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Thread: Ampeg SCT-CL weird oscillation noise

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    Question Ampeg SCT-CL weird oscillation noise

    Friend left me U.S. made SVT-CL to "diagnose" weird noises... Yeah. When turned standby on (so in playing mode), amp produces high-pitched noise along with some hiss. Seems it's not related to the gain/volume settings.

    - SVT has new set of power tubes, some used preamp/driver/inverted ones. I checked all of them on my two testers, power tubes seems 100% OK, other ones too (over 95% emissions and no problems).
    - Swapping AX7s does not change the thing.
    - Preamp board @ big power amp board looks stock - no burnt traces, no bad solder joints, no cap leaks. All resistors test in range.
    - On the tube board 220 ohm resistors were soldered by someone before to replace original 22ohm&diodes, as ampeg bulletin suggest, all test in range.
    - All in/out jacks tested too. Grounds etc.

    I could only get the oscillation noise down when I set the bias super cold (no lights present then). When I turn bias up, the noise gets louder.

    This is the first case I'm dealing with that noise. Where should I look more?

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    Lifetime Member Enzo's Avatar
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    First off you didn't mention 12AU7s.. Two of the three small tubes in the power amp are 12AU7. They are the two behind the bias controls, the third one by the Slave jack is the 12AX7.

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    Most tube testers won't tell you if a tube is microphonic or prone to oscillation due to internal mechanical problems. A tube tester also won't provide any form of positive feedback path that an in-situ tube has in relation to other circuits/tubes/components within an amp. My first suspicion would still be tubes.

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    Good point about tube testers - that they don't measure microphonics and oscillations. I will change them today and see whats going on.

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    Lifetime Member Enzo's Avatar
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    In general, tube testers are good at finding BAD tubes. Like shorted or whatever. Testers are not good at finding GOOD tubes.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by boroman View Post
    Good point about tube testers - that they don't measure microphonics and oscillations. I will change them today and see whats going on.
    Are you getting this oscillation or high frequency pitch (audible range) when you 'dead-patch' the power amp input jack (shorting plug inserted there)? That will isolate the preamp from the power amp. I'd also look to see what your cathode current on each tube is running. That can be measured across the six 10 ohm cathode resistors on the bottom side of the power amp PCB:

    Click image for larger version. 

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    Assuming the cathode resistors' leads aren't caked full of RTV, you can clip your DMM lead on any of the four resistors along the bottom edge, as seen on the last photo. Disregard the scope probe in that image....I had tack-soldered test points on each of the cathode resistors in that set-up to observe the output under drive conditions. Not needed in the basic measurements of the six power tubes. This set-up allows matching power tubes, with nominal current per tube at around 22-23mA, well within the 'GRN' zone of the bias meter LED's.

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    Quote Originally Posted by Enzo View Post
    In general, tube testers are good at finding BAD tubes. Like shorted or whatever. Testers are not good at finding GOOD tubes.
    As I've had to inform many customers, also fellow techs on their learning curve. On occasion I've had to take some heat for telling the truth. But that's life, innit?

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