[John L Stewart]

Thought you may be interested in the results of these heater tests. In a way they are a real eye-opener.

For starters I found the cold resistance of the 6L6GC to be about one ohm. Operating would be 7R. For these 4-terminal measurements up to 390 mA I used a SS CCS I had built about 50 years ago. All taken directly on the tube prongs, no socket to add contact R. Beyond that it was 10 W WW resistors sourced by the 12 volt automotive battery under my workbench.

The amplifier the tubes are installed in is one of two I built, again about 50 years ago. Both were used daily for about 15 years until I bought a SS Sansui. As you would expect the turn on transient is somewhat limited by the on board PT, a Hammond 273BZ. It also runs the 5V4GA rectifier. Used to be 5AR4s but they are now too damned expensive to play with. I bought octal SS replacements for them about 10 years ago.

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while. Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4 A.

Beginning a few years ago I started using NTC Thermistors from Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

The equipment I use to do these measurements is no longer expensive & easily affordable by private individuals. The Scope/Spectrum Analyzers are from Pico Technology of the UK but there seem to be many others out there. I've pretty well parked my dual channel 100 MHz scope.

Still Winter here but Spring is on the way!

Hope all will find this information of interest. Cheers, John