5G9 Tremolux build troubleshooting

Discussion in 'Shock Brother's DIY Amps' started by montana, Feb 2, 2019.

  1. dan40

    dan40 Tele-Afflicted Silver Supporter

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    The readings all look good I believe, so it would seem that your power supply and B+ rail are functioning properly. If the bright channel is now working properly, I would focus your efforts on the wiring of the other channel. There must be a bad joint or wiring error that is affecting only that channel. The normal channel runs from the input jack, through one half of v1 and then mixes with the bright channel at the volume pots. Recheck everything between those two points. I would also give some thought about shortening some of the leads to the preamp tube sockets. There is alot of extra wire length from different stages laying parallel to each other around those sockets. You want the most direct path to the socket without putting strain on the wiring.
     
    Last edited: Feb 8, 2019
  2. dan40

    dan40 Tele-Afflicted Silver Supporter

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    I agree that raising the grid and cathode wires up a bit more would be good and I try to do that in my builds. Most of the old tweed amps seem to have the wires laying flatter because of the tight chassis and the board being at a 90 degree angle to the sockets.
     
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  3. flyswatter

    flyswatter Friend of Leo's

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    When I was suggesting earlier that you move the filter cap grounds to the preamp bus, I did not mean all of them -- just the last one or two in the chain (i.e. splitting the grounds).

    Look at some of Rob Robinette's layouts for various tweed/ BF models to get the general idea.

    Here specifically is what happened with my Brownface 6G3 build (I think it's relevant because it's a similar amp to the tweed Tremolux with a similar tube complement and layout):

    1) I wired up the grounds to all 4 filter caps as a daisy chain and sent it up to the power supply ground star at one of the PT bolts. This arrangement was implied by the Fender schematic (symbols for chassis ground to no specified point) and pictured explicitly on the Mojotone layout for the 6G3.

    Result: The amp hummed and spat like a banshee.

    2) I tried EVERYTHING to locate and eliminate the ground loop. Nothing worked until:

    3) I detached the ground for the preamp filter cap (the fourth in the chain) from the other filter cap grounds and ran that one ground to the preamp bus. Hum was entirely eliminated.

    You've tried enough other things that it sounds like your amp has a grounding issue. Suspecting brand new transformers of failure and worrying about the niceties of lead dress at this particular point are most likely just sidestepping Occam's Razor.
     
  4. montana

    montana TDPRI Member

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    @flyswatter Ahhh, that makes sense now. Ok, I'll do that!
     
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  5. flyswatter

    flyswatter Friend of Leo's

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    Good luck, and just to try it you can alligator lead the grounds in place, then resolder if it works out. No guarantees-- it's just I was in a similar predicament of tried-everything-else/nothing-worked. Then splitting the FC grounds did.
     
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  6. montana

    montana TDPRI Member

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    Tried as you suggested @flyswatter, but no luck.

    I've noticed that the filter caps drain very fast on their own without me having to discharge them. It take 10 seconds after shutting the amp off to drop from 400V to 3V. That seems unusual so wondering if it may be a clue to the hum issue.
     
  7. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Hot tubes drain filter caps pretty fast. The main risk of shock is when you switch on your amp long enough to charge the caps but not heat up the tubes. Or of course if you reach into the amp before the hot tubes can do their thing.
     
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  8. Mexitele Blues

    Mexitele Blues Tele-Meister

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    The screens should be a little less than the plates right? Any chance your power node connections are off?
     
  9. sds1

    sds1 Tele-Holic

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    Have you tried swapping power tubes yet ?
     
  10. bigguy12321

    bigguy12321 Tele-Meister

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    Ok Montana, Sorry for this vague post but your slow drain issue reminded me of something.

    I built a small 2-6av6, SE 6v6 Champ style circuit that had a similar issue. At the time of the build I didn't have a fuse to put in place so I just alligator clipped the three prong power cord to the trans.

    (yes... I know... KIDS DON'T TRY THIS AT HOME...)

    Upon first fire up, the amp worked but had a hum. And when shut I noticed that it took several seconds...maybe 10... to drain down.
    I also had the speaker alligator clipped in.

    Now I told you that so i could tell you this.

    After I put it together correctly with cord and fuse attached and soldered in as well as speaker attached, there was no hum and fast voltage drop.

    I may have also re-soldered the center tap of the high voltage. Anyway, I think that proper soldering of earth grounds and power side in general helped me the most. Voltages were correct in the circuit as it was first wired but maybe the poor center tap and earth (green wire from plug) were shaky.

    Hope it helped.

    a
     
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  11. Nickfl

    Nickfl Tele-Afflicted

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    View attachment 581805

    This is really bad, way two many leads running parallel and close together. Some of them could be a lot shorter, others need to be routed differently so they cross at a right angle. IMO, it is preferable to fly some up away from the chassis rather than have them all this close together. Your V1 socket looks good with all the leads crossing at right angles and spaced apart, all the others need to look like that too, and once they do I think there is a decent chance your problems go away.

    I see you reflowed some of your joints and that improved things, did you also reflow the input jack connections? A bad connection (or bent shorting tab) at the input jack can lead to some nasty hum and buzz.

    The other thing that comes to mind is related to Big Guy's post, specifically I can't seem to see where your PT secondary center tap is and how it is referenced to ground. Ideally the negative side of your power amp filter caps should be directly connected to the center tap, and that should then be referenced to the chassis ground via a lug bolted to the chassis. If you don't have a good connection there, that could explain some of your issues.
     
  12. bigguy12321

    bigguy12321 Tele-Meister

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    Just noticed. Your bias wire (the white with red strip wire) seems to run right next to or on some signal wires. ( I'm assuming yellow is some kind of signal wire)

    Edit... I looked again. They go to the standby switch. Sorry... But...

