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Older paper coupling capacitors always get leaky over time, affecting the tone to the point that your Internet ‘guru’ espouses using only paper capacitors for ‘that sound’. Assuming your amplifier is not sounding as it should, you measure for any DC voltage at pin #2 and pin #7 of every 12AX7. This is definitely wrong, and we can confirm the problem thusly.
He has no idea what the heck is going on, and gives the ‘credit’ for this ‘sound’ that modern amplifiers cannot achieve (with ‘good’ capacitors) to the leaky capacitor. First, disconnect the end of the capacitor that was connected to the ‘positive’ grid.
What is further done usually is to use an isolation probe, where a 1Meg resistor is soldered into the probe casing.
This effectively make the input resistance 11Meg on all ranges.
On the outside, the meter looks like a typical old-fashioned analog meter. Despite the differences in outward appearance, probably 99% of VTVM’s have the identical circuit inside the cabinet.
The heart of every VTVM is the balanced bridge circuit. ), and the execution of this basic principle does not seem to have changed. You should note that the input resistance is constant though out the range switch.
Next, use your isolation probe, and set your VTVM DC voltage scale to ‘150’.
Connect the isolation probe to the ‘open’ end of the coupling capacitor, and the ground clip to the chassis.Power up the amplifier, and watch for any deflection on the meter.If you cannot see much of a meter reading, go downward through the voltage scales (usually this would be 50, 15, 5, and 1.5).The meter movement is located in the cathode of the bridge circuit, and the differential voltage is displayed.In practice, these two triodes are a single 12AU7, which is the most common tube to be seen in every VTVM.Therefore, voltage measurements are far more accurate than with a regular VOM. Probably not, but it will come in handy later, as we’ll soon see.