Fender 5e5a pro red plating 1 tube

Discussion in 'Amp Tech Center' started by gpw5150, Jun 19, 2021.

  1. gpw5150

    gpw5150 TDPRI Member

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    Hi all,

    okay, a little knowledge is dangerous…! But here goes;

    at high volume, after a few seconds 1 6l6 nearest the rectifier red plates and all volume is lost. Amp is immediately turned off;

    follows the socket, not the tube.

    Socket replaced
    Grid screen resistors replaced
    Bias is good

    still happens.

    the caps look good, but before i do anything soldery, is it possible the PI tube cause red plating on 1 valve? My thought is not as the coupling capacitor is ‘protecting’ the 6l6 from high voltage dc. However, it would make sense that 1 side of the push/pull is affected only.

    thoughts please!
     
  2. Intubator

    Intubator Tele-Holic

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    When you swap the other 6L6 in that spot does it red plate as well? If so, looks a problem with that socket, hopefully the tubes aren't damaged. What kind of plate voltages are you getting on pin 3 of each tube?
     
  3. gpw5150

    gpw5150 TDPRI Member

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    Yes, fault stays with the socket. It is not easy to fault find as the fault only occurs playing at high volume. Something triggers the runaway reaction, i am not sure a meter will help. I wondered if one side of the PI was delivering too much to a cap and causing a short? I.e not the cap’s fault.
     
  4. Paul G.

    Paul G. Friend of Leo's

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    Redplating is related to bias, so DC issue. You may be losing bias because you have a wiring issue -- check the connections from the connection to the bias at the 220K all the way through to the grid. Make sure your cathode is at ground. If not there, then it is definitely the coupling cap from the Phase inverter.
     
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  5. cherryburst1

    cherryburst1 TDPRI Member

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    You have to troubleshoot this live to find it. It doesn't sound like you are trained or qualified to do this. I do not recommend untrained/unsupervised individuals attempt to repair something with that much voltage. Usually, you end up doing more damage and increasing the cost of the repair, if you don't get hurt in the process. Be smart. Take it to a tech you can trust before you: A. Get injured B. Cause a minor problem like a intermittent wire/cold solder joint, leaky coupling cap or thermal resistor to become an expensive repair requiring new power tubes and an output transformer......
     
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  6. danlad

    danlad Tele-Meister

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    Could well be bad coupling capacitor. Had one affect an EL34 by causing it to ramp up current until a fuse blew. Just replaced it and all was well.
     
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  7. Intubator

    Intubator Tele-Holic

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    Always good to check voltages before and after the symptoms to compare. I agree with what Paul G said, seems like it'd be a 220K resistor or that associated coupling cap. Here's an original layout with voltages indicated, they'll likely test a bit higher though due to modern 120V vs 110V when the amp was designed.
     

    Attached Files:

  8. gpw5150

    gpw5150 TDPRI Member

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    Thanks all. I have a degree in electronic engineering and have been repairing my own tube amps for ~30 years, however, my main line of work is more around business strategy - I have not had to do major re-works or repairs tbh and only intermittently. My main question was around the PI, whilst it could be a coupling cap, they look fine and danady, I will test them later. However, if the PI is bad, could it redplate a power tube on either of the push/pull sides or would the net effect be the drive of both power tubes? I am limited in knowledge on 12ax7 tubes as PI's. It seems like an easy test to replace the PI, but I don't want to unbias the amp and make a worse problem.....

    Unfortunately. the amp went to a tech to sort this problem recently, the amp has come back with the same issue .....
     
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  9. NTC

    NTC Tele-Holic

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    The thing to do is measure the dc voltage at the grid of the bas tube. As others indicated, the bias voltage is not staying steady on tgat socket. It miggt be good to just replace the coupling cap from the pi to that location to see if that fixes things.

    Is this a real 5E5A or a clone?
     
  10. gpw5150

    gpw5150 TDPRI Member

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    Real 5e5a - the recent 57 re-issue, hence i am surprised at a leaky cap.

    I will check the dc at the tube and see. It is a weird fault as it takes a few chords at high volume to trigger the issue - it does not occur at lower volume or unplayed state. Hence my thought on the PI doing something weird to the coupling cap when signal is passed. **i have now changed the PI to no change in fault.
     
  11. NTC

    NTC Tele-Holic

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    Yeah, a bad solder joint is more likely.
     
