Building A Jones Push-Pull Oscillator

Just as my shelf space for new projects keeps expanding (and quickly running out!), so does my main web site, "The VE7SL Radio Notebook". I have just added a new page to the site describing the construction of my newly-completed Jones Power-Oscillator using a pair of 6L6s.

If you might be thinking of building a 'Jones', you may find something helpful or perhaps find inspiration for a new project. The new page may be found here.

I'm presently building the final version of my RK-39 crystal power-oscillator ... but I have no idea where I 'll put it when finished!

Drake 2NT And The Novice Rig Roundup (NRR)

One of this summer's radio workbench projects was to refurbish my Drake 2NT transmitter for this winter's NRR event. The 2NT was introduced in 1966 with the 'novice' ticket-holder in mind, selling for $129. It was the first and only CW-only transmitter produced by R.L. Drake.


As novice transmitters go, the 2NT was a cut above some of the others out there, boasting interesting features such as a built-in antenna changeover relay, sidetone oscillator, grid-block keying and built-in low pass filter.


The tune-up procedure was simplified as well and with Drake's reputation for quality construction, the 2NT became a popular choice for Novices and Generals alike.





My own 2NT was purchased, along with the matching 2C receiver and speaker/Q-multiplier combo over 25 years ago at a Washington state ham fleamarket. Ever since then they have been patiently sitting on the shelf, trying to catch my attention.

The motivation and much-deserved attention finally arrived this year after enjoying last winter's NRR and hearing several great-sounding 2NT's on 80, 40 and 15m CW. I operated the week-long Roundup with my homebrew 'Longfeller' at 5W output, in order to qualify for the QRP category.

6AG7 - 6V6 'Longfeller'

I have to admit, the Novice Rig Roundup, was one of the most enjoyable contests I have entered in many years! Hearing many rigs from the 50's and 60's sounding so good was just a bonus. It was particularly gratifying to be able to work the midweek daytime activity on 15m ... a luxury that we may not enjoy now for several years if most of the long term solar forecasts are correct.

I eventually ended up with 68 contacts, but at 5W, it was a challenge ... most of my many 'CQ NRR' offerings were unanswered, so it was mostly a 'search and pounce' operation. With this in mind, I soon decided that next time, I'll get the 2NT ready to go and hopefully, with a little more oomph, can attract some callers.

Each entrant received an NRR certificate - a nice touch

Rather than hunt down and purchase the individual replacement electrolytics, especially the multi-unit can capacitor which was impossible to source, I ordered the '2NT re-cap kit' from Hayseed Electronics. They supply replacement kits for several boatanchors and are able to make and stuff multi-section can capacitors to match the original size and specs ... and all at affordable prices.

After cleaning the chassis and all of the switches, re-tubing with new tubes and removing / replacing all of the electrolytics, the 2NT was ready to go. Using a crystal for excitation, the following results were obtained:

         80m 103W input 75W output efficiency = 73%
          40m  92W input 65W output efficiency = 71%
          20m  92W input 65W output efficiency = 71%
          15m  92W input 55W output efficiency = 58%
          10m  92W input 50W output efficiency = 54%

Keydown voltages averaged ~520V while plate current ranged from 170-200ma. All-in-all, right on target and not bad for a 51 year old transmitter!

The 6EA8 modified Pierce oscillator in the 2NT seems particularly 'crystal-friendly', as every old style FT-243 style crystal that I tried sounded great. Even the newer ones from AF4K using a modern HC-49 crystal slab mounted inside a vintage FT-243 holder sounded great and worked perfectly.

My newly refurbished 2NT - ready to go

A couple of days later, I returned to the bench to take some blog photos of the rig delivering power into the wattmeter. When I turned the 'stby' switch to 'transmit', the plate current suddenly shot up (with no keying) and the meter on my Variac supply indicated over an amp being drawn by the power transformer ... all with zero output and no crystal plugged-in!

A look at the schematic diagram revealed that with the grid-block keying system, the final amplifier's cathode is always grounded and a low voltage negative bias applied to the grid keeps the plate current cut-off until being keyed ... with cut-off bias missing, plate current will soar, along with transformer primary current. I suspected that something had gone haywire with the bias supply.

