[ohohyodafarted]

Nick and others with better technical background than I have; please chime in here and discuss "CRT LOAD"

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.

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.

That leads me to believe that CRT places a high demand for 2nd anode current during these bright situations. Therefore can I assume because Chris is using a small crt test jig, that we are never going to see real world demand on this HOT until he hooks up the 21AX and it's associated deflection yoke?

The point I am getting at is this; 120ma of cathode current is no where near the kind of load that will be drawn by the cathode of the horizontal output tube during actual use with the 21AX. Therefore any cathode current readings or temperature of the windings need to be done under full load conditions. When full current is demanded during bright scenes on a 21AX with 22Kv or better, then and only then will we be able to determine if the flyback is going to over heat. And I think from dead cold to full temp under normal loading conditions will probably take at least 30 minutes to achieve.

It is also my gut feeling that because current and voltage are inverse to each other, that a low voltage at the 2nd anode, will cause a rise in current. It is the rise in current that will causes the flyback to heat up. Think of an arc welder. More current (amps)=more heat.

The thing that causes a wire to heat up is the current being drawn through it. Seems pretty basic to me. But I would guess there may be other factors involved.

As for the 6BK4, that sucker (essentially a high voltage triode) 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? 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.

Perhaps Chris' flyback wax melted because it was being driven harder than it needed to be, and the shunt regulator was constantly sucking up the excess and loading the flyback when it produced too much HV.

These are just my gut feelings (and rambling thoughts about a subject I would like to understand better), and to those members that understand this stuff better than I do, please chime in and lets have some good discussion on the cause and effect of real world current demand on the flyback from a crt that is being driven hard, and the effect of the 6BK4 on a flyback that is producing higher voltage than is required. The better we understand the causes and effects that damage our flybacks (mostly pure unobtanium) the better we can protect these irreplaceable components.

Inquiring minds need to know

AS an aside comment; I have a CTC4 chassis that I am working with for the Cheltenham. I turned the chassis over and what did I find? I found the HV doorknob cap with most of it's potting wax compound melted out of the housing. Obviously, the doorknob is leaky and producing a lot of heat as it draws current. This is just another place where unnecessary LOAD is being placed upon the flyback. After the ctc4 series these HV doorknobs were eliminated. I imagine the doorknobs were problematic. My 21CT55 had a bad doorknob and it wiped out the flyback. The doorknob should not have any DC leakage. It's job is to smooth the ripple on the 2nd anode.

On the CTC5 series, all that was used was the conductive dag coating on the exterior of the plastic crt shroud which acted as a filter capacitor between the metal bell of the 21AX and ground. When the 21CY glass tube came out, dag coating on the tube became the filter cap.

So if you have a CTC4 or a 21CT55, make sure your doorknobs are not leaky. I would think it can be a source of excess load on your flyback.