The EH90 is an indirectly heated heptode designed for television service in dual standard sets as an FM demodulator for 6 MHz inter-carrier sound.

There is a paragraph on the EH90 in the 'Mullard Outlook' in an article entitled 'New Mullard Television Valves'. EH90 - Sound Discriminator.

Adoption of the 625-line standard is to include the use of FM transmissions, and the latest dual-standard receivers are equipped to accommodate these transmissions. The Mullard heptode, type EH90, is being used for FM detection in many of these new sets, where it operates as a locked-oscillator discriminator.

The EH90 is used as a dual control valve in this application: the transmitted signals are applied to the first grid, and the local oscillations to the third grid, coupling between the two electrodes occurs by way of the electron stream. The circuit depends for its operation on the fact that the mean anode current in the EH90 will be a function of the phase relationship between signals of similar frequency applied to the control electrodes.

As this relationship varies with the signal frequency, frequency modulations at the first grid are converted to demodulated amplitude variations at the anode, and these are subsequently amplified and fed to the loudspeaker.

The electron coupling in the EH90 is good, as is the frequency-to-amplitude transfer characteristic. The valve is thus very suitable for use in this type of discriminator, and will contribute notably to the benefits accruing from the adoption of FM sound transmissions'.

The side view of the anode shows the wire grids. The rods visible either side of the centre above the top mica are the g3 supports.

The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 46 mm tall.

References: Datasheet and 'Mullard Outlook', Vol. 12, No. 7, December 1962. Type EH90 was first introduced^¶ in 1954.

 
Pin Connections

B7G	1	2	3	4	5	6	7
	g1	k,g5	h	h	a	g2,g4	g3

 
Absolute Maximum Operating Conditions^¶

Heptode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	6.3	0.3	100	30	-1	0.75	1.1	1M	0.95

Thanks to Frank Philipse for supplying the above PDF datasheet

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