I begun to chase DX also on the low bands and realized that five hundred
watts is not really enough. The years using a TenTec solid state amplifier had
made me really like automatic band switching so to buy a manually tuned bigger
amplifier was not an option. Commercially built auto-tune amplifiers are good but they are
expensive. For example, an ACOM 2000A costs about 5500 euros and
you can get a decent car with that amount of money. Thus, I built a new
Picture 1: Generic view to the tank circuit.
I chose the basic concept that was introduced in the local amateur radio
magazine called Radioamatööri (in issue 11/1994). The amplifier is based on
the Russian GU-84 tube. That article suggested using nowadays hard-to-find
stablizer tubes for screen power supply. Solid state is better for this task
anyway, so I chose to make the screen and control
grid power supplies with the G3SEK
circuit, instead. That gives protection to many fault
This article describes the only thing that really differentiates an auto-tune
amplifier from a manually tuned amplifier, namely the tank circuit. The pictures
show only the main principles of each module. Sorry for the poor quality of some
didn't scan the hand-written papers properly. If one wants to make a similar
type of an amplifier, one needs to clarify many details oneself. For an
experienced builder, that should not really cause too much problems.
Continuous Or Steppwise Tuning?
There are two basic ways to automate a tank circuit: continous and steppwise
tuning. The former uses capacitors or coils that are motor-driven. Surplus part
suppliers sell both but the prices are high. A motor-driven vacuum capacitor can cost as
much as thousand dollars. Alternatively, one can use an ordinary capacitor and make the
mechanics oneself, though that is a bit difficult for an ordinary amateur. The
advantage of continuous tuning is that one can fine-tune the tank circuit to
precisely correct values.
Steppwise tuning can be done by combining coil and capacitor values using relays.
The advantage is that the mechanical design is simple and tuning is fast. When one
changes the frequency band the amplifier is tuned right away instead of, say,
some dozen seconds. The disadvantage is that one may not be able to fine-tune the
amplifier for all possible situations due to gaps in the possible value
steps one can take. This can be caused by choosing incorrect component values
right at the start or due to some component values being different to the
(otherwise good) plans.
After a lot of thinking I chose the steppwise option.
I did consider also if I should combine the above-mentioned principles. There
are commercial amplifiers that have separate tune and load variable capacitors
for each band, and then the correct one is chosen with releys. The problem in this approach is that when one buils an amplifier that is supposed to give a
kilowatt out, the price of the variable capacitors becomes extensive. Once, I
was also pondering about a solution in which some base capacitance would have
been set by variable vacuum capacitors and then the rest would have been done
with additional capacitors switched by relays. The idea was that switching
between CW and SSB were to be done by tuning the vacuum capacitors. As e.g.
Jaska OH1MA has made auto-tune amplifiers that only use fixed value components, I also thought that there is no need for any continuously tunable
parts. Mainly, this seems to have been the correct decision, though some of the
fixed capacitors would perhaps have had some good use for small variable
capacitors in parallel to change the actual values to be closer to the specifications.
The Tank Coil
To make a coil for an auto-tune amplifier is no big deal. The band switch is
just replaced by relays. Note that relay control cables have to be protected
well from RF. One should use diodes in reverse direction and some small
capacitors over the relay coils and one should route the cables through areas
with little RF, if possible.
The relay type I used for the tank coil was Russian V2V. To mount such is easy,
just drill a 31 mm hole and a couple of smaller holes for the screws and you are
almost ready. I should have planned and measured the relay positions a bit
better as I had to make a few changes afterwards. The control cables go to the
other side of the shassis which is good for RF protection. Russian V1V relays
are cheaper but I find them less comfortable to work with. The operating voltage
for the V2V relays is 27 volts though at first I run them with 24 volts.
Unfortunately there was one relay that did not connect with 24 volts but really
needed 27 volts as specified. Better to test before assembly.
Originally, I had a great idea to make the tank circuit totally modular so
that each part is easy to disassemble, if needed, see picture 1. The load
capacitor module is at the top, the tune capacitor module is at the bottom, and
the coil module is in the middle. In principle, all was well, but due how some
other components of the amplifier were placed, to disassemble the coil and put it back to
it's place while debugging the amplifier was, in fact, surpricingly laborous.
Well, it could have been worse. I had used D25 connectors to connect the control
cables and that saved a lot of soldering when disassembling the coil.
In his amplifier Jaska OH1MA has planned component placement better and thus service work is
easier (picture 2).
Picture 2: Master piece of art by OH1MA.
