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Thread: Amp low line voltage issues

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    Amp low line voltage issues

    Hi, I played an outdoor gig last week where the power was run off a generator. About 10 mins into the gig my deluxe vm audio kept cutting out. Strangely the bass and other guitar amp didn’t cut out. The deluxe has a power consumption of 180watts and is a hybrid amp with valves in the power amp section. The other guitar amp was a fender super champ x2 with power consumption of 80 watts and the bass amp is a Genz Benz shuttle with power consumption of 400 watts. So what would cause the amp to cut out compared to the other 2 amps?

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    Old Timer Leo_Gnardo's Avatar
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    Ol' fashioned tube amps without any modern stuff inside 'em would probably have no problem other than diminished power & maybe sludgy tone during brownout periods. Anything that depends on a voltage regulator would be in deep doodoo. Most solid state preamps as well as built-in effects devices dependent on the usual plus & minus 15V supplies, would suffer having their power supply regulators quit during dips in power.

    Also amps and other items using switching power supplies, can't count on those supplies operating on low line voltage. In many cases when the power line dips, the power supply goes on vacation.

    Way before the digital age, I participated in a medium size outdoor fest where the whole PA system cut out on every kick drum hit. How embarrassing! They were using dbx160 compressors as limiters across the house mix. Later I found those classic compressors, while wonderful in every aspect, had muting circuits that clamped the audio output to zero if the AC line wasn't up to par. When the power amps drew a lot of current to amplify the kick drum, with each hit the line voltage dropped for a few milliseconds. Had the PA outfit known that, they could have snipped a couple of FET's out of each compressor's mute circuit and avoided the problem permanently. Live and learn!

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    Enjoy. Every. Sandwich.

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    Old Timer J M Fahey's Avatar
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    To *really* know whatīs going on, you might connect your amp to a Variac (no need to buy one, many have one for different purposes) and you gradually dial voltage down by 5V , play loud (or the test wonīt help), lower extra 5V ... rinse and repeat.

    A "normal" amp will gradually become weaker and , as mentioned above, "sluggish" or even farty but shouldnīt really cut off, at least not until fed stupid low voltage such as 70 or 80V

    Now if yours with, say, 100 or 95VAC *abruptly* cuts off ... then itīs not a BAD amp but definitely not to be carried to gasoline gigs.

    I suspect your digital board is the culprit, since the other two are classic/analog ones, even if one is Tube and the other SS.

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    Juan Manuel Fahey

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    Thank you for the information. The fender super champ x2 is another hybrid amp so I think you could be right about the choice of components on the pcb boards. The deluxe vm in my opinion is a great amp but this just lets it down. I am guessing that this issue could happen in solid state too. Is there a trend for cheaper valve amps to have some sort of limiting protection in case the voltage drops too low. I forgot to mention I’m from the uk.

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    if it has a dsp inside,its easy that in case of low voltage the regulator feeding it cuts out,i imagine,maybe it was designed too optimistic.

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