Stewart, Hi. All the FTS tubes made for use in the 5061A/B have the LF coil. Also the tubes removed from FTS 4050 and 4060 etc have the same coil and can be retrofit with adaptors into the 5061A/B. The frequencies are the same. I have a scratch built tester that performs the LF test and allows monitoring and adjusting the tubes temperature and other parameters. Can't see it in the PIX but it has a cradle on the top to hold the tube. Cheers, Corby