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Spare (Blues Jr) Parts Build

Discussion in 'Shock Brother's DIY Amps' started by mcabe, Jun 16, 2015.

  1. Commodore 64

    Commodore 64 Friend of Leo's

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    If you wanted to repurpose the hole for a more useful switch...and it can't be anything in the signal...

    You could make a switch going from diode to tube rectification. You could make a switch changing the bias type (cathode vs fixed).

    You could do a NFB switch, On-off-on with some different resistance values on the On.
     
  2. mcabe

    mcabe Tele-Meister

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    Interesting ideas! This amp has a Vox AC30 like circuit with 2 ECC99 (dual triodes) in the power-amp. I don't have much room to add more components. How hard would it be to change the bias or add NFB? I think the latter would be easier?
     
  3. mcabe

    mcabe Tele-Meister

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    I think a NFB loop would require wiring a variable resistor (to control the amount of NFB) between the output transformer secondary and the cathodes of the long tailed pair phase inverter. Would also need a 4.7K resistor between the NFB connection point (R15 and C12) and ground. I can see how the mod would work, but it involves removing and modifying parts of the ground buss.

    Before I start modding, I think I will get the amp working as designed first. I can always go back and tweak later. So standby switch it is!
     
  4. mcabe

    mcabe Tele-Meister

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    Completed the amp this weekend. Good news is that all the voltages looked good and it started right up without problem. Bad news is that the volume is much too low. Even with volume and master volume wide open the amp puts out about as much volume as my iphone speaker. Clearly something is not quite right.... I dropped a 16 ohm speaker in the amp, which I think the Blues Jr. output transformer should be able to power. I also tried tracing the signal in the amp, but it was not clear where the volume loss is occurring.

    Any common culprits or things to check?


    Here is a picture of the completed amp.
     

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  5. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    I'd look here first.......

     
  6. mcabe

    mcabe Tele-Meister

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    I am really hoping that is not the problem. I'd prefer not to have to buy another output transformer.

    The schematic shows an impedance of 11,600 into an 8 ohm speaker. The Blues Jr. transformer has an impedance of about 6,000 ohms. I always though that mismatches of 2:1 were no big deal. Plus I am using a 16 ohm speaker, so the reflected impedance should be 12,000 which is an exact match, right?
     
  7. davidjon_99

    davidjon_99 TDPRI Member

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    Oops, looked at the wrong info.
     
  8. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    12AU7's usually run at 22.5k:8, so the stock one won't likely be a good match. Poke around with a chopstick just to make sure the signal path joints are good. Also, when you do the voltage check, are you getting 'pops' when you touch each test point?
     
  9. mcabe

    mcabe Tele-Meister

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    I went back into the amp tonight, and checked all of the voltages against some results that a user on ax84 had reported. Everything lined up pretty well, but I definitely had some pops and loss of volume when I probed some of the pins (plates made pops, and grids of the PI loss of volume).

    Not sure what changed, but when I plugged my guitar in there was plenty of volume. This is a loud 4 watts! So either I was working too late last night and didn't have the guitar plugged in properly, or the probing fixed the issue.

    I did wire the bass control backwards, so I am going to fix that and reflow the solder on some other connection points just in case there is a bad solder connection somewhere.
     
  10. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    Great that it came on- hopefully it was as simple as the guitar being turned down or something. I would still go joint-by-joint with a chopstick just to be sure that a bad solder joint isn't still hiding in there that was re-connected when you were checking volumes. I would be a PITA to have the amp sometimes work, and sometimes not :eek:
     
  11. mcabe

    mcabe Tele-Meister

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    Totally agree. Hopefully it was just "user" error. There is a fair amount of hum/buzz too. It changes with the volume controls and is present even without a guitar plugged in. Any tips/ tricks to tracking down noise in a new build?
     
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Since the noise is affected by the volume pot it has to be entering before the pot.

    Since the noise is there with nothing plugged in the input jack it's probably not between the input jacks and the tube grid because that should be grounded out.

    The number 1 suspect is inductive coupling between tube socket lead wires. Have you tried moving your preamp tube wires around? Separating the plate and grid wires (output and input) is important. Use a wooden chopstick to move the wires with the amp running so you can monitor the noise. Be careful and put one hand behind your back (seriously).
     
  13. mcabe

    mcabe Tele-Meister

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    Thanks Rob! I'll see if I can isolate by removing the preamp tube and confirming that the noise goes away. Otherwise it could be rf noise from the florescent lamps or perhaps the MOSFET? I'll definitely be careful working inside the live amp though.
     
  14. mcabe

    mcabe Tele-Meister

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    I was looking at some Vox tone stack layouts online and they seem to match what I have. But the bass control definitely seems to be working in reverse (i.e., when I turn the control to the right bass goes down). It must be backwards, but I am a bit confused.

    Does this look backwards? Controls from left to right are volume, treble, bass, master.
     

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  15. davidjon_99

    davidjon_99 TDPRI Member

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    Do you have the amp plugged into a light bulb current limiter? With my recently built 5e3 I noticed a lot of hum when running it through the limiter vs. going straight to the power outlet. Anyway, just a thought.
     
  16. mcabe

    mcabe Tele-Meister

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    Nevermind. Found a picture of a vintage AC30. My bass pot is wired backwards. I have a hard time with potentiometer layouts. I never know if the images are looking at the front or the back of the pots.

    Here is the vintage AC30:
    http://web.comhem.se/~u70623527/Vox%20AC30,%201960%20bass%20and%20treble.jpg


    Here is the layout I was using: http://www.tdpri.com/forum/attachments/amp-tech-center/271710d1423669690-layout-diagram-ac30-style-build-ac30old-jpg


    Funny, that layout still looks wrong to me.... The vintage amp has pin 1 of the treble connected to pin 1 of the bass pot. The schematic shows pin 1 connected to pin 3. Oh well, don't believe everything you read on the internet I guess.
     
  17. mcabe

    mcabe Tele-Meister

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    I did on startup, but plugged straight into the wall to measure the voltages. I find that the voltages are a bit lower when going though a current limiter. Maybe, I should check to see if the limiter was causing my mysterious volume loss....
     
  18. mcabe

    mcabe Tele-Meister

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    Thanks for the guidance!

    I pulled the preamp tube and the worst of the buzzing went away. Started moving wires around with a chop stick (actually I was using an unsharpened pencil, but same idea) and found that the worst buzz was coming from the wire leading from the output of the master volume. I think it was running too close to the PI output. After rerouting the wires the buzz is manageable. With the volumes dimed and treble turned up, there is still an audible buzz, but it's reasonable as long as the treble is bellow 8 or 9.

    With the treble above 8 or 9, the buzz comes back. This is my first experience with the highly interactive Vox tone stack. I don't think I'll be running this amp with the treble wide open, but what would make the buzz tied to the treble control like that?
     
  19. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The treble control is a high frequency (low pass) filter so it's sending the buzz to ground when turned down.
     
  20. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    Maybe shielded signal cables would be a fix?
     
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