50 W HF amplifier

English version

Christoph Schwärzler, OE1CGS

December 2018
Motivation The PCB as provided includes four other parts,
which will be needed later on for building the two
In February of 2018, I published some detailed transformers T1 and T2. These have to be cut from
instructions1 in German about a DIY amplifier based the PCB at the marked (figure 4) positions before
on a bargain Chinese kit. Since then I received a lot any soldering is done. These parts are the sides of
of feedback and comments. These included the the transformers. The have to be soldered to the
request for an English version of the instructions or respective metal tubes provided in the kit. Use a
parts thereof. soldering iron with 30 to 60 W and a wide chisel (5-
8 mm). Soldering flux helps a great deal. Two sides
With this document, I address the issue, but ask for and two corresponding tubes make up one
the reader’s forbearance that I don’t translate my transformer. Try to have the sides as parallel as
original instructions in full length. Rather I provide possible. It is advisable to wind the transformers
guidance for key issues and actions, which shall only after they are soldered to the PCB.
help prospective builders to avoid pitfalls and
succeed with their project. In doing so, I especially All parts are soldered to the top side of the PCB.
refer to the figures (“Bild”) depicted in my original For the relay this implies to bend the pins (which
document. are intended for through-the-hole mounting) 90°
sideways away from the body of the relay.
Accordingly, this document has to be read in
conjunction with my original document in German, Do not solder the two IRF530 transistors, R4 and
which is published on my website2. I strongly advise R5 at this stage. If you add R6 (see Annex A, very
to read the document at hand in its entire length advisable) don’t solder as well. These parts will be
before starting to build the amplifier. added only after the PCB and the transistors are
attached to their final positions on the heat sink.
Building the kit
The transformers T1 and T2 are classic style with a
The kit3 underlying this whole project is available primary and a secondary winding, where one side is
for less than 20 €. The PCB (Fig. 1) is of good center tapped. This center tapped side is not
quality and in a handful of shipments I had no wound with wire but is provided by the metal tubes
missing parts, sometimes there even are a couple soldered into the structure. For those style of cores
of parts in excess. Not included in the kit is the one turn is defined as going through one tube and
heat sink which is essential and strictly no powered returning through the other. Accordingly the metal
test may be performed without one. Choose your tubes count for 0.5 turns only (see schematics, Fig.
preferred model and vendor, but if you intend to 21).
use my 3D printed enclosure design4 (see below)
the heat sink must not be wider than 110 mm and
higher than 80 mm. The model which I use5 For T1 the other side is wound with 2 turns of the
provides a heat conductance of 1.5 K/W and I provided wire. Starting from one of the small solder
consider this adequate for the PA. I provide a pads to the left side of the transformer (check with
pattern (Fig. 3) for drilling the heat sink on my Fig. 6) the wire is run through the corresponding
website. tube, back through the other tube (1 turn), once
more through the first tube and finally back through
the second tube again (2 turns). Now the wire is cut
If not directly soldered to the PCB, one will also to length, the isolation removed and each end is
have to come up with plugs for the provided soldered to one of the pads.
sockets (PTT, optional: ventilator). I made mine by
cutting the fillets from some BKL plugs (Fig. 2).
T2, the larger of the two transformers is wound
very much the same, but with 3 turns and the wires
Soldering the parts is quite straightforward and soldered to the pads on the right side of the
takes about 2 to 3 hours of time. Please follow the transformer.
schematics in figure 21 and the traces on the PCB.
Yes, there are many SMT-parts but none of them is
very difficult to solder. There are some building The choke (“INDUCTOR” in the schematics, Fig. 21)
instructions on the internet including YouTube is built with two turns on a 2-hole ferrite core,
videos so I confine myself to those actions, which identical to the core of T1. There is a short length
are not obvious or covered somewhere else. of enameled copper wire provided in the kit, but I
prefer to use some stranded wire with a thicker
isolation. Check with Fig. 12.
After all parts (exceptions: IRF530 and R4 to R6) Now check the jumpers. JP1 must be set in order to
are soldered, the PCB is mounted directly to the bypass the filter-PCB. The PTT jumper is left open
heat sink using short M3 screws. Before tightening for the very first test. PTT is low-active and will later
the screws, the two rubber isolating pads for the be triggered by the transceiver.
transistors have to be put in place, holes centered
with the drilled holes for the transistors.
Apply power. There should be no significant current
drawn. Now set the PTT jumper. You should hear
Next prepare the two IRF530. They have to be the relays beeing activated and the current should
handled with some care, as they are ESD sensitive. read some tens of mA.
So try to keep them, yourself and the soldering iron
on the same potential, for example by frequent
Next adjust the bias for the transistors. This voltage
grounding. Ovoid the mistake I made with my first
is best measured between the large soldering pad
kit, when I bent the pins of the transistors down to
to the left of T1 (see Fig. 6) and GND. It is adjusted
PCB level immediately after they left the body. This
with the trimmer VR1 and should be set to a value
way they crossed other traces with different
of about 2.7 V. The current drawn by the amplifier
potential on the PCB with little to no distance – not
should now read about 20 to 25 mA. Check the
good! I advise the following procedure: Keep the
transistors, R4, R5 and R6. None of them should
pins straight until they are over the beginning of the
get hot, even after a couple of minutes.
respective soldering pad, then 90° down and back
90° to the original direction at the PCB level for the
length of the pad. Cut at the end of the pad. This After completing the idle test successfully several
should be achieved by bending down (which is the useful tests can be performed, depending on your
metallic side of the body) 90° approx. 1 mm after interest and equipment. One of those is measuring
the thickened part of the pins end and back 90° to the input impedance which should be better than
the original direction already 1 mm further on. 2:1 on all bands from 40 m to 15 m and only
Always bend all three pins together with suitable slightly higher on 80 m if you added R6.
pliers.
For checking the amplifier at work a suitable input
Insert the white isolators in the holes of the source and a power supply capable of delivering
transistors and tighten them to the heat sink while about 8 A at 13.6 V is necessary. Also keep in mind
aligning the pins to the pads on the PCB. If that a dummy load, attached to the HF-output and
everything is in place, cut the wires at the end of capable of absorbing up to 50 W is strictly
the pads and solder them well to the PCB. necessary now. It is highly recommended to use a
pure sine signal, 50 Ω impedance and adjustable
power from about 0.5 to 2 W for input. Frequency
Only now solder the feedback resistors R4 and R5
is not very important, but starting in the 20 m band
and the impedance matching resistor R6 (see
might be a good idea. If there is no suitable test
Annex A). Solder the power leads and the HF-Input
source available, most transceivers in “Tune” mode
(“IN”) and HF-Output (“OUT”) connectors. For the
should do. Start with 0.5 or 1 W and frequently
later I prefer to solder short pieces of RG 316 (see
check the current to the amplifier as well as the
Fig. 5) as I will use them to connect the filter-PCB
temperature of the transistors and the resistors R4
later on anyway.
and R5. If you do have the equipment (e.g.
Oscilloscope) monitor the output, but be prepared
The kit is now finished. Before performing the to see some non-sinus wave forms.
smoke test, check thoroughly for shorts and
perform a detailed visual inspection.
Low Pass Filters

