Vacuum tubes - DC heaters?  - Page 1
Forum section: electronics / beginners

author: SteveThackery
author_rank: Super Contributor
timestamp: August 20, 2024, 07:23:19 pm
content: I've been checking out a 100 watt tube (valve) amplifier over on the official KICAD forum, and a question occurred to me.Traditionally the cathode heaters are fed from an AC source - almost invariably a winding on the mains transformer. Voltages are typically 6.3VAC RMS, or sometimes multiples thereof on transformerless circuits where the heaters are connected in series across the mains.Anyway, do you think there is any mileage in running the heaters off a DC supply? Nowadays smoothed and regulated DC is straightforward to deliver compared with the early days of valve gear. I wonder if AC-fed heaters can inject hum into the signal path - something that might be avoided with DC heaters.

author: Gyro
author_rank: Super Contributor
timestamp: August 20, 2024, 07:42:26 pm
content: No, or very little, mileage. The little bit comes from very low level signal stages, where you may get some hum reduction. Even then, low level tubes like the EF86 have non-inductive biflar wound heaters to reduce coupling. Even standard tubes have a folded arrangement that cancels hum to a large extent.It is vaguely possible for the hot (but insulated) heater to behave as a cathode, but this possibility is eliminated by elevating the whole heater winding to a few 10s of volts above the highest cathode potential.Methods of producing a DC heater supply, eg. a bridge rectifier or SMPS are far more likely to introduce noise in the audio pass-band.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2024, 08:11:51 pm
content: The main advantage of DC heaters (assuming clean DC) is easier wiring of the leads to the tube socket on low-level stages (e.g. RIAA preamp).Note that RIAA stages have relatively high gain at 50/60 Hz.AC requires careful wiring there, which is covered in tube books.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 20, 2024, 08:25:02 pm
content: Of all the “classic” amplifier schematics that I recall seeing, only the phono preamp or mic preamp stages used DC powered heaters. Not all of time though, there were other tricks to minimize hum.What I also recall seeing is half wave rectification applied to all the heaters while the amp was in standby, for power saving and a faster turn on.

author: Gyro
author_rank: Super Contributor
timestamp: August 20, 2024, 08:38:52 pm
content: Yes, heater wiring always needs to be very carefully implemented, usually using tightly twisted solid core wire. Heater currents don't really have a place on PCBs.Quote from: schmitt trigger on August 20, 2024, 08:25:02 pmWhen you get down to those sort of levels, microphonics can be as much problem as any heater induced hum. Mullard recommended resilient mounting of such stages, again typically with the EF86 pentode.Resilient mounting is / was also used for things like the Nuvistor board on the HP3400A.QuoteAh yes, the old 'instant on' modes on guitar amps and TVs of the era. Sometimes the reduced cathode temperature was enough to prevent tube life reduction due to cathode poisoning (no anode supply / cathode current). That was a real problem in TVs when the standby modes were first introduced.

author: Benta
author_rank: Super Contributor
timestamp: August 20, 2024, 09:12:30 pm
content: Don't forget electromigration on the filaments when using DC.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2024, 09:28:24 pm
content: I have heard that DC shortens heater life in indirectly-heated cathodes in small-signal “receiving tubes”, but have been unable to locate a definitive discussion in my extensive library of quaint and curious volumes of forgotten lore.

author: TimNJ
author_rank: Super Contributor
timestamp: August 20, 2024, 09:39:49 pm
content: Quote from: Gyro on August 20, 2024, 08:38:52 pmHeater current for a typical power tube like a 6L6 is only like ~1A. It's not *that* much. A 2mm wide trace should not give much temperature rise for a few tubes.I designed a PCB based on the Fender 5F6A design about 7 years ago. I routed the AC heaters as a parallel pair of traces. Anecdotal, but it is a quiet amp.

