Cooling the envelope of glass tubes to increase max plate dissipation?  - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: September 28, 2022, 08:01:20 pm
content: How much could water cooling the envelope of glass tubes increase max plate dissipation?, keeping the envelope temperature bellow 100F (40C)And yes I realize there's other limitations like how much the cathode current can flow before the cathode gets destroyed.

author: coppercone2
author_rank: Super Contributor
timestamp: September 28, 2022, 08:59:39 pm
content: well keep in mind the glass is not transparant to IR. Look at a thermal camera at glass and you see a solid object.The glass tube is like having a radiative heating element over the electrodes. How much IR does a glass enclosure radiate? It might not be too large a number but I would not call it nothing. Try something simple like mounting it upside down dipped in a cup of mineral oil. Or punch a hole in aluminum foil or maybe even a paper over the tube so only the glass sticks out and blow a fan over it for testing.You could probobly get something more out of cooling the tube in like a oil bath but tubes are usually designed differently with higher thermal transfer materials (ceramics) if they are meant for active cooling, and I think they put like ceramic pipework that couples to the electrodes directly to actually get a big benefit.I am not sure how you would determine how effective this is, other then over-load testing to see when it burns out compared to a tube that is not cooled.

author: gbaddeley
author_rank: Regular Contributor
timestamp: September 28, 2022, 09:16:28 pm
content: How much is a reasonable improvement? 10% 50%Are you thinking of any particular types? Application?

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2022, 09:30:43 pm
content: A serious reference for questions such as this, originally written in 1960 near the end of the vacuum-tube era, reprinted by Audio Amateur Press in 2000.Robert B Tomer "Getting the Most Out of Vacuum Tubes", Howard Sams & Co. 1960.See pp 96-98 and 145-147 for a brief description of bulb temperature effects.Note that for glass-envelope "receiving tubes", the main cooling mechanism for plate dissipation is radiation (infrared) through the glass.

author: Bud
author_rank: Super Contributor
timestamp: September 28, 2022, 09:40:40 pm
content: Is 40C considered dangerous temp in Latvia?

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2022, 09:54:04 pm
content: PDF link for the book I cited:https://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Tomer_Bud/Getting_the_Most-Vacuum_Tubes.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: September 28, 2022, 10:22:59 pm
content: I wonder what a optical germanium material bulb tube would do

author: TheMG
author_rank: Frequent Contributor
timestamp: September 28, 2022, 11:31:17 pm
content: I can't imagine it would be much difference, as the bulk of the plate cooling is by radiation. There might be a small improvement as a little bit of that infrared energy is absorbed by the glass and re-radiated back toward the plate, but it's probably not much.

author: floobydust
author_rank: Super Contributor
timestamp: September 28, 2022, 11:37:13 pm
content: Power output is limited by the cathode, the filament is sized for the max. cathode current.You can get heatsinks for smaller tubes, I have somePearlbut never really evaluated them.

author: trobbins
author_rank: Frequent Contributor
timestamp: September 29, 2022, 12:02:30 am
content: I'd recommend 1962 RCA Electron Tube Design book - it has an example thermal assessment based on 6L6 output stage tube. Due to the T^4 - T^4 radiative transfer relationship the anode temp doesn't reduce much for a significant drop in glass temp, so there is not much net increase in anode temp obtainable to give a higher anode dissipation capability. Forced air convection is the most practical, and seperating the valve from its neighbours (and not aligning the hotter portion of the anode to the next valves hotter portion) are the typical practical aims. The main benefit from better cooling of the glass is reduced outgassing (and hence lifetime of the getter material). Pushing an anode to a higher temp than nominal has the same detriment to outgassing. It's probably not a great idea to reduce heater power to then allow higher anode dissipation - for a net zero change in total internal dissipation - as the cathode chemistry may suffer from too low a temp.Is your question just for fun, or is there an actual target application?

