the same technique. Because a TV-set uses much more tubes than a radio set, a tube series with a common heater current of .3A was created. Those tubes had the the prefix "P", for
example PCC88. While TV-Sets were manufactured in Europe with those tubes until the end of the tube era, the technique of series heating disappeared rather soon in audio equipment. Main reason for this is a safety issue that should be regarded still today when thinking about using this technique. If the tubes filaments are connected directly to the AC power line, there might be no sufficient isolation between the (ground referred) power line and the respective audio set. In a worst case scenario it may happen that chassis ground is connected to ac line potential. The insulation between heater and cathode cannot be regarded as safe in terms of electrical safety. Therefore, a transformer with well insulated secondary should be the first choice. This safety issue is more relevant for audio equipment because quite often different devices (Amplifier, tape, disc etc.) are linked together. TV-sets are more often used in a "stand-alone" way.
   Another topic that concerns me is DC-heating of indirectly heated tubes. Some recent schematics do use the 6.3V filament winding, connected to a bridge rectifier followed by a large condenser. The DC-Voltage at the condenser is connected directly to the 6.3V tube filament. I would expect to have about 7.5V DC at the tubes heater - I would not feel very good with this simple solution. Do you have any comments on this?? Reduced filament voltage is worth being discussed as well. I remember that many (30??) years ago I worked with tubes in analytical instruments. At that time we used a double triode (E283CC) in an input stage for a photo multiplier. We discovered that this specific tube showed the best S/N ratio when operated with reduced filament voltage (about 5.5V if I remember right) This was discussed with the tube manufacturer (Valvo) at that time. The tube characteristics were confirmed, the

reduced heating was considered to be acceptable even in terms of warranty. However, we were informed that, normally, under heating can permanently damage the tubes cathode (reduced emission?).
Hartwig

  Heater over-voltage is common today. The only motivation I can discern is expediency.  6.3 volt transformers are readily available, so that is what is used.  Just by adding a series resistor to the mix would turn a liability into an asset. The literature of the 50's tells us that the single greatest improvement in tube longevity was won by using a resistor in series with the heater string in TV sets.

Subject: Valve Heaters
  Great Webzine ! I've been lurking since around the third issue but 'til now have not had much to contribute. But on the much-neglected subject of heaters, I thought I'd pen a few lines.

The lower the voltage, the longer the filament will last. Fact. However, thought must be given to the cathode material itself. The following extracts are from "Radio Designers Handbook", 4th ed (circa 1963) ; recommended to anyone designing with valves. Do not be put off by the word "Radio" in the title, it has extensive coverage of audio stuff (I always wondered why pick-up arms were bent at an angle at the end!). A highly recommended book that is so highly regarded it has recently been reprinted (by the RSGB I believe?). Anyway, some extracts from the first five pages of the book (!)

"...... A valve having an oxide-coated cathode has a very high degree of emission as compared to other forms of emitters but requires very great care during manufacture since it is readily poisoned ....."

"Oxide coated cathodes are generally operated at an average temperature of about 1050K