Cooling the envelope of glass tubes to increase max plate dissipation?

[ELS122]

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.

[ELS122]

Actually I now realize this wont help at all in the way I first thought, since the tube doesn't have air inside so the anode wouldn't get cooled by the glass envelope being cooled, apart for the mica spacers that contact the envelope, and the connections to the plate.
but anyway, could I get a reasonable improvement in the max plate dissipation by actively cooling the pins and envelope of the tube?

[coppercone2]

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.

[gbaddeley]

How much is a reasonable improvement? 10% 50%
Are you thinking of any particular types? Application?

[TimFox]

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.

[ELS122]

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.

I read that book, but it talks more about reliability than overloading tubes.

How much is a reasonable improvement? 10% 50%
Are you thinking of any particular types? Application?

yeah 10% would be reasonable I'd say.
application isn't specific, most likely an audio amp since that's the only thing I'd think to want max anode dissipation.

[Bud]

Is 40C considered dangerous temp in Latvia? 

[TimFox]

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

[coppercone2]

I wonder what a optical germanium material bulb tube would do

[ELS122]

Is 40C considered dangerous temp in Latvia? 

no, why? it was just an arbitrary value I wrote.

[TheMG]

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.

[floobydust]

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 some Pearl but never really evaluated them.

[trobbins]

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?

[ELS122]

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?

just for curiosity, always pondered about pushing a tube to it's absolute limits in an amplifier, even built some class B2 EL34 amps with cathode followers driving the control grids...
anyway, I tested it with a random old 6P15P tube I had (RF version of 6P14P/EL84), because it was the only tube I wouldn't be sad about destroying.
in the outside air it would handle 14watts (that's anode + screen dissipation) just before starting to glow on the anode, and in a jar of water and stirring it constantly I could achieve 15watts.
heater voltage was 6.9volts (due to 220V transformer it was high)
the screen was supplied from the anode by a 5.6k resistor, anode voltage was around 230V
used a 200ohm cathode resistor with a pot on the control grid adjusting between ground and cathode voltage.
so it seems kinda worthless, a chimney around the tube would probably achieve the similar gains anyway.

maybe I'll test dropping the heater voltage now.
speaking of that, I found a chart for the 6P15P coincidentally that shows change in plate current, screen current, and transconductance relative to heater voltage.

found it on a thread about heater voltage, apparently many tube amplifiers ran the preamp tubes (12AX7s) at reduced heater voltage ~85% lower for increased linearity.
and some other tubes like 6AL5, and 12AV6 have better linearity at reduced heater voltage.
here's some charts on that:

[ELS122]

well by reducing the heater voltage to an extreme low of 3V I got nearly a whole watt of additional dissipation before red plating. so its now up to 15.5w!!!
wow!

And I now realized that this test is totally pointless, if you just keep the bulb temperature within limits and have a cathode that can withstand the current draw, the tube will live even if the plate is glowing.
you could probably water cool an EL500 and get like 1.5x the plate dissipation while still having a tube that lives long enough to go trough a full song of pink floyd
any more than that and the plate would probably melt. but I doubt the cathode would suffer since it's meant for high current pulsing in sweep circuits so it's like twice as big as any normal tube of the same plate dissipation rating.

so perhaps a better test would be: how much current will the cathode take before EOL'ing, do I check for white dust after use in the tube for that?

[trobbins]

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.

[jonpaul]

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

[ELS122]

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

doesn't the cathode require less heater heating after the tube has been running at high plate dissipation for some time? since it gets heated up by the plate and also by the current passed trough the resistive cathode

[jonpaul]

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_emission

detail recommendations re cooling and filament voltage regulation
RCA TX tubes manual
https://worldradiohistory.com/Archive-Catalogs/RCA/RCA-Transmitting-Tubes-1956.pdf

Tube manual EIMAC
 https://worldradiohistory.com/Archive-Catalogs/EIMAC/Eimac-Tube-Manual-Vol-2-1966.pdf

Power Grid Tube manual EIMAC
http://f1frv.free.fr/telechargement/Eimac%20C&F%20of%20power%20grid%20tubes.pdf

Excellent reading!

Enjoy,

Jon

PS: Memoires...used 7/9 pin minature  tubes in regen radio, 1950s.
1962 811A for  tube Tesla Coil, Electronics Illustrated magazine.

[richard.cs]

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?)

[Alex Eisenhut]

https://www.pearl-hifi.com/03_Prod_Serv/Coolers/Coolers.html

[jonpaul]

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 tetrode

Jon

[trobbins]

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.

[jonpaul]

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 to

Glass envelope <500W
Glass Env >500W
TX tubes > 1 kW
Metal/air colled
Water cooled

etc.

Your thoughts?
Jon

[ELS122]

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.

I meant the cathode getting heater from the space charge emission, although now it sounds dumb but I was under the impression that high emission from the cathode heats it up substantially.
also, what do ya'll mean by "temperature limited cathode"... most tubes dont run temperture limited. and I tested it and reducing the temperature of the cathode does practically nothing to increase max anode dissipation before red-platting, could it make the cathode more happy at high current? from what I've read in many books it's the opposite, reduced heater voltage destroys the cathode especially at high cathode current.