Amateur Radio

Amplifier keying compatibility guide

Will your amplifier damage your transceiver? Keying interface requirements for tube amplifiers with modern radios.

Amplifier keying compatibility guide
Keying line by amplifier
AmplifierKey lineVerdict
* marks a figure taken from the manufacturer’s own documentation that nobody outside the manufacturer has verified — no measurement, no user test, no independent source. It is what the maker says, not what anyone has confirmed, and manufacturers do publish wrong keying specs. Figures without an asterisk have been corroborated by a source with a different origin: a measured community report, a technical article, or a published keying-interface chart.
Radios appear in the picker once their keying-output rating has been captured from the manual. Data capture is ongoing; anything uncertain reads as VERIFY, never as safe.

The keying hazard

Older tube amplifiers (Heathkit SB-200/SB-220, Collins 30L-1, Drake L-4B) use cathode (grid-block) keying that places high voltage (-100V or more) on the keying line. Connecting this directly to a modern solid-state transceiver's keying output will destroy the keying transistor instantly.

This is the single most expensive mistake operators make when adding a vintage amplifier to a modern station. The repair is typically $200-400 at a service shop.

Which amplifiers are safe?

Safe — relay-to-ground keying (connect directly)

These amplifiers use a simple relay closure to ground. Safe to connect directly to any modern transceiver:

  • Ameritron AL-80B, AL-811H, AL-1200, AL-1500
  • ACOM 1000, ACOM 2000A
  • Elecraft KPA500, KPA1500
  • Tokyo Hy-Power HL series
  • Most amplifiers manufactured after 1990

DANGEROUS — cathode/grid-block keying (requires interface)

These amplifiers place high voltage on the keying line. REQUIRES a keying interface:

  • Heathkit SB-200, SB-220
  • Collins 30L-1
  • Drake L-4B
  • Henry 2K, 2K-4
  • Dentron MLA-2500 (check specific version)
  • Most amplifiers manufactured before 1985 with vacuum tube PA stages

Keying interfaces

A keying interface is a small circuit (relay or optocoupler) that isolates the high voltage on the amplifier's keying line from the transceiver's low-voltage keying output. Options include:

  • Harbach Electronics keying interface — purpose-built for the SB-200/SB-220 ($30-40)
  • W7RY keying interface — general-purpose optocoupler design
  • Homebrew — an optocoupler or reed relay circuit; schematics widely available

How to check

If you are unsure whether your amplifier uses grid-block or relay keying:

  1. With the amplifier OFF and unplugged, measure the voltage on the keying jack with a multimeter
  2. If you read zero volts, it is likely relay-to-ground (safe)
  3. If you read any negative voltage, it is grid-block keying (DANGEROUS — use an interface)
  4. When in doubt, use a keying interface. A $30 interface is cheaper than a $300 repair.