Minimum Tube Spacing: 6V6 & 12AX7

[minions]

Does anyone know, or know where to find, information on the minimum tube spacing recommendations for 6V6s and/or 12AX7s?

Thanks.

[sumgai]

minnie,

Actually, I've never seen such specs. I guess in the old days, we just knew that the tubes of each type could be so close as to not quite touch, but further apart was better for getting rid of heat. It's a matter of having the room to work (assembling the thing) versus keeping the unit from becoming too heavy. Back then, most everything was built with a steel chassis. The compromise taken by most manufacturers turned out to work quite well, in terms of no interaction between the tubes, and the heat was easily dissipated. Albeit, compared to today's gear, yesteryear's stuff is obscenely heavy.

Too, in older gear, tubes weren't designed/used to develop so much power as we find today. Back then, power tubes were not so likely to interfere with the pre-amp tubes. Today, the power is nearly doubled from back then, and interference is much more likely. Space between is the usual solution, which correlates nicely with heat dissipation. However, recall that many amps use "tube shields" over their pre-amp tubes for just this reason. In many of those amps, removing the shield(s) might be the very cause of microphonic feedback at higher gain levels. For early examples, look up images of Fisher, Dynaco, Macintosh, etc. Quite compact, I'm sure you'll agree.

I've also seen plates of bare metal mounted vertically to the chassis between the pre- and power amp tubes. Same purpose as the individual shields, just a different way to solve a potential problem. (Or a real problem, if the designer took the easy way out.)

Are you intending to build an amp from scratch?




sumgai

[minions]

I heard on another forum that the spacing should be at least 1.5 times the size of the tube's envelope. Also, someone said to move the power tubes a bit farther away from the PT. Does it cause any problems, or maybe not problems, but setbacks in tone if the PT and OT are too close together? The reason I ask is that moving them close together would be moving DC and AC currents side by side, which is one reason why PCBs don't sound as good as hand wired.

Thanks.