What tubes and how old is the amp?
I assume that VK75 is not a long time in production to undergo capacitor replacements for instance. The fact is that if it "eats" tubes especially one earlier than others than definitely something is wrong.
You may want to check bias if possible. Tube oscilates when it doesn't have negative-enough grid voltage. Check an output tube manual for allowed negative voltage tolerance or call Viktor to find out and measure between the ground binding post and pin#5 with amp on and no tubes using voltmeter set to measure low-hundreds of volts.
I assume that VK75 is not a long time in production to undergo capacitor replacements for instance. The fact is that if it "eats" tubes especially one earlier than others than definitely something is wrong.
You may want to check bias if possible. Tube oscilates when it doesn't have negative-enough grid voltage. Check an output tube manual for allowed negative voltage tolerance or call Viktor to find out and measure between the ground binding post and pin#5 with amp on and no tubes using voltmeter set to measure low-hundreds of volts.