Tech Tips Tuesday Dirty Little Secrets

[@frankwilson2607]

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. Pertinent to this video: When I was a teen I used to fix guitar tube and discrete transistor amplifiers, organs, etc. I recall that the VOX AC100(four EL34 push-pull output the original Beatle amp) had a open wirewound resistor in a frame below a bimetallic leaf switch with contacts that closed when the wirewound heated it up - the current limit resistor was in parallel with this bimetal, mounted insulated from the wirewound heater. Clearly, it was engineered with filament flash in mind - a clever automatic solution - and this video made me finally realize just why it was designed. I should have kept up interest in electronics and done a degree in it but life interrupted and I ended up sidetracked before learning the theory underpinning all the practical repairs I had learned to do. Sigh.

[@xs10tl1]

Indeed.

[@cinnamonhill]

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".

[@stuartbassett3807]

Love all your videos, so informative and helpful. It always amazes me how much you actually know and can remember. :) Sometimes I forget what day it is.

[@rayofcreation3996]

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! 😊👍

[@MichiganPeatMoss]

It's not too late. Restart here. "become a patron". :)

[@marcdraco2189]

@@MichiganPeatMoss I became a patron just out of curiosity because of that amazing capacitor tester - best $2 a month I've *ever* spent. What a great teacher.

[@CharlieTechie]

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.

[@MrCarlsonsLab]

+Tech Chuck Legg Thanks for the kind words Chuck!

[@DavidBurnett1]

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.

[@frankkoslowski6917]

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.

[@andrerouth4253]

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).

[@233kosta]

"This resistor is there... because I can!" *LOVE IT!*

[@holywells]

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!

[@PrinceWesterburg]

Soft start HT = Valve rectifier

[@daleburrell6273]

@@PrinceWesterburg...THE HEATER CURRENT DOESN'T GO THROUGH A VACUUM TUBE RECTIFIER- ONLY THE B+ DOES.

[@6A8G]

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:)

[@MrCarlsonsLab]

You're Welcome, glad you enjoyed!

[@andrews.4558]

Great info, I was not aware of this issue. I am getting back into vacuum tubes after 25 years in modern solid state electronics. For that fix with the MOSFET a Triac could be used for an AC system. The current will jump when the triac fires but should still eliminate the heater flash.

[@idle2600]

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. He's right, that can't be correct. But I am at a loss to explain it. Just goes to show that even after nearly 50 years of experience with a technology, there's always something more to learn. Probably what's kept my interest alive for so long. Thanks again for your very-observant work.

[@MrCarlsonsLab]

Your very welcome!

[@davidausterman5915]

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!!!

[@EsotericArctos]

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

[@mikesradiorepair]

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.

[@MrCarlsonsLab]

+MikesRadioRepair .CBRadio Sounds like those relays would be great. Thanks for your comment Mike!

[@robertcalkjr.8325]

Thanks Paul. Another great Tech Tuesday. I'm enjoying them. Seems like Planned Obsolescence to me.

[@MrCarlsonsLab]

+Robert Calk Jr. I think your right!

[@blueharley2]

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.

[@cny02253]

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.

[@MrCarlsonsLab]

+cny02253 Ya, I agree. Many technologies suffer from this sort of action. Thanks for your comment!

[@lomgshorts3]

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.

[@jimw7ry]

Cycle your tubes once a year. You don't want them to go gassey! Gas is a real problem in large transmitting tubes like 3-500, 3-400 and 4-400 tubes. It a really RISKY buying NOS (new old stock) transmitting tubes from the likes of eBay. They can be gassey and no good even though they have never been used. 73, Jim W7RY

[@diabolicalartificer]

After watching your video I posted a link to it on an UK vintage radio and TV forum. The consensus after a week is that heater or filament flash is a known phenomenon, it isn't a problem and hasn't been known to cause a break. The forum members have a vast number of years of experience between them in the TV and Radio repair trade as well as other electrical engineering fields. See UKVRR - Heater Flash?

[@MrCarlsonsLab]

+Dr Wobble Lots of interesting experiences. It's nice to see people getting along in a forum! I do have to admit, my experience with heater flash isn't a good one. Thanks for your input!

[@ham4ham71]

+Dr Wobble A lot of people will say a lot of things but never provide physical tangible evidence. I am not an expert in EE by any means as my background is SE. I can say if there is a difference, whatever that difference may be, and Mr. Carlson clearly shows this, something is not right and the end road is typically an early failure. This I can guarantee from experience and if it's physics based it's a certainty. This basis is called the repeatability standard.

