The 13VPA is an HF screened pentode with a vari-μ characteristic. 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 13VPA to act as a mixer if required. The five pin based version has g3 tied internally to the cathode.
The top view with the control grid connected to the top cap.
The Cossor box. The price was fixed at 10/6. Resale Price Maintenance was considered against public interest and the 1964 Resale Prices Act ended the practice.
Thanks to Mark from Vintage-parts for sending these images.
The classic envelope is mm in diameter, and excluding the base pins is mm tall.
References: Data-sheet & 1043. Type 13VPA was first introduced in 1935. See also1935 adverts.