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Why Does The Filament On My Output Tube Not stay Lit On My Vibro Champ?

Discussion in 'Amp Input - Normal or Bright' started by JeffBlue, Feb 10, 2018.

  1. JeffBlue

    JeffBlue Strat-O-Master

    528
    Oct 25, 2012
    Southern California
    I made some mods on my amp, 3 prong plug, Mercury Magnetics output transformer, 10 inch speaker mod, 5V4 rectifier for slower warm up and a 5881 for the output. When I play through this amp, everything is fine until the sound fades and drops out. When I look at the tubes, all the filaments are lit except the output tube. The problem is intermittent. Any ideas?
     

  2. fezz parka

    fezz parka The Wiggler of Sticks Strat-Talk Supporter

    Cold solder joint on the heaters.
     

  3. Stevem

    Stevem Strat-O-Master

    885
    Sep 3, 2014
    NY
    Besides checking / reflowing your solder connections You may need to bend the female receiver in the tube socket back in shape ( pins 2 and 7) so they grip the tube pins tight like they should!

    The issue is that as the tube a socket heat up and expand , do the receivers and then they loose contact enough to no allow the flow of enough current to sustain the filament .

    The best fix would be to replace a Fender type output tube socket with there C cup grippers over a Marshall type which grips the tube pins in a different and better way.

    It could also be that your pins on that peticular output tube are a hair undersize from what they should be?

    It's also not unheard of to have the solder connections on the tube itself be bad as I have had a few of these over the years also.
    You need to reflow these connections on the tube pins with a big Ass iron to them hot enough , but this amount of head is basically needed on the pin connections that go to the plate of the tube.
    With pins 2 and 7 a 30 watt iron should allow you to reflow them .
     
    Last edited: Feb 11, 2018

  4. JeffBlue

    JeffBlue Strat-O-Master

    528
    Oct 25, 2012
    Southern California
    Thanks Fezz and Stevem, you guys rock. I have reflowed the solder on pins 2 and 7 and the problem still exists. Probably loose contacts in the socket that need to be tightened. I'll let you know if I correct the issue but I won't be able to tackle it today.
     
    Paperback Rocker and fezz parka like this.

  5. JeffBlue

    JeffBlue Strat-O-Master

    528
    Oct 25, 2012
    Southern California
    I think I have other issues. I brought in 2 NOS 5881s and 2 NOS 6V6s to try on my amp. I installed the first 5881 and both the output and power transformers got hot. I pulled that tube out and replaced with another 5881 and the filament wouldn't light. I then installed the first 6V6 and it started to red plate. I removed that tube and replaced with another 6V6 and the filament wouldn't light. I'm not having fun with this right now.
     

  6. fezz parka

    fezz parka The Wiggler of Sticks Strat-Talk Supporter

    The 5881 draws twice the amount of filament current. Hope the 5v winding is ok in the PT.
     
    JeffBlue likes this.

  7. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

    Dec 22, 2010
    Germany
    Heater winding partly shorted. I am afraid you need another power transformer.
    Measure the heater voltage. If it drops very low you found the issue.

    All the best,
    Robin
     
    Borgatomic, JeffBlue and 8T_BoCO like this.

  8. Stevem

    Stevem Strat-O-Master

    885
    Sep 3, 2014
    NY
    Here's a simple test for a bad / overloaded heater winding!
    Yank out one of the preamp tubes, does the output tube heater stay on fine , or atleast a good amount longer?
     
    JeffBlue likes this.

  9. JeffBlue

    JeffBlue Strat-O-Master

    528
    Oct 25, 2012
    Southern California
    I fixed the problem with the intermittent filament on the output tube.....open/connection on the filament/ground on the output socket. I installed a 5Y3 and a 6V6 and I still get a red plate condition. What value resistor do I need to change for the cathode bias? Thank you for your help.
     
    Last edited: Feb 12, 2018

  10. RJ Squirrel

    RJ Squirrel Strat-Talk Member

    Age:
    63
    38
    Jan 28, 2018
    Frostbite Falls
    The stock resistor is 470 ohms, 1 watt per the schematic. In stock form, the 6V6 plates were dissipating about 13.4 watts, with a wall voltage of 117 volts.

    The 1 watt cathode resistor was dissipating just under 1 watt with a 6V6. With a 5881 tube, the dissipation would have been closer to 2 watts. This may have been the cause for the red plating. Sometimes, when carbon comps are overheated, they go down in value. Another consideration is that the electrolytic capacitors in the power supply may be leaking.