[@diabolicalartificer]

+Ham4Ham Wasn't doubting Mr C or his obvious experience and knowledge, just reporting what other electronic engineers of equal knowledge and experience had to say on the issue, which of coarse should carry equal weight regardless of the lack of tangible physical proof. If you read the thread in question, several posts outlined instances of heater flash and the continued working over many years of the valve. So, this phenomenon whilst a possible cause of heater or filament failure, it is far from a certainty. QED

[@AMStationEngineer]

This idea would have saved at least half of the tetrodes which I was forced to replace in many of the CCA and Continental AM (broadcast) transmitters over the years! (mostly Raytheon were affected by premature filament failure) Now, Svetlana is the only available vendor for many of them! Many thanks, am sending this link to many of my friends!

[@MrCarlsonsLab]

+AMStationEngineer Thanks for spreading the news! Glad your enjoying the video's!

[@tom7601]

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.

[@kevinbyrne4538]

I did something similar with my old tube gear. I used a relay (with a simple timing circuit) to short out a current limiting resistor that I installed in series with the filament supply.

[@MartenElectric]

Ingenuity in simplicity! Thank you for a great video Mr. Carlson

[@MrCarlsonsLab]

+Audiophile Vintage Thanks! Glad your enjoying.

[@allanpennington]

Found an interesting comment on a blog from Eli Duttman entitled NTC THERMISTOR as SOFT START for TUBES..."BTW, the bright turn on flash associated with Philips, Mullard, etc., made tubes is intentional. Philips and the companies they owned (including Mullard) used such construction so small signal tubes would not cathode strip, when used in combination with fast starting, directly heated, vacuum rectifiers." Not sure what Cathode stripping is and if it is applicable to this discussion?

[@Satchmoeddie]

The chemical coating on the cathode will strip off with the electrons and stick to the plates, when the tube's filaments are not heated up all the way and a signal is applied to the tube. They will also strip some at idle too, but not quite as badly.

[@jddr.jkindle9708]

Great demonstration. Within amateur radio RF power amplifiers, many have a Soft Start circuit, which eliminates the in-rush current to the electrolytic capacitors and power tube filaments.

[@td3993]

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.

[@rabidwasp]

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.

[@CharlesM-dp4xe]

Very interesting. I have seen this in the past and always wondered what was going on, it seems the tubes would eventually fail or have a shorter life but never knowing the actual reason ... just buy another tube I guess. Thanks for the insight, very informative and helpful !

[@alecjahn]

Yes indeed, thanks for the thorough and well-paced explanations! I'm a tube/amp newbie, and when I was rebuilding some preamps about a week ago (the paint-by-numbers way :P ) I came across a tube that did this. I think it was a 12AU7, NEC branded, and original to the amp. A quick swap took care of my concern, and now I know what happened! Cheers!

[@MrCarlsonsLab]

+alecjahn Glad you enjoyed the video!

[@idle2600]

Ahem.. See manufacturer's data sheets for "Controlled Heater Warmup Time". This was an effort to raise the resistance of small heaters in series with larger ones as found in series-string heater circuits. I'm sure I remember reading an explanation of this around 50-plus years ago. Just wish I could remember where. Tubes found in a TV tuner with very-low voltage heaters would commonly exhibit this trait. The one clue here was that the bright "FLASH" would only be seen when powering a stone-cold tube. Powering down the heater, and trying it again any time soon would NOT reveal a repeat of the "FLASH". Again, this was a hint that the phenomenon was part of the intentional warmup trait. Just leaves me to wonder if everyone involved with this sort of heater design is long dead, and nobody is left to explain the rationale behind it. Bravo, and keep up the good work! 73

[@MrCarlsonsLab]

Well, the flash is only on one filament, and they can both be used in parallel. This would mean that only one filament has controlled warm-up, which doesn't make sense. I also have batches of tubes (same number and manufacturer) some flash, others don't. I think it's more like "controlled lifetime." Thanks for your comment and input.

[@nomadradio]

Now there's a detail I seem to have missed. Just one of the two heaters. Not sure how I missed that, but this is absolutely wrong. Both heaters should behave the same. Sounds more like a mixup on the assembly line. Puzzling phenomena often boil down to a question of incompetence on the one hand, or dishonesty on the other. Like when a politician says something you know darn well to be false. My vote would be with sloppy identification of heater-wire assemblies as they were loaded into the hoppers. Brings to mind a scenario where 'normal' heater-wire bundles were loaded into the hopper for the 'left' heater, and the CWT heater bundles went into the 'right' heater's hopper. Never participated in the assembly of empty-state devices, but my bets would lean towards sloppiness, rather than a desire to make a product that fails prematurely. I do know that as manufacturing batch sizes shrank during the years of declining demand the defect rate went up. Our 100% incoming-test policy revealed a rising defect rate from the mid-70s until the last remaining domestic factories closed in the 80s. Clearly calls for a strategy to compensate. Or a willingness to replace tubes frequently. 73

[@helmutheller1538]

Chris is right, this is by design. In series heater configuration (300mA) the two heaters must always be in series and so having the current limiting "flash" on only one system is enough.