A quick check of the 2NT's printed circuit board containing the bias system components among others, indicated a discolored silicon rectifier (D5) in the bias supply. An in-circuit measurement revealed that it was indeed shorted, basically supplying raw AC across the filter capacitor and shorting out the transformer winding, quickly elevating the transformer's temperature in the bargain ... not good.

Although the diode only has to handle a small amount of current, it was mounted with a heat-producing 2W resistor straddling its top surface. As well, the phenolic circuit board was noticeably discoloured from the heat. I suspect this was the main reason that D5 eventually failed.

My 2NT was an early one, serial #670, so perhaps this parts-arrangement was re-engineered in later models. Once the diode was replaced with a much smaller modern one, of twice the rating, everything returned to normal. The last thing I would want to do at this stage is to burn out the transmitter's unique power transformer, undoubtedly impossible to replace nowadays.

The next bench project will be to refurbish and improve the stability of my Heathkit VF-1 VFO and to mate it with the 2NT.

'55 QST VF-1 ad

There are several things that can be done to a stock VF-1 to improve its performance. Its wonderful backlit green dial will bring back a lot of memories from my teenage operating days with the VF-1 ... most, but not all of them, warm and fuzzy.

Another small quirk noticed on my 2NT is the meter mount. My early model has the meter mounted so low that the silk-screened 'PLATE CURRENT MA.' label is not clearly visible when looking straight-on.

My #670

Later models show that this was changed by moving the meter a little higher so that the label can be easily seen without having to look down inside. Nothing major, but something apparently missed during the initial panel design.

Later models

I'll have more to say about the 2018 NRR later, but if it's something that you may be interested in trying and you don't yet have a suitable novice-style rig, it's not too late to start looking, building or refurbishing.

You can read many interesting soapbox comments and see some nice NRR station photos from the previous two events here and here, but read with caution as you could easily get hooked. I find pages like this very inspiring and they remind me of the days when QST would publish photos, soapbox comments and exacting equipment descriptions used by each section winner in the annual November Sweepstakes contest.

Winner's gear in '55 CW SS - remember these?

NRR activity really skyrocketed last winter and I expect that it will be an even busier event next time. There are several CW contests that I'm looking forward to this winter and the NRR is right near the top of the list. I hope to see many of you in the 2018 NRR!

VE3AB Netty Electronics

While ordering components for my 630m transverter project, an internet search for some needed FT-37-77 cores brought me to the website run by Earl, VE3AB in NE Ontario.


Purchasing just a small number of parts can usually be expensive, considering the high cost of mailing these days but in this case, the toroids and a few other parts turned into a real bargain.  Earl is a retired Defence Department Manager and Technical Specialist and has one of the most colorful and informative sites that I've ever run across. The extensive, somewhat 'eclectic' site, might at first appear to be a bit disorganized but it really all works nicely once you delve a little deeper ... and there are many layers on Earl's site. It really is quite different and one thing that comes through is his passion for doing things the right way!

It seems that Earl's two main passions are fishing and supplying quality components at good prices. His dirt-cheap shipping to both Canada and the U.S. is something not often seen any longer, as shipping costs often make small purchases totally uneconomical. Forgive me if this sounds like a blatant plug for Earl's business ... it is not. It's just that I found his site extremely interesting and very different from most ... the fact that he is offering something that many hams might find helpful, is my main motivation.

The introductory 'ham radio' page contains, among other things, a collection of Earl's articles describing repairs on many solid state transceivers while his 'Netty Electronics' page describes his small home-based component selling and acquisition activities. His 'ham electronics magazine' page contains articles on working with circuit boards (re-capping, repairing, de-soldering), antenna articles as well as numerous articles on components, all authored by VE3AB. Earl's website and free services are all financed by personally using monies obtained through sales of parts and part radios as he sees it is a service to the amateur radio hobby, which he obviously loves deeply.

Earl specializes in solid state devices of all description, from vintage to modern. He offers a vast selection of parts for older radios and has information on repairing dozens of older solid state rigs. All parts are tested or guaranteed to be working within specs.

