A thought about Tube H-K shorts.

Something that had occurred to me, and indeed heater/filament chokes do feature in grounded-grid RF PAs. However, CRT drive does need good LF response (large plain areas of picture), so by time you've made LF chokes capable of handling whatever several hundred mA, even an amp or so, that the CRT heater takes, you might as well have the familiar low-capacitance mains step-down transformer. That's my guess, anyway. Interested to hear what others rather better versed in this problem think.

It shouldn't be difficult to make a suitable choke. 1mH has a reactance of 6.3 kilohms at 1MHz. And since the current is the same in both lines, a common-mode choke will do the job using a high-permeability ferrite toroid. Even a RFI suppressor common-mode choke from an old switch-mode power supply should work.

I had a scrap wall wart with one of those big black plastic coated beads on it. I've no idea what band of frequencies it was intended to suppress but it certainly made no noticeable difference to the definition of my HKS TV22. That said, it was showing 2M bars quite well without the bead.

Peter

It shouldn't be difficult to make a suitable choke. 1mH has a reactance of 6.3 kilohms at 1MHz. And since the current is the same in both lines, a common-mode choke will do the job using a high-permeability ferrite toroid. Even a RFI suppressor common-mode choke from an old switch-mode power supply should work.

Recently, I opened up one of those IEC block filters (checking for evil Rifas before putting it into service!) whose label proclaimed that the 40mm "polo" within, with a mere 45 + 45-odd turns, was rated 3A at no less than 25mH. My instinctive thought was, oh yeah, come on, pull the other one but a quick check on cheapy component tester confirmed 27mH. Presumably, this falls away rapidly with any DC/LF current imbalance but I was agreeably impressed. I'm sure that the hard-core TV men here would argue for high impedance down to 50Hz but it made me think that a satisfactory heater choke for CRT use wouldn't be so unattainable with cores like this- for typical picture content anyway, rather than stringent test signals. The two windings were each split into two approximately equal sections with appreciable separation around the core circumference, presumably keeping capacitance down, but I wonder how it would compare against a specific low-capacitance transformer, particularly if Trevor's cathode follower idea was used. Food for thought/experimentation...

I had a scrap wall wart with one of those big black plastic coated beads on it. I've no idea what band of frequencies it was intended to suppress but it certainly made no noticeable difference to the definition of my HKS TV22. That said, it was showing 2M bars quite well without the bead.

Peter

I confess that this was just stupid! It was the only ferrite I had so I just thought I'd slip it over the wires without testing it. I've now tested it with one pass through (which is all I had room for) and it has absolutely no attenuation in the low megahetrz. It only starts to do anything at about 30MHz.

Peter

Recently, I opened up one of those IEC block filters (checking for evil Rifas before putting it into service!) whose label proclaimed that the 40mm "polo" within, with a mere 45 + 45-odd turns, was rated 3A at no less than 25mH. My instinctive thought was, oh yeah, come on, pull the other one but a quick check on cheapy component tester confirmed 27mH. Presumably, this falls away rapidly with any DC/LF current imbalance...

Yes, it will fall away. But in the configuration proposed (tube heater fed by an isolating transformer with some capacitance to ground), there should be practically no imbalance current. The only way you can get imbalance is current flowing down this capacitance-to-ground, and it should be at most a few hundred microamps.

I'm sure that the hard-core TV men here would argue for high impedance down to 50Hz but it made me think that a satisfactory heater choke for CRT use wouldn't be so unattainable with cores like this- for typical picture content anyway, rather than stringent test signals.

I can't see why it would need to be high impedance down to 50Hz, because the capacitance to ground in the aforementioned transformer will be high at 50Hz. The choke only needs to be there to keep the impedance high at the high video frequencies, so you don't lose definition with the fast edges being shunted to ground by the non-ideal heater transformer.

Somewhere in the middle, there will of course be an impedance drop (as the choke and the transformer form a series resonant circuit, and we all know how that behaves!) but it may not be a problem- if it is, a third winding on the choke connected to a damping resistor may give some opportunities for a good play.

Yep, I'll put my hand up to some decidedly occluded thinking there, my mind on the back burner whilst doing other things. Of course, as you say, the isolating transformer is still present (to overcome the upset in DC bias conditions in the first place), I was dumbly thinking of a low impedance to ground (in a wideband sense) heater supply. Whereas the iso transformer would presumably have something of the order of a few tens (?) of pF to ground. I've wondered if a typical modern type mains transformer having split bobbin with fairly thick, kV isolation insulation would have fairly low capacitance to ground anyway- and it could probably be easily improved in this respect with something like added insulating ribbing or somesuch under the secondary turns, which themselves would be deliberately spaced away from primary bobbin end. I imagine original low-capacitance CRT heater transformers are a bit of a rare sight nowadays.

A common mode choke will not make much if any difference. Two independent chokes one in each wire may help more though.

Surely the video signal we are hoping to decouple is in common mode on the heater leads so it doesn't matter whether the choke is common mode or two separates.

Peter