Strange Oscillations

Discussion in 'Amp Central Station' started by Milspec, Apr 23, 2020.

  1. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    Some days you just can't win. My trusty SF Twin Reverb (UL variety with Mstr Control) has become well, not so trusty. It has been a great amp that was blackfaced and bias pot added a few years ago, sounds fantastic normally. I had to take it in to replace the diodes after one failed abruptly, but all was good for about 2 weeks and now this.

    I keep the master set at 10 and when I fired it up tonight there was a strong audible oscillation going on. It increased / decreased with the master volume knob, but never fully went away. Then I flipped off the bright switch and the oscillations stopped even with the master turned back up to 10.

    Amp sounds good now, seems to be back to normal with the bright switch off, but I am wondering what the hell is wrong now? My amp tech is a real pro and went through it after replacing the diodes to make sure all the pots were cleaned and played it for quite some time before signing off on it. Played great for 2 weeks and now this issue.

    I thought maybe it was picking up the ceilling fan or something and turned off everything else in the room, but it only went away when I turned off the bright switch on the vibrato channel (I only use the vibrato channel).

    Any ideas?
     
  2. guspac88

    guspac88 Tele-Meister

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    I had an issue with a deluxe reverb where it would oscillate when dimed if the bright switch on the vibrato channel WAS on. Turned out that my input wires (both wires running to the grids of V2) were picking up voltage from other wires as it ran under the board. Replaced them with shielded wires, grounded the shield at the front panel side and it's 100% good now. Early grid wires can be very sensitive, so if your tech repositioned these wires, then a bump or vibration could move them just enough to start oscillating like mad.
     
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  3. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    That can happen for a variety of reasons.

    As guspac88 mentioned, you can even get squeal/noise issues from sloppy lead dress and wiring.

    Try a new tube in your phase inverter socket (if you’ve got an extra one handy) and check preamp tubes for microphonics...

    After that, the next suggestions I’d have involve opening the amp, which is dangerous — even if the amp is off and unplugged (filter capacitors can store high voltages)... so hopefully we don’t have to get to that point.
     
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  4. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    Thanks, I was thinking about trying a different tube there. The suggestion about the wires getting bumped is also a sound one, but I would expect that would have shown up prior to now...I haven't moved that amp since it came home.
     
  5. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    Higher frequencies can often push an amp that’s “ify” into oscillations.
     
  6. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    tubeblow3.JPG

    I learned something new today (saying a lot as I am pretty thick headed).

    I looked at my preamp tubes while still in the amp for any signs of trouble (discolorations, etc.) and all looked fine. Tried the amp and the oscillations came back. So, I pulled the phase inverter tube to install another as was suggested....and found this. The back-side of the tube was discolored and actually looked like a pile of solder more than a burn as it had a hint of silver to it. Definately not a good sign for a tube. Looks to be a no-name Chinese tube as well (I didn't re-tube when I got this amp) so I happily replaced it with a JAN-Phillips 12AT7 that I have in reserve. Problem solved, no more weird sounds and there seems to a little more top end as well...pretty happy.

    The lesson here is that you can't judge a tube without seeing the backside of it. I am also a little concerned about the electricity flow in my room now as there seems to be a lot of issues popping up lately with a couple of amps. Voltage tester shows the wiring is good and the output stays a pretty steady 117 volts, but it still has me wondering.
     
  7. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    WAIT A TICK....isn't that silver deposit on the back of the tube the flash getter? How did it move from the top of the tube down the side like that?

    What the heck happened to this tube?
     
  8. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    Okay, I have learned 3 things now.

    1. You can't really judge a tube via visual inspection
    2. Some tubes are designed with the getter on the side (I had not seen one before, especially in a preamp tube)
    3. I am really an idiot and don't know a hell of a lot on this subject
     
  9. Telecasterless

    Telecasterless Friend of Leo's

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    I think that's a cool name for a band. Or for a song at the very least.
     
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  10. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    I didn't think about that, but you are correct. I was just noting the name "Callico Cats" as a potential choice a few minutes ago.
     
  11. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    Just to close this one out on my end, the replacement tube not only eliminated the oscillations, but increased the volume by a great deal. I had my normal settings but about fell out of chair when I hit the strings. Ended up dialing back the volume on the amp and ran my guitar at about half-way to get the volume back to where I wanted it. Better tone too, clear and articulate...big improvement.
     
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