[@MrCarlsonsLab]

Helmut, you are incorrect. The heaters are not always in series, this is the reason for the center tap at pin 9. Most applications operate these tubes at 6.3 Volts (parallel connection), as this is most standard. So then one side flashes and the other doesn't. That makes no sense. In many tubes (same make) both sides flash, and in others, neither do. It's hit and miss with these tubes.

[@majes1305]

Thanks so much for sharing this, this is one of those things that isn't easily explained, but obviously needs it :)

[@MrCarlsonsLab]

+Majes Alum Your Welcome!

[@Satchmoeddie]

I don't see these modern junk tubes flare up like the old western European tungsten filament tubes did. Nickel chromium heaters generally will never flash or flare up, but just like their tungsten counterparts, before the NiChrome wire gets hot it's still almost like a dead short, for a few microseconds, until the heater filaments get hot. When the heaters get hot the resistance goes up. Old incandescent light bulbs do the same exact thing. Test a light bulb for continuity with a Fluke or even an old Simpson 260 and it measures almost like a dead short.

[@sibsbubbles]

Ah so now I know what this little phenomenon is! Ultimately I was suspicious of 2 Eico-branded 12au7's (Mullard) and just replaced them both (VTVM and a homebrew tube amp). I kinda figured they were not quite made right, but I had no idea this was so widespread, or has a fix for that matter. Thanks for sharing. Love your videos. I've learned a ton from them.

[@MrCarlsonsLab]

Not a problem, glad you enjoyed!

[@johnc8910]

The soft start filament circuit in various forms is very common in both commercial and homebrew high powered, amateur radio vacuum tube amplifiers.

[@Damaraja]

You’re a mensch, Mr. Carlson. Thank you for all that you do 👌🙏🤙

[@ElectronicTonic156]

You could also use a thermal time delay relay to short out the current limiting resistor. It has the simplicity of your switch solution and the autonomy of your mosfet solution.

[@MrCarlsonsLab]

+Eric Wasatonic LOL, You know, just outside the shot to your left at the test fixture scene, I had a 115NO20T on the bench.... But I figured, who is going to go get one of these old thermal delay relays. Maybe like you mentioned, I should have shown it. Thanks for your input Eric!

[@lightningboy401]

+Mr Carlson's Lab, I use thermal relay tubes all the time, lol. I have never used them to ramp up heaters, so that's next on the list. Brilliant idea - thank you!

[@MrCarlsonsLab]

Glad you enjoyed!

[@Satchmoeddie]

I just bought some 115N010s, 115NO15s, 115N020s, 115N030s, and 115N060s. The prices on those have really skyrocketed recently. It's crazy. If you have opened up that Lambda PSU on your bench, you know it has a 115 volt thermal time delay inside it, too. I think mine has a 60 second inside it. I like the way the old octal thermal time delays will let me short cycle something off and back on without waiting a minute for the time delay to it's run full cycle again.

[@daleburrell6273]

@@Satchmoeddie...it's a LOT cheaper to use a 555 timer and a relay-(!)

[@hadireg]

Big thumbs up for all the vids!👍 the close-ups were just awesome! I never knew about those flashes!

[@jedishaw6771]

This is great! I have a tube in my amp that does this. Now I know what to do. Thanks very much!!

[@MrCarlsonsLab]

Glad to help!

[@toddanonymous5295]

Mr Carlson, Another flash of brilliance :). Really enjoy your tech tips. Great using that end cap as a base for the test fixture. Another useful PVC enclosure is an electrical entrance el. I have used them in photoelectric projects. Thanks again for all your great videos.

[@MrCarlsonsLab]

+Todd Anonymous Thanks for the kind words Todd!

[@Geopholus]

I've noticed that in series filament (rectified filament with 60 or more volts across 5 preamp tubes) preamps, that this problem is multiplied, and that with 12AX7A's the chinese rubiy tubes seem to be the best . Not only do they draw less current when on, but the cold resistance is not such a small fraction of the warm resistance. Also they seem to be very consistent from one tube to the next.

[@berndb.5097]

The same circuit, but with a time relay, i use on my halogen lighting. After that the lifetime of the used bulbs were longer.

[@MrCarlsonsLab]

+Bernd B. Thanks for your comment!

[@DennisMurphey]

walking thru your tips and learning so much, i had no interest in electronics back in UnderGrad then got a job in Medical R&D and was really handicapped by the knowledge gap. i was stuck working ergonomic, man machine interface, beam guidance and materials science for Data clarity. this basic stuff should get its way into the STEM program here in the US or Scouts program for Technology. Great basics that can be used to understand circuits and know what and why of so much stuff in everyday use. Modern Day Mr. Wizard stuff that I used to watch as a kid. Thank You for providing this ground level 101 material, extremely well presented and most understandable you little test kits are great too. Now as a Patreon I want to explore all your material and get to where i can solve some on my hobby issues in electronics as well as setup all my test gear for these old radios. very encouraging and energizing for this old brain.

