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Pentagid converters are economical, fairly simple to manufacture and, unlike the triode hexode, did not violate the early BVA cartel prohibition on multiple valves.
However, the oscillator, mixer and amplifier functions all take place in the same electron stream so a high degree of coupling between them is inevitable. One result is that the oscillator frequency 'pulls' strongly in favour of powerful signals and away from weaker ones. Another consequence is that the oscillator frequency is coupled to the signal input grid and will leak out of the aerial unless an efficient tuned filter is interposed. A further problem is that it is difficult to apply AGC to a pentagrid without adversely affecting oscillator performance.
Despite these disadvantages heptode frequency changers remained the norm for many years in low- consumption battery portables which were only intended to receive strong local stations and where their economy of current drain (only one filament, only one electron stream) was of vital importance.
For mains sets, however, the frequency changer of choice was the triode-hexode (a German invention) or triode-heptode. Initially, MOV was amongst the BVA cartel members who suppressed an attempt by the Lissen Company to introduce triode-hexodes into their radios as early as 1934.
However, when MOV eventually climbed onto the, by then unstoppable, triode-hexode bandwagon with their type X41, which could replace the MX40 directly and with advantage. The latter valve was effectively obsolete.
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