What Do Bad Power Tubes Sound Like?

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I was super-excited to get my new Blackstar Stage 60 earlier this month.  But immediately after plugging in and firing it up for the first time, I noticed some strange noise issues. I plugged the guitar in directly, with no other pedals and no effect loop.  On all 3 channels, I heard some very obvious gungy badness on certain notes.  The problem was not evident when listening through the speaker emulated output. 

I made a couple recordings (mic’d with a Sennheiser e609 into Scarlet 6i6 recorded into Sound Forge on the Mac).  You can hear me stepping through the channels, switching on reverb, etc.  Some notes sound fine, and others clearly exhibit the noise. Here’s a couple example recordings:

 

Ruby EL34 Power TubesMy first thought was that it was a mechanical cab vibration, but it’s clearly coming through the speakers, and gets louder as I turn up the master volume.

My second guess was bad power tubes (when vibrated at cab resonant frequencies) so, I experimented with it a bit more. If I damped the vibration of the tubes with my hand, the strange noises were reduced but not eliminated. (ouch- these things get hot!)

To confirm, I borrowed a pair of JJ KT77’s from a buddy’s Marshall amp, and when I swapped those into the Blackstar, the problem was eliminated.

The tubes in the Blackstar Stage 60 are Ruby EL34BHT, labeled IMG matched #1341, PC 44, TC 3300.

When I sent all this info to Blackstar tech support at Korg USA, they promptly sent me a pair of new Svetlana EL34’s, and the problem is now gone.

It’s been smooth sailing ever since. And this amp is righteous!