The 13VPA is an HF screened pentode with a vari-μ characteristic. This exhibit has a clear bulb whereas the 13VPA has a metalised envelope.
The amplifier gain is altered by changing the bias point via an AGC voltage derived from the signal. The greater the signal the more negative the AGC line and thus the lower the amplification of the controlled stage. The AGC or earlier AVC (Automatic Volume Control) has the function of avoiding overloading of the receiver by strong local signals.
The suppressor grid has a separate pin to allow for extra control of the valve this allows the 13-VPA to act as a mixer if required. The five pin based version has g3 tied internally to the cathode.