Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups darrenriley.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Why Does The Filament Drop Out On The Power Tube On My Vibro Champ?

Discussion in 'Glowing Bottle Tube Amp Forum' started by JeffBlue, Feb 10, 2018.

  1. JeffBlue

    JeffBlue Tele-Holic

    824
    Oct 24, 2012
    southern California
    I fixed the problem with the intermittent filament on the output tube.....open/bad connection on the filament/ground. I installed a 6V6 and a 5Y3 and powered up and I still get a red plate condition. What value of resistor do I need for the cathode? I thank you all for your help.
     
    Last edited: Feb 12, 2018

  2. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    Anywhere from 470-680 ohm would be my guess depending on your B+. 3W min, 5W better.
     
    JeffBlue likes this.

  3. Dacious

    Dacious Friend of Leo's

    Mar 16, 2003
    Godzone
    If someone set it up for the 5881 they probably halved the stock one - it should be a 470.

    But the other issue is -- your electro caps, when were they changed if ever? Could be the 25-25 bypass cap is cooked and shorted internally. Result - dead short to the cathode.

    If they've never been replaced, you really need to do this. They're fifty years old and would by causing less than ideal tone. Cap can, bypass caps, bias cap.
     
    JeffBlue and RLee77 like this.

  4. JeffBlue

    JeffBlue Tele-Holic

    824
    Oct 24, 2012
    southern California
    Caps were all changed out before I got this amp. It still had the 2 prong plug. I'm thinking between using a 5881, a 5V4 and using an external cab with the 10 inch speaker (total 2.6 ohms) probably cooked the resistor/capacitor.
     

  5. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    Very possible the cathode resistor cooked, specially if it's a 3W carbon comp.
     
    JeffBlue likes this.

  6. JeffBlue

    JeffBlue Tele-Holic

    824
    Oct 24, 2012
    southern California
    Yep.....cathode resistor is shorted.
     

  7. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    Start with a 470-ohm 5W, if you are going back to 6V6GT, and I think you should.

    A 5881 will make little difference in volume/headroom and only add current draw. I dabbled with one in my Vibrochamp before ultimately going back to 6V6GT.
     
    Wally and JeffBlue like this.

  8. JeffBlue

    JeffBlue Tele-Holic

    824
    Oct 24, 2012
    southern California
    What value cathode resistor should I use if I stay with a 5881? The wife gave me the OK to purchase an Allen upgrade power transformer that is designed for using with a 6L6/5881. My original PT was getting way too hot to the touch and smelled of breaking down. Thanks Mr. Commodore 64.
     

  9. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    imho and ime, that amp wants to see a healthy circuit with a 6V6 in it. A hot PT is an indication of too much current draw. That is not determined by the OT. The bad cathode resistor is a warning, imho. I would suspect everything about this amp. I would assess everything, replace what was bad with schematic value components, install a 6V6 and evaluate the operation.
    If all is proper and there is not enough volume, a high efficiency speaker or speakers will do more than a 5881/6L6 power tube will do for sound production level...Ime and imho.
     
    Nickfl likes this.

  10. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    You are going to need to get a few different resistors. 150, 200, 400, 680 (Parallel 2 for 340).

    But listen, even with a 6L6GC, in cathode bias you are looking at 6.5W output. You aren't going to hear a difference between that and 4.5 watts of a 6V6. Speaker efficiency matters way more.

    Check out page 2, 300V Cathode Biased specs (you'll be running higher than that, but it's a good first approximation.

    https://frank.pocnet.net/sheets/049/6/6L6GC.pdf
     
    JeffBlue likes this.

  11. JeffBlue

    JeffBlue Tele-Holic

    824
    Oct 24, 2012
    southern California
    Gentlemen...........I installed a carbon comp 490 Ohm resistor and a 20uf 200 volt cap in parallel on the cathode and the amp sounds excellent. Sounds like it is running a little cold but clear as a bell with lots of headroom (using the 5881) The previous resistor reads 606 Ohms out of the circuit and that's when everything was over-heating and red plating. Now it kinda sounds like a low power twin (I'll run pedals for dirt) Thanks everyone, I feel very little heat when I touch both transformers.
     

  12. JeffBlue

    JeffBlue Tele-Holic

    824
    Oct 24, 2012
    southern California
    I am presently running/testing a 6V6 in my amp and there is no red plating. I can effectively swap between a 5881 and 6V6 (although the bias may not be perfect)
     

  13. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    All the voltages in the amp's power section should normally be charted before and after you change parts around - and definitely when swapping these power tubes around? You can calculate and then confirm the bias using installed part values and your multimeter.

    I hope you have ditched the 2-prong cord. That needs to happen for safety. There is no reason to leave one installed.
     
    JeffBlue, Wally and Commodore 64 like this.

  14. JeffBlue

    JeffBlue Tele-Holic

    824
    Oct 24, 2012
    southern California
    Yes sir........and thank you. 3 prong cord already installed, Mercury Magnetics ot and 10 inch 4 ohm speaker mod. Wasn't looking for speaker break-up but rather more efficiency.
     

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.