Radio Havana Cuba

Dxers Unlimited for 25-26 November 2006

By Arnie Coro

radio amateur CO2KK

Hi amigos radioaficionados worldwide ! Yes there are radio amateur
operators in the scientific research bases in Antartica, in the most
beautiful tropical islands of the Caribbean and the Pacific, deep in the
heart of heavily populated cities in Japan, India, the USA and Germany.
Operating from small villages in Africa, or working DX from a car while
driving on a British motorway, and yes there are radio amateurs in space
too, having a nice time with their radios at the International Space
Station, where another space suit amateur satellite may be ready soon to
be placed into orbit.You can find ham radio operators practically in
every nation of the world, and their stations may be as simple as a
handheld 2 meters band FM handie talkie, to the most sophisticated setup
especially prepared to communicate with fellow radio amateurs using the
Moon as a passive reflector.

And of course, you can also monitor the ham bands and listen to an
ongoing contest, like the CW Morse Code world competition taking place
this weekend, where my good friend Juan Carlos Molina CO2JD is operating
aiming at winning a good place. Juan Carlos is one of Cuba's best CW
operators,and he enjoys working contests a lot... Then, don't forget
that there are many radio amateurs around the Planet, that talk much
less than others, because they enjoy homebrewing receivers,
transceivers, converters, station accesories like antenna tuners, and of
cours also designing and building their own antennas... Amateur radio is
a great hobby , capable of providing a lot of enjoyment to all of its
participants, but also capable of making possible emergency
communications under the most difficult conditions, something that is
really very much appreciated by every national telecommunications
authority, who, by the way, are the ones that grant the operating
licenses required to run an amateur radio station by an individual.

More about amateur radio later in this weekend edition of Dxers
Unlimited that will continue in just a few seconds, I am Arnie Coro,
radio amateur CO2KK in Havana.

......

This is Radio Havana Cuba, the name of the show is Dxers Unlimited, and
yes, we are on the air twice weekly with the most up to date HF
propagation analysis ... A long time ago I subscribed to the
Regenerative receivers list hosted by Yahoo, and since then have kept a
folder with all the relevant postings, as a useful reference for my
experiments with those fascinating receivers, and on Friday I read what
seemed to be the start up of a new thread on that list amigos...

And what an interesting thread it has proven to be.... It has to do with
the types of vacuum tubes prefered by homebrewers of regenerative
receivers, so I decided to add by two cents worth of workshop practice
on the topic with the following posting that I will now read to Dxers
Unlimited's listeners...

This is the posting to the list sent Friday morning from Havana:

My first ever regenerative used a 3S4 vacuum tube, a 22 1/2 volts
battery for the B + and a 1.5 volts size D cell for the filament...
AND... What a thrill it was to assemble it and then learn how to
use it... This was a "breadboard kit " with the parts clearly painted (
silk screened maybe ) to the board, and the connections were done by
soldering to tie points...

When I finished building it, absolutely NOTHING HAPPENED ?

So I took the assemble kit to my Elmer.... and he was sure that a bad
solder connection was the cause...

Then we went re-soldering all over the breadboard with his 100 Watt
soldering iron and Kester solder...

Ten minutes later , the radio was working perfectly !!!

One triode, 22.5 volts B+, a nice regeneration control by means of a
"throttle capacitor".... and a pair of 2000 ohms headphones...

Then Ignacio , my Elmer, wanted to experiment too, and we wound another
coil ( the one brought by the kit was for the AM broadcast band only )
and that afternoon I was listening to short wave stations from about 5
to 16 megaHertz !!!

More experiments followed, and I then went ahead with another
regenerative, that used an RF amplifier stage for better isolation
between the antenna and the detector...

This was a really sensitive radio...
6SK7 RF amplifier, 6C5 detector, 6SQ7 triode section audio amplifier and
6V6 audio output.

It required a big power supply, using a 5Y3 , and a pair of big 40
microfarads at 450 volts electrolytics and a 10 henries at 150 milliamps
filter choke...

This one hit me a couple of times with its 225 DC volts output !!!

Once sent yours truly almost flying across the garage where my radio
workshop was installed !!!

Later experiments with regeneratives gave way to more complex radios,
the start up of my amateur radio "career", and it wasn't until about the
year 1998 that I began again to build regenerative and regenerodyne
receivers, both vacuum tubes and "sand state".

In the second round of enjoying these wonderful radios, my know-how
about electronics was a bit better, and that allowed to appreciate many
aspects of regenerative detectors that it was impossible for me to
understand earlier in life...

Now, when newcomers to the radio hobby visit me, I always have at a very
visible place of the workshop main workbench a regenerative receiver,
and next to it a regenerodyne...

The regenerative uses a 6AK5 detector , working as a tetrode, with
screen grid control of regeneration , in a Hartley circuit
configuration. The audio comes from a triode-pentode that is very
easily found locally, and capable of producing a really LOUD output from
the loudspeaker...

