overcooling question

[infobot]

Would sound or performance degrade if you overcooled audio tubes ?

I don't mean dry ice / sub freezing point stuff, more barely warm to the touch. Do audio tubes work or sound better warm-hot or within some sort of range ?


I tried a large fan just for kicks on a small tube amp I have while waiting for a better slow speed fan to arrive. Got them pretty cool and started me wondering.

Sorry if seems obvious I searched and couldn't find anything.

[nigelwright7557]

So long as the cooling doesnt affect the heaters temperature it should be ok.

[wakibaki]

Tubes, valves work because of a temperature gradient, from cathode to anode. You can get slip-on heatsinks for conventional glass valves where the anode surrounds the cathode. Sometimes the metallic anode is exposed to permit cooling, still common in VHF transmitters, watercooled valves for transmitters may still be used for all I know, I used to have a book with worked calculations for water flow/kW (at altitudes above MSL, which is also a factor IIRC).

Generally speaking envelope temperature is the principal factor affecting longevity, the cooler the better. You're unlikely to cause problems with room temperature air.

w

[infobot]

I had guesses but, not knowing, it seemed a good idea to ask. Sounds like any sonic or operating gain would too minimal better to extend tube life.

Tube coolers look interesting. Watercooled must really help keep the temp down but probably to be avoided by the novice. Upside down tubes and separate tube rack or something.

Mainly wondered if I'd be throwing anything off like what you two mentioned. Think I'll be fine with anything I'd be likely to use.

[tomchr]

Originally Posted by wakibaki

Tubes, valves work because of a temperature gradient, from cathode to anode.

Eh, no... Tubes/valves work because a metal cathode is heated to a temperature where its atoms are excited to the point where the majority carriers (excess electrons) leave the metal and form a space-charge cloud around the cathode.

The electrostatic field between the anode and cathode causes a current to flow between anode and cathode.

The grid retards the flow of anode current, thus, allowing for voltage control of the anode current, thereby, amplification.

The suppressor and screen grids (for tetrodes, pentodes) have different functions - mainly to reduce the input capacitance (Miller cap) and to deal with the ion cloud created at the anode due to impact ionization.


Forced air cooling was/is used on many transmitter tubes to prevent the tube from melting. Aside from fan noise, I don't really see any issue with cooling audio tubes as long as the cathode temperature isn't affected too much. I don't see much of a point of doing so, but each to his own I guess.

~Tom

[SpreadSpectrum]

You can huff and puff and blow on that tube all day and you won't cool the cathode down.

Remember, the tube is filled with this really cool substance called vacuum that doesn't conduct heat or promote convection very well, and it is really hard to radiate coolness from the glass to the cathode.

The only conduction that takes place from the cathode would be to any pins connected to it. You would probably want to avoid spraying a stream of liquid nitrogen on those pins as it might cool the cathode (not to mention the effects on the glass-metal seal).

[wakibaki]

Tom

Tubes work because of a temperature gradient between cathode and anode. If they were both at the same temperature, then, all other things being equal their emissivity would be the same. Granted, cathodes are coated, but this was not always the case.

This may not be quite the way you are used to thinking of the mechanism by which a tube works (a temperature gradient) but I can assure you that it is not controversial. Certainly not in the context of heatsinking.

w

[Sch3mat1c]

Tubelab is the only person here who would really know first hand if cooling has any effect. This would be because you have to get the plate red hot to make the cathode's temperature rise above limits.

At equal temperatures, I do not believe the leakages (i.e. emission) from anode to cathode equals that from the cathode. The reason is the cathode has a much lower work function. However, it must be true that no net power flow can arise in this situation, otherwise it would violate thermo. (A tube is, in fact, a low efficiency heat pump, as cathode emission is measurable, in the microamps / ~0.5V range. This is, for instance, why open-circuit grids drift to about -1V.)

Tim

[kavermei]

Originally Posted by wakibaki

Tubes work because of a temperature gradient between cathode and anode. If they were both at the same temperature, then, all other things being equal their emissivity would be the same. Granted, cathodes are coated, but this was not always the case.

Well, one could contemplate a tube with 2 identical filaments -- one being used as the cathode and one as the anode. Given enough potential difference (don't call it 'plate voltage' ) and some care (suppressor grid?) I'm sure a workable triode could be made...

[infobot]

Okay from tomchr's reply on was where I ran into problems-not with your explanations, but before posting. My guesses and newness had too many probably incorrect what if's when I tried reading some basics.

Before that, it just sounded more like you guys simplified for my newbie's sake, then the others just took me further.

So far, sounds like there's no "outside the range" where audio degrades. At least nothing appreciable, generally audible.

As to how this happened, amp seemed a little hot and I got jumpy about tube life back when I had few tubes and it was new. It's just a little S5electronics K-12 kit amp, well vented screen top case, but I had a fan so off I went.