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Thread: Motorboating issue

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    Motorboating issue

    Im working on a SE 6L6, tweed style amp and I need a little help locating an issue of motorboating. The amp started out as a 5F2-A clone, but I kept craving for something a little more powerful. I replaced the 6V6 with a 6L6, added a screen resistor on the 6L6, upgraded to a 25W SE OT from Weber, upgraded the filter caps to 40x20x20uF, switched to higher voltage HT taps and switched to SS rectifier. The plate voltage is now around 400VDC, and the amp is safely biased at 22W. I feel that I made the necessary changes for use with a 6L6. However, I've come across an issue of motorboating, an issue that was not present before the changes. It only occurs when the volume control hits about 6 or 7

    I originally ruled out the possibilty of an inadequately filtered power supply. I felt that I raised the capacitance enough. Could I be wrong?

    I don't think lead dress is an issue, seeing as the routing is the same as before. I also ruled out grounding problems for the same reason.

    Could the increase in voltage and power be related to the problem? The input jacks wouldnt be more susceptible would they? Any help would be appreciated

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by EETStudent View Post
    I don't think lead dress is an issue, seeing as the routing is the same as before. I also ruled out grounding problems for the same reason.

    Could the increase in voltage and power be related to the problem? The input jacks wouldnt be more susceptible would they? Any help would be appreciated
    Well, it's worth noting that more power does mean a larger, more powerful EMF wherever it is present in the amp. This could cause oscillation that wasn't there when the amp was lower powered, all else being equal, which it's not. You made changes to the circuit. Motorboating is often handled from a power supply angle (though I'm not 100% on why) trying different, often smaller filter values and/or new grounding schemes can sometimes fix the problem without ever understanding fully WHY it was happening in the first place.

    Chuck

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    Lifetime Member Enzo's Avatar
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    Is there an NFB line? If so disconnect it, any help?

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    Supporting Member Gibsonman63's Avatar
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    For what it is worth

    I had a motorboating problem recently in one of my Blackface Twin Reverbs, which was frustrating because the amp worked fine for the first week or two after I restored it. Out of frustration, I tried the chopstick trick. I was able to fix mine by moving the heater wires, although I must confess I am not sure why it fixed the problem. This has been several months and no further problems.

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    There is NFB line, and I will check that as soon as I get home.

    I will try moving some wires around, and maybe look at shielding in a few places. Can we agree that the power supply is plenty filtered?

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    Power supply filtering sometimes wont help the problem because (possibly), the signal going back to early stages can be small; so attack from the forward path and try reducing one of the coupling caps value to give a higher cut off frequency thereby stopping the low frequency loop.

    I've had this happen to me, and both times using a smaller cap off the final preamp anode sorted it.

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    Supporting Member tubeswell's Avatar
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    Are you using separate cathode resistors for each pre-amp stage? (Common rk on cascaded stages can create LFO type thumping - I found this out from my own experiments).

    (Otherwise I'd shoot for inadequate filtering. Try 50-60uF on the input and output caps of the main filter??)

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