[@MrCarlsonsLab]

I'm glad to assist you in your journey Dennis!

[@davidmckee5659]

Back in the day? What are you, about 34?😂 Seriously, when are you going to show us your tube based time machine? Love your channel sir!

[@CharlesM-dp4xe]

I agree. He looks very young compared to his level of knowlege. I am in my mid 60s, a Ham by hobby and somewhat experienced with older gear, yet his knowlege and logic surpasses my expectations exceedingly. Almost seems suspicious ... hybrid human perhaps, maybe a time traveler, or maybe just really smart, who can say, but he makes excellent videos full of useful information at any rate.

[@jwingo7257]

CharlesM1957 Too funny!...Mr. Carlson is a combination of Spock 🖖 and Tesla in the body of a mild mannered nerd/technician (courteously proposed 😊).

[@MandrakeFernflower]

Paul was the lost timelord

[@klausstock8020]

No time machine. It's all just about slowing down the ageing process with an external resistor.

[@PrinceWesterburg]

I worked for a large, name valve hifi company and now people say I talk with the knowledge of someone in their 80s!

[@adammorris3082]

A very handy tip,Thanks - I've been thinking about this problem very recently and came up with the idea of a 50% stand by preheat through switching the filaments in to series pairs when the amp is switched off(a stand by if you like) and back into parallel at turn on (I'm sure not an original idea) but yes- soft start seems like a much better cure for this problem- again Thanks, All the best Mr Carlson.

[@daleburrell6273]

...the earlier vacuum tube TV sets with instant on, used a DIODE in series with the heater string- that allowed exactly HALF of the AC power to flow through the vacuum tube heaters while the TV set was turned off- and when the TV set was turned ON, the diode was bypassed, and the vacuum tubes received full power. ...on a side note: I remember when these particular TV sets were OUTLAWED in New York City, because they were considered to be a waste of electricity!!!

[@SpectreOZ]

I don't use tubes but enjoyed the demonstration, explanation and solution... thumbs up!

[@12voltvids]

I have done it by placing a light bulb in series with ac input with a switch to bypass once tubes start to warm up.

[@talenttrading]

I love light bulb limiters!

[@lroy730]

I've ben arguing about this for years . I knew it was a build problem . So many people look for the flash as somthing cool , when its like having a tiny ark welder on the filiment .. Great idea , defintitly adding it to my tube testers.

[@jcwirechief9868]

Excellent explanation. I had a tube doing that in my Drake TR4CW and didn't really understand why. I swapped in a new tube and no problem. When using my vacuum tube receivers (R4B) I usually just leave them on all day if I'm going to be using them later in the day. That's to limit the # of filament shocks. I think I need to build a 'step start' for the whole radio.... if I knew how😄

[@tom7601]

Back in the 1960s, before solid-state TVs, some manufacturers had an "Instant-On" feature. They would keep a few volts on the tube filaments, including the CRT, all the time. Of course, this increased your electric bill. Some TVs had a switch on the back that allowed you to turn off the instant feature. It was probably around 3-4 Volts for the 5-Volt tubes and 7-8 Volts for the 12-Volt tubes.

[@tectalabyss]

Fantastic has always,Mr Carlson ! Thank you for your time and hard work. All my best. Bobby

[@MrCarlsonsLab]

+Bobby Tectalabyss Thanks Bobby!

[@andrewchurchley5705]

Going back decades, I’d seen the flash many times, but wrongly, never attached any importance to it. The demonstration shows clearly that the bare, uncoated section of the filament, which is thermally-insulated by the hard vacuum, with nowhere for the heat to go except by radiation, gets the hottest. The demonstration reminded me of the use of a thermistor to protect the filament of a 35mm projector bulb, which could be costly to replace. I never had a single failure after inserting the thermistor in series; yet the bulb life was given as only 10 hours. The thermistor had a high resistance when cold, but a very low resistance when hot. The residual resistance when hot, was small compared with the filament resistance, and so caused negligible reduction in bulb brilliance. I am left wondering whether a thermistor could be used to protect a tube filament(s) in a similar way, with the advantages of reduced space requirement, less circuitry and simplicity.

[@brys555]

Thermistors were used in TV sets where all vacuum tubes heaters are connected in series.

[@daleburrell6273]

...ABSOLUTELY!!!

