The ET "Pro One-9” Bias Probe is an "all in one” bias probe for Noval 9 pin tubes including EL84’s and EL844’s. It is unique in the fact that it shows you both the plate current and plate voltage simultaneously and it is completely self-contained with no switches, knobs or batteries. The "Pro One-9” Bias Probe will read out the plate current in the upper readout and the DC plate voltage in the lower readout. Unlike other bias probes the "Pro One-9” Bias Probe is designed to allow you to play the amp while the probe is installed. So adjust, play, adjust and dial your amp in!