Microphonics that go away by themselves

[jlsem]

Some friends and I were auditioning an expensive (but nameless ) pre-amp for the last three weeks and something happened that I am at a loss to explain. When first played as it arrived, with the stock tubes, it was horribly microphonic. One could literally stand next to it, talk in a normal voice, and hear what you were saying from the speakers. Tapping lightly on the chassis produced a loud ringing sound. Later that week the tubes were replaced with tubes that were selected for low microphonics (12AX7A, 5965, and 6080WB). This helped a little, but not much. Last night, the microphonic behavior was almost completely absent. The only difference in the listening sessions was that the pre-amp had been warming for about twenty hours rather than the usual seven or eight hours. What gives?

John

[7N7]

My 2 centimes: I would suspect that earlier the amplifier was oscillating; it is said that microphony can be a symptom of this.

Some small change in value somewhere as components warmed might have been enough.

7N7

[mgm31]

Yes that's right....

And some other thing is that "new" (NOS/NIB or not) tubes needs a few hours to burn in. For example, the mica parts that maintain in place the anode, cathode and so one have to move a little bit when turning hot / cold / hot /

Another way to explore is to have a look to the schematic.

If a SRPP or like, the filament power supply must be referenced to HT/4 minimum or tube might "ring" and be microphonics.
the other method is to use different half tube for each part of the SRPP.