His pages contain extensive information describing his tests of popular Chinese-made transistors, such as this one describing a batch of dual-gate MOSFET BF981 transistors. His 'test' pages are definitely worthy of your attention and, for many, might be a bit of an eye-opener.

If you are looking for a manual for an older rig, Earl has an extensive offering of free manuals on his site. If it's not there, he can probably get it for you.

He is continually buying up collections of older components, as well as new parts ... if you have stuff to unload you might wish to check with him as so many of these valuable parts end up in the trash as shacks go silent. His extensive list of components can be found here. ... you may just find that special part you've been looking for.

One of Earl's goals for the website is to put up a search-engine for his parts but in the meantime, I found my browser's "Find" function worked well in sorting through the large list.

There is enough interesting information on his pages to keep you reading for hours and I find the random nature of much of it rather attractive. Earl says that changes to the site are coming but I hope they're not too drastic ... I kind of like it just the way it is!

1933 Regen Project

Monday was spent finishing the coil winding for my present bench project, a 3-tube regenerative receiver. The receiver first appeared in the January '33 QST article, "Rationalizing The Autodyne", by George Grammer. I tried to stay true to the original design, both physically and electrically, as much as possible. Where period components were not available, they were manufactured. Several capacitor are 're-stuffed' cases and some resistors have been physically altered to resemble era-appropriate styles. The one deviation I made from the original circuit was to remove the B+ (200VDC) from the headphones by going to impedance-coupled audio, which blocks DC from the headphone circuit. After finishing the coils and doing a quick re-check of the wiring, I held my breath and applied the power.

Much to my surprise and delight, the receiver worked immediately. It took me some time to find the correct control settings and combinations that seemed best but eventually its operation became familiar.

Now although the receiver appears to be working, I'm not 100% convinced that it is working as well as it should. It's difficult to know exactly how it should perform when at its best as it really can't be compared to any of our modern-day receiving systems. Just what did 'good performance' sound like in 1933?

I have some reservations about one of my tubes. It's a '78' from my junk-box stash and has a large red '?' inked across the glass envelope. I may order some NOS '78's or even some more modern '6D6's, to see how they perform. On the plus side, the oscillator seems very stable and there is absolutely no 'hand-effect' on tuning, a common problem with many regens. Peaking up the RF stage tuning causes no pulling of the oscillator but the '78', although a hot-performer in 1933, provides plenty of tube noise as well, if the gain is set too high. I think the main advantage of having the tuned RF stage is to add RF selectivity and eliminate any bleed-through from some of the blowtorch shortwave signals just up the band. Tuning seems best when its gain is set between fifty and seventy-five percent of maximum. The large drum dial, in combination with the small bandspread capacitor, spreads 40m across most of the dial and makes for easy tuning although there is a small amount of backlash.

Another thing I noticed is that it seems to be very receptive to spurious computer birdies, with the laptop sitting just a foot away. Perhaps its unshielded top and bottom are the cause of this as I don't hear any of the signals on my main station receiver.

Comparing it to the two-tube regen in my Paraset, the Paraset sounds much quieter and generally sounds more sensitive ... but it uses more modern tubes, '6SK7's, first introduced in 1938, five years after the '78'. The selectivity seems as good, if not better, than the Paraset, which I consider an excellent performer. The Paraset also sounds just as good as my National SW-3, which was produced in the early 30's ... so the new regen is likely not as quiet nor as sensitive as the SW-3, probably the best simple regen of its day.

Perhaps some newer tubes will make a difference in performance and maybe my expectations are too high. In any event,  the closing paragraph of Grammer's article may be more telling that I originally thought:

"The set as it stands is not perfect, of course; nothing ever is."


**** FOR AN UPDATE OF FURTHER TESTING, AS WELL AS A RECORDING, PLEASE CHECK OUT MY "BUILDING A 1933 REGENERATIVE RECEIVER" WEB PAGE ***

Crystals Go To War

Thanks to a recent posting in the Yahoo ParasetBuilders Group, I have a new-found respect for my small collection of prewar crystals! If you've ever wondered how a rough chunk of quartz gets transformed into an accurate frequency-generating device, this 'cinema-style' documentary shows exactly how it was done ... truly an amazingly complex, yet delicate, labor-intensive process.



