The Mullard triode-heptode, type ECH84, is now appearing in the synchronising stages of dual-standard television receivers in this country. The valve has already been widely adopted by manufacturers producing receivers for 625-line transmission systems using negative vision modulation.
The heptode is generally operated as an interference-cancelling sync pulse separator, and the triode section as a pulse limiter. The heptode function is particularly valuable when interference pulses which are in the same sense as the sync pulses, occur in the video waveform and tend to disrupt synchronisation.
The ECH84 has been designed specifically to supersede the ECH81 in the sync separator application. The earlier type was initially intended for use in AM radio receivers and was only subsequently adopted for synchronising circuits, whereas the ECH84 was designed specifically for the television application. Good operation at low voltages is thus ensured. Another consequence of this specific design is that the linearity of the control grid characteristics of the heptode section is markedly better than that of the earlier valve. In the triode section, too, higher values of mutual conductance and amplification factor have been achieved. These improved properties of the ECH84 are reflected by better receiver performance under fringe reception conditions.