Blues Jr noise after turning it off (video)

Discussion in 'Amp Tech Center' started by sirbergersworth, Sep 30, 2014.

  1. sirbergersworth

    sirbergersworth Tele-Meister

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    Hey all,
    This blues jr has been modded to most of the basic billm mods. green board 1998 or so.
    bias adjustment trimmer
    tone stack mid mod and orange drops to billm spec
    all filter caps replaced and first on doubled in value.
    reverb return mod

    everything sounded good before I did the bias mod. That was the last mod. I put the chassis back into cabinet and biased it.......without speaker plugged in. amp was on for about 10-15 minutes. when I finally did plug speaker in there was little to no output and lots of noise. I tested the resistance of the OT and it was fried. I then ordered a hammond replacement that had similar specs but had a little more iron (PRRI transformer). Put that in and amp sounds great after biasing it with the speaker plugged in. lol
    It sounds great until you turn it off. then you get a fart sound. I think it is a cap discharging. Any ideas where to start?



    I have not fed it any beans btw! lol

    Thanks in Advance
     
    Last edited: Sep 30, 2014
  2. sirbergersworth

    sirbergersworth Tele-Meister

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    Nobody. Bummer.
    Bump
     
  3. codamedia

    codamedia Friend of Leo's

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    Sounds like a discharge to me... I've heard this in plenty of SS amps, but not a tube amp.
    Out of curiosity, have you asked BillM?
     
  4. sirbergersworth

    sirbergersworth Tele-Meister

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    I haven't. I was hoping he would see this thread.
     
  5. Abu Twangy

    Abu Twangy Friend of Leo's

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    Sounds like the power caps discharging to me as well. The upgraded OT may have made it more apparent. Unlike SS amp "whump" it can take a second or two to dissipate.

    I've had it occur with a tube amp in the past--a Sound City 120. If I remember correctly it only happened when I turned that amp off without using the standby switch first.

    Of course the BJr does not have a standby.
     
  6. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

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    I had something similar intermittently occurring with one of my DIY builds. I replaced the OT and still, it happened. It was more of a motorboat sound like the OT leads were swapped.

    When I had the amp open for the 30th time, I noticed that one of the wires on my fuseblock was not soldered in. It was making intermittent connections.

    This would suggest to me that the others are on to something regarding power or discharge.
     
  7. sirbergersworth

    sirbergersworth Tele-Meister

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    I hope to have time to go through the amp tonight.
    Thanks guys.
     
  8. sacizob

    sacizob Friend of Leo's

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    Mine does (have a standby)
     
  9. sirbergersworth

    sirbergersworth Tele-Meister

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    Did you install it or have someone install it?
     
  10. Billm

    Billm RIP

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    Running the amp without a load does no damage--as long as you don't play through it. A brief test without the speaker (until you figure out what you forgot) won't hurt.

    Post-turnoff pops are usually filter cap problems. It's a bit of a pain, but substitution is probably the easiest way to check.

    Before you do that, try the shutdown with volume off, then tone controls off, then reverb disconnected, then master volume off, to see if you can localize or isolate the source of the pops. Then try with V1 removed, then v2 removed. I'm assuming it won't pop with V3 or the output tubes removed, but you can try removing V3.

    Remember that the amp stays energized longer when you start removing tubes and decreasing the discharge paths, so be careful.

    You might also want to install a 470K bleed resistor on the high voltage line--the output from the first filter cap or the red wire to the OT, as on the Pro Junior (about the only feature I like about the PJ). You could test this (carefully) with a 1/2 or 1W resistor and a couple of jumper clips. It might not solve the problem, but it would accelerate the discharge rate a bit, which might help offset the larger filter cap. Gotta say, I've never had this problem modding a BJr.
     
  11. sirbergersworth

    sirbergersworth Tele-Meister

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    Thank you so much for responding. I will try what you suggested and try to locate the source.
    Can I build a audio probe and have a listen? I do this all the time with pedals I am building or fixing but never an amp.
    What cap values do you recommend to use? I was thinking one of the filter caps I removed? Or a high voltage other type?

    Thanks again. I can't believe I am the only one.
     
  12. sirbergersworth

    sirbergersworth Tele-Meister

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    Here is what I have found.
    With only V1 removed, no fart
    with only V2 removed, no fart
    with V3 removed, no fart
    I didnt go any further pulling tubes.
    with reverb tank disconnected, fart but sounded a bit different. lower in pitch.
    Reverb knob does effect the sound with tank in. Doesn't reverberate the fart but does add another squeal of higher pitch after initial fart. Could there be an issue with the tank or the leads. Tank looks bad and has obviously seen some moisture. Amp Chassis itself is very clean.

    I think I may start with a re-tension of all the tube sockets. It passes the ancient Chinese Chopstick Test. I may also run another tank and see if that helps.

    Any other ideas?

    btw, I scrapped the OT initially because it showed zero resistance at the speaker side.
     

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  13. Billm

    Billm RIP

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    Hmm. Did you try replacing V1, since that is the only one where you hear the oscillation? The pops are probably running all through the power and signal circuits, so I doubt that the audio probe would do much.
     
  14. sirbergersworth

    sirbergersworth Tele-Meister

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    I swapped the pre tubes around once and noise was still there. I will try an entire known good set tomorrow. Thanks for following up.
     
  15. sirbergersworth

    sirbergersworth Tele-Meister

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    I replaced all the preamp tubes with known good tubes and the fart is still present.
    I think it is definitely in the preamp. It is now making crackling noises (when on) with the gain over about halfway up to full.
    Ugh!
     
  16. mikiedreb

    mikiedreb TDPRI Member

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    The Blues Jr. master is back !!!!
     
  17. sirbergersworth

    sirbergersworth Tele-Meister

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    Replaced all pre tubes and fart still there. All tubes were known good. I then installed a standby switch that I was waiting for this issue to resolve to install. It is the progressive switch. Install was easy. After install I fired her up. She sounded as good as usual. I played for a half hour and when I went to standby, no fart. I went to off after a second and fart was there. If I leave the amp in standby for 5-10 seconds then go to off, I HAVE NO FART! Yippee.
    I also noticed that when I power off the filter caps immediately drain. (Obviously) lol


    Does that help in tracking down? I am going to pull board again and hit all joints again.
     
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