• 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.

5842 grid leak

regal

Member
2004-01-04 8:41 pm
MD
As I understand it the 6c45pi and SM5842 need a small grid leak, best around 50k. For a small spud amp would it be OK to just use the input volume pot as the gridleak if it is about 6" away from the tube? Or does the gridleak need to be near the tube as possible like the grid stoppers to counter the HF oscillations these tubes are known for.
 
Don't just use the pot as a grid leak, as one of the normal failure modes is for the wiper to lose contact with the track. Add a resistor from wiper to ground too, about 10-20 times the pot resistance. Then you still have some bias when the pot fails.

A valve which requires a small grid leak is clearly expected to have significant grid current so it will need a low impedance source, in order to avoid distortion. I would use a 10K pot, with 100K from wiper to ground.