All the connections are made using copper straps. I found that the copper
plates that are used to cover the floors in front of fire places are just the
right material. One needs to drill 8.5-9 mm holes to the copper straps for the
V2V relay connection screws. The thin copper plate should be drilled so that it
is placed between two pieces of wood. Otherwise it gets difficult. The copper straps are easier to install
correctly when one bends them a bit between the components (see picture 2).
The tune and load capacitor modules are made using fixed capacitors whose
values increase so that each bigger value is, in the ideal case, exactly twise
the corresponding lower value. If for example the smallest value is exactly 1 pF
then the next value is 2 pF and the next one is 4 pF etc. All the necessary tune
and load capacitor values are achieved by combining these small values.
Supposing that our capacitor values were e.g. 1, 2, 4, 8, 16, 32, and 64 pF then
e.g. 97 pF would be achieved by choosing 64, 32, and 1 pF capacitors.
I did not have capacitors with exactly the above-mentioned criteria. Besides
to my great plans,
the actual capacitor values are not exactly similar to the nominal values
anyway. Consequently, tuning the amplifier is not always totally optimal, but
close enough for most practical purposes.
Picture 3: Tune capacitor module.
The capacitors I used for the tune capacitor module (picture 3) have the
following nominal values: 1.5 pF, 3 pF, 6 pF, 13 pF, 25 pF, 47 pF, 100 pF, 200
pF, 390 pF (2200 pf and 470 pF in series), 790 pF (680 pF in series with two 220
pF capacitors that are in parallel). The very smallest capacitors are home brewn
of copper plate. Also Jaska OH1MA starts tune capacitors with 1.5 pF.
The loac capacitors are 7 pf (22 pF and 10 pF in series), 11 pF (in series
two 22 pF capacitors), 25 pF, 47 pF, 88 pF (four 22 pF capacitors in parallel),
170 pF (100 pF and 70 pF in parallel), 340 pF (two 680 pF capacitors in series),
680 pF, 1260 pF (two 680 pF capacitors in parallel), and 3200 pF (this
was originally 2200 pF according to the binary principle, but it was
later found that for topband a bigger value is optimum). Jaska OH1MA
begins with 12.5 pF.
Band selection system is made for Yaesu FT-1000 though the
circuit can easily be modified for other radios. In addition, there is a
possibility for manual band selection.
The system is based on a product called Unified Microsystems BCD-10 Automatic
Band Decoder. The BCD-10 reads the Band Data Port of Yaesu
and based on that triggers a relay that corresponds to the band in use. BCD-10
can't handle much current so in fact it only triggers a small relay that
controls the relays in the RF unit of the amplifier.
Manual band selection is done by breaking the BCD-10 to Yaesu connection with
small relays and inputting self-created five volt control signals to BCD-10
through diodes (picture 4). The picture shows only the principle of the circuit
related to the D pin of the decoder. For each band there are diodes according to
the Yaesu BCD control logic. For example the 7 MHz band is represented by the
following logical values: A=1, B=1, C=0, D=0. Thus when the manual selection
knob is in the 7 MHz position we connect diodes to the decoder pins A and B but
pins C and D are left disconnected. The resistors at the bottom corner of the
picture are related to creating a suitable voltage for BCD-10.That part could
have been done more elegantly.
Picture 4: manual control logic (principle only).
My Yaesu did not provide enough current for several decoders. I use them to
automate antenna selection of my station. So in addition to the amplifier I also
had to make a relay box that
repeats BCD coding of Yaesu providing more current.
Many amateur radio bands are quite wide. A tube amplifier cannot really deal with that having only
one capacitor value setting per band. Thus
I had to have a possibility for more. I chose the following band segments to
have separate capacitor values:
28 LOW MID HIGH
21 LOW HIGH
14 LOW HIGH
7 LOW MID HIGH
3,5 LOW MID HIGH
1,8 LOW HIGH
These have been quite sufficient in practise.
A switch that controls the relays R1 to R5 chooses the segment within each
band. Those relays pass the RF relay control voltages to some dip switches from
the relays that BCD-10 controls. Finally, it is the dip switches that decide
which relays in the RF unit are triggered (picture 5). Complicated? Sorry about
the deficiencies of the circuit diagram. One should, however, be able to
decipher the idea by thinking about the following question: when you have 9 relays,
of which only one provides 27V max 2.5A at any given time (band info), how do you connect additional relays to
divide that voltage to maximally three alternative routes (corresponding to LOW,
MID, HIGH segments within each band).
