Historical clue formulations

• Vacuum tube type — 2021-09-05
• Amplifier of radio signals — 2019-11-24
• Old vacuum tube — 2018-10-14
• Certain vacuum tube — 2018-08-05
• Old TV component — 2013-07-28
• Old vacuum tube — 2011-08-04
• Old TV part — 2007-03-11
• Certain vacuum tube — 2006-07-17
• Vacuum tube — 2005-01-02
• Old TV part — 2004-12-16
• Television tube — 2003-06-13
• Certain electron tube — 2001-08-02
• Old radio tube — 1999-04-10
• Vacuum tube with three elements — 1999-04-07
• Certain tube — 1995-08-11
• Vacuum tube type — 1995-02-05
• Vacuum tube — 1994-02-28