Pentagrid converter

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The pentagrid was a radio receiving valve (tube) with five grids used in the frequency-changer (converter) stage of a superhetrodyne radio receiver. It was universally used in American domestic radios but not very common in European designs.

During the valve (tube) era of radio, frequency conversion in superheterodyne receivers was typcally carried out in British and European sets by a hexode thermionic valve (tube), energised with the incoming signal, plus the local oscillator signal derived from an additional triode valve (tube). Both valves (tubes) would typically be contained within a single envelope and was known as a triode-hexode such as the ECH35.

In the United Kingdom, the valve manufacturers had formed themselves into a cartel (the British Valve Association (BVA)), designed to protect their interests from foreign competition. This cartel dictated (among other things), the price of valves (tubes) and how they were numbered (supposedly designed to make it difficult to identify American equivalents) which were typically half the price (retail) in the USA. However USA types manufactured in the UK by companies such as Brimar sold at the same price as their UK counterparts due to BVA agreements. All manufactures published their own lists of 'equivalents' between their own valves and those of other manufacturers including American types, so cross-referencing was easy, in the UK at least.

Most importantly (for this discussion), they dictated that no more than one electrode structure could be contained within one envelope. This latter point was because the BVA levied a charge of initially £1 per valveholder, to cover royalties on any of its members' patent rights. However pressure from set makers for multi-structure valves to overcome the BVA's charge lead to British and European manufacturers introducing many multi-structure valves and these eventually became the norm. Types such as double-diode-triodes and double-diode-output pentode although common in Europe were unknown in the USA.

If the BVA had had its way this would have meant that a frequency changer would have to consist of 2 valves (with associated holders). The initial answer to this problem was the 'Pentagrid' valve (tube) such as the 6A8 This was a valve that was able to self oscillate at the local oscillator frequency and at the same time accept the incoming signal. The oscillator was able to operate apparently independently because one of the screen grids (grid 2) was connected into what would normally be the oscillator's anode (plate) circuit. The difference frequency was available in the converter's anode (plate) circuit. In Europe an extra grid was added to the pentagrid to form an octode. An earlier version was the Mullard FC4 (mid-1930s) followed later by the EK32 (late 1930s).

Although the pentagrid worked well in the presence of a strong or isolated signal, it did not work so well when it was desired to receive a weak signal that was close to a stronger signal. The stronger signal tended to 'pull' the local oscillation such that the stronger signal was the one that was received. On the other side of the coin, this 'pulling' of the oscillation provided a degree of automatic tuning. Unless suitable filtering in the RF circuit was provided, it was quite possible for the local oscillator signal to be radiated from the aerial (antenna). The 'pulling' of the pentagrid was serious disadvantage only on short wave reception; on the broadcast medium and long wave bands it was of little consequence. British radio set makers abandoned the pentagrid on this account and favoured the use of the triode-hexode.

In North America the All American Five radio was found in every home, and usually used a pentagrid converter. The types were typically, in chronological order, the 2A7, 6A7, 6A8 (metal), the 12SA7 Single ended for series strings, and the 12BE6 miniature. [[ 6SA7]] and 6BE6 versions were also available for radios with power transformers, and car radios for cars with 6 volt electrical systems.


Note: that although this valve (tube) contains 7 electrodes and is thus technically a heptode, this device is usually referred to in technical literature as a pentagrid, to distinguish it from a true heptode. The grid that carries the input signal has to have a screen grid on either side to isolate it from the oscillator grid. The remaining grid is the suppressor grid to combat secondary emission.

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