Universal Heater with a Relay - variant 1

This circuit allows you to exchange vacuum tubes from the "12A*7" family with the tubes of the "ECC88" family without rewiring the heating of a B9a tube socket. Pinouts of both tube families are identical, except for heating. The PCB is mounted vertically, between pins 4 and 5 of the tube socket, and is attached to pin 9 on the other side of the socket. The result is that you can, say, unplug a 12AT7 tube from the socket and plug an ECC88 instead, to compare their sounds.