QRO From The Kitchen
I truly thought I was alone in the somewhat
bizarre hobby-within-a-hobby of repurposing E-waste until I tripped over
AA1TJ’s (no relation) account of an 80 meter transmitter with the
uber-clever moniker of “Das Derelicht”, made largely
from the guts of a defunct fluorescent lamp.
Dumpster diving for fun and profit requires a bit of a reversal in your way of thinking – first you acquire the rubbish,
then you work out what you can build with it! With literally megatons of E-waste bound for the landfill every year, it is our right, nay, our
responsibility to divert a little of it for our own amusement.
I’m really a QRP’er. Honest! It’s
a lonely life, here at the edge of the planet, made that much more
lonely by the insidious proliferation of switch mode power supplies,
which seriously impair most of
my QSO partners ability to dig deep for the weak ones. I had a rant about this in a recent “K9YA Telegraph” article.
While it made me feel better, it didn’t actually result in any more QSO’s.
If I cannot change your predilection for surrounding yourself
with RF-noisy electronic gewgaws, I must give up the hobby or go QRO to
tip the scales back in my favor. So QRO I go. Reluctantly. With much
trepidation, for I have not worked with hollow-state
devices or high voltages for some years. Subconsciously, I
think I must have wanted this project to fail, because I imposed upon
myself the ludicrously low budget of $25 – just 1 percent of the price
of a new amplifier.
The fact that you are reading this hints that, beyond all expectations, the project did in fact deliver the (white) goods.
Bill of Materials
Tube: No contest – cash in some of your cold war dividend and purchase a few GI-6B’s from Vladi, UT5JDS.
At just $7, these are the hands down clear winner in the watts
per dollar sweepstakes. With 350-watt anode dissipation, you can expect
500 to 600 watts output from one tube.
As you will read later, this dovetails nicely with the limitations imposed by our scrounged transformer.
It’s a bit frightening to contemplate the use of a self-described
“microwave oscillator” tube in an HF amplifier, but in practice, they
work out just fine. Being a triode, we neatly sidestep the beginner
jitters around screen supplies and sequencing
of power turn-on. Better still, the GI-6B really doesn’t need a socket.
Priced 4CX250 sockets lately? You’ll understand why your new best comrade is in fact, a Red Army defector.
Plate Blocking Capacitor: While you are
in negotiations with Vladi, price in a few 5 kV capacitors. At $3 ea or
so, they won’t break the bank either, and can be rather hard to find in
the capitalist economy.
Socket: Wait a minute - I just finished telling you that the GI-6B doesn’t need a socket!
Pay attention, will ya! Seriously, it is convenient
to be able to replace the tube without ripping the amp completely
apart. I am deeply indebted to Frits, PA0FRI
for pointing out the ordinary tool clips make good contact rings for the GI-7B/GI-6B.
Frits only used tool clips for temporary sockets, but in practice, I’ve found that they are a perfectly acceptable permanent solution.
Plate Transformer: Raid the kitchen garbage for a defunct microwave oven, preferably a 1 kW, or even 1.1 kW nuker.
Dead it may be, but I’ve only ever seen one with a defunct
transformer – generally, it is the magnetron that fails. Repair is not
generally cost-effective, given that microwave ovens are currently on
sale at my local “Junky John’s” for $49 AUD. You are
going to read all sorts of rubbish about microwave trannies – that they
lack insulation at the earthy end, that it is somehow “too special” for
our purposes, perhaps even that it eats it’s own young, but your own
investigations will reveal that they have just
as many wraps of paper and varnish at the earthy end as they do at the
hot end. Snip the grounded end at the rivet, build a front porch for the
liberated connection out of etched PC board or some other suitably
insulating material, and you are away.
Don’t forget to bung out the magnetic shunts – laminated
rectangles of steel located betwixt the primary and secondary windings.