After viewing the documentary, I can't help but wonder what later health effects some of these workers may have undergone after seeing them handling some nasty-looking chemicals and working directly beside desktop X-ray machines. If you've used old WW2 crystals before, I think you'll enjoy seeing how much work went into their production.

International Radio Restoration Contest

I have recently been made aware of the Socété Québécoise des Collectionneurs de Radios Ancien's / (SQCRA or Quebec Antique Radio Collectors Society) 'Radio Restoration Contest' and have been enjoying some of the published documents describing various refurburations.

Although the group has been sponsoring the refurb contest for over 10 years, this is only the third year that it has been open to international competition. The rules are interesting and are quoted from the SQCRA website:

" ...  participants have one year to restore a basket case radio (the worst it is at start, the more points are awarded for the difficulty). Pictures must be taken before the restoration starts and at all steps of the process. One year later, the participants present their work to the international panel of judges. Pictures taken during the process will help judges better understand the challenges faced by the participants in order to finish their project.

The clubs that don't have a contest can nominate someone or make a group effort to represent their club at this contest.

A documented report containing photos and explanations and optionally a video of the working set from each contestant must be submitted to us. Then judgement and results are compiled to determine a winner and two runners up.

The criteria's for evaluation are available in this document .

Our goal of course is to promote the conservation of the technological / historical heritage, to motivate our common interest, increase the general knowledge of ancient radio technology, gain restoration tips, increase club exchanges, and see what is done in other clubs."

   

Each project is judged on three basic criteria: difficulty (condition when found), restoration (chassis, cabinet, components, overall) and functionality when complete.

The present contest has just ended (March 15) but the judge's comments and project writeups from the previous two contests (as well as this year's project writeups) are available for reading ... and they are both instructive and inspirational as I found several new constructive hints embedded in the descriptions. 

Particularly interesting to me was the sidebar in the writeup article presented by Gerry O'Hara of B.C.'s SPARC Museum. I have been struggling to develop a method of building this period-correct component for several years and the solution looks elegantly simple!

courtesy: http://www.sqcra.org/

There really is enough reading here to keep one entertained for days but the more I read, the more I want to find another old clunker and bring it back to life ... great stuff!

'29 MOPA - Small Step

This morning I completed the wiring of the test-bed MOPA's Hartley oscillator circuit. I was using the 80m tank at the time, wound with 3/16" copper tubing. Oscillator keying is about what I expected ... stable but with some ripple on the note. Even with DC on the filaments, many of these open-breadboard oscillators suffer from RF-modulated notes. Sometime wrapping the power supply or keying leads in a ferrite toroid help or eliminate the slight rasp. On the other hand, some builders prefer to have a note that sounds more '29-like, as in all likelihood, not many notes sounded like pure DC back in the day.
I'm really having second thoughts about my construction method and may just jump to the next (final) construction phase rather than slog through completion of the test-bed model. I've already learned much about the layout by getting to this stage and the extra work involved in completing the test-bed model may not tell me anything new.



I'm using a VT-25 version of the somewhat pricey and hard-to-find type '10' tube. The final version will be built in a similar fashion to my Tri-Tet-Ten, on an aluminum sheet atop the breadboard. Hopefully having the groundplane and shorter leads, will lead to a cleaner note in the final version.

I'm rather dreading this next final phase as there is little room for error. The aluminum sheet must be precisely pre-drilled, as well as the breadboard, and protected at all times (especially when making soldered connections) to prevent any finger marks or scratching on the aluminum sheet. There are still several drilling details to be worked out before I can move forward.

'29 MOPA Progress

Another few small steps were taken this past weekend in the construction of my 1929-style MOPA, previously described in earlier blogs. Now that all of the parts have been either procured or manufactured, I've been playing with developing a suitable layout.
I've always liked the longer, skinnier appearance of breadboard rigs compared to the short, deeper ones and have combined this look with my love of the early 30's style. This was a transitional period in construction methods, as fabrication slowly changed from breadboard to metal chassis construction. The period in between involved mounting a metal sheet on top of a wooden breadboard, taking on an attractive 'art-deco' appearance, also popular in this period. A good example of this type of construction is seen in George Grammer's landmark 'tritet' oscillator rig which I duplicated several years ago, in a much modified version, for 10m CW.