The audio tube is an ECL82, that I am sure has an equivalent in American
vacuum tubes, but can't remember now...

I have also built this radio design using a 12AU7 dual triode and a 6AQ5
audio output , leaving one triode of the 12AU7 ununsed.

This radios are built like battleships, with a thick steel metal panel ,
and heavy wire connections for the detector stage...

The coils are wound using PVC pipe as the coil form, and they fit into
octal tube sockets, so you can change bands easily.

As expected, the first coil I lend to the new builder is the one that
tunes from 6.8 to 7.5 megaHertz ( the 40 meters band bandspread coil )
so they can hear the radio amateurs . Instructions and help to build the
coils for the AM broadcast band and other frequency ranges are also
provided to the prospective builder in the form of a illustrated printout...

You can not imagine the look of the face of the newbies when their
radios start picking up stations !

The power supply I use now uses a locally available 24 volts
transformer, removed from old TV sets. The 24 volts AC are doubled in a
full wave voltage doubler circuit using computer power supply capacitors
and the output voltage is more than enough for the detector stage and
provides good audio output to headphones and moderate audio level to a
loudspeaker.

By using two such transformers ( they are plentiful ) connected back to
back, one can build a + 160 volts DC power supply if the person wants to
have really loud audio output !!! The same ex-TV set diodes and
ex-computer power supply capacitors do the job, except that there is no
need to use a voltage doubler circuit !!!

Experiments with other vacuum tubes have included:
Use of a dual triode cascode regenerative detector ( from a British
magazine ) that published the circuit copied from a Dutch magazine...

This one uses an ECC88 dual triode, but identical results were obtained
with 6BK7, 6BQ7 , ECC85 and other dual triodes, but the ECC81 ( 12AT7 )
didn't work as good as the tubes specially designed for cascode TV set
RF amplifiers...

Classic regenerative detectors with 6C4, 6C5, 6J5, triodes locally
available work OK,but now I prefer to use the 6AK5 working as a tetrode
with the screen grid regeneration control in the Hartley configuration
because it gives a very smooth control of regeneration when you use
regulated DC voltage for both the plate and screen grid electrodes...

Also,the use of a grounded grid RF "isolation" stage seems to be a good
idea too...

The tube combinations are endless, but you can be sure, that all tests
done here for the past 8 years or so, have proven that solid state
regenerative receivers are NEVER AS GOOD as the vacuum tube circuits !!!

.........

Now here is your number one favorite section of Dxers Unlimited, la
numero uno, YOU HAVE QUESTIONS, and Arnie tries to answer them ...
Today's question , you guessed ... has to do precisely with regenerative
receivers. Listener Tom from Ohio , USA, asks why is it that vacuum tube
regenerative detectors seem to work much better than similar ones built
using MOSFET dual gate field effect transistors that are supposed to be
an almost exact duplicate of a vacuum tube tetrode regarding the its
operating parameters... Well amigo Tom , I must tell you that I have
share with you the same opinion... all my tests with at least seven
types of MOSFET dual gate field effect transistors have never given the
same results than those I obtain with event he simplest single triode
vacuum tube regenerative detector, and the worst of these results is
that I haven't so far found a logical explanation for this to happen.
Once I replaced the 6AG7 vacuum tube in a Hartley VFO circuit, fed 12
volts to it instead of the 105 volts regulated, and installed a MOSFET
dual gate device, and it worked perfectly... with much less power output
as expected, but otherwise it worked beautifully , with much better
frequency stability than the vacuum tube provided, something to be
expected as less heat was generated around the tuned circuit...
But similar experiments with regenerative detectors have failed... not a
total catastrophic failure, because the radios did work, but somehow the
sensitivity and the setting of the regeneration control couldn't match
the results obtained with the vacuum tubes... so I pass the question on
to the worldwide audience of this program, it someone can provide us
with an explanation of why vacuum tube regenerative detectors work much
better than those using solid state active devices !!!

And now as always at the end of the show, here is Arnie Coro's Dxers
Unlimited exclusive , but not copyrighted HF propagation update and
forecast... but before starting to broadcast the forecast ,let me ask
for a little patience to all those listeners that have requested the NEW
BROOMSTICK ANTENNA information package... the drawings are still in the
works amigos, and I do want to include some photos too.. Now here is the
update, solar activity at very low levels with two days in a row of ZERO
sunspots, and solar flux hovering around 75 units, with the A index at a
nice and low 5 to 7 units, so, start hunting for low frequency DX just
as you finish listening to this program amigos

Don't forget to send your signal reports, comments about the program and
radio hobby related questions to arnie@rhc.cu or VIA AIR MAIL to Arnie
Coro , Radio Havana Cuba, Havana, Cuba.

Signed Arnie Coro CO2KK