> explaination vv the PCC ie constant current, not constant voltage
All tube heaters work fine constant-voltage.
Some tubes are rated for series-string operation, where the individual tube voltages are not controlled, but the current through all the tubes is (more-or-less) controlled. A particular problem is that heater resistance (ratio of current to voltage) changes from cold to hot, and "hot" happens sooner or later depending on heater thermal inertia. Put a little heater in series with a big heater, the small one heats first, hogs the voltage, gets WAY too much power until the big heater gets hot. If you run different tubes in series, you need to match the thermal inertia. Fortunately the heyday of series-string radios and TVs left us a lot of tubes with controlled heater warm-up.
In a series-string radio or TV, you add up all the heater powers, divide by the line voltage, and that is the heater current. If a particular tube you want to use needs more current, you need to find or special-order a tube with higher voltage and lower current. There is probably a '189 that eats 6.3V 0.36A; but 0.3A was a very popular heater-string current for TV sets (just as 6.3V was a popular voltage for lead-battery and parallel-AC heaters), so they made a 7.6V 0.3A version to eat 0.3A like the other tubes in the string. For single or parallel operation, just feed it 7.6V. In fact since a high-Gm vary-Mu limiter probably wants DC heat, run two in series and feed 15VDC, a readily available power supply.
Also: a tube rated for a certain heater voltage WILL work properly at that voltage, even though the current may not be exactly what the book says. OTOH, a tube rated for series-string operation will work properly at the rated current, though the working voltage may not be exactly what the book says.
Most small receiving tubes have huge reserve of emission. I would not be surprised, if you are not slamming the PCC189 to maximum rated current, if it worked fine with 6.3V at the "7.6V" heater. If you do that AND suck all the current it will give, you could shorten the cathode life. But in a TV set getting a not-weak signal, tuner tube current is low. And an oscilloscope stage usually won't push the tube to the max current because it has to stay well away from overload. (Sadly the audio limiter is one place where we might want to leave these tubes cooking at high current for years, dropping only on peaks. However the LevelLine is not hard on the '189.)