author: tridac
author_rank: Regular Contributor
timestamp: August 20, 2024, 10:18:22 pm
content: At least one early HP tube based spectrum analyser (853 ?) used a solid state regulated dc supply to drive the 2-4GHz bwo heater, the 1st mixer local oscillator. I think that might have been done for stability reasons, or perhaps to eliminate modulation of the lo via the heater. Only know that because the bwo starts to have dead spots in the output, as the tube ages, and the one I got for scrap price with that issue, was fixed by turning up the heater volts by around 0.1v, restoring the bwo to full output across the band. A bit like the old tv crt restoration trickOther than that, can't think of any other reason, but perhaps very low level preamplfiers, tube dc op amps, again for stability or noise. Can't see it causing any problem though...

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2024, 11:25:50 pm
content: Local oscillators in HF receivers often had stabilized heater current for thermal stability. The AC current was passed through a “ballast” tube, such as the 4H4C used in the National HRO-60, to compensate the current against voltage difference. These were PTC resistances in a tube envelope.With modern semiconductors, an appropriate regulator IC is easier with DC operation.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: August 21, 2024, 12:18:58 am
content: I've only ever seen it for the input tubes in some fussy audio preamp designs. I don't see any problem powering heaters with a DC supply. Batteries were use for the A supply before mains.

author: floobydust
author_rank: Super Contributor
timestamp: August 21, 2024, 12:34:31 am
content: Capacitive coupling from heater to cathode does add hum if the cathode resistor is unbypassed, which is the case in some preamp circuits and guitar amp front ends etc.If there is a lone cathode resistor unbypassed then you get hum from AC powered filaments and running them on DC has benefit. Also the wiring radiates a lot of hum, need a twisted pair for AC filaments this has been known for 100 years.For a power amplifier in push-pull configuration, hum cancels in the output transformer so there is no benefit to using DC filaments there.I'll use Schottky rectifier diodes and an LDO to generate DC for filaments.

author: themadhippy
author_rank: Super Contributor
timestamp: August 21, 2024, 12:45:56 am
content: slightly related,is there any advantage to the following heater arrangement? nothing exotic ,just ecc82 ,ecc83 and el34's

author: TimFox
author_rank: Super Contributor
timestamp: August 21, 2024, 12:48:22 am
content: A useful circuit if the transformer secondary doesn’t have a center-tap.

author: themadhippy
author_rank: Super Contributor
timestamp: August 21, 2024, 12:51:19 am
content: But theres no need for a center tap if its only powering the heaters,which is what this is doing

author: TimFox
author_rank: Super Contributor
timestamp: August 21, 2024, 12:54:42 am
content: You need to establish a (common mode) DC voltage on the heater, and not float it.If you ground one side of the heater, you have a common-mode AC voltage on the heater which can couple capacitively to the grid or cathode.

author: MathWizard
author_rank: Super Contributor
timestamp: August 21, 2024, 01:03:13 am
content: I had to make a supply for a vacuum florescent display this winter. And I made some sort of a full bridge oscillator. The rails were between -20, GND, and the DC level of the low side of the load was down around -18V, and the resultant AC was only a few volts. Something like that. But it was fairly high current, up over 100mA if I wanted.If I had to make a VT heater supply, I'd try something like that again, but without any DC bias, or very little.

author: themadhippy
author_rank: Super Contributor
timestamp: August 21, 2024, 01:25:55 am
content: QuoteThat probably explains why my experiments with valves never turned out as i expected

