Help, my amp hisses!
Every now and then I hear someone ask if the hiss in their tube amp is normal. The short answer is "yes".
I just checked the hiss on my VK using my trusty Radio Shack SPL meter. Here's what I found...
With volume on 10, fat switch on, guitar volume on 0, there's a noticeable hiss in a quiet room. I measured between 50 and 55 dB about a meter in front of the speakers. To put this in perspective, normal conversation at around 60 dB is noticeably louder.
At performance volume, which you'll get with the Vibro-King in the range of 3 to 10 on the volume (depending upon guitar, guitar volume, picking intensity, ...) the amp is going to be pushing somewhere between 105 and 115 dB, which is 50 to 60 dB above the noise floor, which is really good. In a decent recording, you won't notice the idle noise. And in a live performance situation, the amp's noise is going to be 20 to 30 dB below the crowd noise.
In other words, it's OK. Tube amps are just not meant to be listened to in a quiet room, cranked, with no input. If you want to crank the amp and not hear the hiss in a quiet room, you might want to try a Hot Plate dialed way down.
That said, some amps do hiss more than others. Hiss comes mostly from the first stage of the preamp, and is amplified by succeeding preamp stages. Therefore, you'll find more hiss in higher-gain amps. Again, there's nothing wrong with that - just play louder than the hiss and it won't be a problem at all.