EL33

The EL33 is a high-sensitivity audio output pentode described by Mullard for AC mains equipment. Our sources place its introduction in 1938 and its typical use in domestic receivers with a single-ended output stage; data sheets also document push-pull operation. It is indirectly heated at 6.3 V and 0.9 A, with parallel AC or DC heater supply. Its octal base has heater connections on pins 2 and 7, anode on 3, screen on 4, control grid on 5 and cathode on 8; pin 6 is unconnected. Specimens with metallisation connected to pin 1 and others without it are described. In documented examples, two upper mica supports have separate roles: one braces the structure against the glass shoulder, the other supports the electrodes. Internal coating and getter deposit can obscure much of the assembly.

For class A, Philips and Mullard specify 250 V on both anode and screen, −6 V control-grid bias, 36 mA anode current and 4 mA screen current. Transconductance is 9 mA/V and internal resistance approximately 50 kΩ; this is not the transformer load, which is 7 kΩ in the example. Philips and Mullard's 1546 sheet give 4.5 W at 10% distortion, while Mullard's 858 gives 4 W under its tabulated conditions. These editions should not be homogenised. RMS input is 4.2 V at the stated output; for just 50 mW Mullard gives 0.33 V and Philips 0.35 V. High sensitivity therefore refers to drive voltage under specified conditions, not a sound quality inferred from numbers.

For two EL33 tubes in push-pull, the tables specify 250 V anode and screen supply, a common 140 Ω cathode resistor and a 10 kΩ anode-to-anode load. Output reaches 8.2 W at 3.1% distortion with 6.7 V RMS input in the manufacturer's convention. Quiescent anode current is 24 mA per tube, rising to 28.5 mA at maximum signal. Mullard also describes triode connection with the screen joined to the anode: at 250 V, 20 mA and −8.5 V bias, the example with a 425 Ω cathode resistor and 7 kΩ load gives 1.1 W at 5% distortion. The lower output belongs to this different arrangement; it is not a defect in the pentode test. The curves include currents, distortion and changes with screen voltage.

Limits also depend on the edition. Both give 9 W maximum anode dissipation, 55 mA cathode current and screen dissipation of 1.2 W without signal or 2.5 W at maximum output. Philips allows 300 V anode and 300 V screen voltage; the Mullard sheets consulted specify 250 V anode and 275 V screen voltage. The separately stated 550 V no-load figure does not replace operating limits. Philips gives 100 V heater-to-cathode, Mullard 50 V. A pinout resembling another output tube is therefore insufficient to establish a safe replacement: supply, bias and load must be checked. Documentary sources: Philips EL33, 10 October 1953; Mullard EL33 Output Pentode, Issue 2, series 858 and 1546 with curves.