Reader Question · Fox-Tango Era
Keying the Ameritron MLA-2500 with the Yaesu FR‑101/FL‑101 Twins
Do the twins need a keying interface to drive a vintage tube amplifier? Short answer: no — and here is why.
The Question
“Do I need a keying circuit to use my Yaesu FR‑101 and FL‑101 twins with an Ameritron MLA‑2500 amplifier?”
— Submitted by KB2IXT
Short Answer
No keying buffer is required. The FL‑101 keys external amplifiers with a genuine mechanical relay contact, and the MLA‑2500’s keying line is one of the gentlest of the vintage era — a low-voltage DC relay coil that simply wants a contact closure to ground. Wire an RCA cable from the FL‑101’s amplifier-control contact to the amplifier’s relay jack and you are on the air.
Why the MLA‑2500 Is an Easy Amplifier to Key
Interface boxes such as the Ameritron ARB‑704 exist to protect the fragile open-collector transistor keying found in modern solid-state transceivers from the hostile keying lines of older amplifiers. Some of those lines really are hostile: the Heathkit SB‑220 presents roughly 120 VDC to whatever closes it, and Yaesu’s own FL‑2100B carries a negative keying line that can spike toward −100 V at relay closure.
The MLA‑2500 (whether badged Dentron or the later Ameritron production) belongs to a friendlier family. Its transmit/receive switching is handled by a 12 VDC DPDT open-frame relay, and the rear-panel relay jack simply wants a closure to ground to energize that coil. The exciter’s keying contact sees low-voltage DC at tens of milliamps — a load so light that the MLA‑2500 rarely appears on any list of amplifiers requiring a buffered interface.
Amplifier |
Keying Line Character |
Buffer Needed with a Relay-Equipped Exciter? |
|---|---|---|
Ameritron / Dentron MLA‑2500 |
+12 VDC relay coil; closure to ground; tens of mA |
No |
Yaesu FL‑2100B |
Negative line (≈ −18 V nominal); can spike toward −100 V at closure |
No for the FL‑101’s relay contact; yes for most solid-state rigs |
Heathkit SB‑220 (unmodified) |
≈ 120 VDC on the keying line |
Marginal for relay contacts; soft-key mod or buffer strongly advised |
Why the FL‑101 Can Key It Directly
The FL‑101 is a product of the era when every serious transmitter was expected to control a linear. Its VOX/PTT logic drives relay transistor Q6 (a 2SC373), which pulls in the transmitter’s internal changeover relay — and a dry contact from that relay is brought out through the rear-panel accessory socket for exactly this purpose. This is the same arrangement Yaesu drew in the factory interconnection diagram for pairing the twins with the FL‑2100B, whose negative keying line is far harsher than anything the MLA‑2500 will ever present. A mechanical contact rated to survive the FL‑2100B will loaf along switching a 12 V relay coil.
Contrast this with a modern rig, where the amplifier keying duty falls to a small switching transistor that can be destroyed by voltage spikes or reverse polarity. The twins simply do not have that vulnerability — there is honest silver-plated metal between the amplifier and the transmitter’s electronics.
Hooking It Up
- Consult the FL‑2100B interconnection diagram in your FL‑101 manual and identify the amplifier relay contact and ground pins on the accessory socket. Wire those two pins to a shielded cable terminated in an RCA plug.
- Connect that cable to the MLA‑2500’s relay keying jack on the rear panel.
- RF path: FL‑101 antenna output to the amplifier’s RF input; amplifier output through a wattmeter to the antenna or dummy load. The FR‑101 receives through the amplifier’s changeover contacts as usual via the twins’ transceive interconnect.
- If you use ALC, run the amplifier’s ALC output back to the FL‑101’s ALC input — cheap insurance for those final tubes and a cleaner signal besides.
- Key the transmitter in a low-power tune condition first and confirm the amplifier’s relay follows PTT/VOX cleanly before applying full drive.
Three Cautions Before You Close the Switch
Verify the keying line with a meter first. The MLA‑2500 is one of the most modified amplifiers ever built — the original Eimac 8875 tubes became unobtainable decades ago, and countless examples have been converted to GI‑7B or 3CX800 finals, sometimes with reworked control circuits. Put a DMM on the relay jack before connecting anything. If you read approximately +12 VDC to ground, the keying circuit is stock and you are in business.
Watch your drive level. The manual’s ceiling is 120 watts of drive, and the FL‑101 at full output is close to that. If genuine 8875s are still in the sockets, remember their grids are famously unforgiving — back the drive off and let the amplifier do the work. It does not need full exciter output to reach its rated power.
No full break-in CW. The MLA‑2500’s open-frame relay is not fast enough for QSK, and hot-switching is precisely how 8875s fail. VOX or manual PTT on SSB, and semi-break-in with a generous delay on CW, is where this pairing belongs.
A Period-Correct Pairing
There is something satisfying about this particular lineup: the FR‑101/FL‑101 twins driving an MLA‑2500 is a thoroughly authentic mid‑1970s station, the kind that would have looked right at home in a 1976 QST photo spread. Thanks to KB2IXT for the question — and if you have a Fox‑Tango era question of your own, send it through the contact page. 73.