Magic Eye Suppressor Grids!

[Chris55000]

Hi!

It's not been mentioned before on this forum I believe, but quite a lot of both the end-viewed side-contact/octal and the more modern side-viewed B9A magic-eyes/tuning indicators had an internal suppressor grid of the display section, connected internally to cathode, EM34s certainly do, and some (but not all!) Y61s/Y63s had one as well - this was usually annotated as " k,g' " on valve-base lists when shown!

Most magic-eye symbols in P.W., R.C., etc., (I draw them that way as well!) tended to show the control/suppressor grids together as a single dashed line for convenience of drawing altho' they obviously were two distinct electrodes!

What was the original purpose of eye suppressor grids?

Can anyone suggest why some valve-maker's were inconsistent about fitting them?

Chris Williams

[DangerMan]

Assuming you mean the helical grid winding that can be seen in the display section of EM34, Y63 etc., this is a control grid with the function of controlling (normalising is perhaps a better word) the beam current of the display.
The current is fixed by its design and not externally controllable except by increasing/decreasing the HT, which will affect it but not by as much as if the grid is absent.
Some early indicators didn't have it, and their life expectancy was even more variable than those later ones that did.
You can imagine the comparison between a zero biased triode and a diode connected across the HT supply... the triode would be more predictable and less affected by cathode ageing than the diode.

Pete

[Mike. Watterson]

It's just a control grid, but connected to cathode.

A suppressor grid is always at LEAST the 3rd grid and near Anode to stop secondary emission from Anode to G2 (screen grid). Without it you get a negative resistance region on a Tetrode.

All most all Eye tubes are triodes with a deflection electrode(s). Basically CRTs. The DM70/DM71 has no deflection electrode or driver triode. It uses a grid with ! cutout and angled cathode filament to vary area of anode hit by electrons.
The Deflection electrode(s) are driven by the separate triode.

[Chris55000]

Originally Posted by DangerMan

Assuming you mean the helical grid winding that can be seen in the display section of EM34, Y63 etc.,

Hi!, Yes, that was the one to which my query referred!

Most articles I read regarding the auxiliary grid in eyes tended to (incorrectly!) refer to it as the "suppressor-grid" (hence my thread title!) - would a "space-charge" grid have been a more correct term, or is that a completely different function altogether?

R.D.H. does mention "some electron-ray tubes have a special construction designed to minimise random changes of target-anode current throughout the useful life of the tube" but in my edition it doesn't say what that is!

Incidentally, American/Japanese and Australian literature virtually never show an auxiliary-grid in their magic-eye circuit symbols!

Incidentally, in Odhams's "Radio, Television and Electrical Repairs", 1961 Edition, there's a circuit given for an FM tuner (no component-values as per usual!) using the American 6AL7GT FM indicator, again with control grid externally connected to cathode & HT- - was this the same display section control/auxiliary grid internally connected on British/European indicators?

Chris Williams

[Leon Crampin]

The EM71 is unusual in having the indicator section control grid brought out to a separate pin. In every application I've seen for this device, the grid is connected externally to the cathode. Has anyone seen an application where this grid is controlled? Some experimentation might be interesting.

Leon.

[AC/HL]

One post moved to a new thread here: https://www.vintage-radio.net/forum/...d.php?t=113873

[kalee20]

Originally Posted by Leon Crampin

The EM71 is unusual in having the indicator section control grid brought out to a separate pin.

Presumably this is the indicator grid (the subject of this thread) rather than the deflection pin electrode, so it would be possible to control both brightness and area by driving the two grids separately. Interesting!

[Synchrodyne]

Originally Posted by Chris55000

Incidentally, in Odhams's "Radio, Television and Electrical Repairs", 1961 Edition, there's a circuit given for an FM tuner (no component-values as per usual!) using the American 6AL7GT FM indicator, again with control grid externally connected to cathode & HT- - was this the same display section control/auxiliary grid internally connected on British/European indicators?

Yes. There were various ways in which the 6AL7-GT could be used. Primarily it was intended for use as an FM tuning indicator, taking both discriminator and limiter grid inputs and displaying both centre-channel tuning (or not) and signal strength. But the control grid, with a negative voltage applied, could be used to squelch the indication when off-channel, hence its separate pinout. This is shown in the GE data sheet, which may be found here: http://frank.pocnet.net/sheets/093/6/6AL7GT.pdf.

Cheers,