McLuhan was interested in the parallels between the vacuum tube as employed (before the advent of transistors) in electric devices like radios, televisions, computers, etc, and analogy as deployed in literature. Here he is in a letter to Norbert Wiener from March 1951:

it may interest you to know that the electron valve (…) represents a principle discovered in 1870 by Arthur Rimbaud and applied to poetry and painting since that time. Your account of the uses of the vacuum tube in heavy industry is an exact description of the poetic techniques of Joyce and Eliot in constructing their works.  Their use of allusion as situational analogy effects an enormous amplification of power from small units, at the same time that it permits an unrivalled precision. Their stripping of rhetoric and statement corresponds to your observation that “it is no longer necessary to control a process at high-energy-levels by a mechanism in which the important details of control are carried out at these levels”.¹ Stephane Mallarmé made this observation about his own poetic technique in 1885. In short, appearances and pedagogical limitations aside, there never is or can be a dichotomy between the top-level perceptions and procedures in the arts and sciences of an age.² (McLuhan to Wiener, March 28, 1951)

He specified this interest further in a letter to Ezra Pound a few months later:

I’m interested in such analogies with modern poetry as that provided by the vacuum tube. The latter can tap a huge reservoir of electrical energy, picking it up as a very weak impulse. Then it can shape it and simplify it to major intensity. Technique of allusion as you use it (situational analogies) seems comparable to this type of circuit. Allusion not as ornament but as precise means of making available total energy of any previous situation or culture. Shaping and amplifying it for current use. (McLuhan to Pound, June 12, 1951, Letters 224)

Another 1951 instance is found in McLuhan’s ‘Dos Passos: Technique vs. Sensibility’:

Ulysses shows a very different conception of history in providing a continuous parallel between ancient and modern. The tensions set up in this way permit Joyce to control the huge accretions of historic power and suggestion in the human past by means of the low-current of immediate incident. The technological analog of this process occurs in the present use of the electronic valve in heavy-power circuits. So that Joyce does not have to step up the intensity of the episode or scene so long as he maintains its function in the  total circuit.

His on-going consideration of the matter is reflected in a series of texts McLuhan published in the mid 1950s:

As a vacuum tube is used to shape and control vast reservoirs of electric power, the artist can manipulate the low current of casual words, rhythms, and resonances to evoke the primal harmonies of existence or to recall the dead. But the price he must pay is total self-abnegation. (Joyce, Mallarmé and the Press, 1954)

The technique of an Eliot poem is a direct application of the method of the popular radio-tube grid circuit to the shaping and control of the charge of meaning.³ An Eliot poem is one instance of a direct means of experiencing, under conditions of artistic control, the ordinary awareness and culture of contemporary man. (Media Log, 1954)

The cathode tube carries ‘the charge of the light brigade’. The tube carries both the charge and the answering barrage. The result is the painting of images by the ballet of electrons. (Notes on the Media as Art Forms, Explorations 2, 1954)

… radio and TV were not just the electrification of speech and gesture but the electronification of the entire range of human personal expressiveness. With electronification the flow is taken out of the wire and into the vacuum tube circuit, which confers freedom and flexibility such as are in metaphor and in words themselves. (Historical Approach to the Media, 1955, emphasis added)

The electronic or vacuum tube first manifested its powers in the acoustic sphere [radio] but did not achieve full expression until TV. (…) Television takes a large step toward reassembling all the elements of interpersonal discourse which were split apart by writing and by all the intervening artificial media. For language itself is a (…) medium which incorporates gesture and all the various combinations of sensuous experience in a single medium… (Educational Effects of Mass Media of Communication, 1956)

His most protracted consideration is to be found at this same time in the 1955 ‘Radio and Television vs. The ABCED-Minded’:⁴

The simplest way to get at Joyce’s technique in language, as well as to see its relation to TV, is to consider the principle of the electronic tube. The paradox of the electronic tube is that it is the [continuation by] means of breaking the conductor in an electric circuit. The tube permits the electrons to escape from the wire that ordinarily conveys them. But the tube controls the conditions of escape. It liberates electrons from the wire but it provides a new context in which they can be repatterned. The cathode inside the tube is one end of the broken conductor and the anode is the other. The anode attracts and receives the billions of electrons that are “boiled” off the surface of the cathode. When a tube is connected into an alternating-current circuit, the anode is positive during half of each cycle. During the half cycle when the anode is negative, electrons cannot reach the anode.⁵ (…) This led to a more effective way than any known before of controlling a large current by a small voltage.  (…)
The grid is the (…) “valve” (…) of the tube. It is located between the cathode and the anode in the path of the electrons. By voltage control the grid acts as [a variable] trigger for the electronic flow. Grid bias⁶ blocking electronic flow is recentralized [or coordinated with the main circuit] by signal voltage. Signal voltage is a trigger that releases full flow of current through the tube. But this flow stops when anode voltage becomes negative. Cycle then repeats. (…)
Thus a tiny amount of energy can be exactly controlled or stepped up instantly to very high potentials.⁷
Now metaphor has always had the character of the cathode-anode circuit, and the human ear has always been a grid, mesh, or, as Joyce calls it in Finnegans Wake, Earwicker⁸. But Joyce was the first artist to make these aspects of language and communication explicit. In so doing, he applied the principles of electronics to the whole history of culture. The entire cyclic body of Finnegans Wake is suspended between a predicate and a subject.⁹ The cathode-anode aspect of metaphor and language Joyce first extended to syntax. He took the charge of meaning out of the wire of direct statement into the vacuum-tube of the self-contained poetic drama of his “all nights newsery reel” (FW 489).

Later in this same essay:

Metaphor means a carrying across. (…) There is necessarily discontinuity in metaphor. There has to be a leap from one situation to another. (…)  Joyce carried these (…) proportions into every gesture and situation in the WAKE. That is why it is always radiant with intelligibility when seen or apprehended. Here Comes Everybody is his cathode, Anna Livia Plurabelle his anode, and Earwicker or Persse O’Reilly his grid or triggerman. We are the main circuit into which this electronic tube is connected. We can thus see that the functions of a work of art, or electronic tube in the social circuit, are manifold. Principally, however, the tube provides a means of control which can step up the feeble signal voltage to greater intensities of manifestation. The tube permits multiple re-shaping of the ordinary current of small talk and gossip for many kinds of work. It is no exaggeration to say that all things in this “funanimal” (FW244 – Ed.) world were current for the tube of the WAKE. All the social currents that ever were, Joyce can easily adopt in the vacuum tube, or head, or glass house of the sleeping giant Finnegan.