Picture 5: Band segment selection.
Jaska OH1MA controls the RF relays using a micro processor. A fairly simple
computer program chooses everything (frequency band, segment within a given
band, the inductor relay, and the capacitor relays within each segment). It is
a good solution. In fact, it is much better solution than mine. There are quite
cheap but yet powerful micro processors available, such that you can even
program with such a simple language as Basic. I chose the dip switches just
based on them being free for me (thanks to my favourite radio club).
The main idea is in picture 6. The control signal of each band segment comes
to a veroboard having rows of dip switches. Each row of dip switches corresponds
to a band segment. The dip switches choose if the relay that
corresponds to the dip switch in question triggers or not. The diodes are
essential to guarantee that a band segment does not put voltage to another band
segment. Let us imagine, for example, what happens if the first and third dip
switch of the 28 HIGH segment are connected and for example the first and fifth
dip switch of the 28 MID segment are also connected. If we have 28 HIGH selected
then without diodes also the 28 MID segment would get voltage. Thus we would
trigger relays 1, 3, and 5 even though it should only be relays 1 and 3. It was
this types of problems that I had to find when a few diodes were shorted.
Picture 6: The diodes of the dip switches.
The control unit of the amplifier employing the logic described above is
shown in picture 7. In addition to what has been shown here, there are also some
indicator LEDs that show for example which of the band segments is selected.
These LEDs are a great help when trying to correct several types of soldering
errors and when finding blown diodes.
Picture 7: Control unit of the amplifier.
To commission this kind of an automatic amplifier is quite laborous. The
principles are the same as when tuning a manual amplifier. Some tune for
maximum power, some seach for a dip in anode current. The main difference is that
one cannot just press the cw key and start turning capacitor knobs. Each change of
tune or load capacitor values is done by switching on or off one or more of the
dip swithches when the transmitter is not on. When one is at a border of some
big elementary capacitor value then this may mean touching quite many dip
switches. It is a good idea to tune the amplifier first into a dummy load and
only when the ballpark values are found do the final touches with real antennas. The good part
of the story is that when you
have finished, you don't have to touch the knobs again. It's automatic.
The Russian V1V relays take about 110 mA and V2V relays take about 150 mA.
There can be roughly ten of both types triggering the same time. Thus one needs
about 2.5 A for the relays. I installed the transformer to the control unit
mainly because of having limited space in the RF deck. This amplifier has
external anode power supply. At first, the amplifier provided 1 kW out with 1800
volts at anode. When the Finnish power limit was increased to 1.5 kW I upgraded
the amplifier simply by building a new 2700V anode power supply.
It is a good idea to verify the functionality of all RF deck components before
installation. I had one suspicious fixed vacuum capacitor and one relay that did
not quite work with my original control voltage. One should be careful with the
diodes. It took me a long time to find the problem when a particular diode looked
exactly like all the others, except that it was a zener. Debugging blown diodes
was a story of it's own. There are differences in D connectors. There are good
ones and then there are such that loose pins when soldering. Free D connectors
became very expensive, if I value my time in any way.
It would be good idea to plan beforehand both the electronics and the
mechanics. Less surprises so. Using printed boards instead of veroboards would
have saved time. Soldering diodes for dip switches is really, really laborous
when one uses a veroboard.
And finally, you usually need a much bigger RF deck than what you at first
So, was it worth doing? That is a good question. This amplifier is not as
good looking, small-sized, and quiet as an ACOM. But it does provide the necessary
power. The basic structure is simple and reliable. If there ever are any
problems, I can fix them myself.
To reach the original 1 kW target cost about 2000 euros. Better deals at hamfests could have
saved hundreds of euros. The most expensive item group was the vacuum
relays, they cost about 500 euros. In addition there are spare parts worth about
700 euros. I have actually needed one of the spares. Despite of having being
built like a tank and being easily able to provide double the power I use, the
grid of my original tube was mechanically broken after a few years. It is that
nasty tingle tingle sound that you hear while shaking a broken tube that finally
revealed me why I occasionally did not get any power out despite of having high
anode current. Luckily, I had bought quite many spare tubes with reasonable
prices, so this is no big deal. Despite of some recent price increases, price to
performance ratio of the Russian tubes is still superior to anything else on the
market. Having said that, I do admit that nowadays I would like to have
something more - I would like to get one of the modern solid state legal limit
auto-tune amplifiers. And having said that, I also admit that the price to
performance ratio of those is still so bad that I will, for years, easily find
more sensible things to spend my money on.