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In Reply to: RE: Don't bother.... posted by MLin on December 17, 2023 at 07:18:22
One correction, with the help of my wife, I confirm only the one KenRad VT-231 on the right channel is microphonic.
Follow Ups:
Any 6SN7, but it seems especially some of the best vintage ones, like the KenRad, is apt to be somewhat microphonic. I use ring dampers on the linestage voltage amplifier tubes in my MP1, for what it's worth. The 6SN7s in the circlotron output stage are less likely to be the problem, so far in my experience. Also, place the MP1 such that it is not likely to be "rattled" by the speaker output SPLs.
I just tried two more KenRad 6SN7GT tubes; one is also microphonic but fortunately one is not. I left the good one in the MP-1 for now. So, between the 4 KenRad NOS 6SN7 tubes, half of them are microphonic. Very disappointing.
Maybe I had been lucky, I've used many NOS 6SN7 tubes. Everything from 5692, VT231, GT, GTB, GTA and these two are the first microphonic ones. They are so bad, the noise is clearly audible 6 feet away from the speaker when all I did was turning the volume knob or lightly tap the pre-amp chassis, not even tapping the tubes.
It makes me wonder if it's the tube socket?
I have a few but have not used them for many years. I don't recall their being particularly microphonic. They are more robustly constructed, internally, compared to other 6SN7s, so I might have thought they would be less likely to ring. Where are you placing your preamplifier in relation to the speakers, and is the preamplifier well isolated from other sources of air borne and directly transmitted vibrations? All of that can make some tubes that are "mildly" microphonic, useable. I agree with Ralph that tube dampers don't help much, but the O-rings that I use seem to help at least somewhat. Currently running RCAs with the gray glass; I think VT231s.
Thanks Ralph and Lew. I can confirm that it's the tube that's microphonic and has nothing to do with the tube socket. It was just my wishful thinking that maybe I did not insert them firmly and tightly into the socket that contributed to the ringing noise.
The MP-1 is well isolated and has the full damping upgrade from the factory. These two KenRad tubes (one 6SN7GT, one VT231) are just badly microphonic - turning the Shallco volume pot was enough to make them ring. They do have very nice sound though so I am thinking about putting them in the V2 position of the MA-1.
Agree with the 5692 tubes being robustly constructed and not likely to ring. I actually remember reading somewhere they were made for military purposes and intended to be used in equipments situated at artillery base.
I've had that happen, where the clicking of the attenuator is enough to set off the tube. In the MP1,the pot is located close to the R channel linestage input gain tube, which makes for a problem in the case where the tube is microphonic to begin with. But in some cases, the tube stops ringing once you're set the gain to suit your taste.
Microphonic tubes do. Damping rings do very little to address this issue too. So the best place for such a tube is in an amplifier where there is less gain downstream.
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