Blues Jnr - dime the master or not?

Discussion in 'Glowing Bottle Tube Amp Forum' started by spenno, Jul 2, 2019.

  1. spenno

    spenno Tele-Meister

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    Hey all.
    I've recently picked up a nice ltd Blues Junior (seafoam with creamback). I've been delighted with it so far - it's exceeded expectations. I wish I'd ignored the naysayers sooner; it's plenty loud enough, sounds much better than expected, and has behaved very well. Until...
    So something I'd always read about was the 'trick' of opening up the master volume on the BJ to release the tone and control it using the volume. After playing for half a year with both vol/master on 4, I thought I'd try a gig with the master maxed out, and the volume on 3ish. Definitely a different sound - I'd say fuller and more pleasing to my ears. So I did the same at the last gig and within a few minutes there was a bad squealing when played at any reasonable volume (not when idle, only playing).
    I replaced the three preamp tubes and output tubes in turn to find the culprit to no avail, tweaked anything that looked tweakable to a numbskull (ensured speaker/reverb were plugged in) before sending it back to Fender, as it's well within its warranty.
    Within two days it's been repaired and is on its way back to me with the fault report stating note 'shorted EL84 and open S.G. resistor replaced'.
    So even though the problem still existed with new valves, can I assume this is anything to do with the 'trick' of maxing the master volume? Or just a wild coincidence? I assume this is a widely used thing, so is it just left to chance?
    I have read all about the mods for this amp and had planned on getting a load including the bias mod. But even though I love the little guy, there's no way I'd contemplate mods with a BJ until the 2 year warranty had expired.
     
  2. Marc Morfei

    Marc Morfei Tele-Holic

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    I used to have a BJ, but never ran it that way. I ran it as you did, vol and master on about 4.5, just on the edge of getting some grit. Pushed it over with pedals as needed. But I know others advise to open up the master. Diff'rent strokes. I tend to never have anything on 10, ever.
     
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  3. Commodore 64

    Commodore 64 Friend of Leo's

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    Assuming SG is screen resistor tube failure can cause that, if the failure results in excess screen current. Could go the other way, too.

    The MV on the blues Jr. doesn't change the voltage on the tube. It doesn't change the operating point of the tube. It just changes how much signal gets to the grids of the power tube and thence gets amplified. I'm not sure diming an amp is materially different that idling it from the power tube's standpoint. I notice a few mA of difference on a big strum of open chords if I've got something hooked up to monitor plate current. But nothing huge.

    http://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf

    Looks like screen grid resistor is 100 ohm, 1/2w resistor. I use 3W screen grid resistors. Totally possible you just blew a screen resistor from general use.
     
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  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Those amps are notorious for blowing screen grid resistors, but it has nothing to do with how they are used. Set the controls how you want and rock it out.
     
  5. Matthias

    Matthias Tele-Afflicted

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    Yep! Mine did that!
     
  6. Matthias

    Matthias Tele-Afflicted

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    Double post
     
  7. Keefsdad

    Keefsdad Tele-Holic

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    I find they sound good unless you crank the gain REALLy high and the master low. That really sounds bad.
     
  8. Beachbum

    Beachbum Friend of Leo's

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    I have this theory that seems to work well for me. I play with the knobs until it sounds good.:)
     
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  9. MuddyWolf

    MuddyWolf Tele-Meister

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    Sometimes its not the heat or electrical "juice" that blows up a cranked amp. It can also be the intense vibration of the speaker rattling apart your combo amp. Or sometime the mojo of the righteous licks is too much and the amp blows up. Thats happened to me a couple times.
     
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  10. Chiogtr4x

    Chiogtr4x Friend of Leo's

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    For me, since 2000, I run my Volume at 4 (5 max), Fat switch ON, and Master wherever I need it for the room ( never past halfway) for what I'd say is a big, clean tone ( here it's enough volume for clean lead or rhythm) , just shy of breakup.
    Then I use one of two OD"s two push distortion in a few directions:

    Blues Driver- which maintains a 'smiley face EQ' and pretty damn close to a cranked Fender amp tone...

    Barber Direct Drive- a midrange boost, thicker clean tone with a good crunch and sustain- not fizzy, good '70's rock tone ( IMO)
     
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  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Yep. This happens to me constantly.
     
  12. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    :lol:
     
  13. Deebs3

    Deebs3 Tele-Meister

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    Running the master high seems too noisy for me, just use the dials as you feel like.
     
  14. Teleguy61

    Teleguy61 Friend of Leo's

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    Hot Rod Deluxes also are guilty of this.
    Seems it's common in the modern Fender PCB amps, as some of the parts are too light.
    1/2 watt screen grid resistor is not enough.
    Had a HR Deluxe that fried a screen grid resistor, which took out a tube, which caused a very distorted signal (this was on the gig), which took out one of my orange frame JBL D-120F speakers.
    Needless to say, the HRD took a hike pretty quick, as even after repair, I didn't feel I could trust it.
    I am interested in a Blues Jr, but this possibility keeps me hanging fire.
     
  15. spenno

    spenno Tele-Meister

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    Thanks for the tips guys.
    So if I get some mods further down the line, is there an improvement/tweak that can be made to the existing screen grid resistors? Sorry, I’m not clued up on these or what they do, but must admit I have already lost a little confidence but love the amp enough to want to persevere.
     
  16. Joe M

    Joe M Super Moderator Staff Member

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    I have always maxed the master on any amp that I've owned with one and controlled the overall volume with the gain and guitar volumes. Works best for me. But, as others have said, do whatever sounds best for you.
     
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  17. spenno

    spenno Tele-Meister

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    My original post isn’t really about which sounds best. Rather whether diming the master could contribute to the issue I’ve had.
     
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  18. Joe M

    Joe M Super Moderator Staff Member

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    No
     
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  19. Wally

    Wally Telefied Ad Free Member

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    FWIW, the Blues Jr and Pro Jr. amps are set up to run those EL8’s at or near 100% of max plate dissipation, ime. That is very hot biasing for fixed bias. I have wondered if they are shooting for Class A push/pull operation???? At ay rate, get ready to replace power tubes regularly even if they don’t blow.
     
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  20. twangking

    twangking Tele-Afflicted

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    Yes. Crank the master and remove it out of the equation. Run it at three. With my bridge pickup it'll break up and start to bark.
     
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