It
is true that the magnetizing current is rather high in a nuker
tranny. If you suffer from cold feet in winter, it is recommended that
you build the power supply in a floor-mounted separate enclosure so that
you can warm your toes on it. Otherwise, find
two roughly equivalent transformers, and wire them up series
primary/series secondary. This dramatically reduces the standby current,
and probably pulls the combo well back from the hairy edge of
saturation as well.
Sadly, microwaves are obviously spec’d out in “marketing watts”, a
unit of measure not bound by the physical constraints of our universe,
so there is not much point in hosing up a second GI-6B in parallel to go
for the full gallon. That’s also why we’re
such fussy garbage-pickers, sending our meal back repeatedly to the
kitchen if it isn’t at least a 1 kW nuker.
You’ll probably find that the secondary voltage is a tad high for our purposes – around 2200 VAC if yours are like mine.
There is plenty of anecdotal evidence that the GI-6B will
cheerfully withstand this, even full-wave rectified, but if you are
lucky, you may find that turns can be removed from the secondary. No?
Then consider adding turns to the primary (usually easier).
Anything you are able to do on the primary side will, as an added bonus, also reduce the rather high magnetizing current.
Rectifiers: You guessed it – dead microwaves again.
You’ll need about four carcasses, so put the word out amongst
your friends. Don’t have any friends? Join your local “Freecycle” group
(q.v.). You will be amazed how fast the dead can move (towards your junk
box), and in what numbers!
Filter caps: There are two ways to go
here – you can liberate lovely 1 uF oil-filled caps from microwave ovens
that will probably provide you with years of trouble-free service, but
you will need to find about 20 of them to
parallel up. As an added bonus, each comes with an integral bleeder
resistor - but – even 20 in parallel probably isn’t enough bleeder for
our purposes. Alternatively, if there are any PC CRT’s left on the
planet that haven’t already been bulldozed, they generally
contain at least one 220 uF, 450 volt electrolytic. Series up about 8
of these, and you’ve got a really nice filter bank. Of course, you then
have to come up with equalizing resistors. Your call.
Blower: Yep, microwave again. Like most external anode tubes, the GI6B’s
gulp the air. In large doses. Conventional wisdom states that
only an (expensive) squirrel cage blower can do the deed – that blade
fans simply cannot handle obstruction. Sadly, conventional wisdom is
nearly right. A cardboard mock-up, including a chimney
around the fan blades and the anode cooler yielded – nothing! It
was only when I mounted the tube axially, and cut a “wind tunnel” the
same diameter as my blower blades into my enclosure, that I began to see
any air movement across the anode
cooler. Obviously, we must screen this opening with
hardware cloth to keep prying fingers from finding our 2 kV supply, but
mine was going into a 19 inch rack, so I’m pretty safe on that score. I
also mounted the microwave blade fan as close
as humanly possible to the cathode of the tube. The proof
of the puddin’ is obviously in the eatin’: I see less than 2 degrees C
temperature rise on the anode cooler after a 15 second “brick on the
key” episode. I’m calling that a win! And, no,
I’m not going to tell you how I made those temperature measurements –
I’ll reserve that for an episode of “Mythbusters”. Or maybe a remake of
“Dumb & Dumber”.
Filament Choke: For a change of pace,
we’ll raid a dead CRT or television for this one. PY2WM put me on to the
potential source of balun material in his brilliant treatise:
http://py2wm.qsl.br/balun/Balun_with_free_ferrite.pdf
It would appear that the flyback core, with its almost supernatural permeability, would serve us best.
I glued the two halves of my victim end to end in order to keep the “gozinta” end well separated from the “gozouta”.
The swastika-like end result no doubt raises a few eyebrows, but performs well.
Filament transformer: I think mine came from a defunct AM/FM stereo receiver – I don’t really know.
The GI-6B wants a rather modest 12.8 volts or so at 2 amps, which
shouldn’t be too hard to come by. Don’t ignore OEM or even PC power
supplies. We’re not slighting the sexual orientation of the filaments –
they really are AC-DC!
