45W RF SSB CW AMP (EBAY) DIY Build = 55W Upgrade

This is a guide to build & improve this $14 DIY Ebay kit as the OEM docs (also incl. here) are borderline Chinglish.

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This project does the OEM build, discovers some poor biasing and improves the overall outcome making the Amp more suitable for QRP use with a more sensible input range.
The improvements will permit more power and better spectral purity, but the Amp seems to 'like' the 20 meter band viz. purity with about a 20 dBc down on the 3rd harmonic, 27 dBc down on the 5th. A LPF is required to meet 43dBc down Tx harmonic specifications.

I've also prepared an LTspice simulation (see the files link) using common models as a schematic capture of the circuit with the optimum part values which match the supplied PCBs silkscreen for a better build. The supplier includes excess SMT parts (which makes this variant possible) in the kit; excepting the 2n2222 transistor I ended up using for the new input stage.

I suspect the excess parts are to 'cover' all the different PCB variants of which there are several, albeit I got no 680pF capacitor, but 2 x 200pF SMT units inste

Okay, without  getting too detailed:

You ideally need an Oscilloscope good to 100Mhz with 10: 1 probes and an FFT display in math. Also a good 50W, 50Ω load for testing the power output. A DMM is expected as is a variable amplitude & variable freq. CW signal source.

To build the better Amp (the hacked one) use the LTspice schematic in the files link to match component values to the supplied PCB silk screen parts. Note the BOM text file as well.

Note the logs below detailing an alternative build mod#2 that improves the purity somewhat esp. for 40 meters. THE Mod#2 LTspice file carries all the details there.

Now I HAD to swap a 2n2222 in for the input transistor as i accidentally killed the OEM unit while experimenting. 9V across the base-emitter can do that 8(. Note the 'dead bug' installation of the 2n2222 in the pictures

You can try it out without swapping the input Tranny Q3, it should be fine as is. Your input gain beta might be a bit higher though.

Also note the wire jumper installation replacing the T1 transformer and the 470Ω through hole resistor used to 'jumper' the Q3 base to SMT R4. That through hole resistor can be substituted by a 510Ω OEM SMT part if u solder wire leads to it for mounting.

This allows you to try both emitter and source follower  Q3 configurations by just moving the wire jumper from the collector to the emitter.  I found either worked similarly. Note the R10 feedback is removed altogether ( becomes the 510 Ω thru hole DC bias resistor) as is the C16 HF gain compensation and the T1 transformer.

A couple DC bias & feedback values are changed for the 2SC1971 Q2 in order to improve its spectral purity. This draws around 0.6A of current at 14.5V and heats that transistor , so be sure the 4-40 hardware, insulating washer and insulating pad are installed firmly to the heatsink. A 120mm Fan (80CFM) mounted on the H.sink fins  is required for max power use over more than 5 mins.

The L1 RFC, T2 and T3 transformers are wound as follows:

1) T2 = 7:3 turns ratio (pri: sec) of the thinner magnet wire supplied. 4:1 inductance ratio

2) T3 = center tapped 3 to 7 turns (pri:sec) of the heavy magnet wire. 1:4 inductance ratio

3) L1 RFC is 9 to 10 turns of the heavy magnet wire.

Having a current and voltage controlled linear PSU on hand is helpful when first powering up to prevent over current damage due to improper assembly errors.

The J9 solder pad jumper on the bottom needs to be shorted in order to use a single  12-15Vsupply.

I'd suggest setting the supply current limit to 1A if that is available to you. Otherwise a 10 Ohm 3 to 5W power resistor in series with the supply might be in order. You can test the voltage drop across it should not exceed 6V when 1st powering up. If that is good you can remove it and apply direct power. Otherwise check your build for shorts or reverse polarity on the transistors or 1Kuf electrolytic.

Calibration Discussion:

Be sure the Transistors are well mounted and insulated from the heat sink before doing this step.

The key item here is RV1 & RV2 need to bias the MOSFET gates to  Vth which is about 3.35V for the IRF530 supplied.  Anymore causes current drain & heating. I have observed that once Vth is reached the current drain goes up.

RV1 should be turned fully COUNTER clockwise before starting, RV2 should be fully  clockwise before starting so that the Gate to ground resistance for Each MOSFET is a few ohms, measure with your ohmmeter b4 applying power.

Apply power.

