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Thread: Peavey Mace/Deuce VT fx loop sensitivity mod?

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    Peavey Mace/Deuce VT fx loop sensitivity mod?

    Hey forum, first post here. Nice place you guys have. I've read alot of good info in these threads.

    Old Peavey Mace/Deuce/Classic VT's have an effects loop, pretty cool for an older amp like this. After tweaking and spinning knobs on these things for a few months I've just come down to using them as power amps. Problem is the J6-power amp in, isn't sensitive enough to run my effects straight into this input. Is there a way to modify this circuit to increase the sensitivity of J6?

    I thought about lowering the value of R117. Or using an equal value pot in its place?

    Thanks in advance for any input.

    https://photos.app.goo.gl/up7VCVuwDZyqNaLA6

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    Last edited by 76jimmy; 07-12-2019 at 01:14 PM. Reason: Spelling

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    'Fraid not, there's negligible signal attenuation across R117, so reducing its value will not result in any perceivable increase in sensitivity.
    However, there's some heavy attenuation at the power amp input, see 2nd page https://el34world.com/charts/Schemat...mace_deuce.pdf
    I suggest to swap the 1k and 4k7 resistors over (IR13 and IR14? Kinda blurry).

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    Last edited by pdf64; 07-12-2019 at 03:49 PM.

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    Wonder if you could repurpose the op amp that drives the Line Out to boost the signal to the power amp? Maybe make the Line Out jack into a second Power Amp In for smaller signals.

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    Thanks for the suggestions PDF and glebert. Maybe a dual gang pot wired in with some resistors to allow adjustments? Trying to understand how this is attenuating the signal. Is it because ir14 is grounded? I'm an amateur at audio electronics but I'm always looking to learn more. As far as I understand the signal is getting reduced by those ir13 and ir14 resistors? Not sure what values to use in there place?

    I've been using the direct out to Daisy chain to another idenentical Peavey Deuce VT head.

    If I did figure out how to adjust input sensitivity at j6 that would be ideal.

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    Quote Originally Posted by 76jimmy View Post
    ...Trying to understand how this is attenuating the signal...
    The 2 resistors form a potential divider, in the same way that a regular volume control is a variable potential divider https://en.wikipedia.org/wiki/Voltage_divider.
    I wouldn't remove the resistors totally and take the function out to a volume control pot, as they may also be acting to maintain stability; so leave something there.

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    Quote Originally Posted by 76jimmy View Post
    Trying to understand how this is attenuating the signal.
    I've edited that part of the drawing by tipping up IR13, putting the power amp input at the top instead of to the left. Notice how IR13 and IR14 look like a potentiometer (volume control) set at less than 20%. You can replace those two resistors with 5K pot and have more than 5x the sensitivity available. Log or audio taper would be easier to control than linear taper.

    EDIT: Pete beat me by not wasting time with MS Paint.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	volcon.png 
Views:	6 
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ID:	54290  

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    I'll do some experiments with these ideas. Thanks again.

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    Quote Originally Posted by 76jimmy View Post
    I've been using the direct out to Daisy chain to another idenentical Peavey Deuce VT head.
    \
    So you are saying you like your music really loud and you like hauling very heavy amps?

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    Yes... It's been a lifelong problem. Cheap, loud amps make me smile.

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