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Author Topic: Tube VFO  (Read 12088 times)
KB1WSY
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« on: August 04, 2012, 06:05:22 PM »

I'd like to build a tube VFO. How about a diecast aluminum box, about 1/8" thick, housing the key oscillator components and mounted on top of a larger, standard, thinner, aluminum chassis, with the oscillator tube sticking out of the back to avoid thermal drift issues? I'm thinking of something like this:

http://tinyurl.com/cv8jp4e

Is there something about thick diecast aluminum that makes it hard to work? Will it bust tungsten carbide drill bits? Can I use my Greenlee punches to cut the tube socket hole?

Haven't decided on a tuning mechanism yet but it will either be taken from surplus military gear (ARC-5 or BC-221) or built from scratch using one of several vintage dial mechanisms that I have in the junk box (Eddystone 598, National, or similar).

Apart from the mechanical rigidity, I'm trying to keep it simple: an oscillator stage and a single buffer stage, with 3.5-3.6 MHz as the primary range (for CW) and optionally a 7.0-7.2 MHz harmonic selected at the output of the buffer. This in turn will drive my pending "Low Cost Transmitter " (sic) for operation on all non-WARC bands 80-10. The buffer stage would be mounted on the chassis outside the diecast box. The power supply on a separate chassis.

This is yet another project that forms my "Novice+" station. The "plus" sign is because Novices weren't allowed VFOs....

Tnx es 73 de Martin, KB1WSY
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AC5UP
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« Reply #1 on: August 04, 2012, 06:35:37 PM »

Is there something about thick diecast aluminum that makes it hard to work? Will it bust tungsten carbide drill bits? Can I use my Greenlee punches to cut the tube socket hole?

Aluminum is easier to machine than brass or steel and for simple holes the thickness just means it takes a little longer to drill. As for using a chassis punch, could work but my intuition says it won't. Murphy's law and like that. Might be wise to do a little web crawling at Greenlee to see if there's an FAQ on the topic. If you're thinking only one or two larger holes a rat tail file will get the job done. There are also such things as side cutting drill bits or burrs so a drill press could be rigged to open up a pilot hole to any size.

As for the idea of a thick-wall enclosure being more stable on temperature delta...... Yeah, I think that could be an advantage.
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W4OP
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« Reply #2 on: August 04, 2012, 08:36:27 PM »

There is no nicer metal to machine than diecast aluminum. It is very brittle and mill or lathe and drills cut beautifully.

Dale W4OP
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KA4POL
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« Reply #3 on: August 04, 2012, 09:49:22 PM »

From the specifications: Excellent mechanical properties, easy for cutting and die casting.
See: http://www.aludiecasting.com/support-data.php
Sounds like an interesting project, good luck.
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G3RZP
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« Reply #4 on: August 05, 2012, 01:01:48 AM »

I've used the UK equivalent of Greenlee punches on diecast aluminium boxes with no problems. If it gets too tight, just leave it for a while, and the the punch will gradually move the metal, then tighten up again.

An ARC5 transmitter tuning cap and oscillator coil are good choices. The ARC5 used a Hartley oscillator: personally, I prefer a high C Colpitts and some people swear by the Vackar, which I've never found any better than a high C Colpitts. The Clapp has too much variation in output over the tuning range for my liking. What tube are you thinking of?
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KG4RUL
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« Reply #5 on: August 05, 2012, 06:23:34 AM »

I've used the UK equivalent of Greenlee punches on diecast aluminium boxes with no problems. If it gets too tight, just leave it for a while, and the the punch will gradually move the metal, then tighten up again.

AND keep the punch liberally lubricated at all times.  The product I use is: http://www.riogrande.com/Product/Liquid-Bur-Life-Lubricants/117985?pos=2
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KB1WSY
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« Reply #6 on: August 05, 2012, 06:30:00 AM »

An ARC5 transmitter tuning cap and oscillator coil are good choices. The ARC5 used a Hartley oscillator: personally, I prefer a high C Colpitts and some people swear by the Vackar, which I've never found any better than a high C Colpitts. The Clapp has too much variation in output over the tuning range for my liking. What tube are you thinking of?

