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Tube grid loads down input signal in Vox AC30c2, wut?

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  • Tube grid loads down input signal in Vox AC30c2, wut?

    This amp was clearly designed by people who hate service techs. Even the schematics are nearly unreadable despite being created in the age of CAD. And I have a feeling this particular amp hates me personally as it seems to violate laws of physics.

    I've got a 200 Hz signal generator plugged into the "high" Top Boost input. Measuring at the grid (pin 2) of the first preamp stage with the power off, I get 250 mV RMS. Turning on the power (but not standby) the signal level drops to zero as the tube warms up.I turn off power and as the tube cools the level slowly recovers to 250 mV. If I turn off power and pull the tube, the level immediately goes back to 250 mV. With power on and standby on there's a bit of signal, but only 30 mV RMS.

    What gives? The grid electrode is supposed to be very high impedance. I've tested the passives and they're in spec. Heater voltage is +/- 3.50 V DC. I've tried a few different tubes and checked them all for shorts.

    (On the schematic, tube pins 1,2,3 are on the left.)
    Click image for larger version

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  • #2
    If C11 is shorted or the cathode of the input triode is somehow otherwise shorted to ground (stray solder bead/wire clipping, bridged solder pad, etc), then the grid will be forward biased and become low impedance, with the voltage from your signal generator dropped across the grid stopper resistors.
    Have you checked that you've got a positve voltage at the cathode of approx 1.5V?

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    • #3
      Have you checked the output impedance of your signal generator?
      It should be 50 Ohms if the signal leads are wired correctly.
      A 1kHZ signal source is more usual for measurements.
      The input impedance on pin 2 will vary, dependant on the input selected. The valves grid, has a very high impedance but reduced due to a combination of R5, 6, 7, 8 & 9.
      On pin 2 itself is in excess of 5M Ohms and can be disregarded in this circuit.

      Quite a common input configuration that I have come across in many makers amplifier front ends. Nothing complicated here.
      Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
      If you can't fix it, I probably can.

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      • #4
        Is your generator signal free from DCV?

        What's wrong with the amp?
        - Own Opinions Only -

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