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2024, 02:40:42 am
content: Rectifying AC doesn't make much sense: the advantage is small while the cost is relatively large (and, not that rectifiers and capacitors are exactly expensive themselves, but like I said, the advantage is just that small).The advantage is larger for low-level stages, guitar pickup, phono, etc., where the ripple makes a bigger difference. The cost of a simple rectifier-filter is more acceptable, too (i.e., rectifier, cap-input filter, RC filter to heater -- R necessary to adjust voltage down (can get some extra filtering out of it too), wasting some power).DC bias also helps, to reverse-bias the heater-cathode diode. The heater still has some emission, and this helps reduce current flow in general -- thus reducing ripple input as well.You could go fancier and use an SMPS, but 6.3VDC is somewhat uncommon, and if you're going to go the trouble, why not make the 300V or whatever with it too? But now you're doing an entire power supply design for very little amplifier, and EMI is a real problem.Tim

author: Andy Chee
author_rank: Super Contributor
timestamp: August 21, 2024, 04:30:38 am
content: Quote from: T3sl4co1l on August 21, 2024, 02:40:42 am5VDC is very common these days. I wouldn't imagine it's too difficult to build or modify one for 6.3VDC operation?

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2024, 04:46:42 am
content: Quote from: T3sl4co1l on August 21, 2024, 02:40:42 amI am not sure what the Ef 7.5 line is, but heater to cathode voltage also affects reliability.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2024, 05:30:25 am
content: E_F should be filament voltage, though I don't get why it would go up >6.3V (makes sense) but then down again at 7.5V? Otherwise, that's about what I understand though, staying on the positive and highish side of H-K ratings. I wonder what exactly the failure mechanism is being plotted... is that just regarding H-K failure (short, or H-H open)?Quote from: Andy Chee on August 21, 2024, 04:30:38 amDepends. Flyback almost certainly; forward maybe; LLC probably not.Tim

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2024, 07:56:30 am
content: Quote from: Andy Chee on August 21, 2024, 04:30:38 amCompact 5V SMPSs these days use (coincidentally) 6.3V rated capacitors, so capacitor replacement would probably need to be included a modification.

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2024, 08:17:52 am
content: Quote from: David Hess on August 21, 2024, 04:46:42 amNo, I'm not sure why the Ef 7.5 line is for as it is significantly higher than 6.3V +10% to start with. Any H-K voltage above 100 - 200V (tube dependent) is outside the datasheet maximum specs anyway, so extending the graph to 300V is somewhat academic too.The heater - cathode insulation limits are rarely an important consideration though. The only time they can be at risk in thing like high voltage SRPP stages or maybe cathode followers, where it might be advisable to raise the heater DC bias to some mid point (with due consideration of other tubes on the same heater supply).I've never found the need to implement DC heater supplies on any of my designs, even low level. Raising the DC bias of the heater winding center tap (real or artificial) to 20-30V above the highest cathode voltage has pretty much guaranteed silent operation. Microphonics a stray magnetic fields from transformers are far more likely to be a problem (and sometimes confused with heater coupling).On RF stuff, local decoupling the heater supply to the relevant chassis ground with low inductance capacitors [and series ferrite beads] can be important to stability, but this is due to capacitive H-K coupling at high frequency rather than 50Hz. [EDIT: This can also be relevant to high frequency tubes, like the E180F and 6S45P, being pressed into audio amplifier service, but these often also need grid, cathode, and even anode stopper resistors to tame them too].

author: Xena E
author_rank: Frequent Contributor
timestamp: August 21, 2024, 08:20:21 am
content: As stated DC heating of preamplifier tubes is possibly the only real advantage for sound quality.There is another more satisfactory method that I've seen that is a revival of the methods used in soundtrack lamp exciters in vintage cinema equipment, that is to use HF heating, it can even be used with directly heated tubes, the disadvantages of DC heating are thereby negated.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2024, 08:28:43 am
content: Yup.There is some risk of IMD in DHTs, where the nonlinear cathode-grid transfer function causes some mixing, but this can be avoided by 1. choosing a frequency far above 20kHz (say a few 100 kHz), so that not only signal but harmonics as well are well attenuated at comparable frequencies where IMD might occur, and 2. using low voltages -- since typical DHTs have quite low gain, e.g. 50V grid input required for full swing, and the e.g. 2.5V RMS of the heater is a tiny fraction of that range, and therefore looks like a nearly-linear section of that curve. That is, applying the calculus principle of continuity of derivatives, where the heater voltage swing is small in comparison to the overall range of the transfer function, and what curvature it has (nonlinearity, distortion), thus the distortion and mixing of that signal is small. Conversely, types with high filament voltage may be discouraged for such service; this mainly means transmitter types anyway, for which the power level is much higher, or for RF purity reasons, so you might want to avoid the issue entirely and stick with mains/DC for them.Also easy way to get isolation; if you need to float something with a relatively fragile heater, well, pretty easy to add more windings to a small ferrite transformer and be done with it.Tim

