Motor boat

[lemdawg]

I have a Marshall clone 2x12 combo a 6v6 25 watt jtm45 wanna be. Recently it started to motorboat. It only happens when I play it is quiet if just instrument plugged in and turned on. I have opened that amp and can't see anything obvious no swollen or leaking capacitors, I replaced the tubes with no fix. Any tips on isolating the bad components

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[phatt]

Hi lemdawg, and Welcome.
           I'm not the expert here but If I recall maybe check the supply filter caps as I think signal can bleed back through the supply if filter caps are not up to spec. Be patient, Others here will know more.
Phil.

[g1]

Please post the schematic.

[lemdawg]

Hi lemdawg, and Welcome.
           I'm not the expert here but If I recall maybe check the supply filter caps as I think signal can bleed back through the supply if filter caps are not up to spec. Be patient, Others here will know more.
Phil.

Thanks for your help


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[J M Fahey]

Post schematic and if possible a gut picture, showing internal layout and wiring.

Motorboating is often caused (specially if "just appeared" ) by dry capacitors, because they allow low frequency signals to "travel backwards"  from current hungry power stage into sensitive preamp sections, but it may also be caused by poor grounding, signal travels back through imperfect ground connections or layout.

Your amp may also be oscillating at inaudible high frequencies, you won´t hear *them* but it works like a so called "blocking oscillator": oscillation is strong, blocks next tube, stops, starts again, you won´t hear the ultrasonic oscillation but the on-off "thump" , repeating regularly.

*Really*  , the best tool here is a scope, because we are dealing with too high or too low frequencies to be heard, we are dealing with their "side effects", but hey, we´ll try to do our best with what´s available .