[@MrUbiquitousTech]

Excellent info and explained very well! I remember once in a while seeing a tube do that and thinking it wasn't long for this world

[@TRXLab]

Excellent video here, as always Paul. Have a quite similar protection in my amp as well :-) Very good and valuable tech tip. Take care

[@MrCarlsonsLab]

+TRXBench Thanks Peter!

[@Daniel-ib5bx]

+TRXBench Agreed I learned alot in this video.

[@waynecarlson3736]

OK You got me totally addicted watching your repairs of everything, some of which I now have. Cannot find a Jackson 591

[@stefanscholz2509]

Thanks for pointing this effect out, as most viewers will not know about it This is why heater filament windings on transformers were made "soft", high inductance or resistance. To limit the inrush current old commercial designs also user uranium dioxide resistors (NTC characteristics) in filament circuits, or on older power amps I own from the 30's they have a preheat winding (2.5V) on the filament transformer winding, which is operated by turning the main switch. AFAIK this was basicly to prevent unloaded DC buildup when applying power, due to direct heated rectifier tubes vs indirectly heated power and pre amplifier valves, which took longer to conduct. This also added highly to the reliability, which a commercial PA system was designed for. For non commercial, mass production, the inrush effect did not matter, as you stated "as long as it reaches the stated useable time" or warranty time (6 month from sale to the end customer), the mfg was fine. And, like Philips/ Valvo (Mullard/ Amperex, ...) being one of the biggest tube makers in the world, you needed the replacement purchase business to keep the factories going.

[@dhpbear2]

I'd guess this circuit would be appropriate for tubes that DON'T exhibit the flash problem as well. There IS an initial in-rush of current whether or not it flashes (16:06).

[@Toobzilla]

ive been meaning to ask where you learned all that you know. vacuum tube technology has been out of favor for a lot of years, so i thought, "hes not old enough to get it on the job, so he must be a serious tinkerer. but just now you mentioned making a test tool to deal w pallets full of vacuum tubes that had heater flash so. that either means im way older than i wanna be or you absobed a lot of good info in your formative years!

[@B4TS1]

I use something similar for old series set radios. The 60 watt light bulb with a switch across it built on a 4 inch steel electrical box. After about 15 seconds warm up, I throw throw the switch for full power. Had the flash problem with the 35W4's blowing the filament prematurely and this cured the problem.

[@MrCarlsonsLab]

+ClockWatcher Great solution! Glad to managed to save those 35W4's. Thanks for your story!

[@charleslauter5035]

When I was in the NAVY the Destroyer (ship) I was on had hundreds of 12au7 tubes that were the shortest lived tubes. I replaced the with type 6189 type tube. They lasted much longer.

[@MrCarlsonsLab]

The 5814 was also very good. Thanks for sharing your story Charles!

[@jenniferwhitewolf3784]

Best reason to engineer DC filament supplies with a soft start feature, that ramps up the voltage over a period of time. I usually choose about 45 seconds, and do not start the high voltage supply up until the filaments have been on full for about 2 minutes. Also observe the heater to cathode voltage limits in the tube manual. In some cases you may have to create several filament supplies and operate them electrostaticly floated at a specific positive or negative voltage in order maintain the specification for any given tube. In a typical circuit of mine a differential cascode triode set has a pair of cathodes near ground, and another pair at about +125. In this case, a high impedance voltage divider is created to bias the filament at about +67 volts. this puts the cathodes of all four elements at far less than the +-100 volts specified for the tubes used. The long tail current source for the differential set above also consists of a series stack of parallel triodes similar to the tubes used in the diff amp above it. Again, the design centers of where the cathodes are below ground, and a single filament supply biased to a point midway between the cathodes is used. If the volage between the cathodes was near or exceeds the heater to cathode specification, additional filament supply will be needed. In some amplifiers, I need 3 floated and biased filament supplies in the front end circuit, and 2 separate supplies for the power tubes, each bias point driven by a buffer to 'follow' the instantaneous voltage at the cathodes of the low impedance power high current tubes in the Circlotron topology balanced bridge output stage.

[@daleburrell6273]

...SOUNDS COMPLICATED-(!)

[@willykuntz7520]

I'm also ignorant,learning a lot just watching this guy!

[@circuitsmith]

I've been working with tubes (TV and Hi-fi, later T&M) since the early '70s, and before that watching my father and uncle (hobbyists) as a child. I'd seen cases of heater flash but never gave it much thought or knew what to call it. In my years working in repair shops I think I saw some American brands that had flash. Of course there were many brands of tubes back then that are now gone and forgotten (Philco anyone?) Thanks for teaching me something new about tubes! It's been a long time.

[@gordonwelcher9598]

I can see that the filament that has the flash has a loop in it with no coating before it connects to the pin. The one with no flash has the coating brought right up to where the attachment is made.

