Help on my Green Board Blues Junior

Discussion in 'Amp Central Station' started by Bomberman977, Jun 23, 2020.

  1. Bomberman977

    Bomberman977 TDPRI Member

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    Hey all, I’ve been trying to fix this problem on my Blues junior for a while now. I usually play with the master at about 10 and the volume around 1-2 for home use. At low volumes I have no issues but when I bring the volume to around 4 (with master at 10) I get a nasty hiss distortion type sound. Not sure if you can hear the full effect on video(https://drive.google.com/file/d/1q20xPjZbwplhgytGGtIR_OtS1UJapxAJ/view?usp=drivesdk). I tested another speaker to make sure it wasn't that. To add, I also tested the speaker resistance and its correct. I also played with the speaker out of the chassis, and the fuzzy hiss still remained making me think its an electrical issue, not a weird vibration. Tried other guitars and cables. And also the tubes are about at about 6 months of “light” play, so newish. This prompted me to test all the resistors in the thing and I replaced a couple that were acting strange, while I was at it I replaced all the electrolytic capacitors and direct wired the tube outputs of the preamp and power tubes, but left the phase inverter tube still with the ribbon cable (see pic, Fyi I also did a Frommel modification for the faulty reverb on green boards which is those resistors and yellow wire across the middle). I thought maybe the hiss was some issue with a connection in the ribbon cables. All these things combined did tremendously cut out noise from the amp, but didn’t solve the hiss issue at louder volumes. Ive checked the power transformer with a volt meter and its stable. I tested the amp input cable and it has continuity. Quite honestly I’m stumped and wondering what the issue could actually be. Maybe the amp is breaking up too early, but the hiss doesn’t sound like breakup to me. Next, I guess would be testing all the other capacitors? Any ideas, I’ll take anything at this point :)
     

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  2. saltyseadog

    saltyseadog Tele-Meister

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    Long shot answer to a long shot question but, there is a mauve/purple wire that goes from the speaker jack board to iirc v13? anyway you can see it there in your pic. When I did my BillM mods to my green board BJr' many years ago he warned that the wire is very easy to break so be careful when doing the mods. When I finished I also got some hissing sounds and remembering BillM's advice thought maybe that was the problem. In my case when I checked the wire was not broken but being as it was so thin I resodered it put it all together and the hiss had gone. Now I don't know if that fixed it or not as I say the wire was not broken but it is worth checking.
     
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  3. Bomberman977

    Bomberman977 TDPRI Member

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    Thank you for your input, I looked at that wire and saw it was also not broken. I re-soldered it anyways like you did. I still get the same hiss when playing low notes. Just as a comment the amp is dead quiet when on, its only when I play low tone notes (open E string, A string higher on the neck, etc.) does it become an issue. Also its much more noticeable on my neck pickup. It's quite a weird hiss almost crackle noise that is produced. I just have no idea what could be causing it at this point. Thanks again.
     
  4. SouthAustinite

    SouthAustinite TDPRI Member

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    Had similar problems with a green board Peavey Classic 30. Three hours later I gave up and thought to push the tubes up further, problem solved.
     
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  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I couldn't hear anything in that sound clip.

    Do you have a different speaker you could try?

    Any odd voltages on the power tube pins?
     
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  6. Bomberman977

    Bomberman977 TDPRI Member

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    I bent the pins of the tubes a bit to make them fit tighter, I also tried my old tubes that have more hours on them and the issue still persists. Its like a fizzy crackle sound coming from inside the amp I think and translating into a hiss fuzz on the speaker.
     
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  7. Bomberman977

    Bomberman977 TDPRI Member

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    I did try another speaker and had the same subtle but present fuzz noise. Do you mean fluctuation in voltage from the tubes or something else?
     
  8. Matthias

    Matthias Friend of Leo's Gold Supporter

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    Weird fuzzy overtone when it should still be clean and nasty noises when pushed? Mine did that after a power tube went and took the screen grid resistor with it.
     
