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Ampeg J-12 Jet too quiet

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  • #16
    Alrighty... I modeled the amp in Spice, though I used el84 power tubes because that's what I have right now in this program. On the up side el84's bias up and mu similar to 7591's so the results should be similar enough. My conclusion...

    There's something wrong with the amp. With the exception that it may have a grossly inefficient speaker it should, indeed, be about as loud as any other amp with a pair of small bottles. It could be a leaky cap, a drifted resistor, blah-blah-blah, etc. But first on my list of suspects is the trem circuit or maybe just the roach. If the resistance for the circuit paralleling that 5.6M resistor via the trem bug isn't being maintained at a low value any series resistance will act as a voltage division with the volume pot resistance. To maintain a low resistance in the roach the lamp must be lit and the LDR inside in proper working order. Otherwise the amp will suffer greatly reduced gain.

    If your trem is busted or weak I think you may need a new trem roach. A simple test would be to jumper the 5.6M resistor and see if the amp gets a louder clean tone. You'll probably need to lower the volume setting on the amp to get it clean with a 100mV singnal plugged into the front. It turns out that Fliptops offers the roach now (for a hefty price!). Or if you never plan to use the trem then just disconnect it and experiment with much lower values for that 5.6M series resistor. Maybe just jumper it, though that might be too much gain.
    Last edited by Chuck H; 01-23-2020, 02:10 AM.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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