What is the device called that goes between Amp and key output on the xmtr?

Discussion in 'General Technical Questions and Answers' started by KI5OMM, Jun 30, 2021.

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  1. KI5OMM

    KI5OMM Platinum Subscriber Platinum Subscriber QRZ Page

    I don't know hardly anything about amps but I am trying to learn. I know you have to protect your transceiver from the key line voltage from the amp. I understand there is a device that regulates this but I do not know anything about it.

    Could someone kindly explain this to me? I want to purchase an amp (possibly older, used) and I don't want to damage my rig.
  2. WA2EIO

    WA2EIO XML Subscriber QRZ Page

    Older amps (tube type) were keyed by a higher voltage than modern ones. ( in some cases up to around 150VDC or so.) The older rigs usually used a relay to key the amp, so the higher voltage wasn't a problem. Newer rigs usually use a transistor or IC and they can not handle these high voltages, so an 'amplifier key line interface' or 'buffer' is needed. I have no interest in any of thes untis, but here is some info:




    73, Ron WA2EIO
    Last edited: Jun 30, 2021
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Before investing in "anything" to buffer or isolate your rig from an amplifier, it really pays to know exactly what the rig is and exactly what the amplifier is.

    Most amps manufactured in the past 30 years use low-voltage, low current keying and absolutely "nothing" is required to use them with most modern transceivers.

    Some older amps use higher voltage keying, or even negative polarity keying, that is incompatible with solid state key lines in modern transceivers -- but I'd recommend "modifying the amplifier" to change that, to make the amp compatible with "anything," rather than adding an external Band-Aid as an isolation accessory. Modifying an old amp for low-voltage, positive polarity keying is generally not difficult and a lot of "old" amps have already had that done by a previous owner.

    Find out exactly what the hardware is before doing anything.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    If the amp is like an old Heathkit, or a Collins 30L1, then here is a circuit:

    Most modern Amps don't need any interface, just a cable.
  5. KI5OMM

    KI5OMM Platinum Subscriber Platinum Subscriber QRZ Page

    That's good advice. I don't have my eye on a specific amp yet so I can't really say what I will end up with. My rig is an older Yaesu ft-900. I'm going to spend some money on a hexbeam first anyway before making the jump to an amp. Thanks all for the advice and tips!
    WN1MB likes this.
  6. W5RKL

    W5RKL Ham Member QRZ Page


    The Yaesu FT-900 is an excellent transceiver to drive most types of linear amplifiers without requiring a relay adapter between
    the FT-900 and the amplifier relay circuit.

    I don't see where you stated you have the FT-900 user manual and have read it. The FT-900 user manual's pages 10, 11, and 12 cover
    connecting various types of amplifiers to the FT-900. Page 12 states the maximum AC and DC voltage and current ratings of the FT-900's
    internal relay switching circuit.

    If you don't have the FT-900 user manual, you can download it from my website by clicking the link below:


    Go to page 10 and study the connections to various "types" of amplifiers. Page 11 shows the various connectors and the wiring
    connections to each. Read and study page 12 titled "Connecting a linear amplifier" so you understand what is required to connect
    various types of amplifiers to the FT-900.

    For any amplifier you choose, you should spend time reading the amplifier's manual BEFORE you attempt to connect and operate the
    amplifier to/with the FT-900. If you have any questions about topics in the manual you don't understand, ask for help in understanding the
    topic you are having trouble with.

    Mike W5RKL
  7. KA5IPF

    KA5IPF Ham Member QRZ Page

    Good choice, always spend money to maximize the antenna first.
    WN1MB likes this.
  8. W9WQA

    W9WQA Ham Member QRZ Page

    jjfwiw,,,you can get a low volt 50 ma relay that will switch line volts at 10 amps!!
    can be direct driven by most rigs keying line. ez fix
  9. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page

  10. KI5OMM

    KI5OMM Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks for your link to the manual. I do have the factory manual and I have browsed those pages that you mentioned. I figured it would be wise to ask some folks who know what I need to connect to a Heathkit amp or some other older tube type amp as that's what I am leaning towards at the moment. I like old iron and so I will probably go in that direction at first.

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