    If it were my amp I would do the following:
    1. Replace the 1 watt cathode resistor with a 3 to 5 watt resistor in the 470 to 500 ohm range.
    2. Replace the electrolytic capacitors in the amp, if they are original or have not been replaced in the past 15 years or so.
    3. Test and replace if necessary, the 1K, 1 watt dropping resistor in the power supply rail.
    4. If you don't have the hands on experience to perform the amp maintenance, take it to a tech. The preceding information will provide insight to the tech as to the nature and possible causes for the problem. This also gives you an idea as to what may be needed.
    5. Measure the wall voltage to determine the actual AC voltage supplied to the amp. Depending upon measured voltage build a bucking transformer to bring the wall voltage down the 117 volts that the amp was designed for.http://www.geofex.com/article_folders/vintvolt/vintvolt.htm This would be a nice project for anyone with a vintage amp.
     
    Last edited: Feb 12, 2018
    JeffBlue likes this.

  11. RJ Squirrel

    RJ Squirrel Strat-Talk Member

    Age:
    63
    38
    Jan 28, 2018
    Frostbite Falls
    I really need to be more cognizant of my grammar.:oops:
     
    JeffBlue likes this.

  12. JeffBlue

    JeffBlue Strat-O-Master

    528
    Oct 25, 2012
    Southern California
    Thank you very much Mr. RJ Squirrel, you are most knowledgeable. And thank you Robins.
     

  13. JeffBlue

    JeffBlue Strat-O-Master

    528
    Oct 25, 2012
    Southern California

  14. Stevem

    Stevem Strat-O-Master

    885
    Sep 3, 2014
    NY
    You still need to address the issue of over loading the 6.3 volt filament winding in the PT.

    Once you replace that resistor install the 6V6 output tube and measure your AC heater voltage and note it down, then install the 5881 output tube and make the same check,if you find that the heater voltage had dropped off to 6.1 volts or less then you will at some point burn up the amps power transformer!

    I have to ask why it is you want to run a 5881 output anyway, as they do not really add any more volume to the amp from what I have found , but they make for a a bit of a fatter tone which by the way you can get by installing a JJ brand 6V6 and not have the issue of burning up your PT.

    What these amps want is a better 8 to 10 watt output transformer in them if you want a bit more head room and more low end firmness!
    These amps stock output transformers and power supply even in the best condition can only muster 4.5 watts RMS and roll off a lot of low end at 100 hz on down , atleast from what I have seen from my 82 reissue BF vibrochamp hooked up to my test gear.
     
    JeffBlue likes this.

  15. RJ Squirrel

    RJ Squirrel Strat-Talk Member

    Age:
    63
    38
    Jan 28, 2018
    Frostbite Falls
    The power transformer should be OK with a 5881. Fender used the same power transformer (125P1B) in the AA964 Princeton amp. http://thevintagesound.com/ffg/schem/princeton_aa964_schem.gif

    I'm still curious as to wall voltage. If it's in the 125 volt range like at my house, it will cause undue stress on the amp.
     
    JeffBlue likes this.

  16. JeffBlue

    JeffBlue Strat-O-Master

    528
    Oct 25, 2012
    Southern California
    I just got the okay from the wife.......I can purchase a Vibro Champ upgrade poor transformer from Allen Amplifier. It is designed for use with 6L6/5881 tubes and should eliminate my overheating/over-amperage draw.

    Attempting to source the new cathode resistor and cap.

    For some reason the 5881s sound better to me than the 6V6.
     
    Robins likes this.

  17. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

    Dec 22, 2010
    Germany
    5881s are absolutely amazing in many guitar amps, I really love them.
    The Sovtek WXT5881 is absolutely outstanding and most sturdy.
    Of course it is all about personal taste.
    Measure the heater winding first and make sure you don´t spend any money without needing it.
    I am still pretty sure it is a partly shorted heater winding - again, the voltage drops massively if that happened to your transformer.
    I wish it is still good.

    All the best,
    Robin
     
    JeffBlue likes this.

  18. RJ Squirrel

    RJ Squirrel Strat-Talk Member

    Age:
    63
    38
    Jan 28, 2018
    Frostbite Falls
    FWIW, Fender used this same power transformer in the AA1164 Princeton Reverb. It was at this point that Fender started experiencing problems with the power transformer. Heater current in the AA1164 Princeton Reverb is 2.1 amps compared to the 1.5 amp current draw of the Vibro Champ with the 5881.

    The output transformer could be a different story. The current rating may not be sufficient for a 5881 and the load would not be optimal. The 5881 wants to see about half the load that the 6V6 wants.

    Cathode bias resister should be at least 3 watts to allow for sufficient heat related de-rating, 5 watt would be better.
     
    JeffBlue likes this.

  19. JeffBlue

    JeffBlue Strat-O-Master

    528
    Oct 25, 2012
    Southern California
    The last time I checked, I was reading 6.5 volts AC on the filament wire taps. The power transformer was getting way too hot to the touch, you could smell it. Thanks Robin.

    Remember......I installed a Mercury Magnetics output transformer that should handle the 5881. I will search for a 5 watt carbon comp resistor. Still trying to figure what value. Thanks RJ.