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Trouble shooting help?

Discussion in 'Shock Brother's DIY Amps' started by Mark the Moose, Jan 26, 2021.

  1. Mark the Moose

    Mark the Moose Tele-Afflicted

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    What might cause an amp to suddenly get very quiet and very distorted? Princeton Reverb circuit. It was a 35watt 6l6 monster, switched the output transformer to take it back to 6v6 tubes. I changed V1 and V4 to brand new JJ's. Sounded great for a day.

    I can't see any obvious bad solder joints. I tried pulling and replacing V1, V2 and V4 with no change. My tube sockets are super cheap, in swapping tubes could I have lost a socket?
     
  2. Nickfl

    Nickfl Friend of Leo's

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    What's your bias set at? Did you confirm the tubes are not red plating? Did you try other power tubes in the amp and/or try these tubes in another amp? Just because they're new doesn't mean that the tubes aren't the problem so I'd rule that out first.
     
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  3. Mark the Moose

    Mark the Moose Tele-Afflicted

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    My DMM is not precise enough to get a proper bias reading. Plate voltage and B+ were both sitting around 375volts, no red plating, tremolo sounded good. I have another 6v6 amp I can swap with and make sure the issue doesn't travel with the tubes.
     
  4. Peegoo

    Peegoo Poster Extraordinaire

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    Pull the tubes and inspect the pins for dark spots or other signs of arcing in the sockets.
     
  5. jsnwhite619

    jsnwhite619 Friend of Leo's

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    What is the PT secondary and what rectifier are you using? 375v is really low for a Princeton Reverb with a 5u4 or gz34. 415v-420v is more the expected range. If you're at 375v B+ and a mostly stock circuit and PT secondary, that son of a gun is probably biased really hot - like smoking hot!
     
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  6. Mark the Moose

    Mark the Moose Tele-Afflicted

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    It’s currently using 540v secondary leads, but have 680v leads available. I think I’ll try switching over and see what happens.
     
  7. Milspec

    Milspec Poster Extraordinaire

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    Loss of volume and quick distortion.....output transformer.
     
  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    How bad *is* your DMM? Imprecise bias would be a lot better right now than no bias. Are you comfortable with how to do it? Rob tells us here:

    https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

    This is also a great excuse to add some 1-ohm 1% cathode resistors on your output tubes. Makes measuring plate current directly a piece of cake; any meter that can measure mV will do. A solder lug, star washer, and keps nut on the socket mounting bolt give you the perfect spacing.

    upload_2021-1-26_20-1-19.png

    You measure VDC (in mV) across the 1-ohm resistor -- Ohm's law means mV = mA, so bingo, you know your cathode current, which is close enough to plate current that your bias calculations (in Rob's nice simple 'current' calculator) will be within a few percent.
     
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  9. NTC

    NTC Tele-Meister

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    I wouldn't change to the higher voltage taps without checking bias at the current voltage. Check the other voltages again as well. Perhaps ohm out the OT again just to rule that out. And start shopping for a better DMM. You deserve it.
     
  10. Nickfl

    Nickfl Friend of Leo's

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    IME, you should be able to get an accurate bias reading with any serviceable meter. How are you taking your bias reading?

    If you are using the output transformer resistance method, don't try to measure the plate and center tap to ground and then do the math, instead attach one lead to the center tap and the other to the plate, much easier to get a sufficiently accurate reading that way because you're only measuring a few volts of difference rather than measuring a high voltage.
     
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  11. thechad

    thechad Tele-Meister

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    Just to cover all the possibilities before you go digging in your amp too far, did you try swapping guitars/patch cables? Just to rule out an upstream problem...
     
  12. jsnwhite619

    jsnwhite619 Friend of Leo's

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    That would be closer to what I'd expect with a 540v PT. A stock Princeton Reverb would be a 650v I think.

    You might try wiggling and trying all the preamp tubes in their sockets and see what happens. I tried one of Rob's mods on my 5e3 a few years ago and thought it just completely killed the tone and output. When I undid the mod, the problem was still there. I don't know if it was solder in the pin or just loose and stretched out, but a bad connection on my V2 tube was to blame. Not the tube, but the socket was jacked up. You had to wiggle the tube - any tube - just right to make contact and work. Otherwise it sounded like a .5W-1W amp on the OD channel. If it was working and now it doesn't, it may have gotten hot enough for a bad joint or something to show itself.
     
  13. Mark the Moose

    Mark the Moose Tele-Afflicted

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    I tested the power tubes in another amp and rolled through some extra 12ax7s with no change. From here, I think I have a battle plan, feel free to add to or re-order.
    1. Order a better meter. Mine does not do decimal places nor does it measure current.
    2. A little voice (Magnum PI style) keeps pointing me back to the cap on the bias board. I feel I may have over heated it. I have another so I think I'll swap it out.
    3. Test tube socket pins for continuity top to bottom. These are the cheapest tube sockets imaginable, so I have little confidence in them.
    4. Reflow some solder joints. I think they are good but it only costs me a few minutes.
    5. With the new meter, work out any possible voltage/current issues.
    6. Change the OT. I have a bigger OT with 6l6 tubes that I just pulled out, would also have to change the bias resistors.
     
  14. Mark the Moose

    Mark the Moose Tele-Afflicted

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    My DMM doesn't include decimal places (not very precise) and has no amperage measurement abilities. New one will be here Friday. The 1 ohm cathode resistor, would a 2 watt be sufficient?

    I did find a bad solder joint on the bias pot. Once resoldered the amp is working properly again, though I am still concerned about low voltages. I'll know precisely on Friday when the new meter comes, but is it likely that low voltages will yield a high current level at the tubes or is it just reflective of using the lesser PT taps? I'm tempted to connect the 680V taps to see what the result is, any thoughts?

    Much thanks to everyone who chimed in with ideas on this project.
     
  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I know what you mean about decimal points, but imprecision is better than no 'cision' at all. :)

    As for current measurement, you don’t need it for either Rob's OT resistance method or the 1-ohm cathode resistor method.

    And for the 1-ohm resistors, 0.5W is fine, and will fit better in the space between pin 8 and a solder lug.
     
    Last edited: Jan 27, 2021
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  16. NTC

    NTC Tele-Meister

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    Wait on the higher voltage taps until you get everything working well and reliably.
     
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  17. Linkjr

    Linkjr Tele-Meister

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    .
     
  18. kbold

    kbold Tele-Afflicted

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    Buy one that also measures capacitance. Quite handy.
    Get one that measures capacitance with the probes (some measure caps by plugging them into sockets on the meter, but this is then not possible to measure caps in-circuit).
     
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