  12. dan40

    dan40 Friend of Leo's

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    Is this a vintage amp that has been working fine up until now? Has the OT or PT been replaced recently? I agree with everything the fellas mentioned above but I remembered this article by Randall Aiken that I thought I would mention. In the paragraph titled "how much current" he notes that with higher than normal plate voltages, there is no bias setting that will prevent redplating at certain points in the tube's curve unless the OT's primary impedance is adjusted upward to compensate. This condition was noted in certain 100 watt Marshall Superleads that ran 1.7k OT's at plate voltages approaching 500vdc.

    I only mentioned this in case you have recently replaced the PT or OT in the amp. If the amp is original then this condition would not apply in your case.

    https://www.aikenamps.com/index.php/the-last-word-on-biasing
     
  13. Paul G.

    Paul G. Friend of Leo's

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    Redplating -- bias. Bias -- DC. Coupling caps are also referred to as decoupling caps because they isolate DC between stages, so I don't see anything that the PI can be doing to cause redplating as long as the (de)coupling caps are OK.
     
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  14. Paul G.

    Paul G. Friend of Leo's

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    I'd put a meter on the grid of the socket that's giving trouble, turn the amp on and chopstick my way back to the bias input while making sure your bias voltage stays constant. Vibration may be triggering it by disturbing a broken solder joint, or a resistor that looks ok externally but is broken internally or at the point the lead enters. So poke the grid stopper (if you have 'em) at each end and at the middle. Wiggle the wire from the 220k resistor. Poke the 220k resistor at both ends and the middle. Lift the end of the coupling cap at the 220K and measure for DC at that end of cap. Poke the cap with the meter still connected to the meter. Good luck.

    Of course, voltages can kill ya, so follow correct safety practices, one hand in your pocket, no screwdrivers, etc.
     
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  15. gpw5150

    gpw5150 TDPRI Member

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    Thanks dude, will acquire some chopsticks from the kitchen - my wife will be annoyed.....please don't tell her.

    Vibration is an interesting one, I do wonder......I could just re-flow all the related joints I guess, but that is a good call. It is a tricky one as there is no issue below about 8 on the vol, but it is a damn loud amp with a big 15 shifting some air.....and there is no issue if the amp is just left alone at high volume, not being played.
     
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  16. gpw5150

    gpw5150 TDPRI Member

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    Okay, after a number of tests today, i think it is a weird leaky cap issue.

    so,

    bias is good
    Power tube 1 redplates as the signal increases - i.e i can control the redplating via the guitars volume.

    Shaking/vibrating the amp shows no effect.

    i think that the cap that de-couples power tube 1 has a bizarre fault, there is zero dc until the volume increases where dc seems to be exposed to the tube causing the issue, this is not something i can easily measure with my kit at home, but at least I have a load more info for the tech.
     
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  17. cherryburst1

    cherryburst1 TDPRI Member

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    As grid current increases with power draw, there should be a small but equal drop in bias voltage across both grids equally.
    Have a tech. put a voltmeter across the the outputs of the grid leak resistors where they connect to the 1.5k grid resistors. + lead to resistor of suspected faulty circuit. With no AC signal, look to see trace voltage difference across the GL resistors. Use a signal generator to supply a 1kHz tone to amp. Increase volume monitoring meter. Any presence of significant voltage difference will be + voltage leaking from cap. Of course, you could just replace the cap and be done with it. If the cap is at fault, the problem will be solved. I would replace everything in that path from both PI plates to 1.5k resistors. If the signal is collapsing on the other drive side, it would also cause more current draw on the suspect 6L6. If the problem goes away when 2 caps and 4 resistors are replaced, you're done.
     
    Last edited: Jun 20, 2021
  18. cherryburst1

    cherryburst1 TDPRI Member

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  19. Intubator

    Intubator Tele-Holic

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    Glad you figured it out! I'd personally be inclined to replace a few if not all of those coupling caps if they're deemed questionable or IC brand... Maybe the filters as well if you have it all out of the cab anyway...
    Photos?
     
  20. gpw5150

    gpw5150 TDPRI Member

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    Orange drops.

    TBH, this amp has IC’s on the filter circuit, but custom made Fender yellow’s in the tone circuit. I don’t want to play with the amp too much and lose the sound.

    I have changed to a new tech, i think the last guy new a fair bit, but jumped to conclusions. The new guy is at least 70 years old, plays a tweed super and was a radio engineer. Think he is thorough.


    0687290C-BFD0-4487-8146-1BA2E9D3C749.jpeg
     
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