author: Xena E
author_rank: Frequent Contributor
timestamp: August 21, 2024, 09:40:30 am
content: Audio amplifiers using DHTs back in the day, most often relied solely on a filament winding on the line transformer that was centre tapped to the signal ground through grid biasing arrangements.In an experimental small (sub 1W) class A Push Pull amplifier that I was shown by a student, directly heated miniature FSU rod pentodes were used throughout.The whole demonstration was powered by a 12V SLA battery through an HF SMPS giving the 1.2V HF filament supply, and the 90VDC B supply.The push pull output tube filaments were powered directly from the HF transformer of the PSU the regulation for both (HF) A and (DC) B+ was taken from monitoring the 1.2Volt HF A supply.The bias for the output stage is generated via a CCS whose load is the filament of the voltage amplifier (20mA ISTR ?), all very elegant: and sounds superb as well, with no vestigial HF on the output.

author: tridac
author_rank: Regular Contributor
timestamp: August 21, 2024, 11:18:58 am
content: Have seen that before, but better stiil, use a 500r trimpot, whatever, then you can adjust to compensate to any imbalance...

author: SteveThackery
author_rank: Super Contributor
timestamp: August 21, 2024, 03:39:52 pm
content: My thanks so everyone who has contributed. I've found it to be a very useful and enlightening discussion.

author: TimFox
author_rank: Super Contributor
timestamp: August 21, 2024, 03:52:53 pm
content: A corollary question to DC or AC heater power choice:Vacuum-tube heaters or filaments have strong temperature dependence: the cold resistance is much less than the resistance at operating temperature.When driven directly from a transformer secondary or "stiff" DC supply, there is a very high inrush current, that stresses the heater at power-on.In operation, it is good practice to maintain the heater voltage to within +/-10% or better.There are many ways of reducing this effect, for example:1. A resistor in series with the heater wastes power during operation, but reduces the initial current. (AC or DC)2. A NTC thermistor in series with the heater or transformer primary will reduce the initial current, but wastes less power during operation after it heats up. (AC or DC)3. If a normal voltage regulator circuit is used to stabilize the heater voltage at 6.3 VDC (or whatever), its current limit can reduce the initial current while maintaining the voltage at normal current. (DC)#3 is perhaps the most accurate method with simple components, assuming DC power.When tubes were dominant, small-signal tubes were inexpensive and extending their lifespan was often ignored.I remember 12AX7s, when in production, were < $1.00 at retail. Have you priced an NOS Telefunken 12AX7 recently? (Maybe $500 ea.)By the way, an important limit is the voltage between heater and cathode (for indirectly heated cathodes), which is why the heater cannot be allowed to float, and important for cathode follower and phase-inverter operation, where the cathode is at a substantial voltage above ground.This varies from tube to tube: some are good to +/- 200 V (peak), but many have much lower insulation voltage ratings.The rating (specified on the tube data sheet) is often asymmetric, or with a limit on the DC value (for heater positive with respect to cathode) less than the peak value.

