with the amount of components you've already changed, there probably aren't that many more to check. With an old school amp with an eyelet board, you can tap around on the board with a wood stick while strumming a guitar (or a signal generator if you have one), but I don't know how effective this would be on an amp like yours with a PCB board.

    Here's a link to a short video I made recently.

    https://www.youtube.com/watch?v=4E8Hs9uXq2Q
    Thanks for the reply! Yes I went back through with a magnifying glass and found no suspects. Those that were suspicious to me I sucked out the old solder and applied new solder. Cool video!! I have an few oldie amps myself. Nothing like them!!

  4. #4
    Forum Member Michael Smith's Avatar
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    Re: Blues Jr (Cream Board) Overdriven Issue

    You can try to plug a guitar signal into the reverb return jack to see if the distortion still appears in the output section of the amp. If it's clean, then you will know it's somewhere in the preamp section. You may have to crank up the volume if going into the reverb return since you won't have the preamp gain stages. I didn't look at your specific schematic, but was relying on my knowledge of Fender circuits in general.

    Short of using an oscilloscope to see where in the entire circuit the distortion first appears, I don't know what else to suggest. I assume you already tried a different guitar cable, or even a different guitar.
    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

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    Re: Blues Jr (Cream Board) Overdriven Issue

    Quote Originally Posted by Michael Smith View Post
    You can try to plug a guitar signal into the reverb return jack to see if the distortion still appears in the output section of the amp. If it's clean, then you will know it's somewhere in the preamp section. You may have to crank up the volume if going into the reverb return since you won't have the preamp gain stages. I didn't look at your specific schematic, but was relying on my knowledge of Fender circuits in general.

    Short of using an oscilloscope to see where in the entire circuit the distortion first appears, I don't know what else to suggest. I assume you already tried a different guitar cable, or even a different guitar.
    Thanks for the recommendation! I’ll give it a try when I get back home. I do however have a oscilloscope, tone generator and a dummy load but the problem I have is I know how to connect it all but do not really know what I’m doing when it comes to signal tracing. Been looking all over YouTube and people go down rabbit holes in explaining setup. So I’m up for whatever I can learn.

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    Re: Blues Jr (Cream Board) Overdriven Issue

    Good news! I learned a ton watching some YT videos and some others as well. problem was in V1. Sounds weird but hey, can’t make this up.. So here goes.. Pin 7 on V1 was not getting anything. Pulled the board down and pretty much swapped out the resistors to V1. Testing the resistors as they were removed, the R1-1M and R2-10k were no good and did not measure any resistance. I also, checked continuity from R2 to V1 Pin 7 and it was good. But, on the socket side it was not making contact with the tube. So I retenciones the socket. After replacing the resistors and putting it all back together, is still sounded distorted.. But here is where it gets weird for me.. I originally tested the amp with a new speaker and it sounded distorted. Now after fixing up V1 and trying that same new speaker again, the distortion was gone and it sounded very full and warm. So I guess it was not getting clean signal and pushed dirty signal through the amp to the speaker. Hence that old saying, garbage in, garbage out.. but this pointed me to the issues and relook the speaker.. Thanks!!

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