Mystery of the whistling amplifier


I replaced the manufacturer-supplied low-power tubes (two 12AU7 in the pre-amp section; four 12AT7 in the amp section) in my VTL integrated amp with NOS Telefunkens from Vintage Tube Services. After a few months I started hearing hissing and whistling through the speakers. It happens regardless of the source (DAC or the phono preamp).  I read here and elsewhere that this was probably caused by about-to-fail preamp tubes. Here're the things I've tried to address this:

1.   I took the 12AU7s out and switched them between their sockets. I also reseated them in their original positions. These measures made the problem go away for a while. Then it came back.

2.  I put the manufacturer-supplied tubes (JJ Elec ECC81) back in, and that "solved" the problem for a day or two; then it didn't.

3.  A Youtube video recommended scraping the pins shiny with an x-Acto knife blade. I did this, and it worked for a while. Doing it again, perhaps more thoroughly, also worked for a while, and seems to have reduced the overall level of the sound. I also hit the pins with Deoxit after scraping.

I listen to this setup several hours a day. Sometimes, like right now, there's no unwanted noise. When it is happening, you can't hear it through loud passages at all. But of course I want to fix this.

I'd welcome any suggestions or diagnoses before bringing this to the dealer.


john_g
I thought more along the lines of applied physics that geoffkait indulges in.


Cute discussion starts on page 2. (from paragraph 2.2)

http://www.physics.princeton.edu/~mcdonald/examples/tirepump.pdf
I'd buy another set of tubes and try them.  If they don't fix it, no worries, tubes don't last forever.   This sounds like it will be a trial and error fix since a lot of things can cause this.   Cracked solder joints can do this, so a reflow of the affected circuity is in order.  Check out the capacitors in the local area, as those are suspect given the symptoms.   Let's call that Plan A, Plan B is to take it to a reputable dealer or, better yet, back to the factory for a fix.