author: Bud
author_rank: Super Contributor
timestamp: August 21, 2024, 04:49:02 pm
content: I remember something along the lines that DC heating may cause uneven emission and uneven utilization of the cathode material and lead to reduced operational life of the tube, because of the uneven potential of the cathode along the heater, since the heater has one end grounded, at zero potential and gradient potential increasing toward the other end which is under a higher potential. To mitigate this, heater voltage polarity was flipped every time the device is turned on, or set it randomly.Old farts here may confirm or provide a better explanationEdit: to an extent this effect can be visually observed on VFD displays if the heater is not center biased. The display will have a noticable brightness gradient along the tube.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2024, 05:02:25 pm
content: Quote from: Gyro on August 21, 2024, 08:17:52 amThat is what I mean. Reliability should have dropped and been lower than the 6.9 volt line. I suspect the graph was just mislabeled.The relevant text from Getting The Most Out Of Vacuum Tubes - Tomer is:There are many heater failures that appear to be simplythat and nothing more, but which in reality are the endresult of heater insulation breakdown. This is particularlytrue where a relatively high heater-to-cathode voltage ex-ists. Until a few years ago, all indirectly-heated cathode-type tubes had a heater-to-cathode rating of approxi-mately 90 volts. This was based on the known limitationsof the insulating material used on heaters. There has beenno change in this material over the years, yet heater-to-cathode voltage ratings have steadily increased. Thisdoesn't make much sense—from an engineering point ofview—even though it does permit the design of many cir-cuits not possible under the older, more conservative rat-ings. As a result, we have far more breakdowns of thistype in the field today than we had some years ago.Quote from: Xena E on August 21, 2024, 08:20:21 amTektronix used regulated DC heaters at least once in their 1A7 differential vertical amplifier which used pairs of 8393 medium-mu triode Nuvistors in parallel for 10 microvolt per division sensitivity. The DC heaters kept AC ripple out of the most sensitive part of the signal path, and regulation controlled voltage drift. The heater current was trimmed to balance the differential input stage.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2024, 05:05:07 pm
content: Quote from: Bud on August 21, 2024, 04:49:02 pmThat applies to directly heated cathodes, where the heater itself is the cathode. Indirectly heated cathodes which became the dominant technology have no such issue.

author: TimFox
author_rank: Super Contributor
timestamp: August 21, 2024, 05:07:40 pm
content: Yes. Another term for indirectly-heated cathode is “equipotential” cathode.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 21, 2024, 05:30:51 pm
content: Some RF tubes like Klystrons have datasheet specifications for amplitude and phase pulling vs. heater voltage. In radar applications, this manifests as 60 Hz and harmonics in the received spectrum. Typical mitigations are running the filaments at the same frequency as the radar's pulse rate. This still produces spurious emissions, but they lie at the Nyquist frequency of the received spectrum and can be ignored. High power tubes have rather high filament drive requirements (7.5V, 33A) so it's rare to find DC driven filaments since the filament supply must pass through the high voltage pulse transformer used to power the cathode.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 21, 2024, 05:48:53 pm
content: Where low power tubes were once used in domestic portable radios a range was developed that allowed economical (for the time!), powering from dry cells.The filament current that these devices needed was so relatively low that designers had to be mindful of the fact that the plate current added to the current through the filament and not only were there potential gradients this inequality of filament current could also cause limited life span, this was a particular problem in sets that had 5 tubes say, with nominal 1.5V heaters that ran in series across a 7.5 Volt supply.If the schematics of the day are inspected those sets so designed often had bypass resistors wired across sections of filaments to counteract the effect.

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2024, 07:12:31 pm
content: Quote from: David Hess on August 21, 2024, 05:02:25 pmNo, that graph makes no sense to me either, the text doesn't really help either - Yes, it follows that increasing heater-cathode ratings without change of materials would increase breakdown events, but why does it show the lowest failure rate (irrespective of incomprehensible heater voltages) at ~90V h-k rather than 0V. I think it's just drawn wrong!

