Since the AVC is such an extremely high Z circuit, it also makes a big difference on what the condition of the 6BF6 is ...especially gas. You need to get a tube with "lowest gas" not just measuring "good" on the gas test. Also since the 6BF6 tubes are hard to find, a 6AV6 works too ...and ismuch better in my opinion. 73 Kees K5BCQ