The Heathkit TT-1 is Heathkit's flagship dynamic tube tester — a true mutual-conductance (Gm) tester rather than the simpler emission-style testers of the era. Roll-chart and pin-selector switches set up each tube with its correct plate / screen / bias voltages; the meter reads transconductance directly in µmhos. Also performs gas tests, shorts tests, and a life-test cathode-emission check. Built as a two-piece — main tester + separate roll-chart drawer.
Mutual conductance (Gm) is the figure of merit for active tube performance — emission testers (like the [Supreme 385](../Supreme%20Instruments%20385%20Automatic%20Tester/CLAUDE.md)) measure only whether the cathode is still emitting and don't catch a tube whose Gm has fallen below useful. The TT-1 catches weak tubes that pass emission. Common service issues on this unit: the roll-chart drive mechanism, the high-voltage selenium rectifier (replace with silicon + series resistor for proper voltage drop), the switch-bank contacts, and the meter zero adjust. The TT-1A is a slightly later revision with minor circuit improvements.
A multi-volume deep dive on this instrument is in planning. When the first volume lands in the source project's 02-inputs/volume_sources/, this page upgrades automatically — no website code change required.