Why a damper isolation transformer?
 

I understand that Damper tubes can develop a H-K short, and cause problems. But both Triad and Thordarson (26F70) made isolation transformers to isolate the damper filament from the rest of the chassis, and alleviate the H-K short issues.

But why use a transformer, when replacemnt of the tube would likely solve the problem without modification or added expense?

Perhaps some chassis were more prone, or the H-K short could cause many more problems? They were first made in the 50s, so probably not a color-TV specific problem....

Any ideas?

Early damper diodes (like 6W4) probably didn't have quite enough H-K insulation, as compared to later ones. Look at a late version, you'd see a spiral of whitish coated wire? between the heater and the cathode.

The isolation transformer was probably a solution to avoid having to replace the tube every few months, which consumers would not like at all...

Since the heater power for the majority of the tubes was grounded to the chassis on one side of it's line , connecting the damper's heater with one side grounded was said to put too much potential between the now grounded heater and the operating at much higher voltage cathode . This caused many HK shorts in damper tubes and you'll find some sets with a separate damper heater winding incorporated into the main transformer for this very reason . The aftermarket transformer was used as a repair for sets without such a separate winding that were repeatedly shorting damper tubes .

Even some later sets like to eat dampers. Case in point I have a Silvertone CTC-15 clone that if the osc. coil is adjusted too far off of synch lock will short it's damper REAL fast. It had a good damper but all lytics were bad and killing the B+ when I got it. Damper tested good before test where it gave shrunken raster than faded out so I spun my wheels for months trying to get HV with a blown damper (after the recap). It ate 2-3 replacement dampers while trying to go through the H osc. adjustments. And since then it has killed at least 1 more in operation (that one did not short, but it also fooled me by testing good unless you held the test button for 8-30 sec and allowed it to slump into the bad range.).

I never used 6W4's in sets, newer or older. The only tubes I carried was 6AX4 and 6AU4 and the series counterparts.
You couldn't use the Raytheon Japanese 6CJ3/6DW4 in Zenith color sets. The heater current wasn't right. The regulator and damper tube heaters were in series. In those sets, you had to use a GE or Zenith branded 6CJ3/6DW4. :scratch2: