Sometimes the magic eye valves just wear out. Sometimes you can squeeze a bit more life out of them by increasing the supply voltage, but a good plan would be to get hold of a replacement which you are sure is good, and try that instead. As far as reception goes, remember that these sets were designed for use with an external aerial. Even a few metres of wire outside the house makes all the difference.
The phosphorus in the Magic Eye tubes fade over time, so I wouldn't judge the AGC on that. You can still measure the AGC from the 6H6 with a voltmeter to see its range when tuning in various stations. Having switch-mode power supplies radiating nearby wont help.
someone may have a spare Magic Eye tube they could sell you? on this site if you ask otherwise I've purchased a few new Toyo 6U5 off ebay in the past.
Hi Fred, as others have said, the green phosphor in magic eyes does wear out over time. I've heard the figure of '1000 hours' bandied about as a ballpark life for them - so its far more likely that you will buy a radio with a dead eye than otherwise. The AGC adjusts the shadow angle in the eye, not the brightness. The brightness is a function of HT voltage, and so boosting that can sometimes lead to a dead eye being marginally viewable again. Conversely I have also heard that reducing the HT voltage on a new magic eye can increase its lifespan.
One other thing to check is the plate resistor (between pins 2 and 4 on a 6U5/6G5) - sometimes (actually, most often) this is hidden away inside the socket at the end of the flylead where it plugs onto the eye itself. These are known to go open or high, causing a far lower (or none) voltage on pin 2 than the eye would normally need - and this can reduce or even eliminate visible green. So its worth double checking that. I typically just replace this resistor as a matter of course, regardless of its condition. 'just in case'.
I have read, although I don't immediately recall where, about experiments with simple HT voltage booster circuits providing a bit more life for a magic eye - but if you do this, make sure its just for the eye and not the rest of the radio. I note that the Emerson D140 takes its HT from a point after a choke AND the field coil. You could (carefully) experiment with disconnecting the HT voltage to the eye from the main HT rail and trying it back before the choke, as the voltage at that point will be higher. This would be something to be done carefully, so if you're not confident with these circuits and high voltages then leave well enough alone and just find a replacement eye.
You could buy a good used second hand eye - if you can find one... or something new old stock - but make sure its tested and working because otherwise its almost certain you'll get a dud. I've heard stories of people buying 'tested' used eyes and getting a dead one, and the seller saying they ohmed out the filament and it was ok... which is not a suitable way to test most valves, let alone an eye.
Like Richard I have also used the Toyo 6E5 to good effect as a replacement. They are not cheap (well, actually they are - but the postage is not)... but they work well. I think when I bought them it cost me over $60 for two, shipped to NZ. Now I see its more like $70 for two including shipping and Grab Snatch and Take. They came well packaged and are essentially brand new.
Another option is the Soviet-made 6E5C - this is, I believe, a 6E5 with an octal base, so some rewiring would be necessary. These can be had cheaper from Russia or the Ukraine because the postage works out a little cheaper generally... I did buy and test one of these although I never used it in anger. Mainly because I put it 'somewhere safe'... its still somewhere safe... and will be until I finally either remember where that is, or stumble across it while looking for something else.
One potential problem with the 6E5 as a substitute for the 6U5/6G5 is that its more sensitive and so its operation might not seem perfect. Its likely to fully close on strong stations and not allow you to 'see' the peak tune - but you should be able to hear the peak anyway - magic eyes are more about the look than the value they provide to actually tuning a radio (in my not-so-humble opinion, anyway).
Not much to add here but if you do use a 6E5 a simple resistive 2.5:1 divider will bring the sensitivity correct. The 6E5 closes with -8 volts and the 6U5 with -22 volts. Make sure the resistors are high value 5.6M & 3.9M so that the AGC line is not compromised.