WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I had a similar problem for a project. Here's the solution I devised. How you'd actually implement it would depend on the delay you need. To increase it, add more decade counters. That way, you wouldn't need to worry about quiet/silent intervals in the program material.