10-20 watt out amp 1 or 2 Watts in

Discussion in 'Amateur Radio Amplifiers' started by K8TDN, May 30, 2019.

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  1. K8TDN

    K8TDN Ham Member QRZ Page

    Looking for user recommended amps can be assembled or kit. I have seen some on eBay but w would like someone that is using one to give some input. I don't have lots of test equipment to adjust the amount but could research and see what I need. I just want to boost my 1 to 2 watt rig a little.
    Thanks I advance

    Terry Nichols K8TDN
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    What frequency range do you need to cover, is this a VHF/UHF 2 watt rig or an HF QRP rig?

    There are definitely some kits and finished designs for 10dB gain linear amps with output power limits in the range you're talking about but any decent linear amp will work as well. A full legal limit linear will remain linear and amplify a 1 watt or 2 watt signal to deliver 10-15 dB of gain or so. That's the whole point of 'linear' when discussing amps, they may have a high maximum power rating but the faithfully reproduce low level signals without distortion as well, it's essential for SSB use. That said, a small limited power, likely solid state, amp will be smaller and take up less space on our operating desk if you really don't need higher power.
  3. K8TDN

    K8TDN Ham Member QRZ Page

    This is for HF
  4. KX4OM

    KX4OM Ham Member QRZ Page

    Take a look at the 10W amp by QRP Labs:


    It was extensively tested by Allison, KB1GMX before the kit went commercial. It has 26 dB gain, so it would require a 50 ohm resistive attenuator pad to be used with a 1 or 2 watt output rig. The 50 ohm pad is beneficial in other ways, as it establishes a well-defined input impedance. An attenuator is very easy to build; 3 resistors. There are calculators on the web. Allison's comment in the assembly manual:

    "With sufficient drive, the amplifier is capable of 20W output at 13.8V supply with +15 dBm input, over the range 3-30 MHz"

    Ted, KX4OM
    Last edited: May 31, 2019
    K8TDN likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's a very good deal.

    The heatsink alone without the amp is worth that much.:)
  6. KX4OM

    KX4OM Ham Member QRZ Page

  7. K8TDN

    K8TDN Ham Member QRZ Page

    When you say attenuator should I build to say a 5w level instead of 2w to be safe. I am trying to find the diy section to build.
  8. K8TDN

    K8TDN Ham Member QRZ Page

    I just want to know what test equipment will I need to assemble and test the kit after assembly to be sure it is working ? I don't have o-scope
  9. KX4OM

    KX4OM Ham Member QRZ Page

    The output power level of the transmitter needs to be known for two reasons. One is to calculate the attenuation needed. There is a 3dB difference between 2 watts out and 4 watts out at the normal 50 ohms impedance or your rig, for example. That factors into the calculation for the resistance values in ohms of the three attenuator resistors.

    Also, it matters from the resistors' wattage rating, as they will be dissipating the extra power as heat. These must be resistors that don't have any inductance. In the past, 2W carbon composition resistors were available, but they are pretty rare these days, and even 1/4 watt carbon comps are hard to find. What I use are metal film resistors for 1/8 to 1/2 watt applications, and metal oxide resistors for 1W, 2W, and to a lesser extent 5W, when they can be found. The availability of the different values is less as the wattage is greater. All of the available resistors are cheap from Mouser or Digi-Key. The shipping will be more than the cost of the resistors, even with USPS Economy shipping. It's always good to buy extras for different attenuation pad possibilities. For example, if you need attenuation to go from 2 watts to the 15dBm (0.0316W) needed by the amp as drive, that is 33 dBm - 15 dBm, or 18 dB of attenuation. That is a plug-in number to the website calculation for the needed resistor values. If you need to attenuate 4 watts from your transmitter, twice the power as before, that is 21 dB of attenuation needed. You can see that doubling the power is a 3dB increase. Note that "dB" is a ratio, while dBm is an absolute power value. 1 dBm in a 50 ohm system is equal to 1 milliwatt, rounding off the decimal places. So, 3 dBm is 2 milliwatts, and 10 dBm is 10 milliwatts. Thus a 1 dBm output to a 10 dBm output is a 10 dB change. Play with an online dBm to watt calculator.

    You may likely need to parallel some or all of the three resistors to get close to the values needed for the attenuator. For these purposes, standard value resistors are okay.The resistors in a Pi configuration will have the first and third legs the same value and the top of the Pi resistor will be a different value. The 1st leg and the top resistor dissipate most of the power. So, you might be paralleling two 1W or 2W resistors for 1 and 2, and probably even a 1/4W one for the number 3. There are calculators online for paralleling resistor values to get an exact (or close) value.

    You can get a value for the power output of your rig by building a very simple RF voltage probe and using a 50 ohm dummy load. Check the web or use a handbook for the circuit for the probe, and how to use it.

    Ted, KX4OM
    K8TDN likes this.
  10. KX4OM

    KX4OM Ham Member QRZ Page

    For a CW rig, an oscilloscope is not strictly required. If you are building a kit like the one I mentioned, which is a pretty sophisticated linear amplifier, the linearity of the SSB in particular needs to be looked at. For CW, a linear amplifier is not required. In that kit, CW will be very accurately reproduced, but a simpler amplifier that operates in Class C will work. There are a lot of amplifiers in that category that can work fine. Diz, W8DIZ used to sell one, but it has been discontinued. For reference, you can see the circuit at:


    Another very popular amp was designed by WA2EBY. This website has information:


    Also, the amplifier board used with the Bitx transceiver project is simple. There is a lot of information on that one on the web.

    Ted, KX4OM

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