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  9. Bomberman977

    Bomberman977 TDPRI Member

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    I previously measured the resistance of two of the screen grid resistors as 47ohms which was normal, but now that I checked them they look like they are only at 23ohms which is odd. I wonder if this is the root of the issue, would this value make that noticeable of a difference in sound? I think that screen grid resistors reduce high frequency parasitic oscillation which could be the higher frequency hizz fuzz noise I am hearing. This could be why its not filtering out all of the noise and the amp is still somewhat usable (47 vs 23 ohm). But also I'm no electrician haha.
     
    Last edited: Jun 24, 2020
  10. Matthias

    Matthias Friend of Leo's Gold Supporter

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    I’m not sure! I just know what Fender replaced in mine. I had a quick look at some explanations because I was curious and it looks like you could be on the right track. Too much gain at the wrong part of the circuit, maybe? That’s already past my amp knowledge!
     
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  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    are they supposed to be 47 ohm? The schematic version I'm looking at shows 100ohm.
     
  12. Bomberman977

    Bomberman977 TDPRI Member

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    https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-Schematic.pdf

    I'm not sure if R35 and R36 are actually screen grid resistors (bottom middle of schematic). Anyways these are the ones that are measuring incorrectly at 23-24 ohms instead of the 47 which I think it says on the schematic. Am I looking at it right? Are these the possible culprits?
     

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  13. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    If something measures half the expected value, what could that indicate? :D

    That's also the schematic I pulled up, which isn't the same version as yours I don't think. I checked this one, which has r35 and 36:

    https://www.thetubestore.com/lib/th...r/Fender-Blues-Junior-III-Schematic-Rev-D.pdf

    but notice the dotted line? That indicates they aren't on the tube board. Also, in my experience, screen resistors are up on the main board, just a bit to the left of center.

    I wouldn't worry about measuring individual resistors *until you get a voltage chart and see if anything is off on the tubes themselves*
     
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  14. Bomberman977

    Bomberman977 TDPRI Member

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    Yes I see that, so this is actually a blues junior 1 Revision C so its a little bit messier/different of a circuit than a BJ3 and also has a completely different numbering system which they changed in 2001 I believe (don't quote me on it haha). I think on your schematic of the BJ3 R35/36 translate to R54/55. So I have no idea if these resistors are even related to the issue I'm experiencing, but its possible. I'm not really sure if the R35 and 36 are even screen resistors. How would I go about getting a voltage chart for the tubes? Tube tester?
     
  15. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    And, if you have any doubts...use your meter to see what pins those resistors to connect to. You can look at the traces to get an idea, then confirm with the meter.
     
  16. Deebs3

    Deebs3 Tele-Meister

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    i would double check your guitar lead, try a few. Was the problem with my green board BJ all along even though i had a new lead.
     
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  17. Bomberman977

    Bomberman977 TDPRI Member

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    Would playing with another guitar change this? I also tried other cables. I found the same problem with both.
     
    Last edited: Jun 24, 2020
  18. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Voltages on the tubes means just that - put tubes in amp, and measure the DC voltages on each pin.
     
  19. Bomberman977

    Bomberman977 TDPRI Member

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    Thanks for the help again, I really appreciate it. I made some quick measurements with the amp on measuring DC at each pin with a multimeter. Not sure how pins are numbered but looking at it from the picture I started from the bottom left and circled clockwise to pin nine. Here are my results. Not sure If there is anything abnormal here or not. Not sure what to look for 7FF78D6C-D7F9-4FDC-AD40-6E66BF7B80F3.jpeg
     

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  20. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    You got the numbers in the right order, I believe. There's a lot of wrong numbers there though, which would certainly contribute to your issue, but I wouldn't expect the amp to work at all with the data you've provided.

    For example, you have to have voltage on pin 1 of v2 and pin 3 of the power tubes for the amp to function.

    It might be time to get it to a tech if it's still not behaving properly at this point. Or...put the amp away and do some additional homework and come back to it later. No one likes hearing that reply, but with some of the things you've said (how to measure voltage, not sure which resistor is connected where) you might be out of your depth on this one at this point as you've done more than just the basics and aren't getting reliable results.
     
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