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2024, 08:01:56 pm
content: Indirectly heated tubes have a reasonable heater thermal mass that tends to reduce thermal shock, the low cold resistance of several parallel heaters tends to load down the transformer secondary at switch-on too at least for the 6.3V ones, rather than the series chain connection 300mA ones. The exception were the 'quick heat' tubes that would flash alarmingly at the termination end.Most alarming are the directly heated Thorated Tungsten filament tubes, where the filament undergoes a physical transition at ~900k due to the Miller-Larson effect*discovered in 1944. This causes the crystal structure to re-orient as it passes through the transition temperature, leading to necking, cracking and ultimate failure. Although the filaments are rated for around 30k hours, each cycle reduces life by around 0.2%! The only way of mitigating this is to keep the temperature above 900k [Edit: and limiting inrush current to reduce the stress on the increasing number of defects from when you do cycle them]. Not a happy situation for owners of big, expensive, and potentially rare DHTs.*Morgan Jones, Valve Amplifiers, third edition, pp 261.

author: floobydust
author_rank: Super Contributor
timestamp: August 21, 2024, 08:51:17 pm
content: Diodes (selenium) and large value electrolytic (filter) capacitors were expensive back in the day.A critical application needing DC filament power is in condenser microphones.For DC filaments, designs pulled it from B+ through a dropping resistor, making a lot of heat. It also contributed to cathode current, so the heater and cathode were pretty much at the same potential, and cathode resistor could be small. e.g. Altec, Telefunken or Neumann U47 (VF14 60V 50mA filament, undervolted) and many other mic's used the same scheme for low noise.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2024, 10:56:01 pm
content: Quote from: Gyro on August 21, 2024, 07:12:31 pmI trust that part of the graph. Biasing the heater positive relative to the cathode prevents emissions from the heater to the cathode, which would otherwise damage the insulation. The graph is based on empirical results.

author: CaptDon
author_rank: Super Contributor
timestamp: August 22, 2024, 02:38:13 am
content: I have no explaination for the following, but repeated experiments showed the same results. 12VAC RMS VS. 12VDC Observation, the lamps operated on D.C. had a much shorter life. The lamps were lit for an 8 hour shift each weekday (actually 8.5 hours daily). The A.C. operated lamps showed marginal darkening of the glass and a broken filament which seemed to retain its original thickness at end of life. The D.C. operated lamps showed intense darkening of the glass and the reduction of filament thickness was visible to the naked eye. This experiment was undertaken do to customer complaints of short bulb life. It was amazing how fast a 12VDC regulated supply could kill a 14.4 volt bulb!! But all of this is confusing because we all know of cars operating at a somewhat variable 12 to 13.8 volts D.C. in cars 20+ years old and the dash lamps are still working fine!!! Maybe fixed vs. variable voltage is some sort of clue? Controlled testing directly opposing reality?? Bulbs in question #53, very common in older cars and motorcycle instrument clusters, and in this case arcade game back lighting.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2024, 03:02:47 am
content: Quote from: Gyro on August 21, 2024, 08:01:56 pmParticularly Mullard types were/are famous for this; to be clear, it's a normal / intended effect, for parallel operation. Such tubes should not be used in series chains, IIRC.QuoteHmm, I'm not aware of any phase change in the W-Th system. Although I don't find a phase diagram offhand, but, it's present in small amounts so is probably either an insoluble impurity or solid solution, and either Th (or ThO2) itself. Pure Th has a phase change (FCC to BCC at 1360°C). Pure W has no phase change, it's the same crystal up to MP. Jones seems to misrepresent this here -- it appears Miller and Larson published in the 50s, regarding creep, not phase change:https://en.wikipedia.org/wiki/Larson%E2%80%93Miller_relationI don't see anything for either creep or phase transformation for W-Th mixtures, buthttps://inis.iaea.org/collection/NCLCollectionStore/_Public/32/053/32053126.pdf?r=1&r=1is interesting for creep, in the same temperature range, of a couple other alloys. Creep and recrystallization proceed normally at higher temperatures it seems, which is certainly evident in tungsten lamps (especially the blocky recrystallization and sublimation in a failed halogen bulb), but creep should be modestly relevant for thoriated cathodes. (Literal "filament" cathodes, wires and ribbons, are strung up between spring supports, so stress and creep are relevant; very large or specialized transmitters may have self-supporting cathodes, with creep nearly irrelevant?)I would be inclined to think thermal cycling causes differential stresses, and perhaps activation of grain boundaries/migration (perhaps under electromigration forces too? maybe not with AC though). Maybe that spreads into a stress riser that eventually snaps the filament, maybe it's governed by creep (might the creep timer be reset every time it's cooled down?), no idea.This probably all isn't too germane for Jones' purposes, more just to say something on the topic, than to give the academic rigor that vacuum tube designers and manufacturers would need, so I suppose it doesn't matter all that much.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: August 22, 2024, 04:19:06 am
content: tungsten does all sort of weird stuff1) if you snap it, it can splinter like wood2) if you heat it too much, the tip of a tungsten welding electrode turns into basically,, if you took a tree trunk, cut it into a cone, and left it in the sun and rain for 15 years. It starts turning into a 'blooming onion'.I am convinced its actually a type of lumbermetals not supposed to do thishttps://www.reddit.com/r/Welding/comments/an5atq/my_friends_tungsten_split_after_he_broke_his_cup/Its higher temperature effects, but I am convinced there tungsten is a tricky materialI think tungsten actually comes from trees. I don't think they mine it, its probobly extracted from the center of a specific pine cone.