[@JoseSilveira-newhandleforYT]

Many years ago, we used a diode in series with the 6.3VAC output of the transformer and used a delayed relay (about 10 sec) to short it. Just another way to protect AC filaments. For DC filaments you can use same setup, but with a string of 4 or more diodes in series, to drop the voltage enough to avoid flash and then the delayed relay shorts them all.

[@MichaelLloyd]

I'll be darned... I didn't expect to see heater flash but I just witnessed it twice. I was testing the tubes from a SIlvertone 1484 guitar amp that I am restoring. The tube compliment, all Silvertone branded tubes, is (2) 6L6's (no flash), (2) 6FQ7's (no flash), (2) 12AX7's (no flash), and (2) 12AX7 / ECC84's both had filament flash. It was very obvious. So much so that I thought I would see zero emissions when I tested them. All of the tubes tested very strong so that's good... I came back to watch the video to make sure that I was seeing the same thing. Thanks again!

[@MrCarlsonsLab]

+Michael Lloyd Your welcome Michael!

[@Satchmoeddie]

A lot of Silvertone tubes were rebranded Mullard or Philips tubes. Starting back in the 1950s companies like Fisher, and even Carvin Guitars used European preamp tubes. Sears Silvertone was no exception. It's a crap shoot in their guitar amps, but the Silvertone hi fis, stereos, and outboard powered speakers for hi fi, usually had Mullard or Philips tubes in them. Fisher tended to use lots of Telefunken tubes. Simpson, Daystrom, Allied, Eico, HP, Tektronix and other test equipment companies also liked using the European tubes. Amperex tubes were usually made by Philips. Norelco was American Philips. I don't know why Philips was branded Amperex here. Later on Sylvania & Philips ECG were one and the same, (kind of,sort of).

[@W1RMD]

I've recently picked up an old transformerless phonograph amp that uses a pair of 50C5's and a 12AX7. The filaments are wired in series and the 12AX7 flashes worse than this! All three tubes use 150mA filament current each, but due to extra strain on the 12AX7 filaments, the 12AX7 filament voltage goes up to 20 volts for a few seconds until the 50C5's warm up! I'm not sure of the make of the tubes. All three just say "CBS" and "made in USA" on them. If I slowly increase the filament voltage this flash doesn't happen. I'm going to try this circuit. Thanks again Paul!

[@anthonytidey2005]

I'm not sure weather you are talking about modern made valves (tubes) or older pre 1970. I started work in Marconi Instruments (English Electric) in the late 60's started on mainly valve test equipment. I do not remember any case of heater (filament) flash. As I remember most valves were mainly English Electric (EE) or Mullard but there were others I cannot recall. I'm sure if we had this problem especially EE would have felt with it. Thanks for anothe great, well described video

[@curtchase3730]

Excellent video! You are very knowledgeable on electronics. It seems just about anything that uses electricity has issues with high current draw when powered up. Capacitors, power transformers, filament light bulbs, motors, and the list goes on. Some devices NEED full current to kick em into gear, but most electronics would be happier if that "inrush" current could be suppressed a bit, like your flashing tube example. Items that use modern cheap switching power supplies should be left alone though. They arn't too fond of a slow voltage rise when powered up. But, old skool iron transformer equipment driving tubes would be a good candidate for some kind of ramp up to full voltage instead of slamming it w/full mains power IMO. An example: I have, yes have an old Heathkit solid state stereo amp that has no speaker protection circuit or relay like most modern amps to. When I powered it up, the speakers would thump something terrible. Ya, I learned if I switched the speakers off, waited about a minute, then switched them on again, no thump. But I had to remember to do that everytime. So, I built up a simple delay timer circuit and using a hi wattage lower ohm resister in series with the main lead from the transformer to the rectifier driving the B+ to the output transistors. When the amp was turned on, the relay would be open, forcing power through the resistor slowly charging the B+ capacitors for a few seconds. After the time out, the relay would close thus providing full power from the transformer! NO more thump! Amp is pushing 50 years old now! I made the mod around 20 years ago and amp has never had a failure!

[@jimadams2473]

Great topic. I am looking at my guitar amps for heater flash in the tubes right now. Thanks for the great simple tip.

[@MrCarlsonsLab]

Your welcome Jim!

[@priestblood]

Great video I only have a couple of vintage rf signal gens with tubes even though I can't see them heat up as there sealed in. I will fit the pre warmers it a lot cheaper than replacement tube, Thanks Paul

[@MrCarlsonsLab]

Thanks Wayne!

[@ajaxlepinski3285]

Mr. Carlson ROCKS!!!! Love your vids!!!

