Is there a strategy in preamp tube placement?

Yeah largely sized tube elements combined with lots of gain is a challenge. It’s also hard for those flimsy mica spacers to tightly hold down the grid/plates/cathode spatial relationships enough versus the gain factor, especially after all these decades. Ken Rads in particular often have a nasty little audible internal rattle you can excite with a tap to the glass (this is with the tube in your hand, not plugged in). Those tubes are REALLY gonna give you a headache in preamps.

Even in 12AX7 the short plate variants are sometimes thought to be less susceptible to microphonics. 12BH7 as a 12AU7 sub is another example where the larger plate structure (12BH7) leads to big headaches with microphony.

Later versions of 6SN7 had better bracing and plate designs to mitigate these issues.

Thanks @lowrider57, I actually did purchase the Ken Rad from Brent Jessee.  Great guy to deal with.   My first set of RCA was purchased from him also.   I was going to hear from him if he's able to replace those KR that I sent back with something low noise and non-microphonic, but I haven't heard back yet.   He actually never mentioned any extra fee for noise and microphonic testing.   I wouldn't mind paying the extra if he can do that.

Anyway, reading some of the comments here, maybe I should just forget about the KR for my preamp.  The RCA black plates that I have are very robust.  No noise or microphonic, and sound pretty good.  Maybe I should just stick with those for now.  

@xcool 

The noise testing service is somewhere on the Brent Jessee site, $10/pair I think. You have to request it in your email to him.

With 6SN7s from the 40's or 50s, I always order noise testing. In Brent's description he may say a certain tube is low-noise, then it's not necessary to pay an extra fee. As @mulveling stated, later generations of tubes used different internal designs to mitigate microphonics. Some military-grade tubes are "ruggedized" with extra support rods and micas to withstand shock and vibration. With 6SN7s, there's a chance these tubes may be less microphonic.

Thanks @lowrider57 for the information on the fee.

I'm still fairly new in tube rolling, and learning a lot from this thread.  Thanks!

These days I’d also be leery of counterfeits of tubes that are now 70 and more years old. The best sounding KRs are the black glass variant. You can’t see what you’re buying.