I've been combing the vintage literature and currently favor a project from QST, February 1962, "An Easy-To-Build V.F.O." by Lewis G. McCoy, W1ICP. If you are an ARRL member the article can be consulted in their online archive. It does indeed use a high C Colpitts in one half of a 12AU7A to output 80m frequencies. The second half of the 12AU7A is used as a cathode follower for isolation. The second tube is a 6AU6 buffer/multiplier: there's a switch that shortens the multiplier output coil, doubling to give the 40m frequencies when desired. The voltage-regulated power supply is on a separate chassis. In terms of parts, nothing fancy or hard to find (that's a relief). I already have the coils specified in the original circuit, or I could go with the original coil from an ARC-5 transmitter.

The other circuits I found tended to be either too complicated (I am trying to keep projects to within my limited competence) or too simple. There are quite a few one-tube circuits floating around, but they usually have two disadvantages; (1) output on one frequency only and (2) wimpy output, thus requiring a separate buffer/amplifier stage. In a more sophisticated setup the single frequency at say 5 MHz + heterodyning would be "de rigueur" but I am not there yet. Yes I found the Vackar projects but thought I should go with something more "mainstream" as I continue to learn, slowly, about radio.

This is going to be my third project, and I am barely started on the first two (RX and TX) but there is no harm in beginning the VFO planning now even though it may not get started until Christmas time!

I don't much like the original mechanical design of the "Easy-To-Build VFO" project. It crams all the circuitry into an upside-down standard 3" tall aluminum chassis, used as a box with a metal plate bolted to the top (what would normally be the bottom). This upended chassis is, in turn, bolted on top of a steel chassis for stiffness. It would work fine, but forces a compromise on the size of the dial (because the "box" is only 3" high). The original design uses a Millen 10039 which is one of the few dials that are small enough to work with this design.

OK let's go with the diecast box (and/or the original chassis from an ARC-5, stripped except for the tuning mechanism and coil). The diecast box is not cheap ($45 or so) and probably only needed if I build my own tuning circuit/dial assembly rather than recycling an ARC-5. If a diecast box is used, it will be perched on top of a standard aluminum chassis. The chassis+box would provide enough "front panel" height for a nice dial, and give room for downstream components that may not fit inside the box. The chassis would be deep enough so that the 12AU7A, sticking out horizontally behind the box, would be in no danger of breakage if the unit were shoved into the wall. (There are quite a few designs that have a very exposed valve sticking out behind the unit, with no protection, and I don't like that much.)

BTW the only reason I went for diecast is that there don't seem to be commercial supplies of 1/8" thick standard aluminum boxes/chassis, they all top out much thinner unless you go up to rack-size stuff. Having heard of people's struggles actually building 1/8" thick boxes from scratch, I didn't really want to go there myself. A popular alternative is to use a standard .06" thick box, and stiffen it with plates of 1/8" aluminum, but the diecast option seemed neater and I think probably much stiffer overall.

The biggest practical diecast box I could find for this project is something like 6"x6"x4" inside, perhaps not big enough for the entire two-tube VFO. Is it just the oscillator portion that needs very high mechanical rigidity? Can I get away with putting the buffer/doubler (and its coil) on the chassis portion rather than inside the box? My gut instinct says "yes" although I suppose mechanical vibration of the doubler coil could be an issue, but nothing like as bad as mechanical vibration of the main oscillator circuit. My thinking was to mount the buffer/doubler tube on the apron behind the diecast box, with the doubler coil underneath the chassis, conveniently close to the output socket and keeping the wiring neat.

Connections. This design is meant to be connected directly to the TX's crystal socket with a 2-foot length of RG-58 which is actually part of the tuned circuit, having about 50pF capacitance. I haven't found male connector that fit into an FT-243 crystal socket but I could improvise by sacrificing a crystal and usings its base as a plug (I have a couple of unwanted VHF type 8 MHz crystals).

Keying. There are fierce advocates on both sides of the "key the oscillator, don't key the oscillator" issue. For the moment I've basically come down on the "don't key" side because it greatly simplifies the whole chain of equipment, albeit making it hard/impossible to get full QSK. The QST "Easy To Build" VFO just has a simple "Spot/Standby/Transmit" switch and also has provision, in the power supply, for external switching of the B+ if desired.

