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969 Ampeg SVT being snappy

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  • #1

    969 Ampeg SVT being snappy

    Replaced output tubes. Originally the problem was a hum from loading of the driver circuit which came from a parasitic oscillation (about 7KHZ), which would come and go as I would adjust the bias balance pot. I replaced that, along with re-flowing all the solder connections on the driver board and exercising all the tube sockets as well as cleaning them and the amp is quiet and well behaved. However, occasionally it will let out a loud SNAP! like something is arcing (and probably is). No other noises are associated with it. It just sits there nice and quiet, then suddenly it might snap once or three times in a row like a clock. It might take 2 or 3 minutes before it happens again. It usually snaps at least once after warm-up and being turned off of standby. I tried tapping on it with a rubber hammer all around, on the chassis, on the box and I can't get it to snap from anything I do. It happens the same if the volumes are turned all the way down.
    I've just finished two solid days on this beast and it's disheartening. My brain is done. Any suggestions?
    svtpreamp.pdfsvtpoweramp6550a.pdf
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  • #2
    1969 Sorry

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    • #3
      Originally posted by booj View Post
      1969 Sorry
      Did it do this before the new output tubes? I would turn the lights out and watch the output tubes for arcing. Line voltages are a higher than they were in 1969 and new production tubes not as robust. Do you have a variac that you can use to reduce the line voltage?

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      • #4
        Do you mean scary-loud snaps? That does sound like bad tubes or arcing sockets.

        Just so I understand your description, you're not talking about loud cracking noises like you get with old CC screen resistors, are you? I had a 70s Fender Twin that was giving up annoying rare and intermittent cracking sounds. Ripping out all of the CC screen resistors and replacing them with mox resistors solved the problem.
        "Stand back, I'm holding a calculator." - chinrest

        "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

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        • #5
          On second thought, you really don't want to fix that amp. Just send it to me and I'll make sure that it gets a proper disposal.
          "Stand back, I'm holding a calculator." - chinrest

          "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

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          • #6
            From the speaker? Or heard through the air from the chassis?


            I too like turning out the lights and watching it. Arcing power tubes is a good start, but I;d also have the chassis out so if a spark snaps under the board you can still see it reflecting. Of course inside the transformer won;t be visible.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Thanks for your reply's! The sound is coming from the speaker. It did it before I re-tubed it. I thought it was me chop - sticking it. That's why I re-flowed the solder. I've since dis-connected the old can type caps and replaced them with some Axial e caps. That sound was very distinct like a static discharge. Maybe it was a cap getting ready to go. Even if not they were 44 years old. I'll try it out tomorrow after I've triple checked my new wiring.

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              • #8
                So.. the latest is, I noticed a bit of hum on the output (100mv) and checked the hum adjust control. It was broken. One lug was shorting to the center lug / ground. Replaced that. Now the hum sits at about 25 mv. Still kind of high.
                The snap sound is still there. I tried the lights out thing and could not see any arcing even though I heard the snap sound several times. I was looking at the bottom of the main chassis, and also looked at the top of the driver board. I also cleaned the 44 year old flux off the driver board (foil side) and did some re-touching of the solder re-flow.
                I thought I had a clue when I turned off the light to play in the dark. I heard what sounded exactly like the snap sound I was getting. Maybe the problem was in the AC line? I swapped power outlets and ran through an extension cord. No fix. I'll clean the dirt of the top of the driver board and get back with ya later.

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                • #9
                  Cleaning the driver board did nothing. I did take a look at the preamp chassis with a chopstick and got some BVVVTTT!! action. Got it turned off quickly. I think that's a clue for something.
                  I have put the old outputs back in due to handling. Did nothing for the snap problem.

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                  • #10
                    I got the snap out of it. The problem was in the preamp section. Also discovered the bvvvt! problem. The snap was due to a wire coming from the tone circuit inductor which was too close to a power supply connection. The bvvt! problem, (I think) was caused by a loose ground connection of a shield wire from one of the input jack sets.

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