How hot can/should the tubes get?

[Paul Marossy]

I'll agree with Doug on this one. It's just three little preamp tubes...
On my Firefly, the back is open and the front has about 65% free area underneath the grille cloth, which allows free communication of air thru the enclosure.

Just to clarify: On an amp with six preamp tubes, four power tubes, a rectifier tube, and transformers, things get quite hot under the chassis! So, I think a fan is a good idea.

[puretube]

3 thoughts:

A combo-speaker is way "louder" than a fan ("microphonically" speaking);

A speaker moves about the same amount of air as a fan, and does so proportional to the output/dissipated power;

An upside down amp ("hanging tubes") is the worsest construction for a power-amp: the innards (above the tubes) should get the cooling;

[Ammscray]

DON'T cool tubes with a fan, tubes like to be hot!

I must respectfully disagree. A common thread in all the manufacturing data I've ever seen is that tube envelope temperature is a first order predictor of short life - the higher the glass temperature, the shorter the life.

Obviously, if you never turn them on, they last forever, but the trick here is to get the tubes to live long enough to be useful in actual operation. I think we would all be disappointed in re-tubing every gig - or every ten minutes, which is a distinct possibility with no airflow.

High temperatures boil off any residual gas adsorbed in the envelope, contribute to gas current, and can lead to thermal runaway in otherwise good tubes. High gas current tubes don't sound particularly good to me, but there could be someone who likes the tone.

The temperature of the glass is not necessarily related to the internal working temperature too directly. Between the plates and glass is a pretty high vacuum. The plates get rid of heat only by radiation. The change of absorption/reflection of IR at the glass surface changes trivially with temperature. It is true that if you get the glass up near melting, it will re-radiate and further raise the plate temperature, but the tube is in a bad way then. But the glass temperature is not a big effect on what goes on between the cathode and plate. That's what good vacuums are for.

Like was said though, make sure the bias is right with no cherry-plating or anything but putting a fan directly on the tubes of an AC30 or older Marshall can affect the sound...I've heard it myself...there's been some great threads on this over at the plexi palace forums

Everything in a tube amp affects the sound. The most common comment that my amp-tech friend here in Austin gets is "My amp burned out during a gig last night. It really sounded great just before it blew out. Can you fix it so it sounds like that all the time?" I have a personal momento of that - my tech friend saved me an EL34 that got hot enough to melt the glass and the vacuum actually sucked in a dent in the glass before it vented.

If you believe that you can hear a difference in tone depending on the temperature of the outside glass, there is no particular reason for a fan blowing directly on the tubes - what you do need, though, is some amount of air flow over the tubes. The amount of flow isn't large, which is why I mentioned two ways to get slow flow with available fans. Sure, fan vibration in the chassis might be transmitted as microphonics or modulation of the signal by mechanical movement of grids relative to cathodes and plates - but you oughta not mount fans so they act as shakers anyway. A smooth, slight, quiet flow of air is what you want.

I won't quibble about whether you or anyone else can hear a difference in tone with fan cooling - there are people who will swear that they can hear the difference in sound depending on the oxygen content of the copper wire carrying sound signals, and the metallic composition of the solder in joints the signal goes through. They might even be right. One quite-famous Austinite swears that he can hear the brand of his effects batteries, which direction his guitar cords are plugged in and the order in which the screws were put in the back of his speaker cabinet. Maybe he can.

I can tell you that I have tested it both ways in a blind test with a few pretty critical musicians and no one has ever been able to do better than 50/50 at just guessing whether they were using an amp with a fan or without unless they could actually hear the fan running. If there was no fan noise, they were 50/50 on whether the tone was better or worse for both fan and no fan.

It *was* funny that I got 100% good guesses when I had a fan running out of sight and *outside of the amp(s) in test*. I loved that one.

Right, I realize all that, I'm not even going to get into who hears what...I'm basing my statements on my own experience, not on what's supposed to be, or could or should be...let me clarify: I've been playing my old Marshalls and Oranges, both with plate voltages above 500, and my Voxes without fans for almost 28 years live and in the studio in pro situations...

I've never blown an amp, tube, or 25 watt greenback speaker for that matter...just lucky? I don't think so, I push 'em very hard...it's my observation that it's operator error that makes amp problems (or most anything else) happen, NOT how hot the amp gets...

If your amp is running properly all the way around, whether you set it up or somebody qualified did, you shouldn't be scared to play it the way it was meant to be, playing music is supposed to be fun without worrying all the time...

I think sometimes people get too carried away with worry about all the things you're NOT supposed to do just because it says it in the book, instead of having a good time...I like to have a good time, and not worry (about anything) when I'm doing it...

 just take good care of your equipment and you shouldn't have too many problems, that's the best you can do

Oh I forgot to mention, on the "amp sounds the best right before it blew up" theory...I'll never believe that one, an amp sounds it's best when properly set up with the right tubes and biased, etc, etc...there's no magic to making an amp sound unreal, just know-how

[Paul Marossy]

"An upside down amp ("hanging tubes") is the worst construction for a power-amp: the innards (above the tubes) should get the cooling"

Agreed. That would be great if you could design it that way right from the start. But, when you get a vintage amp, you are stuck with the aforementioned scenario of a hanging tube garden. So, IMO, some kind of cooling via increased airflow across the tube is better than nothing...

[puretube]

fully agreed, Paul: this is a case of not cooling the tubes, but getting the tube-created heat away from the rest...

