I'm using E88CCs in a cascoded differential pair as the input stage of a fully-balanced power amplifier. Twin muting relays pull the signal ends of the grid stopper resistors to 0V until the valves have warmed up, and there is an initial time delay to allow the heaters to get up to temperature before the HT is turned on. During this time, the grid acts as the anode of a diode, and current flows from 0V through the muting relays, into the grids and down the tail resistor to the negative bias rail. This is not totally unexpected and shouldn't cause a problem because the grid current flow should cease as soon as the HT rail comes up. At least, that's the theory - and in most cases this is exactly what happens. However, I've found that with certain individual valves, grid current continues to flow for a while even after current starts flowing from the HT rail, and this means that there is an audible "thump" when the muting relays open.
I need to find a solution to this that doesn't involve testing and selecting valves that don't exhibit the problem. Has anyone else come across this phenomenon or got any ideas about how I could get around it?
I need to find a solution to this that doesn't involve testing and selecting valves that don't exhibit the problem. Has anyone else come across this phenomenon or got any ideas about how I could get around it?