[miniman82]

Originally Posted by ohohyodafarted

I looked up the 2nd anode load for a 21AXP22. It said 500 microamps per gun maximum or 1.5ma total 2nd anode load. (I am assuming full brightness). At 25KV that converts to 37.5 watts (if I did my math correctly 25,000v x .0015A = 37.5 watts) That is certainly enough to warm up the flyback, but I would think the CTC4flyback might be able to dissipate that much heat safely.

500ua per gun is probably a bit overly optimistic; not that the tube couldn't handle it as it certainly would be bright with that much available current, just that I seriously doubt any flyback ever made for an early color TV could produce it. My Director gets pretty close, it has the strongest horizontal section of all my sets and a bright picture even on a so-so CRT.

That doesn't mean the flyback necessarily has to get hot in the process though, if everything is tuned the way it should be it's a pretty efficient magnetic coupling device. The horizontal output tube has to bear the full load of anything downstream of it, so that means yoke, HV, other circuits like AGC and gating pulses, AND the rectifier filaments. If the yoke has leakage or some other problem for example you might not necessarily see it on the screen, but it's a load on the horizontal circuit nonetheless. Same goes for HV leakage paths to ground and surrounding circuits, dusty/dirty HV cages and old HV wiring all contribute to losses.

These are all reasons why each restoration I do begins in the HV cage and horizontal circuit.

It is my impression, from discussions concerning blooming under heavy video demand in bright scenes, that a very heavy demand is placed on the flyback during these situations. 21CT55 uses a modified ct100 chassis and the horizontal drive section of the CTC2B chassis is woefully inadequate to provide enough current to the flyback during these bright situations. As a result you can get picture blooming and fade out on the crt.

Same processing circuits, much different HV setup with better flyback and larger horizontal output tube. They made a compromise when they designed each set, though if you ask me they made a mistake with the CTC-5 not a compromise.

As for the 6BK4, that sucker is designed to limit the upper end of the voltage range by shunting the 2nd anode when the voltage gets to high. So that leads to the next issue. If you have a flyback that is producing (for example) a consistent 30KV and the 6BK4 is set to start regulating at 25KV, isn't the 6BK4 placing unnecessary continuous load on the flyback?

It usually doesn't take much regulator current to bring it back to earth from a free wheeling situation (like reading the anode with the 6BK4 out of circuit), but anything lower than a regulated 22kv or so usually strains the tube and it starts red plating. The Admiral I just picked up was set to 18kv, and let me tell you there were all sorts of interesting colors coming from the plate of that 6BK4. Probably got my annual dose of x-rays.

Wouldn't you be better off bringing down the output voltage of the flyback so the 6BK4 wasn't loading the flyback output. Sort of like putting your foot on the accelerator of your car and regulating your speed by applying the brakes. You would be better off to ease back on the accelerator instead.

Unfortunately it's not that simple, if you reduce the power produced you also reduce headroom. The HV winding makes what it makes as a function of how much power is put into the flyback primary, so it would be a much better idea to limit the power going INTO the flyback instead- AKA a pulse regulator as some later Zenith sets did. By limiting the amplitude of the pulse on the primary side of the flyback, it only makes what it needs instead of over producing and being dragged back down by a 6BK4. The RCA prototype does the same thing, with a 6BQ6 pulse regulator in parallel with the 6CD6 output tube. I often wonder why many more sets didn't use this regulation scheme, I'm willing to bet there'd be a lot less burned out flybacks if they had.

FWIW, most early color TV's don't make enough HV to be dangerous. I run my Wingate with the 6BK4 disconnected all the time, because even then it struggles to make 22kv on a good day with a tailwind. I wouldn't try it with anything earlier though, the first ones were absolutely lethal with HV production. Whatever you do, do not allow anode voltage to rise past the upper limit of what the CRT can handle.