You really only need a external relay with older amps than have higher keying voltages like a SB200, SB220, L4B etc that have not been modified.
That isn't always true.
Older amplifiers generally do not have back-pulse cancelling diodes. When the relay line opens, voltage can spike to 50-100 volts or more. While this Dentron is a 12 V relay, Dentron never looked at voltages closely. The 12V supply comes from a 12V winding that is a capacitor input, and actual voltage is around 18 volts. Current is around 175-250 mA, depending on the TX light used. About 125-150 mA is the relay coil, which is rated at 100 mA but is operated over the rated voltage. The rest is amp current.
While the steady relay voltage and steady current is safe for his particular rig, the back-pulse from the Dentron when the relay opens exceeds 50 volts. Oddly, the backpulse voltage from Dentron's also varies with the particular lamp used in the transmit indicator! With no bulb, the peak voltage can be 80-100 volts. With a high current bulb, it is much less.
This probably will not hurt the Yaesu transistor, but it is only rated at 60 volts. The Dentron has no diode across the relay coil, and one should be added. $.10 spent in the Dentron can save wear and tear on the keying transistor.
We should all remember many radios use small reed relays or small transistors to key amplifiers, and just because an amplifier says it has 12 volts doesn't mean it is always + 12 volts with no nasty spikes. :-)