If you are really stuck, you could bung the secondary turns out
of one of the baby 600-watt microwave transformers you accumulated along
the journey, and replace them with a 12.8-volt low voltage secondary.
There would be ample room for another “control electronics” winding for relays and suchlike.
Bias Board: My circuit was a blatant theft from:
http://www.nd2x.net/oz1dpr/schalt.jpg
because the parts required were all in my junk
box. No parts made of unobtainium here! I suspect my pass transistor
was once a TV or CRT horizontal oscillator device, but I’m not really
sure. Feel free to make substitutions – the circuit
is quite tolerant of that.
Meters: Mine were QA rejects from my
former employer. Since you are unlikely to be so lucky, I might suggest
plant pot moisture meters.
If it amuses you, you may leave the original calibration of
“moist” to “wet” in place, or freehand some new scales. For a distinctly
high-tech look, I am told that Asian Ebay sellers periodically put
their DVM’s on special for the princely sum of $1.99
each. You are unlikely to better this price, analog or digital! Of
course, now you
really need a “control electronics” DC supply – no way I’m reaching in there to change those 9 volt batteries!
Plate RF Choke: You guessed it – microwave again.
I unravelled a blower motor winding for wire, and spooled it onto a ½-inch or so diameter dowel.
I may have got a bit carried away, as my choke came in at over
400 microhenries, vastly more than we need, but no nasty resonances were
noted, at least like where I like to operate, so I’m calling it a win.
Cheese Slicer “Tune” Capacitor: Sorry,
you are on your own here. I prised mine from the fingers of an old
geezer too long in the tooth to homebrew any longer, but if you are too
proud to beg, you can always have
a go at building your own:
Plate Inductor: I happened to have a
chunk of B & W Minidux in the junk box, so used that, but it’s
nothing you couldn’t replicate with a bit of 2-inch PVC pipe and a bit
of household wire.
My amp is an untuned input, so a band change, while not trivial,
is as simple as changing a tap – but not to be undertaken while half
asleep!
A roller inductor might be a better option, but would almost
certainly blow out my modest project budget. A band switch capable of
this kind of power is clearly out of the question! Interesting
economics, eh? When playing with 7-dollar finals, it’s actually
less expensive to build 9 monoband amplifiers than to purchase a band
switch!
Multi-gang “Output” Capacitor: I
suspect that the aforementioned defunct stereo tuner might have donated
this – I don’t really know. It is about a 6-section variable capacitor,
and, with all sections paralleled, yields about
1000 pF at maximum – not quite enough for 160 meters, but certainly
more than adequate for 80 through 10 meters. As Bill Orr, W6SAI,
amplifier guru extraordinaire, was quick to point out in the “West Coast
Handbook” (“Radio Handbook”, most editions), we’re
back to 50 ohms at this point in the circuit, so the plate spacing
requirements are not onerous.
Relays: Ordinary control relays are quite adequate at HF – study any 70’s vintage boat anchor, and you’ll see what I mean.
I’d like to say that I scavenged these from a dead microwave as
well, but in reality, the beefy relays in a nuker always seem to be
SPST. We want SPDT. I have no idea where mine came from, but suffice to
say, they’re a common item in the scroungers marketplace.
Chassis/Sheet Metal: You
guessed it – microwave again. The presence of both an outer and an inner
skin (cooking cavity) presents both some challenges and some
opportunities. We can let components “dance on the
ceiling” if we choose, knowing that their mounting bolts will not show,
but some of the soldering is reminiscent of keyhole surgery, so special
care must be taken to avoid cold solder joins.
The sheet metal between the power supply compartment and the
cooking cavity makes a very nice plate/grid shield, which we sadly
violate, of course, when whittling our wind tunnel through the lot, but
no signs of instability were noted.
Although steel, the microwave oven carcass is of light enough
gauge that there is no danger of burning out drill bits - an absolute
pleasure to work with
Besides, how many other enclosures color
coordinate with your kitchen décor, and come with bragging rights: “Amp
here is a Samsung, OM – strictly an appliance operator here”.
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