The new MOPA will take on a similar appearance, with an aluminum plate over a mahogany breadboard. The tentative parts layout is shown below, minus the metal plate. I used the photograph to plan the wiring and to make layout notes.

Amplifier on left - oscillator on right

Although this early 30's style is visually attractive and offers the advantage of shorter and more efficient ground connections, it is a bit of a nightmare to construct. Many of the parts must be both mounted and insulated on the groundplane, while other connections must go through and under the breadboard chassis ... but I think the overall result will be worth it.

In order to mount all of the tuning capacitors above ground, a set of hardwood spacers were turned on the lathe. The spacers have been coated with two coats of spray-on lacquer to finish them in appearance as well as to keep moisture from penetrating

.

One of the hardest vintage components to source are era-appropriate fixed resistors. Even when found, so many of the original ones have drifted in value and cannot really be trusted to stabilize. Accordingly, I have  manufactured all of the ones needed, using more modern brown-body resistors.

First, the color bands were sanded off and then reproduction "OHMITE" labels were made, very similar to the original style.

The labels were then glued to the resistor bodies and given an orange color-wash using highly diluted acrylic paint. It would be nice to find good originals but these should work as a suitable substitute and will certainly maintain their value for many years.

The next job will be to fasten all of the components to the test-bed, wire it up and test everything out before stripping and rebuilding the finished version ... lots of work still ahead.

My old single-tube Hartley is starting to look not so bad after all!

BK Bustle

The AWA Builders Group has been buzzing with activity over the past few days as many '29ers get prepared for the upcoming Bruce Kelley QSO Party. If you've been following the blog of late, you'll already know that the BK is the year's premier operating event to celebrate radio as it was in 1929....only self-excited oscillators are allowed (no crystals!) and only tubes that were first introduced in 1929 or earlier. Party activities commence this Saturday afternoon at 2300Z and run for 24 hours....same for the following weekend, only add 160m to the action.

You may hear plenty of activity in the nights leading up to the Party as various stations burn the cobwebs out of their handmade creations around 3550-3570 in the next few nights...you might give them a call if you hear them as most will be delighted to know that they're getting out of their backyard.

A few weeks ago, in the same spirit of preparation, I began work on a small amplifier for my Hull Hartley transmitter. Ever since entering the BK Party, I have been held hostage by the monster winds which always seem to show up here during early December. Self-excited oscillators connected to blowing antennas create an unusual-sounding signal...somewhat musical....up to about 30km, but after that, somewhat dreadful, as shifts in frequency between keying elements can be measured in tens of Hertz or more and make it difficult to copy a weak signal.

I put the amp together in the original 'ugly construction' style of the 20's, which I'll call 'ugly breadboard'. For the most part, period-appropriate components were used, as it was to be a 'proof-of-concept' project....to see if and how it would work and what changes might need to be implemented in a 'finished' version.

Amplifier stage followed by Hull Hartley exciter on far end.

The original circuit, shown below, was published in the February 1931 QST and penned by George Grammer (W1DF), the ARRL's Assistant Technical Editor. Since I already had the oscillator section, I built the amplifier only, making a few modifications.

Courtesy: http://www.arrl.org/

My changes involved removing the B+ from the exposed plate tank coil by putting C7 between the plate and the tank circuit, grounding the coil tap point and shunt-feeding the B+ directly to the plate. As well, the operating bias was changed from the original combination of grid-leak and external fixed-bias (always a pain) to grid-leak bias only. Since I planned to let the oscillator run and key only the amplifier stage, no external (extra) bias was needed. The original system of feeding drive via capacitive coupling from the oscillator tank circuit was maintained as was the 'plate neutralizing' system. With the plate tank no longer carrying B+, a wide-spaced variable capacitor was no longer required for C3. These changes are illustrated below:

Courtesy: http://www.arrl.org/

Testing went smoothly and with the help of a tuned RF-sniffing wavemeter (the actual one I built when I was a brand new 15 year old ham!), the amplifier neutralized beautifully. It was unconditionally stable, in spite of the numerous clip leads surrounding the board connecting to the separate Hartley oscillator in a somewhat haywire fashion....it really was a mess. As one other '29 builder (KK7UV) so perfectly described his latest workbench project, it "looked like the transmitter was on life-support!"