You can CAREFULLY bias each FET slowly using RV1 & then RV2 watching the current drain on a 0-10A setting of your DMM. Once it goes up (from the  quiescent half amp or so)  by 20 or 30mA per MOSFET you can stop raising the gate voltage. If you go too fast you can spike the current thru the FET and blow it with a non current limited supply. A reasonable current limit is around 3A here for safety. Eventually expect to draw a bit more than 5A at full...

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Modified BOM to match PCB

plain - 3.17 kB - 08/28/2017 at 15:56


hack mod power output.jpg

Power output across 50Ω load at 5Vpp in, 14Mhz. 14.5V, 5.4A

JPEG Image - 880.46 kB - 08/28/2017 at 04:34


Hack & load test.jpg

New input stage & rebiased Push pull driver with rewound T2

JPEG Image - 892.26 kB - 08/28/2017 at 04:33


Mod input hack.jpg

New input stage

JPEG Image - 959.69 kB - 08/28/2017 at 04:33


OEM testing.jpg

Power applied

JPEG Image - 844.21 kB - 08/28/2017 at 04:33


View all 17 files

  • 1 × See BOM text file in files section

  • Improving 40 meter purity

    mosaicmerc10/17/2017 at 05:03 0 comments

    I did a significant mod #2 , note the LTspice.asc file containing all the changes (annotated as text)  in the

    files link.

    In effect altering the front end attenuation to take up to almost 1W input to drive the 50W out, as well as rebiasing the  input transistor to a pure emitter follower that provides DC bias directly to the secondary transistor amplifier as well as the superimposed  CW signal.

    A secondary advantage is a single negative feedback loop encompasses both the first and second transistor stages due to maintaining the phase with the emitter follower input tranny. So better purity is assured across both stages with one loop and no capacitive coupling.

    40 Meter  purity is improved and the overall component count is significantly reduced.

    Wideband limitations: I believe the winding ratio of the T3 needs to be the same but turns must be added for the primary to  have a minimum impedance that won't  cause supply rail ripple at the lower HF  (sub 12Mhz or so) . There's no room in the binocular core to test this out.

    LPF is essential for clean CW.  Some of the bands are so distorted that it causes the BIRD 43 to read significantly off the Vpp to wattage calculation.

    Note the Comparison purity  of MOD#1 vs Mod#2 below:

  • Power boosted to 55W

    mosaicmerc09/21/2017 at 01:25 0 comments

    With the temperature comp. in place I did a max sustainable  power test into a 50Ω load.

    55W is ok @ 20 Meters driven by 14.5Vcc with about a 4Vpp input signal. A higher than 4Vpp input overdrives the early gain blocks and adds more harmonics.

    This is CW btw.

    That's a 22% power boost for little cost. No doubt the EBAY sellers would love to advertise a 55W Amp for $14 . I wouldn't be surprised that they figure this out from this project and make the cheap mods to do it and sell more value for the same price. Wouldn't you buy a 55W temp. compensated module over a 45W module for the same $$?

  • Thermal Runaway Limiting

    mosaicmerc08/30/2017 at 23:24 0 comments

    For continuous use temperature compensation on the output transistors is required to prevent thermal runaway.

    This was achieved with a pair of SMT 47K, 3960 Beta NTC thermistors direct soldered to the source pins for ground and thermal reference. The thermistors were jumpered per the image to the RV1 and RV2 joints with L6 and L7.

    The Results with no cooling fan @ 27C ambient.

    After about 30 mins to hit steady state:

    1. DC bias to the gates reduced by 110mV (thermistors at work)
    2. Overall current draw dropped from 4.69A @ 14.5V to 4.59A
    3. Overall RF out dropped from 45W 'cold' to 43.5W steady state
    4. Output transistor TO-220 TAB temperature held at 92C peak.

    Note the images below for assembly and thermal performance.

  • Calibrating my RF meter and some Input vs Output Amp results

    mosaicmerc08/28/2017 at 17:58 0 comments

    When testing the output of the Amp into the 50Ω load, I discovered my Coaxial Dynamics 100W, 2-30Mhz Slug for my Bird 43 was reading 30% hi.

    This is how to dismantle a stock bird slug:

    BUT..a Coaxial dynamics Slug leaves NO room to leverage the top metal disc off. I had to drill the edge of the knurled grip & label disc with a 2.5mm PCB bit and then use a small flat  'trimmer' screw driver to raise it after heating it with my hot air gun to 300C, the glue was quite soft/tacky by then.

    Then removing a hex nut gave full access to the trimmer pot.

    Now my 100W slug can be tuned to 100W or 50W just with the trimmer pot!

    I set it to 50W as that is best for the power range I am working with right now.

