Is there a strategy in preamp tube placement?

Questions to you guys with the Ken Rad with microphonic. How do you guys deal with this issue? or do you simply just live with it? Thanks.

Yes, I can confirm Ken-Rads tend to be the most microphonic and can have come-and-go noise issues. Vibration control tweaks, tube dampers - these have no effect on such a problematic tube. The only "solution" with such a tube is to move it further downstream. The more upstream it’s positioned, the more gain it has stacked after it, amplifying all that noise crap (or in the case of microphonics, making a stronger feedback loop!). Having high sensitivity speakers acts like extra downstream gain, and exacerbates the issue. The only exception is a tube right before a volume control, which acts like a large negative gain in most systems, but it’s rare to find a slot like that (maybe cathode follower output in a tube phono stage).

With Ken-Rads, I’m only consistently able to use them in power amps. Almost never in preamps, no matter the slot. And DEFINITELY not gain slots in the preamp. A preamp slot in an integrated amp doesn’t count as an "amp slot".

@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.