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Vibro King - Round Four

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  • Vibro King - Round Four

    OK guys... Hopefully this will be the last round. I have thrown out everything thus far and started from scratch. I left the amp alone for a while and took a fresh look last night...

    When I turn the amp up to 8 or higher, after a few seconds of play, the sound starts to worble much like the tremelo circuit would sound. Once that starts I can cut the volume on the guitar and there is a cyclic popping sound - psuedo motorboating. I have changed all the filter caps with orginal product, Ill. Cap. note that I don't play the amp this loud, I was just evaluating it after getting it home. It was a steal, I got it for a cow and a handfull of magic beans. The amp plays and sounds fantastic at normal volume levels, 4 or lower.

    When I plug into the effects return jack, bypassing the preamp and reverb, the amp operates normally cranked.

    Removing V2 causes the issue to cease. By applying pressure to pin 2 on V2 (at the base of R46) with the chopstick, it causes a loud low frequency (bassy) roar. The same happens when I apply pressure to the opposite end of the shielded cable (at C19). It stops the ticking briefly. No amount of lead dress altering affects it. I re-flowed the solder at both places.

    I sprayed deoxit in the socket and worked a tube in and out several times then purged it. I checked the pin sockets with a strand of wire, all are very firm.

    I checked R45 & R46 - 2.2M while out of the circuit and I also changed C19 & C2.

    The voltage at V2 pin 1 is 363. The voltage at V2 pin 2 is 160 (dropped across the 2.2M), I checked R59 (10K) is also in spec. That V is higher than the schem shows so I checked R67 – 10K in P/S section. It looks to be in spec. I also used a “mini grabber” jumper to jump out the shielded cable (C19 to pin 2) to ensure good continuity. No change.


    As an added note, the ticking changes patterns and interval frequency randomly. Also, if I cut the TMB stack pots to 1 the problem goes away, but so does the tone! The Mix (R2) control in the rev circuit also affects it so I changed C18, the coupling cap between R2 and V2 pin 3. This seems to be a gain related problem. I even tried a 12AU7 in V1 to lower the gain hitting V2. I tried a 12AY7 in V2 to lower the gain further… No change.

    I hope that this is more accurate and systematically obtained information that will help.
    Attached Files
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