author: trobbins
author_rank: Frequent Contributor
timestamp: September 29, 2022, 03:05:39 am
content: Many have used anode red-plating as a short term test condition, and it was one crude method to set bias in 807's (adjust to red-plate then back off a bit). All fine for testing as valves are very robust, but they suffer in a slow insidious way due to outgassing, and chew through their prospective service life, and you may get runaway if the input grid current rises and the grid leak resistance is too high.

author: jonpaul
author_rank: Super Contributor
timestamp: September 29, 2022, 05:23:40 am
content: Hello all,Tube glass must withstand the vacuum/ atmosphere pressure differential, and be compatible with the Kovar terminations and seals. Pyrex and special glass. Other material suggestions are not practical.Transmission tubes to 5 kW are cooled by forced air, glass tubes like Eimac, Varian have special sockets and chimneys.10 kW..up to megawatts, (UHF TX, radar, particle accelerators) use ceramic metal tubes, conduction cooled with heats sinks or water jacket, pioneered by Varian.RCA TX tube handbooks for oldglass tubes list the plate dissipation vs airflow.Finally the filament voltage is related to the maximum cathode current, à well know formula.Most tubes required specific filament voltage within a 5..10 % band.Large Transmitting tubes require 100s of watts filament power and tight control of the filament voltage at the pins.réduction of filament voltage reduces the rated max current ( see température limited cathode)Hopefully this will clarify the tube cooling situation.jon

author: jonpaul
author_rank: Super Contributor
timestamp: September 29, 2022, 08:48:35 am
content: Hello ELS122: many thanks good question if plate heats up cathode:Check the specs of the tube in question. filament temp can be 1000 C or more.Plate will be ~ 250C max.Thermodynamic 101 problem.Will differ for direct/indirect heated cathode.https://en.wikipedia.org/wiki/Thermionic_emissiondetail recommendations re cooling and filament voltage regulationRCA TX tubes manualhttps://worldradiohistory.com/Archive-Catalogs/RCA/RCA-Transmitting-Tubes-1956.pdfTube manual EIMAChttps://worldradiohistory.com/Archive-Catalogs/EIMAC/Eimac-Tube-Manual-Vol-2-1966.pdfPower Grid Tube manual EIMAChttp://f1frv.free.fr/telechargement/Eimac%20C&F%20of%20power%20grid%20tubes.pdfExcellent reading!Enjoy,JonPS: Memoires...used 7/9 pin minature tubes in regen radio, 1950s.1962 811A for tube Tesla Coil, Electronics Illustrated magazine.

author: richard.cs
author_rank: Super Contributor
timestamp: September 29, 2022, 12:48:47 pm
content: Having seen a a heavily abused PL509 driving a tesla coil fail by softening the glass until it bubbled inwards and let air in, it would seem that cooling the glass would probably allow more short-term abuse by eliminating that failure mode. That was running with a bright orange glowing anode, so presumably even if the glass hadn't failed it'd have been trashed by outgassing, overheating of the cathode, thermal stress, or something else within a short time anyway (an hour maybe?)

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 29, 2022, 01:13:09 pm
content: https://www.pearl-hifi.com/03_Prod_Serv/Coolers/Coolers.html

author: jonpaul
author_rank: Super Contributor
timestamp: September 29, 2022, 04:53:33 pm
content: Tesla Coils are often designed for very short duty cycle.30 sec...200 sec.Continuous op will stress and burn various parts. Often fans are used to cool the tubes (or spark gaps)The tube plate ratings can specify the max dissipation and color eg dull red.tube Pyrex or high silica glass is built to run 1000s of hrs at rated power.Above a few kW tubes are water cooled or metal ceramic.EIMAC 4CX15000A tetrodeJon