    The heaters do look a little close to signal wires here.
    IMG_1368.JPG


    Also, did you do any work on the filter cap grounding? I had a bad hum that turned out to be where I had a terminal strip for a common ground for two filter caps that was riveted to the chassis. I found it by chopsticking all around till I could affect the noise. I put a #8 bolt in place of the rivet and viola, quiet.

    I'll never rivet those again...

    As much as it sucks, these things always come down to lead dress, stray wire strands, and grounding.

    a
     
    Last edited: Feb 11, 2019
  13. flyswatter

    flyswatter Friend of Leo's

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    Sorry to hear it didn't work. At least you eliminated one more possibility. 10 seconds for caps to drain down to 3V (and then hover around there until you drain them manually by shorting them to ground) sounds normal to me. It's a common misconception that filter caps remain charged with deadly voltage for a long time after you turn an amp off -- this certainly can happen on defective amps -- but if it is properly functioning, the heated tubes will drain the voltage as long as the cathodes are properly connected. The reason we always check and drain them anyway is that the risk factor of being wrong is too great.
     
  14. montana

    montana TDPRI Member

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    Alright, lots of good things to try here. Thank you!

    I'm going to tackle shortening those leads. Maybe even flying the heaters overhead could help.

    Will check and reflow around input jacks.

    Will check and reflow the ground connections of my heaters (they are at one of the power tubes, btw)

    And I have tried swapping out all tubes.
     
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  15. trouserpress

    trouserpress Tele-Holic

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    Show us your soldering iron.;) Can you adust the temperature?
     
  16. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Lots of good ideas here, and Montana, you get points for being patient and responsive to suggestions. But progress has eluded us, right? We gotta get that pretty amp working....

    Let's review. If I've followed along, your remaining problems are
    1. A loud hum *at warmup* that fades to quiet hum after a few seconds
    2. Board 'rings' microphonically when tapped
    3. Guitar sound is distorted and weak or squirrelly? on both channels?

    Are there other issues I'm forgetting?

    Lead dress, soldering, and ground issues all cause hum, and in general hum has dozens of causes, so maybe we need to figure out why the hum is loud only during warmup. That has to be a clue. Filters? Heaters? Smart folks?

    As for the ringing board, tap it with a fingernail or metal tool while the amp is off. You're trying to see if any component or wire (or board mounting screw) rattles audibly and can be damped with a little pressure.

    You asked once if you should try a different rectifier tube. All tubes are suspect for microphonics, I think. Have you swapped them all out?

    Are there any clues hidden in that voltage table? What about the fact mentioned above the screens are hotter than the plates? Clue, or no? If the voltages are all OK, does this rule out some problems we *don't* have to look for? Smart folks, looking at you again. :D

    Montana, did you do any mods?

    I see you did an adjustable bias. What bias voltage do you measure at the 220K junction? What dissipation do you measure / calculate for your power tubes?

    You did the highlighter trace I think. Did you grab your schematic and an ohmmeter to verify that grid and cathode resistances measure expected values to ground and that plates are correctly connected to the proper B+ nodes? General resistance / continuity / discontinuity testing is helpful. Are grounds 100% grounded? Do things that aren't supposed to be grounded leak to ground? What about continuity on all backside or invisible wiring?

    Finally, do you have another amp? Can you do an audio signal trace through this circuit? Sorry if this is TMI, but you're close, and we gotta round up *all* possible suspects.
     
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  17. montana

    montana TDPRI Member

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    Thanks so much for the support.

    @King Fan Those 3 existing problems are correct.

    I’ve poked and pushed on the board trying to get it to give up a ghost. Any connection or problem area, but nothing obvious.

    I will need another night with more time to work through shortening the leads.

    But I traced continuity from electrical cable ground to the inputs, pots, preamp, power amp, including heaters. All check out.

    Yes, swapped out rectifier and all other tubes once—no change in problems.

    This is interesting though...my bias voltage at the 220K junction is -27V—that should be right according to the schematic—but I put a bias probe on the power tubes and I get 34.5mA!! This appears to be a way off, no? I’ve retraced the wiring here again with everything looks correct.
     
  18. King Fan

    King Fan Friend of Leo's Ad Free Member

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    We need somebody smarter than me on voltages, but that schematic -27V is only approximate — and then only if you’re running schematic B+.

    Have you adjusted your adjustable bias? What is your B+ if you do? What are your B+2 and B+3 — after the choke and after the drooping resistor?

    Does one of your channels work just fine? Or...? Sorry, just trying to make sure I’m in the area where the problem may be. :)




    Sent from my iPad using Tapatalk
     
  19. montana

    montana TDPRI Member

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    Yep, got both channels working now. And the hum is moderate. The standout issue to me is the weird robotic tone I'm getting. Not sure if it's related to the hum or independent.

    Here are my existing voltages:

    voltages.png

    B+ = 402V (schematic says 370V).
    B+2 = 400V
    B+3 = 339V

    I lack the experience understanding the how and why of voltages here too, but if I calculate the dissipation (388 x .0345 = 13.38 watts) I'm over the 12 watt limit of a 6V6. Although my power tubes aren't red-plating.

    I adjusted the bias pot to get 25mA on the power tubes and the plate voltage goes up to 408V. That gives me 10 watts on the tubes.
     
  20. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    Can you adjust the bias pot to get to about 19 or 21mA?
    Or here, play around a bit with Rob's calculator to get where it sounds best and when you're positive you've got the current in the safe area -
    https://robrobinette.com/Tube_Bias_Calculator.htm
     
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