Power supply. The original schematic shows two OA2's in series, to give the two required HT voltages (225 and 75) from the 500V CT secondary. But the text refers to an OA2 and a *OC2* which seems more logical given that the latter has an operating voltage of 75V. Boy, the print in those tube data charts is small! Had to get out my magnifying glass! If you put an OA2 and OC2 in series, you get a voltage divider network with 225V coming out of the top and the OA2 underneath dropping it to 75, and the OC2 on the bottom completing the circuit. That's a nasty typo in the schematic although I assume the vast majority of homebrew builders would have spotted it straight away.

I have a theory question. Why is it that VFOs from this era are usually limited, by design, to a relatively low frequency output such as 80m or even, quite commonly, 160m? Mathematically, every time you multiply a frequency, you surely also multiply the drift/instability by the same factor. I would have thought, for instance, that on 10m you could get serious problems given that any stability issues with your 80m oscillator would be multiplied by 8.

AND keep the punch liberally lubricated at all times.  The product I use is: http://www.riogrande.com/Product/Liquid-Bur-Life-Lubricants/117985?pos=2

I was wondering about that. I bought a few "new old stock" punches from the 1950s (because they are sizes that aren't made any more, and besides they were cheaper) and they came with copious amounts of grease pre-applied. The modern ones that I bought had no grease at all. I'll make sure to use the lubricant!

Homebrew OTs will note, with approval I hope, that with this third project I am tentatively moving away from slavishly following the original ARRL designs and may even be showing some signs of original thought and homebrew "scrounging" ethos. I won't apologize for the length of this post ... although I probably should ... there are lots of things to figure out and I do find the advice from other eHammers invaluable. In another few months I hope to be communicating with you in the way Hiram P. Maxim intended!

73 de Martin, KB1WSY
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KE3WD
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« Reply #7 on: August 05, 2012, 08:20:43 AM »

The idea of using more AL mass to control temperature can work if you intend to leave the unit turned on and operating 24/7, such that temperature will more or less stabilize.

It also has its detriments, though, consider how long it may take to come up to that temperature at turn-on.  Then consider the differing ambient temperatures the device may be under when cool, such as ambient winter temp vs summer, etc.  More thermal mass also means longer heatup cycle. 

Early designs for military and other high stability stuff usually incorporated a filament heater with thermostatic switch, rather light thinwalled but insulated enclosures. 

TIP:  Aluminum machines very nicely with the use of common Kerosene as cutting fluid.  Be advised that there are quite a few different AL formulations out there, though.  Diecast is not going to machine as nicely or as pretty as say, T6061 or the likes. 

If you are after stability, by all means consideer the Permeability Tuned Oscillator circuits over the Variable Capacitor types.  PTO, using moving ferrite inside coil often means coming up with mechanical designs to move the ferrite, but is not a big issue IMO.  Such designs always start with a good knowledge of, believe it or not, the notebook drawings of DaVinci.  Still valuable source for ideas today, source reading for the young robotic designers, or should be.


73
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KB1WSY
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« Reply #8 on: August 05, 2012, 08:55:03 AM »

The idea of using more AL mass to control temperature can work if you intend to leave the unit turned on and operating 24/7, such that temperature will more or less stabilize.

My main purpose in using thick AL was to get *mechanical* stability. Thermal stability would be achieved mainly by mounting the tube(s) such that the heater is outside the box. I take your point about leaving it running 24/7 and we will eventually find out whether that is needed.

A few months ago I renovated an Eico Model 315 RF signal generator that has an oscillator/buffer circuit running from a voltage-regulated power supply. It is on a thick copper chassis totally encased in a steel box with no ventilation whatsoever, and the box gets quite hot to the touch after it's been runing a few hours. Experimentally, I have been leaving the sig-gen turned on for days on end. After the first 2 hours or so there is no drift at all -- at least not when measured with the rather broad selectivity on my little Sony digital shortwave set. Given that my proposed BFO is a similar design, but with even *more* provision for avoiding mechanical drift/instability, I am hopeful of getting a stable result.

