Tube noise mystery, continued

In this discussion a while back, people offered me many helpful suggestions as to how to deal with the whistling and hissing noises a VTL integrated amplifier was making. The short answer was that restoring the manufacturer-supplied JJ Elec tubes in the amp and preamp sections made the noises go away, leaving me with six fairly expensive NOS Telefunken tubes on the shelf. This was puzzling, since Andy at Vintage Tube Services had told me that old tubes could be expected to last longer than modern ones.

This February the amp started making new and scarier noises:  loud cracks and pops, sometimes after it was turned off. So I took it into my dealer, Stereo Unlimited in San Diego. They hooked it up to a couple of speakers and heard the same thing. Bob called Bea Lam at VTL and described the problem; she said to send it up to them. So I left it there and couple of weeks later got the call that it was ready. The problem had been a bad power tube (EL34), which was replaced. The only charge to me was shipping back and forth to Chico, about $90. I was given plots of full- and low-power frequency response and noise levels, all of which looked perfect.

I must say I think this is pretty extraordinary service and support, from both VTL and Stereo Unlimited, on a three-year-old unit.

But of course I got curious about the NOS tubes. Would they now work without the whistling and hissing? I took off the case and tried first the 12AU7 tubes in the pre-amp. No extra noise. A couple of days later I put the Telefunken 12AT7's in the amp. It's been fine ever since, and sound great. All's well that ends well!