Steve Anderson wrote:Yep, the heater cools down quite quickly, that's why that red/orange glow stops. But the cathode in relative terms has a much greater thermal inertia or mass - hence the reason for all tubes/valves taking dozens of seconds to warm up and get going. It's the reverse when the heater is disconnected.
I should of really timed it better but did find it interesting i kept wondering why it was still working with the heater disconnected ,most of the time you have both circuit power supply and heater both switching on and off same time ..this time just the heater power was removed as i wanted to test the circuit while on and the crt off or heater off ...never saw the effect before i was surprised ....
In most equipment the HT supplies will discharge quicker than the cathode cools, but in a CRT often the supplies will keep the thing going for quite a while as most CRTs draw such a small current. An average valve/tube table-top radio will draw 40-60mA from 200-300V, a 'scope type CRT is usually much less than 1mA.
Well its an interesting effect indeed !
One problem with the induction heating idea is focusing the heating to the cathode only, you do not want the grid to get hot such that it too starts emitting electrons. This was one of the design challenges in later and smaller valves/tubes. To get a high mu (amplification factor) the cathode-grid spacing needs to be very small. If ever you take apart an ECC83 dual triode you'll see what I mean. That has a mu of 100 and was about the limit, though the ECC803 did manage a mu of 130.
I don't recommend breaking a valve/tube deliberately, they do contain some nasty toxic chemicals.
Steve A.
I have been thinking about this for a while the induction coil first idea was a flat coil around the base of the glass just below the cathode where you see the wires connections to parts of the tube problem i was thinking here it would also heat the various connections to the control grid anodes and such may be too much ? but you can control the induction you don't have to melt the thing perhaps there would be a sweet spot where the heater connections warm up enough to get the thing to boiled off electrons in thermionic emission but may be very slow to get going...Valves tend to be warm so i would think other parts of the valve can take being warm up also in this idea ...
Idea 2 is to actually to heat metal pins to the heater at the connections with small induction coil thermal temperature make its way up the wire to the broken heater which would be better as you could control it but i suppose you could connect those pins to longer rods and heat them with a flame if you had to Macgyver it as a last resort ...after that i am running out of ideas ....Laser pointed at the heater !!!!!!!
I do have a little crt with a broken heater but these unproven ideas don't need it to be broken just a method without using voltage to the heater if it works or not ...need to make that induction heater !
My original thoughts were to try this on a triode or such wired up as a oscillator or amp and test .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.