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    Hiss in one channel/ST 70

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    Pat R.

    Posts : 54
    Join date : 2015-02-26
    Location : SE Pennsylvania

    Hiss in one channel/ST 70

    Post by Pat R. on Wed Apr 08, 2015 6:19 pm

    New build ST 70. Having a hiss in one channel.. Just picking brains before I dig in, plus this might help someone else.

    Hiss/light crackle noise in right channel only. Hiss is there with preamp unplugged. Swapped all tubes from right to left, one section at a time. No change in hiss.. all tube sockets new. All tubes tight in socket. After doing a left right swap with output tubes and drivers then finding no change I swapped one of the outside tubes on the board with the center tube. No change. Plugged in preamp. No Change.. Hiss does not change with volume..

    I'm wondering if it's a cold solder joint somewhere.. (ya know them old eyes ain't what they used to be even with the BIG cheaters on)



    Also of note.. pulled output tubes No hiss.. replaced them, pulled drivers, no hiss..


    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Hiss in one channel/ST 70

    Post by GP49 on Wed Apr 08, 2015 6:58 pm

    Here's a trick I sometimes used:

    Listen to the amp with stereo headphones.

    Using a jumper wire, jump from a point in the signal path of one channel to the exact same point on the other channel. BE SURE IT IS EXACTLY THE SAME POINT IN THE SIGNAL PATH! The input to a stage of the amplifier, isolated from the B+ voltage of the prior stage by a coupling capacitor, would be a good place.

    Now, with the jumper in place bridging the two channels, if the hiss is now combined in both channels of your headphones, the source is BEFORE that point in the circuit. Move to an earlier point in the circuit until the hiss is in the "bad" channel only; at that point you have gone past the faulty stage and can localize your troubleshooting.

    I did this with stereo TUBE amplifiers but would not try it with transistor amps...might blow up something.

    An alternate method to try would be to GROUND the signal path through a capacitor (0.47uF/600V film ought to work), at various points through the circuit. If the hiss is still there, it is probably from AFTER the point you are grounding; if it is gone, probably BEFORE that.

    Pat R.

    Posts : 54
    Join date : 2015-02-26
    Location : SE Pennsylvania

    Re: Hiss in one channel/ST 70

    Post by Pat R. on Wed Apr 08, 2015 9:26 pm

    Good tip.. I'll give it a try as soon as I can find the windex for my glasses..

    Thanks!

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