Actually, I have another one of those Eico Model 315 signal generators (it is in poorer condition so I used it for parts), it's the blue thing in the background of this picture: http://tinyurl.com/8o7qhm9. I could, theoretically, rebuild this as a VFO. I just feel more like building something entirely from scratch and anyway, the Eico display dial is huge and therefore a bit "overkill" for a transmitter driver. I will probably save it for a future receiver project, it's a fantastic dial.

73 de Martin, KB1WSY
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G3RZP
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« Reply #9 on: August 05, 2012, 10:18:46 AM »

Box making with 1/8 inch thick aluminium is best done with angle or square stock, screwed and tapped at the corners. I have made boxes from 1/4 inch aluminium plate, with screws holding the 6 pieces (4 sides, a top and a bottom) together. Those are RIGID! Using 1/4 square aluminium rod will do a good rigid job, and should be doable for much less than $45. You'll probably need to use 4-40 hardware.

But if you can find an old ARC5 transmitter chassis, that will be nicely rigid, and will have low thermal mass. Mount it behind the front panel, and use a couple of pulleys to drive a dial mechanism moved further away from the tuning control so you can have a bigger knob for frequency control - the little knob is not so good for readily netting.

A good system is to use a Colpitts with something like 500pF mica caps from grid to cathode and cathode to ground, and then as small a cap as you can use for reliable oscillation from grid to the tuned circuit. That way, you swamp out variations in valve capacity. Something like a 5763 or a 6AG7 will give plenty of output at the second harmonic, using 150 on the screen and 300 on the plate. Yes it gets hot, so you keep it away from the tuned circuit, but it has the advantage that because of the heat, the capacitance changes happen pretty quickly, keeping the drift down. One trick I found useful in one TX was to run the oscillator with cathode bias and use a diode to short the tuned circuit on receive. That kept the tube at pretty much the same temperature. Another trick was to arrange that on receiver, the VFO shifted 100 or more kHz away but kept oscillating - that kept the components at the same temperature..

Keying. Generally, it's best not to. There were schemes of 'differential keying' which turned the VFO on marginally before the PA and off after the PA to minimise chirp, but I never made any of them work.

Why 80m? Some had 40m fundamentals so the drift was only multiplied by 4 on 10 metres. But unless your VFO was as stable at 40 in percentage terms as it was on 80, it didn't get you very far, and in fact, because changes in valve capacities would be the same, the effect was that the 40m VFO was more than twice as unstable as the 80m one! Some wartime transmitters used a fundamental VFO right up to 15 or 18MHz - BC191 in the US, T1154 in the RAF. Lousy signals from them up there - drift and chirp and T7 note! However, the Wilcox-Gay VFO for the RCA ET4336 (WW2 again!) used an 807 oscillator in an ECO and multiplied. Nice piece of equipment, and pretty stable.

Another advantage  of the Colpitts with a pentode and a tuned circuit doubling in the plate is the increased isolation. By correctly proportioning the screen and plate voltages, it can be made very much less susceptible to voltage change, too. Best done empirically.

I'm not a great fan of ferrite or even dust iron tuning mechanisms. I know Collins and Drake used them - it can be a bit cheaper than a really good capacitor. If you want not to have to individually calibrate each VFO, and not spend a fortune or capacitors having exactly the same capacitance ratio
and pF at the same angle, it's worthwhile. (Gang capacitors with matching between sections of 2% are expensive and 1% VERY expensive. That's matching between sections, not between capacitors, which is even worse). All of which is why the BC221 wavemeters were individually calibrated.

The drawback of an ARC 5 capacitor is that it's a lot of turns if you want bandspread. One way is to use the pulleys and extra dial and to get a mechanical step up to the readout dial so you cover 80 metres  with between 5 and 10 turns of the knob.

Good luck with it all.

73

Peter G3RZP
 


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KB1WSY
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« Reply #10 on: August 05, 2012, 11:19:28 AM »

A good system is to use a Colpitts with something like 500pF mica caps from grid to cathode and cathode to ground, and then as small a cap as you can use for reliable oscillation from grid to the tuned circuit.

As the "Easy-To-Build VFO" ARRL project article says: "The tube capacitances are practically swamped out by the use of large (1,000pF) silver micas as a capacitive voltage divider from grid to cathode (to ground, my alteration) of the oscillator tube."