The following checks are best done with a current- The amplifier without any suppression of harmonics
limited supply at 12V and set to a maximum does not qualify for operation under most legal
current of 2 A. If no current limitation is available systems (like FCC Part 97). It has to be followed by
insert a high power 10 Ω resistor in the power line (a) suitable Low Pass Filter(s), before any signal
and monitor the current. may be radiated by an antenna.

Terminate HF-out with an adequate dummy load In chapter 3 (“Aufbau der Tiefpassfilter”) I present
(better save than sorry) and leave HF-in open or, suitable 7-element Chebyshev Filters for operating
better shorten it. from 40 m to 20 m. I designed a PCB for 2 filters
including the relays to switch those filters in and
out (Fig. 8). The filter PCB has the same dimension Measured data
as the amplifier PCB and can thus be stacked on
top and mounted with the same screws (M3 x 35 Chapter 6 (“Betriebsparameter”) deals with some
mm). It runs on the same 12 V power supply as the measured data for the finished PA.
amplifier and is controlled manually by a simple 2-
way switch. In case more filters are needed, two or
Table 1 shows the most important parameters,
more of the filter PCB can be stacked on top of
measured with 2 W of input signal power and a
each other. Connect either “A” or “B” to the “+” pin
supply voltage of 12,8 V. The gain is between 12
in order to select the respective filter. If none is
and 13 dB and the efficiency something between
connected the HF-signal bypasses both filters.
40% and 45%.

The PCBs are double sided for through-the-hole

Harmonics suppression (Table 2) is (with the
soldering of the parts. The exact parts values and a
possible exception of the 3rd harmonics for the 20m
schematic (Fig. 7) for 40 m/30m and 20 m filters
band, where the antenna and/or tuner get a job)
are given in the original document. As are the
sufficient for interference-free transmissions.
number of turns on Amidon T68-6 cores to wind the
required inductors. The capacitors should be high
quality like mica-capacitors. From my website The handling of the amplifier is rather
Gerber-files for the PCB can be downloaded or straightforward. Connect HF-in (from the
alternatively I provide pdf-files for etching them transmitter), HF-out (to the antenna), PTT (from the
according to the toner-transfer or some other transmitter, typically some ACC port which has to
method. be active low) and power. Select the filter
according to the band you want to work and power
on.
Measured and published data for the filter
characteristics confirm that the low pass filters
successfully suppress harmonics and keep the The PA must be run into a SWR not significantly
impedance at 50 Ω. exceeding 2:1.

