Is there a strategy in preamp tube placement?

@xcool

I own two pairs of Ken Rad VT-231 black glass and a pair of clear glass (1945). When purchasing these tubes or others from the 40’s, it’s imperative that the dealer offers testing for noise and microphonics. Andy from VTS, Kevin Deal, Brent Jessee (for a fee) all noise-test their tubes. But there’s no way to tell if a tube will become microphonic at some point in its lifetime. All three pairs of my KR’s have remained noise free. But one of my beloved Tung-Sol BGRP’s went microphonic after 3 years. I've been using these tubes in gain stages of my preamp and amp.

There are many causes of noise from a vacuum tube and microphonics is only one of them. You can test for the other kinds prior to purchase but it is hard to predict whether a tube will be microphonic or not in your home system, because circuitry, cabinet damping and/or direct tube damping, proximity to speakers, typical listening levels in terms of SPLs, socket design, etc, all contribute to the problem or to its prevention.  That said, some tube types are just more prone to the problem than others. In the case of octal base triodes, like the 6SN7, 6SL7, etc, those are just more likely to be microphonic.  Some say this is due to the relatively large size of the glass envelope and the fact that the plate structures are more or less prone to vibrate in the large volume environment.

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.