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Master volume Twin Reverb blowing fuses

Discussion in 'Amp Tech Center' started by jondanger, Sep 24, 2016.

  1. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    As others have suggested it's well worth replacing all those white caps inside the amp while you're there. That thing is remarkably clean inside.

    Is your pull boost pot in working order? If not I can point you to a replacement for it if needed, they're remarkably pesky things to track down.
     
  2. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    I did find an acorn in the back of the cab, aside from that - fresh.
     
  3. Thin69

    Thin69 Friend of Leo's

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    Very nice inside, looks all original.

    Your bias is original and is a balance pot so you can balance the two pairs of power tubes. As BobbyZ suggested adding a bias level pot to the existing balance pot is very worthwhile mod and gives you to set overall bias level in addition to balancing the two sides.
     
  4. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    For posterity, here is BlueGlow's instructions on how to add a bias adjust to the bias balance circuit.

    http://www.blueglow.de/TechPaper/BalancePlus.html

    My grounding resistor is 15k, so I guess I'll go with a 10k trimpot and 10k resistor, provided I can get everything else working okay.
     
  5. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    The pull boost pot did seem to be working right. When I was testing for "any sound at all" plugged in via the limiter, I checked the pull boost, and definitely noticed a gain boost. Had me wondering if it actually sounds good despite rumors to the contrary. Then it had me wondering if I could make it footswitchable! Then it had me wondering what I've gotten myself into . . .
     
  6. Thin69

    Thin69 Friend of Leo's

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    I did mine exactly like that. No need to drill holes in the chassis for the pot! Those values for the trimpot and resistor should work fine.
     
  7. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    Sure. One of the downsides of the pull boost thing is that it can add noise. If you simply ran the pull boost wires out to a foot switch and back it'd be a horrible mess (probably). If you used a DPDT relay you probably do it without adding noise.


    Oh, man... Been there!
     
  8. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Okay, so I replaced all the filter and bypass caps and . . . same voltages, same issues, except now I have 7vac on the pilot light. I tried a known good pair of 6L6GCs in V8 and V9 (middle power tubes) and got about 317v starting plate voltage, but as the amp warmed up it started decreasing rapidly. I get the same thing with the Phillips 6L6WGBs that were in it when I got it, but the starting plate voltage was around 295 with those, eventually settling around 245.

    With the power tubes pulled, I have plate voltage of 467, and I'm not blowing a 2 amp fuse without the limiter. The amp calls for a 3 amp, just being extra cautious until I sort this out.

    So it seems like I'm dumping current somewhere after the power tubes, but I don't know where to look. Any ideas? Are we getting to the voodoo seance part of the process yet?
     
  9. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Let me revise those heater voltage numbers. I don't know why I had 7vac to start. Now I have 2.78 vac from one side of the pilot light to ground, and 5.57 vac between the two with power tubes pulled. With the power tubes in, I have 2.23 vac from one side of the pilot to ground, and 4.45 vac from one side of the pilot to the other.
     
  10. Thin69

    Thin69 Friend of Leo's

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    I wonder if your power tube grid voltage is opening up. At 65% max plate dissipation my grid bias voltage is -43V on my SFTR. If the grid voltage opens up and approaches 0V the plate current will go through the roof and dramatically drop plate voltage. What is your grid voltage (DC)? It's pin 5 on the power tubes.
     
  11. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Grid voltage is -30 on V7 and V8, and -25 on V9 and V10.
     
  12. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    I just pulled the grid resistor on V10 out of the circuit by lifting the lead on pin 7. It's a little high but not out of control. Reading 1670 ohms. Spec is 1500.
     
  13. Thin69

    Thin69 Friend of Leo's

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    That does sound low on both sides and I believe would create excessive plate current but I don't really know to what extent. I think something is amiss with your bias circuit. Does the amp still have a 15K resistor from the bias pot to ground?
     
  14. Thin69

    Thin69 Friend of Leo's

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    Grid resistor is fine. There's almost no current through this resistor so really not a voltage drop across.
     
  15. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    It does still have the 15k bias resistor.
     
  16. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    The schematic has -52v on the center lug of the bias pot. I've got -30v to start, then creeping down to -26v after about a minute of the amp warming up. I think you're right that something might be amiss there . . .
     
  17. Thin69

    Thin69 Friend of Leo's

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    Can you check the voltage at the back of the bias pot where the 3.3K resistor connects to it.
     
    Last edited: Sep 28, 2016
  18. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    I've got -30v there. But I'm also a little bit low on the other side of that resistor too. Schematic has -63v and I have -35v. This is the spot where the diode, the 33k resistor, and the 100uf cap come together.
     
  19. Thin69

    Thin69 Friend of Leo's

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    Definitely too low schematic shows -52VDC for that location. Did you change the white 80uf bias caps and do you have the +terminals going to ground?
     
  20. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Yep, the bias board looks like this now:

    [​IMG]
     
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