Another trick was to arrange that on receive, the VFO shifted 100 or more kHz away but kept oscillating - that kept the components at the same temperature.

I hadn't thought of that and have never seen it mentioned before. Sounds easy to implement, just switch a fixed capacitor across the LC circuit during receive.

As for keying:  I have been getting a chorus of advice from OTs that achieving QSK with this vintage of homebrew equipment is difficult ... so am putting that on the back burner. My Morse skills are still under par, so anything that can help the ergonomics of switchover and provide more time to concentrate on copying and sending would be good. But I still have several months of station-building to work on the CW skills before trying the first QSO.

73 de Martin, KB1WSY
« Last Edit: August 05, 2012, 11:21:16 AM by KB1WSY » Logged
N0OKS
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« Reply #11 on: August 05, 2012, 11:32:59 AM »

The aluminum alloys used for die casting have silicon in them. Maybe up to 10%. 6000 series aluminum, 6061 and 6063 have almost no silicon in them. These are called wrought alloys.

Die cast aluminum is hard. Try scratching die cast aluminum and wrought aluminum with a nail to see the difference. You can pound a nail through wrought aluminum, You will probably break, crack, your die cast aluminum box if you try to pound a nail through it. You can bend wrought aluminum. Cast aluminum will break if you try to bend it.

See http://en.wikipedia.org/wiki/Aluminium_alloy for more info.

Mark, NØOKS
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G3RZP
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« Reply #12 on: August 05, 2012, 11:40:56 AM »

Two 1000pF in series means 500pF across the tuned circuit. This may be a bit much, but a 100 - 500pf series cap to the tuned circuit effectively taps the tube and reduces loading, thud increasing the working Q of the tank circuit.

So do you have access to a scrap command tx? That could make life so much more easy......

Some years back, I did a 'fun' tx. 6SJ7 VFO on 1.75 MHz doubling in the plate circuit, a 6AG7 buffer or doubler to 7MHz and an 807 PA. Covered 80 and 40. Plus a 6SL7, a 6H6 and 6V6 to look after arranging an automatic change to transmit when the key was pressed and an adjustable delay on flipping back to receive after the key was released. Ran about 15 - 20 watts input. What's more, all out of the junk box....including the separate PSU!
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KB1WSY
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« Reply #13 on: August 05, 2012, 12:22:47 PM »

So do you have access to a scrap command tx?

Not yet, but as I noted, this project is months away and I might even be able to get to a hamfest or two by then. So far this has been jinxed by family and business commitments. Although my state ARRL's annual convention is coming up, I won't even be in the country. However there's a flea market across the river at MIT on August 19th which I hope to attend. The alternative is online, ARC-5 TXs turn up frequently on eBay and a beat-up one should be found for under $50 which is the upper range of what I expect to end up paying (if I go the ARC-5 route).

That could make life so much more easy......

Ah, but you see, "easy" is not for me. Otherwise I'd buy everything off the shelf at HRO and spend less money too! I'm already feeling like a bit of a cheat for using a lightly modified BC-453 as part of my homebrew receiver. Real hams build complete, decent-performance receivers from scratch....

I know this must sound quite mad, but it is a relatively benign lunacy. My XYL might beg to differ ... but has been very supportive so far. We'd already been married for 24 years when I suddenly reawakened this Ham Radio monster! That was quite a surprise!

73 de Martin, KB1WSY

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G3RZP
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« Reply #14 on: August 05, 2012, 03:23:04 PM »

If you can't find a command tx carcase, then you need a double bearing - front and rear - variable capacitor. Something around 100 pF swing, and a ceramic coil former. I'd suggest also some old APC type trimmers. If you get stuck, it is quite possible that the RZP junkbox may be able to help, although you would have to wait until I go to the US again next year - unless you have a relation from the UK visiting or one from the US coming here.

There's a myth in ham radio - at least this side of the pond - that using polystyrene capacitors and dust iron or ferrite  cores gives a compensation of temperature coefficients. Well, sort of. When the analogue FDM (Frequency Division Multiplex) schemes were going on, there were developed ferrites and polystyrene film capacitors that would together give more or less a zero temp co. At low frequencies - <100kHz. But as a general rule, t'aint so!

73

Peter G3RZP
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