I am trying to find an AC probe for my HP-410C. I have a probe from a USM 116 VTVM. The 410C uses an EA53 diode in the probe and the 116 uses an EA52. From what I have found on the internet, these two tubes seem to be similar, base connection, heater voltage, frequency range, etc. Does any one know if I could use the probe from the USM 116 on the 410C? David KC2JD