    So as an FYI using a 14Mhz signal @14.5Vcc. Wattage measure via the Bird 2-30Mhz 50W slug.

    10:1 scope probe, scope to 100Mhz bandwidth.

    3.75Vpp =  50W out as per the Bird 43.(138Vpp on the Oscope, 50 Ω load), 4.82A draw

    3.22 Vpp = 40W out as per the Bird 43.(126Vpp on the Oscope, 50 Ω load), 4.43A draw

    2.74Vpp = 30W out as per the Bird 43. (111Vpp on the Oscope, 50 Ω load) , 3.93A draw

    2.23Vpp = 20W out (Bird 43)  (92Vpp on the Oscope, 50 Ω load) , 3.31A draw

    1.42Vpp= 10W out (Bird 43) ( 68Vpp on the Oscope, 50 Ω load) , 2.52A draw

    1.12Vpp=5W out (50.4Vpp on the Oscope, 50 Ω load) , 1.96A draw

    Pls note the 3rd harmonic is about 20dBc down. Even harmonics are reduced  as this is a push pull amp.

View all 4 project logs

  • 1
    SMT parts

    Install all the SMT parts first as per the BOM file here, do not place C16 and there is no C33 supplied.  Do this by dressing one pad of every component with a bit of solder, then holding the part with a tweezer on the pads, reflow the solder joint. then apply solder to the other pad(s) of the part. You will have excess SMT parts. You can replace Q3 with a 2n2222 as I did, optionally.

  • 2
    Fit the TO-220 Transistors, Q1, Q2, Q4- note pics in file section

    Bend the transistors' leads up 90° just before they thin down...use a bird beak pliers to stabilize the pins near the case to do this.  Bending at the thin part of the lead is about a mm off. Place the PCB on the Heat sink and mark the holes left for Q1,Q2 & Q4.  Then Place Q1,Q2 & Q4 Tabs on these marks, leads up. Fit the PCB down so the leads pass thru the pad holes and place 1/4" (6mm) spacers at either side of the PCB as per the picture in the files section. Tack solder ONLY the left pin of each transistor so that its TAB  hole is easily accessible from the top of the PCB. Then mark these tab 3mm hole centers with a sharpie for a drill (2.5mm, or 3/64") and 4-40 tap.

  • 3
    Heat sink Prep (6"x6"x6mm base suggested) fins should be about 1.5" or 20mm deep.

    Drill (2.5mm or 3/64") and tap (4-40 or 3mm) the screw holes. Obtain 1/2" pan head screws to match.  Place the PCB with the
    transistors on the heat sink and align the TO-220 TAB holes to the tapped heat-sink holes. Melt the solder joint on the transistor to make any adjustments. Install the 3 screws and remelt the joint again to stress relieve. Now solder all the remaining transistor leads.

View all 6 instructions

Enjoy this project?



Mike wrote 08/22/2019 at 15:36 point

Hello mosaicmerc

New to this site and my first posting. Can someone confirm the value of R12 in the input attenuator. Diagram shows 5R6 but if this is a 3db attenuator at 50 Ohms my calcs say it should be nearer 18 Ohms.

  Are you sure? yes | no

james21170 wrote 09/16/2018 at 18:22 point

Rudolf, I've simulated it in LTspice too and can't get a Peak to Peak any higher than about 20v Peak to Peak.

I'm obviously doing the same thing wrong that you are. 

Did you get it working in the simulator or the actual amp itself.?

The only difference on my schematic was to use two resistors instead of the potentiometers like yourself.

I did make a pot schematic in LTspice but whenever i use it for this model it refuses to work.

Also, i couldn't find any information about the ZTX849/2SC1971 to add it to the LTspice program.

Could this one transistor make such a massive difference?

I've done the mod on the PCB but again I can't get anymore than about 30v Peak to Peak on the scope when connected to a 50 ohm load.

Regards, James.

  Are you sure? yes | no

reuterr wrote 04/20/2018 at 17:43 point

Hello mosaicmerc,

I have tried to LTspice17 (and LTspice4) simulate your file "45W CW SSB EBAY DIYmod2.asc".

First I replaced the 2 potentiometers with 2x 2k4 resistors to make it work.

At the 50 Ohm output I got 22 Vpp = 1.3 W instead of 90 Vpp = 20 W, seen in yor file

LTSpice_Simulation.png.  And 50 W = 142 Vpp at 50 Ohm. See at

What I am doing wrong?

Regards, Rudolf DL5FA

  Are you sure? yes | no

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