Metroliner telephone article 4084

You're in the right area of the set.

Ok, first, for those just tuning in: Color TV sets or early vintages (and maybe today, I guess), needed pretty tight regulation of the high voltage to the CRT. Swings in voltage affected convergence, picture size, color balance, and everything else.

At 25kv, the current draw on a color CRT was only a few miliamps, which was good because the high voltage power supply couldn't put out more than a few ma anyway.

The early sets (15 inch experimentals and 1953-54-55 production) used a few devices to hold the high voltage constant. Motorola used a cold cathode regulator, but most everyone else used the RCA shunt regulator setup. As it's name suggests, the shunt regulator is basically a tube with the plate at the high voltage, the cathode at ground. the grid varied the transconductance (?) of the tube, which effectively changed the load it presented to the power supply. By holding the CRT current draw + the shunt regulator current draw to an equal number, the high voltage would stay the same, since the HV supply dropped very quick with loading. I think RCA got it within 500volts, and I suspect they could have gotten better, with some effort.

The tube the the industry (aka, RCA, the only ones who stuck with color TV that early on) was a weird 'beam triode' (yes, triode) called the 6BK4. I think the 6BD4 was the developmental-early version? It was rated out to about 30kv at the plate, and had a mu that was exceptionally high (I was to say 2500). This tube was the size of a large glbutt, i.e. 'GB' envelope, octal base.

The 6BK4 was the de facto high voltage regulator in the industry, even into the 70's. Everyone used it, even if they didn't use the 3A3 high voltage rectifier or the 2AV2 focus rectifier. Towards the end, GE had a few 'Compactron' knockoffs, but they spec'd the same.

As was known as early as the 40's, any tube in a TV set, if driven at a high enough voltage, would make X rays, though soft ones. RCA, etc often quoted 16kv as the hard and fast number above which X ray generation would begin. 1B3s, etc had warnings, but B&W sets rarely went above 16kv (even large screen ones) and the current was very low in any case (1 - 2 ma).

Color sets, of course, went higher and had more current draw, and thus the high voltage sections were enclosed in a heavyish box. This was standard practice for RCA sets into thr 60's, though CTC-16 broke with it by placing the 6BK4 in open air with a shield around it, and the 3A3 on top of the flyback transformer, UPSIDE DOWN.

At some point, someone with lots of time on their hands noticed that some sets emitted a virtually harmless amount of X rays under some conditions, generally in a downward direction.

This, naturally, caused hysteria.

A few types by GE were singled out (I don't know if they ever were proven to be any better-worse than others), and the 6BK4 itself got 'modified' in the form of thicker, leaded glbutt. The 3A3 got this too, and GE's was actually lead cased, and a bit bigger, which was good since it now had a warning that it was lead and bad for you, in addition to the x ray warning.

The government, of course, got involved, and eventually, a circuit to render the set 'unwatchable' in the event of HV regulation failure, became a requirement. This was where H Stat, and other HV protection got its start.

By the end of the 60's, RCA came up with pulse regulation (which clamped the flyback pulses on the low side of the transformer), and the coming of the solid state tripler meant the HV section wasn't going to be an 'x ray generator' anymore.

Interestingly, TV repairmen must be x ray proof - I'm not aware of any increased cancer rates for them (at least that could be traced to x ray exposure), though they virtually hovered over the HV sections on TVs all day long...

A few other HV regulation schemes came about in the 70's, the most famous was Zenith's magic capacitor, which was magic in that it tended to fail full open, allowing the high voltage to skyrocket, which then caused the HV to arc through the neck of the CRT to the yoke, neatly slicing the neck off. Well, at least it rendered the set unwatchable ;)

Big recall operation, too. I think it nearly sunk them.

I'm not aware of the pattern the 6BK4 threw, though the defective GE tubes might have thrown a U, given the claimed problem was misalignment of the elements.

Frankly, I have a few sets with 6BK4s, and I don't worry. As long as the HV is adjusted to the book value, there's nothing dangerous about the sets anyway, and in any case, most older sets can't get the HV up over 30kv much, so they're not like the dentist's X ray machine.

And you're not supposed to sit in front of the TV anyway ;)