• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

When to use a grid leak resistor?

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SY said:
There has to be some sort of DC path to ground or to to some reference voltage that has a DC path to ground. So, there's always some sort of grid leak if the grid isn't returned through a transformer or set at some reference voltage.

Thanks for chiming in, SY:)
Hmm..:xeye:
In a linestage where there is usually a pot or attenuator on the input, a grid leak resistor could be skipped then since there is a dc path to ground via the pot?
 
In that case, the pot IS the grid leak.

As a matter of course, I will always use a resistor from the grid to ground anyway, just in case there's an intermittent between the wiper of the pot and its resistive track. It's not needed in normal operation, but could potentially save all of the downstream equipment in case of a fault.
 
SY said:
In that case, the pot IS the grid leak.

As a matter of course, I will always use a resistor from the grid to ground anyway, just in case there's an intermittent between the wiper of the pot and its resistive track. It's not needed in normal operation, but could potentially save all of the downstream equipment in case of a fault.

Thanks gain SY.
And now it is time for a "leak".:D
 
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I believe the deal is that a floating grid will collect charge. This collected charge causes the grid voltage to rise, thus, eventually resulting in positive grid-cathode voltage. This will cause a large current to flow in the tube, which will likely destroy it. The grid leak resistor is there to bleed off the charge. An input volume pot is likely to serve the same function, but a 1Mohm resistor is not that expensive so why not just put it on there and be done.

There are some cases where the grid leak isn't strictly needed. But you will always need a DC path from the grid to some sort of ground.

~Tom
 
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