[@g0fvt]

Hi, a very interesting and thought provoking video. I believe that a significant part of the problem is when heaters and HT are sourced from a common transformer. Typically an old school radio will use a thermionic rectifier, so on power up you have a quite large transformer ONLY seeing the heaters as a load and able to provide considerable current into them. Although I have not done any experimentation I would suspect that a solution may be to power the heaters from a relatively small dedicated transformer. IE one that is incapable of providing many times the heater current required by the equipment. Thanks for the video!

[@MrCarlsonsLab]

+g0fvt Thanks for your comment!

[@rsattahip]

Remember the instant on TV's in the 60's and 70's in which the filaments were run at a reduced voltage when the set was plugged in and turned off? Did that extend or shorten their lifespan? I occasionally pick one of these up at a garage sale etc. to tinker with. Thank you

[@AIJenkins]

Always a good learning experience at Mr. Carlsons lab! I thought for sure he would use a 555 timer for the delay on the second circuit. The tube was more “period” though. Nice to know 💯

[@clytle374]

good video. I keep thinking that those really fine leads are okay as long as they kept them short enough so that the pin keep the temperature down until the rest of the element warmed up and lowered the current.

[@nelsonarmor]

When I repaired vacuum tube equipment in the 1970s era, I noticed some older equipment had ntc resistors in series with the filament chain to prevent inrush currents to extend the life span of the tubes. There were also vacuum tube TVs and amplifiers that had the “instant on” feature with the simple switch/resistor in the filament chain as Mr. Carlson described. The switch was integrated with the on/off-volume control. Excellent presentation as always sir.

[@vonzigle]

Thanks for the excellent video! Over many years I've seen the flash phenomena in tubes and just thought it was the nature of the beast... It did not occur that this could be a defect that would shorten tube life; very interesting!

[@MrCarlsonsLab]

+vonzigle Your welcome!

[@nathanas64]

Love this series. So glad I discovered it !

[@howardleopold9887]

The better vacuum tube equipment had means of limiting turn on surge with either thermistors or “surgistors”. The surgistor was a wirewound resistor attached next to a normally open bi-metallic contact across it. This was connected in series with either the power line or the tube heaters. When the power to the equipment was turned on the resistor would limit the initial surge while heating the bi-metallic contact. After a few seconds the contact would close removing the current limiting of the resistor, and the contact would stay closed due to the slight heating of the current through the bi-metallic contact arm. I was very surprised that nobody seemed to be aware of this common solution to this old problem.

[@goodun2974]

Heathkit W5M audio power amp monoblocks were factory-fitted with surgistors at some point because of power transformer failures on early production units, related to high filament/heater-current draw at turn-on.

[@noelguerrero2770]

Good info! Thanks. Now, what about using a high wattage resistor in series with the filament transformer? I would install a "warm up" mode switch in a convenient location to bypass the resistor. Or maybe incorporate a time delay circuit with a relay to bypass the resistor. More like the speaker time delay for transistorized amplifiers to eliminate the "thump". Thanks again for the great video.

[@MrCarlsonsLab]

+Greenfield 54 Sounds reasonable! Thanks for your comment!

[@kenabi]

soft-start circuits are great. wouldn't be hard to figure out something for the ac variants to automate it.

[@karlbirkirflosason8190]

Very informative video. I've watched a bunch of your videos, and learned a lot, but this is my first comment. How would a hobbyist go about finding out the component specifications in that mosfet circuit? Let's say for a 60's vintage tube radio with 5 tubes with 6,3v heated filaments. I get a lot of the physics and the math, I'm just not sure what I should be referring to or where to start. Thanks a lot for your videos! I also really appreciate your thoroughness in your electronic work.

[@daleburrell6273]

...I know I'm late- but first off- the circuits in this video, used DC for the heater voltage!! A "5 tube radio" uses AC for the heater voltage, and the FET circuit would NOT work with AC- but the "resistor and switch" circuit might work.

[@balthromaw6305]

i made a circuit for my PCL-82 tubes using a mosfet power resistor and a few smaller resistors with a capacitor to make a delay circuit.. When power is first applied the mosfet is off... allowing the power resistor to send half the current into the 16V filaments to preheat them, after a short delay the capacitor charges and switches on the mosfet allowing now the full 16V to enter the filament , by now the tube was fully preheated and no heater flash occurs. Wanted an automatic function so I can simply turn on my amplifier and not have to worry about any other steps

[@arnegammelgaard7471]

Dear mister Carlson Isnt there a risc of cathode posining, leaving the tube with low glow on instantanly, i remember at simular solution in CRT television way back in the seventies, made to cut out the heating period to achive picture on as one pushed the "on" bottom on the remote. That cost alot of CRT tubes back in them days!

[@SeanBZA]

NTC thermistor can also be used, and it would be simple to make your FET solution work on AC as well, so you can use it for AC sets to reduce rewiring.