author: floobydust
author_rank: Super Contributor
timestamp: August 22, 2024, 04:27:35 am
content: "The Influence of Notching Upon the Life of Miniature Lamp Filaments"Notching is the phenomena in which a saw-toothed surface appears over portions of the filament. The notching grows due to the electro-migration of tungsten ions and becomes more noticeable after long operation. Notching depends on the lighting condition, either DC or AC. At AC voltage, the notching occurs near the area where the filament is supported by the anchor or is connected with the lead-in wire. These areas have a temperature gradient. At DC voltage, since tungsten ions move in only one direction, the notching occurs over the entire filament. Therefore, the lamp life at DC voltage becomes shorter than at AC voltage because of more severe notching. In order to reduce the notching, a rhenium tungsten filament, which has a higher temperature recrystallization point, can be used."sourceedit: tungsten seems to be crystalline in structure, as well as the fact it oxidizes too. SEM pictures are pretty interesting.Blown filament (vacuum loss they say)https://www.sciencephoto.com/media/601254/view/blown-tungsten-filament-sem

author: Andy Chee
author_rank: Super Contributor
timestamp: August 22, 2024, 05:39:57 am
content: Quote from: floobydust on August 22, 2024, 04:27:35 amWhat is the lowest AC frequency at which severe notching becomes pronounced?For example, what if one were to reverse DC polarity of the filaments at every device switch-on power up? Does that count as AC?

author: Xena E
author_rank: Frequent Contributor
timestamp: August 22, 2024, 08:17:53 am
content: Quote from: T3sl4co1l on August 22, 2024, 03:02:47 amYes, heater flash was one of their traits, particularly the 12AT7, 12AU7, and 12AX7 series. The phenomenon of heater flash is now one of audiophooleries urban legends that signifiesbetterquality.One spectacular 'flasher' is the 6AL5/EB91. Theseweredesigned for use in 300mA controlled warm up chains, however, had so little thermal mass would light up a room at switch on.They were not particularly known for open heater failures but subject to a common fault of h/k insulation breakdown.In bright emitter electron tubes, embritlement and wasting of the filamentary type cathode was always a factor in life determination. When the minature low power battery tubes were introduced it was difficult to produce the filament with any consistency in tungsten, the answer that some manufacturers adopted was the use of nickel.The problem with nickel filaments being that it was more sensitive to supply voltage variations and it's narrower temperature/resistance range meant that overvolting damage occurred more readily.