Enclosure At the end of my original document I present some

modifications and future ideas:
A bespoke enclosure for the amplifier and one filter
PCB was designed (Fig. 11). The 3D model is
Annex A
available for downloading for private use4. Such an
enclosure can be printed on any 3D printer. The
Measurements and comparisons to comparable
design offers very convenient mounting and
amplifiers imply that there should be a resistor of
servicing of the electronics, especially since all
about 18 to 22 Ω and capable of withstanding at
connections are fitted on the back plate (Fig. 13).
least 1 W soldered at the position “R6” (Fig. 20).
However in the kits I built there was never such a
A 3.5 mm audio plug is connected to the PTT on the resistor included, so I had to buy this one
amplifier, so T/R switching can be controlled by the separately.
transmitter via a standard audio cable.
It is only with this resistor present, that the imput
The enclosure includes protective clamps for the impedance of the PA gets 2:1 or better. So this
connections on back side and spacers between modification is highly recommended.
amplifier and filters. M3 nuts on the inside are used
together with screws to tighten the back plate with
Annex B
all the electronics to the protective hood. See
figures 12 to 14 for instructions how to put the
Sometimes the IRF530 included in the kits seem to
parts together.
be of minor quality. You might want to exchange
them with original parts bought from your preferred
There is on ON/OFF switch situated at the back as dealer.
well as fuse holder (use 10A fuses). A switch with
ON/OFF/ON positions can be mounted on the
Annex C
front side (drill/cut a hole according to your part at
hand) to activate the suitable filters for the bands in
Replace the IRF530s by one power MOSFET like
use.
the MRF186. The amplifier PCB is ready for this,
but the values of R4 to R6 probably have to be speaking countries, so they might not apply for
modified and the choke changed to 1 turn. your area.

I have not done so until now, but this modification All in all this PA can be built for slightly less than 50
is top of my list and I will report on my website as €, where a significant part of the cost has to be
soon as I have built and tested such a modification. attributed to the ferrite cores and the mica
I expect this to bring the PA significantly closer to capacitors. Maybe there is the most potential to
50 W (maybe at some slightly higher supply reduce cost even further.
voltage). The downside is, that a MRF186 will cost
at least about 10 €, which – in respect to the
Art.Nr. Art. Nr.
original kit – significantly increases the cost. Conrad Reichelt
Bezeichnung
DIY Kits 70W - -
Annex D Heat sink V 7331G
Resistor 22 Ω/2W 2W METALL
1474539
This annex deals with using more than one filter 22
RG-316 coax cable 1179842 -
PCB. Basically these PCBs can be stacked and
Switch ON/OFF WIPPE
serially wired. Thus it is easy to include more bands 1587515
1801.1146
and potentially automate the switching process.
Switch ON/OFF/ON WIPPE
Anybody step forward? 1587513
1808.1103
Fuse holder 1587496 PL 126001
Annex E Fuse 1576515 TR 10A
Strain relief KAZU 42
3,5 mm stereo-connector LUM KLB 4
In this annex some ideas for modifying the
BNC-connector 1564888 UG 1094U
connectors and including status information are
PL-connector 1579410 SO 239 SH
presented. PCB for LPF www.aisler.net
www.itead.cc
It might be a good idea to have a LED on the front Amidon-cores T68-6 (6x) T 68-6
panel indicating power on and another one showing Enameled copper wire KUPFER
605604
0,8 mm 0,8mm
when the amplifier is activated (TX). For the later
Relay 12V,
pin 1 of the relay on the amplifier PCB could be 629500 HFD2 12V
2 states (2x)
used, together with a series resistor of about 760 Diode, 1N4148 (3x) 564851 1N 4148
Ω. Connector 2 pol. PS 25/2O W
Connector 2 pol. PS 25/2U W
Annex F Connector 3 pol. PS 25/3O W
Connector 3 pol. PS 25/3U W
Mica-capacitor CY 22-2 xx
This is a bill of material (“Materialliste”) for all parts
different values CY 22-3 xx
used in the PA. It is also available in extended form
for downloading from my website6. However the
quoted sources serve predominantly the German

1 http://www.oe1cgs.at/wp-content/uploads/2018/02/50W-HF-PA.pdf
2 http://www.oe1cgs.at/50w-hf-verstaerker/
3 Many sources. I bought from and can recommend: https://www.aliexpress.com/item/DIY-Kits-70W-SSB-
linear-HF-Power-Amplifier-For-YAESU-FT-817-KX3-Ham-Radio/32807365037.html
4 https://www.thingiverse.com/thing:2768290
5 https://www.reichelt.de/profilkuehlkoerper-75x100x40mm-1-5k-w-v-7331g-p22279.html
6 http://www.oe1cgs.at/downloads/