Surprisingly little drive was required from the Hartley and it was run at just 200VDC. The amplifier efficiency was around 50% which could probably be improved upon with careful attention to biasing values, shorter leads and more efficient antenna coupling. On 80m, the only band that I wound a coil for, keying was clean and very nice (an advantage of being able to run the oscillator at low level). Without too much effort, 15 watts out was easily obtained...far too much for the BK Party.

Tapping down on the tank coil for 40m operation proved to be a different story as the note sounded like a buzz saw...very raspy and nasty. To really undertake a fair test, a dedicated 40m tank coil would have to be used as well as a  much 'cleaner build'.
The rat's nest of unwanted coupling of both RF and AC ripple was showing through on this higher frequency.

For multiband operating events, such as the BK, things have to be efficient enough to make band changing and tune-up procedures quick and simple and in its present haywire form, the amplifier would just not be 'contest-friendly'.

All in all it was a very valuable building exercise as a LOT was learned. I am now convinced that for a true MOPA (Master Oscillator Power Amplifier) combination to do what is needed will require an all-in-one style of construction....both oscillator and amplifier on the same board, with short leads and little chance for unwanted parasitic-coupling between stages. With separate coils for all bands and calibrated dials, tuning and changing bands would be very much easier than trying to couple two separate units together. I can see why the struggle to get the original '29ers away from their simple Hartley's and TNT's and over to MOPAS went on for several years.






So once again I am at the mercy of the wind-gods and hoping for two back-to-back Saturdays of quiet BK-friendly air...in early December...on the west coast...by the ocean...yeh, right!

More Resistors From China

I suspect that, overall, very few hams actually take the time to read ham radio blogs. Of those that do, there is probably only a small percentage that take the time to read any post-blog 'comments' that might be made by others....so...if you missed the comments regarding my recent e-Bay resistor purchase, the following information may be of interest.

Steve, G1KQH, commented that he had found an even better deal on resistors from China at Banggood, in Guangzhou. I have heard good things about this company, which sells a large variety of items from electronic parts to the latest quadcopters. Steve reported a good buy on 1%, 1/4W resistors...50 different values and 50 of each....2500 resistors in total. The offering may be found here.

Courtesy: Banggood.com

Steve adds: "The values of resistor are: 10M ,4,7M, 2M, 1M, 390K, 330K, 300K, 220K, 200K, 180K, 150K, 100K, 82K, 68K, 47K, 33K, 37K, 20K, 18K, 15K, 10K, 9K1, 8K2, 6K8, 5K6, 4K3, 3K9, 3K3, 3K, 2K7, 2K4, 2K, 1K5, 1K2, 1K, 680R, 470R, 300R, 270R, 180R, 91R, 68R, 56R, 47R, 20R, 10R, 6,6R, 1,8R, 1,2R , 1R.
They are five banded resistors inc one for the tolerance.
Arrived quick and you can use Paypal on Banggood, so no problem getting your money back if things go wrong."

With the free shipping, this is a super price and could be very difficult to beat! In addition, they offer smaller 1/4W resistor kits, a nice kit of 1/2W resistors, SMD kits and capacitor kits.

G1KQH maintains an interesting amateur radio blog, well worth a read. Thanks for the info Steve.

'29 Style Transmitters...What To Build? - Part 2

When it comes to putting a transmitter together for the BK, there are a lot of choices! My advice for a first-build '29 rig would be either a simple TNT or a Hartley-style oscillator. Both are easy to get operating and, when correctly optimized, are capable of putting out a nice-sounding signal.

For a first-time build, with the main objective being to have something ready in time for the December BK, I would not be concerned about overall appearance or period-appropriate parts. For now, the only thing that must be period-appropriate is the tube...improvements can come later.

