Soda Jerk

I had the same problem. Is the head second hand? I replaced the PCB beneath the valves with a handwired Point to point circuitry. Contemporary valve heads tend to be really cheaply put together when it comes to the transistors and the cheap, shitty chinese PCB's they use. Handwiring it yourself will cost you nothing more than 30 and a Sunday afternoon. It will make it much more resilliant and give it a boost of clarity in your tone.