author: trobbins
author_rank: Frequent Contributor
timestamp: September 30, 2022, 12:22:23 am
content: For a typical audio output stage glass envelope valve, with anode dissipation of circa 15-40W, 1962 RCA p.261 discusses a thermal assessment on 6L6G where Tk ~ 800degC, and Tp~400degK for 18W plate dissipation, with the heater wire itself up around 1200-1300degC for this indirectly heated cathode example. Glass external surface temp can peak to about 250C hot-spot and there are a few 'cooler product' papers that expand on that.Due to the large difference in temperature between each of those parts (filament, cathode, plate, glass), a significant change in temp of one part (such as anode) has insignificant effect on hotter parts (cathode and filament). Its worthwhile doing some T^4 maths to get a feeling how that works.Also the current through the cathode material, and hence any resistive loss (W) due to that current, is definitely negligible compared to the heat flow from the filament through the cathode and out to the plate and beyond.Transmitter tubes by their nature are very different beasts than the glass envelope tubes that 99% of us typically comes across - imho there is little insight to be gained from assessing their performance, and for the novice there is a lot of misconception that could result.

author: jonpaul
author_rank: Super Contributor
timestamp: September 30, 2022, 05:58:38 am
content: Bonjour Trobbins: ACK, lots of confusion in this thread re tube types/power.But some Class A high end build amps with TX tubes, amd the general question re cooling of power tubes is interesting.We could classify the Q/A to apply toGlass envelope <500WGlass Env >500WTX tubes > 1 kWMetal/air colledWater cooledetc.Your thoughts?Jon

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2022, 09:57:54 pm
content: Yes, normal receiving-tube operation is "space-charge limited", rather than "saturated emission" (which is temperature limited).The formation of the space charge "cloud" produces a retarding field at the cathode, which reduces the cathode current below the saturation level, and greatly decreases the sensitivity of the cathode current (at constant electrode voltages) to the cathode temperature.The Child-Langmuir Law (with V3/2dependence of the current) is a description of the space-charge limited cathode current.An interesting exception is the 5722, a small diode run in saturated-emission mode, to generate shot noise whose value can be calculated from the measured cathode current.In operation, the filament current is adjusted to obtain the desired cathode current.https://frank.pocnet.net/sheets/201/5/5722.pdf

author: jonpaul
author_rank: Super Contributor
timestamp: October 01, 2022, 06:53:26 am
content: Air cooles mémoireCirca 1963, I built a zero- 400V 250 MA reg lab power supply (like the classic RCA tube app note, or Heathkit). I Modified with three beam power tubes (6BG6, like 6L6 with plate top cap) in parallel.Cooling with a 5" Rotron muffin fan blowing directly on the three tubes.With full load 250 MA and low V out max plate dissipating 100W total.It still runs fine.Jon

author: trobbins
author_rank: Frequent Contributor
timestamp: October 01, 2022, 08:43:11 am
content: I found it interesting to look for references related to how much cooling was provided by direct radiation from the plate to nearby surfaces at ambient temp, versus being absorbed by the glass envelope and then the outer glass surface being cooled by convection and radiation, for commercial audio output stage tubes (ie. up to circa 40W plate dissipation). The glass transmission and absorption is a function of radiation wavelength, and very dependant on glass thickness and type, but the glass can be surprisingly thin and the glass is typically not too exotic, so imho a fair % of heat transfer can make it out by radiation for hotter anodes as more energy is contained in the shorter wavelength portion of the black-body like spectrum curve.Some manufacturers tried to enhance the radiation emission performance of the plate (ie. type of black coating), and some tried to enhance the uniformity of temperature over the structure by improving conduction within the structure (using a copper cored metal sandwich), and some tried to increase the radiating surface area of the anode structure using larger 'wings' where the anode temp was hottest, and some tried to use the largest practical glass surface area. There are some nice photos on the web of red-plating anodes which show the hottest region of the anode structure, and is a guide to constructers to orient any line of valves such that the hottest anode faces point away from neighbouring valves.

author: jonpaul
author_rank: Super Contributor
timestamp: October 01, 2022, 01:31:51 pm
content: In the 811A Tesla coil of 1960s, indeed the plate had fins and was black or dull gray, the center of the large surface was a dull red after some minutes.Jon