Fender Hot Rod Deville hot bias after installing new tubes

Discussion in 'The Squawk Box' started by egiljae, Oct 12, 2017.

  1. egiljae

    egiljae Junior Member

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    Hi guys!

    Had a gig the other day, when all the sudden my Fender Hot Rod Deville 2x12 turned black. There was a burnt smell to it, so I switched it off and moved to another one on stage. Upon inspection, the fuse had blown, and one of the output tubes was toast.
    Decided to not replace preamp tubes, so I only ordered two new 6L6-GC's. After installing them and warming up the amp, I'm not able to get the amp to bias under 102mv!
    I suspect there is something messed up, and looking at the capacitors, one of the 22uF ones (C35) looks like it's leaking.

    Now, would the bad capacitor result in the output tubes running glowing hot?

    Thanks!
    [​IMG]
     
    Last edited: Oct 12, 2017
  2. Matt_Krush

    Matt_Krush Senior Member

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    No, it won't cause that issue. But should be replaced....In fact replace the other Illinois Caps there as well (they tend to not be the most robust longest lasting caps on the market).

    What is the Plate Voltage and Grid Voltage?
    What 6L6's did you use?
    What value is the bias pot?
     
    JohnnyN and frankv like this.
  3. egiljae

    egiljae Junior Member

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    Ok, so I did some measurements:
    - Plate voltage (pin 3): 496V
    - DC screen voltage (pin 4): 495V (Is this the grid voltage?)
    - Bias pot, turned all the way to the left, measured to 62,1V (right pin) when in standby.
    Not sure if I can measure it's resistance when it's not off the PCB, but it reads 1,3 Ohm across the pins.

    The tubes are Electro-Harmonix 6L6GC matched, it says: Lp: 57 and Gm: 4,900 on the package.
    I measured voltage at the caps, and I noticed they discharged really fast when I switched to standby, from 245V to 30 in a few seconds.


    Oh, just a note. I changed the tubes when I bought it second hand, since it had the same tube blown then. Has been rock solid for years, and I can't remember biasing problems back then.
     
  4. Matt_Krush

    Matt_Krush Senior Member

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    Bias pot should be 25k (but yes you'd have to lift it to measure it accurately)

    R77 shows 68K.
    Check that.
    You may have to change that resistor to get the range down. (lower value)
    You can try piggy backing/parallel connecting a 120K with the 68K..that would bring it down to 43k and perhaps get you down where you need.

    It may take some experimenting..Or you can try a different set of tubes.
     
  5. egiljae

    egiljae Junior Member

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    Tried piggy backing a 120K, but that only made it worse. With the 120K in parallel, bias was at 120mv.
    Then I tried using one of the old tubes and one of the new ones, which lowered the bias to 75mv.
    I assume the new set of tubes is very conductive, and I will ask for a new pair.

    Is there a replacement kit for the capacitors, or any recommendations for which ones to get?
     
  6. Matt_Krush

    Matt_Krush Senior Member

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    hmm...if decreasing the R77 value brought in more current, add 22k to it in series to raise it.
    ***There are multiple schematics for the Hot Rod Deville...what value is R77 on your board?

    Caps for power filter? F&T are good.
    Use the 500vdc rated if replacing 47uf with 47uf.
    **You could opt for the 80uf to tighten up the bass/lowend if you desire.
     
  7. egiljae

    egiljae Junior Member

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    As far as I can tell, R77 is 100K. The amp is a 230V version if that somehow clarifies.
    Ordered new F&T caps for all the axial ones:
    • 2x 22uf 500V
    • 2x 47uf 500V
    • 2x 100uf 350V
     
  8. egiljae

    egiljae Junior Member

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    Last edited: Oct 29, 2017
  9. Leña_Costoso

    Leña_Costoso Senior Member

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    Sometimes, when a tube goes south in bad way, it takes out other components.

    I suspect your bias circuit is ok. It was before, and should have enough range, and then some, for just about any tube set you'll throw in there.

    However.... lets say the tube shorted internally (it happens!). You may have toasted a grid stopper, or the isolation resistors on the grids (or both). Those would be R64, R59, R68, R65. Also check R61 and R62, those are the screen grid resistors. Screen grid resistors are known to blow when a tube shorts.

    And by all means change that leaky capacitor. And... the radial cap in the lower left of your picture looks like its beginning to bulge on top... when they go Jiffy-Pop on top, its time to change 'em before they go splat.... just sayin!
     

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