Mr. Carlson has such a wonderful and clear didactic style. My Dad was a Seargant Tech at WWII end with a radio operator's license He built his own current-limited power supply, lights and all, just as Paul demonstrates elsewhere. He used to fix all kinds of fifties-sixties-era HiFi stuff for his friends while I played in the basement while he worked. Some of it 'rubbed off' on me.

"This resistor is there... because I can!"

Great tips. I once had a 1950's Allen 3-manual church organ which used 12AUX7 tube oscillators, one for each note. They were built into 6-note chassis and the chassis were built on to a large floor rack. There were three racks so you can imagine the number of tubes involved. I never counted them but when I switched the organ on the lights in the house dimmed for a second. It took about as much current as a clothes dryer. But it sure shook the house when I "pulled all the stops".

Just for info: I have always used a "soft start" set up for all of my vacuum tube circuits. Some have been manual, with a switch and resistor, and others have been automatic with a time delay relay and resistor. Both types have always been successful in eliminating any filament or heater flash and in saving all of my favorite (and rather expensive) tubes used in my audio circuits. I also use the same type of soft delay in the B+ power supply circuits for all of my pre-amps and power amps. Good luck and happy listening to everyone!

Using a constant current source is very gentle on the heaters and is easy using an LM317. The heater supply AC is rectified to DC that gives the CCS the voltage compliance headroom it needs to work properly. Another technique is a negative temperature coefficient thermistor - it does require a bit of experimentation and does drop a bit of voltage even warmed up. With newish tubes with good "emission" running at just below the rated voltage doesn't seem to hurt performance. I was told that the "flashy" heater tubes were meant for series connection off the mains voltage using a ballast resistor that drops the excess voltage and limits the inrush current. This was popular with cheap radios and TV sets where a penny saved equaled more profit! I completely agree with Mr Carlson that gentle treatment of the heaters helps prolong service life. The one possible exception might be directly heated triodes where the flash helps blast off cathode poisoning (there were several tube "rejuvenators" that worked on this principle).

Good stuff, and yes, even back 30 odd years ago, (when tubes were supposedly cheap ....), I did this. On my Hallicrafters Rx and Tx, I installed a big wire-wound resistor across the power switch (each rig is supplied by an individually switched power board back in my rack). So when the main switch for that particular piece of equipment is turned on, there is a soft start for all the heaters/filaments, before the power switch on the actual equipment is turned on to bridge the resistor and bring full power to the tubes. Good also for re-forming the capacitors - in that case, the HT switch is also turned on some minutes later with the power switch still 'off', so there is half LV and something like half HT in that mode. Not as good as running through a Variac, but a quick precautionary tactic.

My hat's off to Mr Carlson for spotting something I have surely missed for many decades. Seeing the 'flash' was something I associated with european-made versions of the twin triodes, or american-production parts marked with the european-standard "ECC83" type number. In all those decades I never once looked closely enough to observe just one (only) of the two heaters exhibit this trait.

I have a Swan MK I H-F amplifier. How I save my expensive 3-400Z tubes is to use a Variac rotating the control on the Variac slowly upwards taking about two minutes to reach full line voltage. This way you do not shock the tube's filaments. I've had the amp for over 25 years, the tubes have not gone soft or lost power at 15 or 10 meters. I do have a matched pair of 3-400Z's stored in a cool, dry place, but they may never get used unless I get rowdy and drive the amp to 3.5KW with 100W drive from my Drake Twins. Your way for smaller tubes is great, I will have to remember this.

That was awesome Mr.Carlson. Wish I was twenty five years younger I'd go back to the electronics course I'd dropped of from and complete it successfully with so much inspiration that's coming from a person such as you. Your videos rekindle the passion for this art and science. Thank you so much and best wishes to all those aspirants out there. Good luck, cheers! 😊👍

A Temperature stable Vacuum Tube. A paradigm shifter in light of many preconceived ideas dating back to the age of drifting oscillators. Realy enjoied this little demonstration.

I'd love to see "Tech Tips Tuesday" come back sometime. These were really great short info videos :)

I recall from my teenage years in the 60's dismembering old radios and TVs (In those days, TV repair shops would happily give away scrap sets to hobbyists). I would frequently see an arrangement of series connected heaters with a thermistor to "soft-start" them.

Some of the RCA CTC series tube chassis used a thermistor in the filament chain to provide a soft start. When we got a call for a dead TV, the first check was the thermistor if it was open, very common, we'd remove the body of the resistor and twist the leads together. That way, we didn't have to remove the chassis to solder in a new part. There was another thermistor in the degaussing circuit that could fail, but it rarely happened.

Awesome video. This is why I soft start all my tube RXs with a variac but I love this analysis and demo with automatic circuitry. Love it!!!

Hi Mr. Carlson I have seen this often in valves - firstly, when I was little, I noticed the flash in one of the valves in our B&W telly. This was a transformerless design with all the filaments tapped across the mains via heavy duty resistor to absorb the difference. The telly ran for approx nine years like this. When I git a bit older, I read in the 1950's hobby magazine that it was known about & "didn't seem to hurt the valve". In 2014 I built the Mullard 10+10 amplifier using two EF 86's & four ECL86's. Three out of the four ECL86's flash on startup so it will be interesting to note how long these filaments last. I have always assumed that the flash was normal - partly 'cos of our telly & partly because of the article. Thank you very much for posting this - it's the first time I've ever heard of it being a problem:)

What I have been doing for years is using an older style time delay relay like was used in commercial/industrial equipment back before PLC's became the norm. The kind with a octal tube socket base and a timing adjustment knob on top. Most are rated with 10 amp relay contacts so they handle a pretty good load. I ended up with hundreds of them years ago at a auction of a local relay manufacturing plant that was moving operations over seas. Went there to buy a Bridgeport milling machine and ended up coming home with the Bridgeport milling machine and hundreds of these little time delay relays along with a bunch of other goodies. Took me a while to find a use for them but I found this filament flash problem a great place to use them. The ones that have a time adjustment range of 1-60 seconds is perfect for this application. Just mount a octal socket, break the filament feed line and run it through the relay and install the dropping resistor across the relay contacts.

Once again something new you have bestowed upon me. Never had heard of filament flash, thanks for the education. I could listen to you for hours, you are very knowledgable.

You can solve the problem by placing an inrush current limiting thermister in series with the power cord, which actually drops in resistance as it heats. This also protects solid state diode circuits, though only when turned on cold. It will always protect the tubes, though, because they only flash when cold. My 11 tube RCA Color Television dot/color bar/crosshatch generator uses one of these, and always turns on gently. Amazingly this piece of equipment has ALL of its original components, which test and work perfectly.

Outstanding video. This issue has always reminded me of the Phoebus cartel back in the 1920s . That was suppose to have gone away, but instead I think the practice of these sorts of shenanigans multiplied.

Thanks Paul for touching on the main problem I encounter when buying old radios. I cringe when I hear a seller saying he plugged in his Grandfather's old radio and it didn't do anything. Cold filaments & discharged leaky capacitors put a tremendous surge load on an already marginal power supply. Better if they would sell that radio completely untested and let me bring it up on a variac. I have gotten some with exploded electrolytics that popped metal foil ribbon under the chassis in the high voltage area.