[@MrCarlsonsLab]

+SeanBZA Ya, the problem with that is noise. Any disruption "In-balance" of the AC heater string equals buzz or hum. (In most cases) I imagine you are thinking of using two fets? The thermistor rely's on resistance across the device to keep it hot, that resistance would reduce the normal operating heater Voltage.

[@zoxwolf2133]

You are the best elec teacher i know :)

[@tonywalker8250]

Fascinating. I'm off now to try and impress someone with my new found knowledge on heater flash :)

[@Zone1242]

Damn, now I have to replace one of the tubes in my (German made) headphone amp. A little knowledge, etc. Thanks for the great video. Cheers, John

[@MrCarlsonsLab]

+Zone1242 Glad you enjoyed John!

[@marvs9340]

I added a Wuerth Surgistor to my Ham tube tranceiver for this very problem. Not sure if the Wuerth product is still available .

[@jamesbomar3903]

Thank you for a very interesting and educational presentation. I learned a lot today.

[@frankgeeraerts6243]

Good Advise .........people today like automatisation but the simple way is often more reliable , in this case a switch and resistor . One has to notice that while " preheating " vacuum tubes one has to take in consideration the design of the HV power supply in order not to stress components like capacitors and their designed max voltage ratings. ( max voltage and max voltage surge ) A good rule of thumb is to use a voltage at 70% of the voltage rating of an electrolytic capacitor. Without a load on the power supply one may exceed this max voltage , ( special attention with choke input filtering is needed ) I saw well renowned amplifiers with two 250 V capacitors in series ( sometimes without a resistor/voltage dividing network ) with 490 V HV supply under load ( and when tubes are fresh !!!! ) As well as 450 V capacitors ( low grade quality ) with 435 volt HV rail ( no room for line voltage fluctuations, voltage spike at start up or ageing of the capacitors , they tend to lower /loose their max voltage rating when they age ) Besides the value is often not the bette or best one , economic considerations or audiophile overkill, rarely the right amount and quality... To extend the life of your ( precious NOS ) tubes .....: 1) SOFT start of the filaments ( MC INTOSH used to install an NTC resistor on his AC input which performed a slow start up for the filaments as well as the HV .......) Was this to simple ? 2) PREHEAT the filaments for more than 30 seconds ( Powertubes like KT88 for more than a minute ) BEFORE applying the HV SHUT DOWN the HV before the filament supply to drain the HV. Install a bleeder resistor if your amplifier does not have one . 3) USE vacuum tubes always within the limits f their electric design .... 4) DO NOT OVERHEAT OR UNDERHEAT the filaments , apply the correct voltage for what the tube was designed.....6.3 V and not 5.9 or 6.9 !!!! 5) Correct line voltage !!!!!!!!!!!!!!!!!!!!! On the funny and flashy side of vacuum tube history : " The BBC did never shut down amplifiers .......think about it ." AT LAST.............Pull up the reference voltage of the heaters of small signal vacuum tubes ( dividing network on the HV /small +small smoothing cap ) in some designs , it will avoid noisy tubes and reduce the heater to cathode voltage difference not to exceed .....( this precaution is often forgotten by modern manufacturers and leads more than once to repeating problems for which there seems no evidence other the heater to cathode isolation related problems )

[@wadehicks9270]

That's a great idea I've seen that in my 35 year old guitar amp and thought sure be nice to stop that really good info thanks for sharing.

[@MrCarlsonsLab]

+wade hicks Glad you enjoyed Wade!

[@nor4277]

Nice to learn new stuff about tubes,I notice your poor soldering sponge is about dead if you would like I send you a new one ,thanks for sharing .

[@DANTHETUBEMAN]

I really like the new sensor 12AX7 tubes for Marshall super lead / Plexi type guitar amp circuit. I'm getting the mid tone and it cuts out some high end. I'm lucky they make a currant tube I like. There El 34 tube does what I like also.

[@SmoothSentiment]

I Love This Guy. I have learned so much in the short time I've been a subscriber.

[@themainproblem]

Thanks Paul! Your video's are just the greatest!

[@MrCarlsonsLab]

+Thema inproblem Thanks for your kind words!

[@ericgee6585]

Always Interesting. How about a switch and a 40w bulb in series with the amplifier, or whatever. And no, I haven't tried it. Love your work. Thanks.

[@MrCarlsonsLab]

+Eric Gee Thanks Eric!

[@robertziolkowski118]

Are you saying the old US tube Mfg's made better tubes than the ones coming from Europe now that everyone seems to want? I was in the tube business with Tung-Sol and later with RCA and it didn't seem to be a concern back in the 50's to 70's. Wonder if it was due to poor welding of the filaments. Blasting away the tungsten with too much heat. I enjoy your videos on the old Ham gear W2HER

[@MrCarlsonsLab]

That is a very good possibility Robert.