Look at the
KT66 data sheet.
At the top of the sheet, it says the maximum diameter of the tube is 53mm. At the bottom of page 3, the Installation section says if the tube is run at max ratings, the tubes should be spaced 9cm (90mm) center-to-center. So GEC spaced these 30w tubes' centers at about 1.7 times the envelope diameter. That leaves 37mm (almost 1.5") between adjacent tubes.
If you space the tubes too closely, air convection can't carry away heat fast enough, and everything gets hotter in that area. The generally-quoted rule is 1.5 times the envelope width, center-to-center. For high-dissipation tubes, it's wise to increase the spacing further.
Obviously, if you're using 12w tubes (maybe the stock EL84's that were on your chassis?), the lesser heat means closer tube spacing is allowable. And EL84's might be spaced yet closer than 6V6's because the narrower envelope means more free air between the tubes than if 6V6's were installed in the same position on the chassis. An
EL84 data sheet says the envelope has a diameter of ~22mm; with a 1.7-times-diameter spacing, the centers only need to be ~37.75mm (~1.5") apart, center-to-center.
If you lose cooling efficiency due to tight tube spacing, you may need to replace cooling efficiency some other way. Maybe a fan will help. I've never tried heatsinking a tube, though that's theoretically an option.
You may be able to cheat spacing
a little by ensuring the cool sides of the envelope face each other. The hottest parts of the anode are often where the seams in the plate are, because extra material here can be formed into fins to radiate a little more heat. These will be the areas where the grid support rods
are not, so orienting each tube so the grid support rods are facing each other places the cool sides of the tubes closest together. See the picture below for an example.
Topic: Power tube spacing (Read 2961 times)