EH2

The EH2 is a variable-slope heptode for superheterodyne receivers, usable as a mixer with a separate oscillator or as an RF/IF amplifier. Technical accounts describe a hexode-like structure with five grids: received signal on grid 1, oscillator on grid 3, grids 2 and 4 separately connected screens. Grid 5 is a suppressor, added to improve internal resistance and operation at relatively low plate voltage. This explains why some early Philips Miniwatt sheets still say “Hexode”, whereas application accounts call it a heptode.

Heating is indirect, 6.3 V, 0.2 A, AC or DC, with series or parallel supply under the manual’s conditions. The available Philips Miniwatt sheets are dated 12 April 1937, documenting characteristics and applications at that time without automatically establishing a commercial introduction date. Capacitances include input-grid 5 pF, plate 11 pF, grid 1–grid 3 0.2 pF and plate–grid 1 below 0.0015 pF.

One mixer point uses plate 250 V, screens 100 V, grid 1 −3 V, oscillator 14 V RMS on grid 3 with grid resistance 0.5 MΩ. Plate current is 1.85 mA, combined screen current 3.8 mA, conversion conductance 0.4 mA/V, internal resistance 2 MΩ. With screens 80 V, grid 1 −2 V and oscillator 10 V RMS, currents are plate 1.8 mA and screens 3.5 mA, with the same conversion conductance. Automatic control acts on grid 1; increasing negative bias to the tabulated values reduces conversion conductance below 0.01 mA/V.

RF/IF amplifier operation is different. At plate 250 V, screens 100 V and grids 1 and 3 both −3 V, tables give plate current 4.2 mA, combined screens 2.8 mA, transconductance 1.4 mA/V and internal resistance 1 MΩ. Ordinary transconductance is not conversion conductance. Application accounts recommend applying the same control voltage to both grids, feeding screens from a divider and decoupling supply lines. External metallization has its own contact, normally earthed.

Application circuits show an EBC3 as a separate oscillator and also allow a triode-connected EF6. The account describes operation down to 5 metres wavelength and coupling to EH2 grid 3 through 20–50 pF, preferring 50 pF for multiple-band reception. On short waves it recommends oscillator frequency above received frequency and very short oscillator-circuit connections. These instructions belong to the illustrated circuits, not a promise of such performance from every assembly using these valves.

Keep limits tied to the document. The 1937 Miniwatt sheets specify working plate 250 V, zero-current 550 V, plate dissipation 1.5 W; each screen working 125 V, dissipation 0.5 W, zero-current 400 V. Maximum cathode current is 10 mA. Accounts differ in some limits: German allows plate 300 V, English retains 250 V; heater-cathode provisions also differ. Do not combine whichever limits are highest: follow the appropriate specification as a whole for a particular circuit.

The valve has a European eight-side-contact base, also called Ct8/P8A, with grid 1 on the top cap. Drawings distinguish plate, both screens, grid 3, cathode/suppressor, heater and metallization; a top cap therefore does not automatically mean plate connection. Technical drawings give maximum diameter 32 mm and overall height 90 mm. One archived Philips specimen bears a small aeroplane drawing on its coating: a feature of that specimen, not evidence of military use or a special version. Documentation: Philips Miniwatt EH2, sheets 5821–5823 and curves, 12 April 1937; EH2 Heptode application accounts in German and English, pages 92–97; Dutch EH2 Heptode sheet, pages 22–23.