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 13VPA to act as a mixer if required. The five pin based version has g3 tied internally to the cathode.
The perforated cylinder is the electrostatic screen. The top cap connection can be seen to pass into the top screen to connect to the control grid.
The Cossor box. The price was fixed at 15/6 50% higher than the original price (paper label over original box) additionally the valve was subject to 5/1 purchase tax. In total a price of just over a Pound - expensive. 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.