ctc12 clone beam current

[miniman82]

If you mean .93ma with a dark screen and HV set to max, that's your problem. Most of the Sams I've read for roundies specify .8ma with a dark screen, HV set to whatever the schematic calls for. If you're pulling any more than that, chances are you're getting close to melting the 6BK4 as you're likely exceeding it's dissipation. You see HV schematic information has more to to with the regulator tube than it does the chassis.

For example, the 6BK4B (which is what most will have in their chassis) is rated at 40 maximum plate watts. Taking into account a 20% tolerance, this tube can be expected to dissipate somewhere around 32 watts continuously. Typically it comes to around 25 watts though, RCA designed the shunt circuit conservatively to maximize the shunt tube's life.

So for your example with a dark screen and the HV pot at 'low' and drawing 1.64ma (assuming a 23kv anode), that comes to 37.7 watts- easily above what RCA would have designed it to do, and dangerously close to the tube's maximum dissipation. I strongly suspect that if you look at the tube's structure under those conditions with a darkened room, you'll see the plate glowing which can be seen reflected off the bottom part of its structure (the part containing the grid and cathode, it's usually shiny so you can see stuff in it).

For the other example: .93ma@23kv=21 watts which is much better but again, outside what I would consider normal for a shunt tube which should be closer to .8ma. Obviously whenever you have something on screen, shunt current should drop as the CRT beam current picks up where the regulator left off. You should check to make sure the shunt tube's cathode current drops as brightness increases, if not the regulator circuit is suspect. Also take a look at your B+boost voltages, as that's where your regulator is taking its reference voltage from. If B+boost is jacked, so will regulator cathode current as the grid receives it's marching orders from whatever B+boost is doing.