[samplesj]

How much cool down is generally used when people are comparing two tubes.

If its not to hot to handle can it be switched within a minute or so of killing the power (giving caps time to discharge)?

I'm mainly asking about signal tubes, but what about rectifiers and power tubes?

With some tubes I've switched its been obvious, but I've got a couple I'd like to do faster switching with so I can better compare.

Jeremy

[anthony a.]

for signal tubes, i usually wait a minute for them to cool down and then disconnect power cable from source and remove the tubes using a soft cloth.  the cloth is not necessary, but yo should give them a quick touch to see if they have cooled down, and if they have, you could just take them out with your hands.

[jeffreybehr]

I wait just a few seconds before pulling front-end tubes and maybe 15 seconds for power tubes.  Temperature doesn't matter if you use an OveGlove, but you'll need a safe place to place the hot tubes while not damaging anything.  I use a cookie-cooling rack from the kitchen.

I NEVER disconnect powercables before pulling tubes.

[Jon L]

This is all amp and tube dependent.  I wait at least 10-15 seconds before I swap tubes, but some tubes really need to be in the upright position all the time, i.e. Svetlana 300B, which has filaments that tend to warp under heat and cause shorts. 

Some dictate that tubes need to be cooled down before swapping, but heck, we're trying to do A-B comparisons here.

[lcrim]

This is a good topic and I've already gone to school on it.  Jeffrey's point about having a place to put the hot tubes after changing out has often occurred to me too late.  Jon's wisdom about not turning certain tubes on their side is valid as well. 
I do mess about with signal, preamp and phono section tubes pretty often and they aren't nearly as hot as power tubes for the most part.  I shut off the unit but I don't unplug it and wait for the sound to die, which usually indicates that the caps have discharged.  Swap out the one or ones  I'm after and turn things back on.  Aural memory is fleeting but I still believe that you can move quickly enough to make some conclusions even if it takes a few times.

[samplesj]

Thanks for the replies so far.  This matches what I did with the JJs and Sovteks I've got on hand, but now that I've got a few "nicer" tubes I wanted to make sure I'm not damaging them.

I wrapped my squeezebox cleaning cloth around them to get them while they were hot, but the silcone glove/mit is an even better idea.  Thanks.  The cookie rack is also interesting since I don't have to worry about rolling or scortching something.

Like Jon L said we're trying to A-B here so the 15 minute cool downs I saw suggested elsewhere is pretty much a show stopper.

The JJ 12ax7 vs Sovtek 12ax7 was easy to hear.  However even though this may cost me my membership in the golden ears club, I'm not hearing a DRASTIC difference between a short plate mullard 12ax7 and JJ gold pin 12ax7 in my HagTech Chime.  Of course these are more recent vintage short plates and not the older long plates with dual getters so maybe it would be more noticable there.  I believe that for a few of the test tracks I used the Mullard had more body (several violin pieces I though I heard much more), but I want to switch a bit faster to check.

I've got some long plate Mullards and Brimars coming so maybe its just this newer short plate Mullard isn't in the same league.

[richidoo]

Recifiers tubes should not be turned back on while they are hot. They usually arc and that is not good for them     

If your tube amp has diode rectification, then only the heat and tube stress issues apply. The sound of the amp will change when it is fully warmed up, heat soaked and stable. Sometimes the subtle difference between two input tubes might be overshadowed by the non-stabilized heat, if you can hear that well. In the long run, comparing immediately AB is not productive. You gotta listen to your favoirite music over at least an hour so your brain adjusts to a tube sound. Then switch to the other and listen to the same stuff for an hour. Notice any subtle subjective differences you hear in all tha passages of music, and write them down. Then go back to the original tube and see if the differences you wrote recur. It can take a while, but is more accurate on subtle things than quick AB. Because it 's music, not a test signal. Music varies so much, you gotta hear the part on lots of your fav music before you really know if it is preferable overall.
Have fun!
Rich