Edwin Armstrong on electron tube positive bias

Some inventors are centuries ahead of their time.

The first 100 years have already passed since Edwin Armstrong, not only inventor of FM radio,  published in 1917 (re edited from 1914) a paper mentioning kindly how the operation of the triode vacuum electron tube is not very well understood...

In modern time (and backwards in time too), it seems technology goes so fast that sometimes details are left behind.

God is in the details, as master composer Johan Sebastian Bach used to mention...

The Armstrong paper is cited at the end of this post.


A small part of it, relevant to the Pleiades V series preamplifiers operating with less than 6 volts anode potential thanks to positive bias grid neutralization, will be copied on the following paragraphs.

As it has been mentioned on euroelectron posts, electron tubes can operate as very sensitive and low noise preamplifiers with very small battery anode voltage (as well as reduced heater voltage although it seems this can be done anyway), when positive bias neutralizes the grid. The first point on the Armstrong's paper is copied below, being the most relevant.

At the time (1917) the word Wing was used instead of Plate or Anode and Audion instead of Triode.

The modern terminology and comments will be given in parenthesis.

So let us hear the master inventor...

(End of the paper's second paragraph...)

The characteristic shows that, starting with the grid and filament at zero potential difference, a negative charge imposed to the grid produces a decrease in the wing (anode) current and a positive charge imparted to the grid produces an increase in the wing (anode) current.

(So far so good, this is what we know too on 2017).

Edwin Armstrong continues...

This is the fundamental action of the audion (triode) when used either as an amplifier or as detector. The reason for this action will appear upon examination of the behavior of the audion of the type shown in Fig.3.

(Before moving on to the very important 3rd paragraph, it seems fit to clarify that on figure 3 is drawn a triode which has 2 anodes, one on the far left of the glass valve envelope and one on the far right).

Edwin Howard Armstrong...

The wings (anodes) of the audion (triode) were placed symmetrically with respect to the filament, but only one grid was employed. (between filament and right plate). It was found that, under similar conditions of filament temperature and voltage of the battery V2 (plate supply battery), a considerably smaller current was obtained between the filament and plate on the side in which the grid was inserted. In both measurements the grid was left entirely free of any connection with the rest of the apparatus. Obviously the grid obstructed the flow of the thermionic current. Investigation showed that this was due to the charge accumulating on the grid when exposed to bombardment by the electrons passing from filament to wing, the electrons pass readily enough into the grid but cannot easily escape from it, and as a consequence of this, negative electricity piles up on the grid. The potential assumed by the grid when exposed to this bombardment may be several volts negative with respect to the negative terminal of the filament, it may be the same as the negative terminal, or it may be positive with respect to the negative terminal, but it will always be negative with respect to the potential of the field in the plane of the grid which would exist if the grid were removed from the bulb. The negative charge on the grid, therefore, impedes the flow of electrons from filament to plate, causing the decrease in wing (plate) current. The placing of a positive charge on the grid from an external source tends to neutralize the negative charge on the grid, thereby permitting an increase in the wing current. The addition of a negative charge to the grid increases the deflection of the electrons and produces a further decrease in the wind current...

So this is the third paragraph of the amazing paper written 100+ years ago.

This is a very helpful and revealing explanation on why the Pleiades preamplifier operates at even less than 6 Volts on the plate when the grid is neutralized from its self assuming negative potential. This is done by connecting a resistor of the order of 4.7MΩ from anode to grid on the Pleiades front end preamplifiers. The electron tube used is the EF183 triode connected.

Armstrong describes that the negative potential is caused by electrons passing from filament to wing (anode).

It was found by accident while experimenting with the Pleiades V6 on breadboard that the grid is negative even without any plate source connected to the apparatus.

At the time of Armstrong there may not have been voltmeters of very high input impedance, so direct true measurement of the grid potential may not had been feasible.

Now it may be argued that Armstrong was experimenting with a tube that had more gas, less evacuated, than modern tubes.

The Pleiades experiments were performed on modern vintage ECC82 and mainly EF183 vacuum electron tubes.

It is striking performing the following experiment. This is by carefully (fuse in series with battery is important for safety), supplying 6.3V only to the cathode heaters while a high input impedance (Z=10MΩ) voltmeter is connected between cathode and grid.

As soon as the cathode starts heating up, a large voltage of millivolt order keeps increasing until the temperature rises to its peak value.

If we then measure grid to cathode current, (we in fact short circuit the grid to the cathode using a micro ammeter), a substantial grid to cathode current is caused by the measuring procedure itself.

So it is observed that just the boiling of electrons out of the cathode can induce this grid (cathode) voltage and current.

The same phenomenon occurs by repeating the same measurements to plate instead of to grid but it is much less pronounced as the plate is much further from the cathode than the grid.

Acknowledgement is due to Hliana for insisting that the grid must be positively biased.

Reference:

Operating Features of the Audion - E. H. Armstrong - Annals N. Y. Acad. Sci. Vol XXVII, pp. 215-243, 2 August, 1917