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  1. #1
    Les Paul Forum Member JeffBlue's Avatar
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    Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    I have made some mods on my amp, 3 prong plug, Mercury Magnetics output transformer, 10 inch speaker mod, 5V4 rectifier for a slower warm up and a 5881 for output. When I play the amp, all sounds great and functions fine until the sound fades and drops out. When I look at the tubes, all of the filaments are lit except the output tube. The problem seems intermittent. Any ideas?

  2. #2
    Les Paul Forum Member
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    I guess it's either the wiring, the socket or the tube. Re-flow the heater connections to the socket, re-tension the connections within the socket and if that doesn't work try a different tube.

  3. #3
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Quote Originally Posted by springhead View Post
    I guess it's either the wiring, the socket or the tube. Re-flow the heater connections to the socket, re-tension the connections within the socket and if that doesn't work try a different tube.
    Thanks Springhead, I have reflowed the solder joints. I need to re-tension the socket connection. I did spray contact cleaner on the tube pins and reinserted into the tube socket.

  4. #4
    Les Paul Forum Member Jeff West's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Check if you can whether that power tube does the same in another amp, or even a tube tester. It is very possible for a tube to have an internal connection that opens as it heats up, then is restored after it cools down. This can happen with the heater, or with other elements. In the latter case, the heater stays lit of course. Not guaranteeing that’s your problem here, but it could be.

  5. #5
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Oh NO!.......I have other issues. I brought in 2 NOS 5881s and 2 NOS 6V6s and installed the first 5881 and both the power and output transformers got hot. I replaced the 5881 with another 5881 and the filament wouldn't light. I then installed the first 6V6 and it began to red plate. I then installed another 6V6 and the filament wouldn't light. What is going on? This thing is driving me crazy.

  6. #6
    Les Paul Forum Member
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    I'm assuming this is a single-ended cathode biased amp, correct?

    First: Double check not just the connections, but the wiring itself. Make sure all wires are correct, and also look for little solder hairs that could be making cross connections.

    Second: Replace the socket to make sure there's no internal. New ones are like $2.00

    Third: Do the math for bias current, and figure out proper value for cathode resistor. Replace with a new resistor and cap (again, cheap). Save the old parts if you're sentimental.

    Lastly, 5881 and 6V6s are not normally interchangable.
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  7. #7
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    I fixed the intermittent filament on the output tube....... open/bad connection on the filament /ground on the output tube socket. I replaced the 5Y3 and another 6V6 and I still have a red plate condition. Thank you for your help.
    Last edited by JeffBlue; 02-12-18 at 05:00 PM.

  8. #8
    Les Paul Forum Member
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Red plate is pulling too much current. I'd replace the output tube cathode resistor/cap combo with new of like or similar circuit values and voltage/wattage.
    2007 CR8
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  9. #9
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Quote Originally Posted by thejaf View Post
    Red plate is pulling too much current. I'd replace the output tube cathode resistor/cap combo with new of like or similar circuit values and voltage/wattage.

    Thank you for your response, I will search if I have those values or go and buy some.

  10. #10
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    The cathode resistor is shorted.

  11. #11
    Les Paul Forum Member
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Lift the output tube cathode cap - unsolder one end. It may have gone short,. You can then (with power off) measure the cathode resistor and see if it is ok. It's more likely to be the cap than the resistor, unless the latter has cooked.

    You can use the amp without the cathode cap.

  12. #12
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Quote Originally Posted by springhead View Post
    Lift the output tube cathode cap - unsolder one end. It may have gone short,. You can then (with power off) measure the cathode resistor and see if it is ok. It's more likely to be the cap than the resistor, unless the latter has cooked.

    You can use the amp without the cathode cap.
    Thank you Mr. Springhead. I measured the cap connected.

  13. #13
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Gentlemen...........I installed a 490 Ohm resistor carbon comp resistor and a 20 uf 200 volt cap in parallel on the cathode and the amp sounds excellent. Kind of like a low power twin (perhaps running a little cold using the 5881) Both transformers are barely warm. I'll install a 6V6 later to see if there is still a red plating issue. I want to thank everyone who has replied and offered their advice. You guys are great to hang out with here on this forum and I am constantly learning here. Yep.....you guys rock!

  14. #14
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Presently running/testing the 6V6 and there is no more red plating. I can effectively swap between 6V6 and 5881 (bias may not be perfect)

  15. #15
    Les Paul Forum Member
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Good news! So was the old cap short? i.e. did the resistor measure ok?

    490 isn't a standard value, and would make it a bit cold. Or do you mean 390?

    Either way, glad you got it running.

  16. #16
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Yes, it is running cold, but I got the amp working! BOOYAH! The original resistor only appeared to be shorted while in the circuit, but read 606 ohms when removed. This is all a major milestone for me, I needed to have a victory.......I CAN do this. I have a Monotone Studio One amplifier kit that I built and isn't functioning correctly. I may have the confidence to tackle this kit again and for that, I thank all of you who had replied and offered your input and experience. I love hanging out with you guys on this forum.

    I will be changing the value of the resistor at some point. Thank you.

  17. #17
    Les Paul Forum Member
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Glad it worked out for you. Also, IMHO if you're running it a bit cold and happy with the sound then I wouldn't change it. Just because the bias is not running warm or pushing "textbook" values is not necessarily a bad thing (e.g., these are guitar amps, not HIFI systems).
    2007 CR8
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  18. #18
    Les Paul Forum Member JeffBlue's Avatar
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    Re: Why Does The Output Tube Filament Not Stay Lit On My Vibro Champ?

    Quote Originally Posted by thejaf View Post
    Glad it worked out for you. Also, IMHO if you're running it a bit cold and happy with the sound then I wouldn't change it. Just because the bias is not running warm or pushing "textbook" values is not necessarily a bad thing (e.g., these are guitar amps, not HIFI systems).
    I love a good blackface clean tone which is why I love Fender amps. My Vibro Champ sounds like a low powered Twin and sounds great. When I use my pedals for dirt, it still sounds fantastic. I may experiment with different values later, but for now, all is well. tHank you.

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