OT Don't read this (was Re: Altair MBL source)

[Tony Duell]

> 
> I've just remembered that Magic Eye valves/tubes have (I think)
> an amplifying triode in the same envelope as the indicator.  And I

They do (and sometimes 2 triodes, for the dual-sensitivity indicators 
like the EM34). 

> think they're internally linked together.  Does anyone have any
> data on these gadgets that would show this sort of information?

Sure. The data I have on them shows the triode anode is directly 
connected to the control electrode of the indicator section. That's then 
brought out to a pin. 

Conventioanlly, the target is connected to HT+, the triode anode/control 
electrode to HT+ via a resistor [1], the cathode goes to ground, and the 
signal you want to display (often the AGC line in an AM set) goes to the 
triode grid.

[1] This resistor is notorious for going open-circuit. Something to check 
if the magic eye seems dead.

-tony