As well, I would not be overly-concerned about running the maximum power of 10W input. If you are able to get a type '10' or the equivalent VT-25, or a pair of 45's, then the legal-limit is easily within reach. Utilizing something smaller, at just a few watts output, should not be considered a deterrent, as BK-operators all seem to have very good ears. Just 2 or 3 watts will guarantee plenty of contacts no matter where you are located!



Shown above is a fine little TNT built by Kevin, WB2QMY, in New York. It uses a very affordable UX-201A triode, originally manufactured in 1925 for radio receivers. Although Kevin's TNT puts out barely 2 watts on 80m CW, we had no trouble working each other in a recent BK QSO Party. If you build it, they will come!

If you prefer to tackle a TNT, here is the information you will need. This circuit appeared for several years in the ARRL Handbook's transmitter-section. I suspect that it was probably built by thousands of young hams in the late 20's and early 30's and affordably introduced most of them to the magic of radio. For more building details, including how to keep high-voltage off of the main tank coil, see the information on my website describing the TNT project.


Should you be interested in tackling a Hartley, here is an interesting circuit described by Nick, WA5BDU.

Courtesy: WA5BDU

Nick's circuit uses the readily-available '27 type, popular because of its indirectly heated (modern style) cathode. Nick fully describes his project here.
Such a transmitter would readily lend itself to a parallel arrangement of two or more triodes, such as the 27 or the 45 and would develop good power levels inexpensively.

ABØCW has designed a Hartley oscillator using a pair of 27's in parallel and uses them to drive a small amplifier. As described on his website, the oscillator would make a fine stand-alone transmitter with a simple link-coupling antenna circuit.

Courtesy: ABØCW

You can find a list of '29-style related building links, as well as a gallery of transmitters constructed by others, at the bottom of my TNT web page here.

And....circuit ideas, help with parts and lots of BK-chat can always be found at the  AWAGroup.io where the focus is mainly on building and operating.



'29 Style Transmitters...What To Build? - Part 1

Most hams in the late 20's and early 30's seemed to be using simple transmitters...one or two tubes at the most. No doubt the poor economics of the time made it difficult to build anything really elaborate but it didn't seem to stop them from getting on the air with whatever they could put together.

New rules for amateur radio signal stability came into effect in 1929, making that a pivotal year for amateurs, resulting in several new ideas for 'modern' transmitters hitting the publications of the day.

All of the simpler, one-tube transmitters, were self-excited oscillators capable of pretty good sounding signals when operated correctly and when the wind wasn't blowing the antenna around. Being directly-coupled to the antenna meant that any variations in antenna impedance caused by antenna wire movements, would result in  the note rising or falling in frequency by several hertz ... with the resultant 'musical-sounding' note. You can usually hear lots of these during the windy winter BK Party!

One of the most popular first-time transmitters in the late 20's was the one-tube TNT (Tuned-Not-Tuned) style, as it was such a simple design.

With a low parts-count, it was easy on the pocketbook. The grid coil was broadly resonant near the plate circuit's frequency, resulting in enough feedback to sustain oscillation. The grid coil really needs to be optimized (a simple procedure) for peak efficiency and note quality. You can listen to the note quality on my own TNT while transmitting on 40m.

The Hartley oscillator is just as easy on the pocketbook as the TNT, and in the opinion of most, capable of an even better-sounding note once properly optimized.

This transmitter is optimized for best note-quality and plate efficiency by finding the correct tap point on L1. Hear my own Hartley's tone while transmitting on 160m.

The third popular transmitter of the 20's was the TPTG (Tuned-Plate-Tuned-Grid).

The TNT, mentioned above, is a variant of the TPTG and similar in operating principle. Although the addition of a second variable capacitor added to the TPTG's building costs, optimizing performance was somewhat easier since the grid circuit could be more readily adjusted for the oscillator's 'sweet-spot', without having to add or remove turns on the grid coil.

All of the triodes mentioned in my previous blog will work well in the above circuits. Keying is normally accomplished as shown below, by connecting the balanced filament resistor on the directly-heated cathode to ground.

It is important to know that construction need not be fancy to get on-the-air for the BK and there is no reason why modern components cannot be used, along with the era-appropriate tube. If you're still undecided, I'll give you some building details to consider next.