[petemoore]

I think one reason Mfr's don't use fans more is because of the maintanence, cost and they are vibratey and make noise.
 If you have tubes sockets into PCBs, I strongly recommend a cooling fan. This Will increase the life of the amp.
 I've run Marshalls and seen them run for years [of course many others], and Im' not inclined to put a fan in there.
 I don't like the idea of something vibrating an amp except  the speaker cabinet, which is purported to be part of the sound in combo's as well as stacks.
 I've seen cooling fins, take a piece of copper or other thin conductive long rectangular sheet, slice fins off of one long side so it looks like a comb, bend the unsliced part [about 1/2''] to 90 degrees from the slices, and stwrap this around the tube, repeat as you like. You may have seen this on furnace flew pipes [and in hardware stores] for increased heat recovery from the exhaust flow pipe...ugly/effective.
  Cool Tube: Clean.
 A very clean surface transfers thermal energy much more effectienly...for those of you with motorbikes, you can extend the life of your air cooled valves by keeping the top of your motor pristine clean.
 When the ambient temperature is closer to that of the tube surface, the amount of thermal transfer [even with the same amount of flow], and the amount of flow, both are decreased...enter switched fan theories...less maintanence, cleaning etc.
 I could be running tubes hotter than optimal even in the Marshall, but I can touch for a second only without burning my finger, the base is cooler, I like that I have experience with using Tubes, if things get 'that' hot there may be trouble.
 I DO have a fan in the Dynakit, even though I point to pointed over the PCB around the tube socket [the tube fried the PCB traces...not that uncommon], I still like having the fan in there...I had years of misbiasing/bias mods/running hot/drifting bias...it was designed for lower voltage of the era...late 50's, I should have dropped the input voltage down to match...live and learn!!!
 If heat seems like it could be a problem, a cooling fan is by far the easiest way to go to get running temperature down some.

[Ammscray]

I think one reason Mfr's don't use fans more is because of the maintanence, cost and they are vibratey and make noise.
 If you have tubes sockets into PCBs, I strongly recommend a cooling fan. This Will increase the life of the amp.
 I've run Marshalls and seen them run for years [of course many others], and Im' not inclined to put a fan in there.
 I don't like the idea of something vibrating an amp except  the speaker cabinet, which is purported to be part of the sound in combo's as well as stacks.
 I've seen cooling fins, take a piece of copper or other thin conductive long rectangular sheet, slice fins off of one long side so it looks like a comb, bend the unsliced part [about 1/2''] to 90 degrees from the slices, and stwrap this around the tube, repeat as you like. You may have seen this on furnace flew pipes [and in hardware stores] for increased heat recovery from the exhaust flow pipe...ugly/effective.
  Cool Tube: Clean.
 A very clean surface transfers thermal energy much more effectienly...for those of you with motorbikes, you can extend the life of your air cooled valves by keeping the top of your motor pristine clean.
 When the ambient temperature is closer to that of the tube surface, the amount of thermal transfer [even with the same amount of flow], and the amount of flow, both are decreased...enter switched fan theories...less maintanence, cleaning etc.
 I could be running tubes hotter than optimal even in the Marshall, but I can touch for a second only without burning my finger, the base is cooler, I like that I have experience with using Tubes, if things get 'that' hot there may be trouble.
 I DO have a fan in the Dynakit, even though I point to pointed over the PCB around the tube socket [the tube fried the PCB traces...not that uncommon], I still like having the fan in there...I had years of misbiasing/bias mods/running hot/drifting bias...it was designed for lower voltage of the era...late 50's, I should have dropped the input voltage down to match...live and learn!!!
 If heat seems like it could be a problem, a cooling fan is by far the easiest way to go to get running temperature down some.

Some good info there Pete thanks

I do keep my tubes very clean and fingerprint-free with alcohol  (not the drinkin' kind) and I do believe that makes a difference...

And, I had the same experiences with you with my tube hi-fi (lots of tube stuff in this house!) it's a whole different ballgame using 50's and 60's tube audio nowadays...well worth the effort and the very expensive tubes, without that gear I probably wouldn't listen to much music I'm so spoiled with those damned tubes

no I don't run a fan on the hi-fi stuff it does get alot hotter than the guitar amps but I haven't had any problems, in again 35 years+ of listening on tube hi-fi...I'm not condoning NOT using a fan there to cool the back area, it definitely is a good idea in lots of cases but in mine it's on all day and I've been running the same quad of 7591's for 10 years now! Boy do I love the sound of that tube (please somebody make a good one?)

[Paul Marossy]

Regarding the tube cooler thing, that is a good idea, but it can't be just any old unfinished metal. I had a few discussions with Bill Perkins from PEARL about his tube coolers. The surface contacting the tube glass envelope should be as dark as possible, otherwise, you will create something of a "radiant energy trap" that is quite likely to increase rather than decrease bulb temperature.

This is due to several reasons.

1. With DIY tube coolers, there is most likely going to be a very small area of the tube cooler that is actually in contact with the glass envelope compared to the PEARL tube cooler.

2. Second, using shiny metal like aluminum, decreases the emissivity (in this case, emissivity being the ratio of the radiation emitted by a surface to the radiation emitted by a blackbody at the same temperature) . Shiny sheet aluminum has an emissivity on the order of about 0.05 compared to the nearly 0.95 emissivity of the the almost black body PEARL tube cooler. The shiny aluminum surface reflects some of the heat energy back to the source, and this results in a lesser rate of heat being radiated away the glass tube envelope, probably increasing the temperature inside the tube. The dark surface of the Pearl tube cooler absorbs more of the heat so that it is radiated away from the tube more effectively than a shiny surface.

So, if you should decide to make some tube coolers, I would suggest painting the surface in contact with the tube with a hi-temp black spray paint. Blackened copper would probably work OK, too.

If you want to read more about these PEARL tube coolers, follow this link:
http://www.pearl-hifi.com/06_Lit_Archive/01_Audio_Notes/PEARL_Tube_Coolers.pdf