Recent discussion on the RSGB LF Group reflector about high-powered LF / MF amplifiers brought an interesting response from Tom, DK1IS, and his unique solution.

It's no secret that a Class D / E amplifier using switching MOSFETs is a popular and reasonably inexpensive method of generating some serious RF on the LF and MF bands. Equally well-known is their propensity to gobble-up FETs should the amplifiers encounter much reactance in their output load. Most builders include some form of protection for sudden over-current or unwanted SWR excursions which will shut down the amplifier before any FETs can self-destruct. Those that don't usually end up replacing FETs.




I would venture to guess that over 90% of the transmitters now being employed on LF or MF are using switching MOSFETs in a Class D / E design but there are some amateurs using vacuum tubes to do their heavy-lifting ... and with good results.

DK1IS's beautiful homebrew amplifier is shown below. Tom provided the following description:

Hi Wolf and group,

nice to hear that someone else is thinking about this approach! I´m
content with my homemade tube PA for LF and MF which has provided
reliable service since nearly 4 years now. Only some thoughts about this
concept - I hope not to bore all those hams who are happy with their
semiconductor PAs:

Years ago I had a MOSFET PA for LF, Class B push-pull with 250 W RF. It
worked well at constant conditions, but when I had to retune the antenna
due to larger QSY or made antenna experiments there always was the
danger of blowing up these nervous semiconductors. After 4 or 5 times
changing the MOSFETS I decided to build a new PA - with tubes! Looking a
little bit anachronistic this PA is absolutely good-natured. Designed for
broadband service on LF and MF it makes no problems when changing the
antenna coarse tuning from one band to the other even when the fine
tuning isn't done yet. With my former MOSFET-PA this would have been
impossible.

I wanted to have a linear PA - this usually means class B. You have to
decide between narrow band and broad band (like an audio-amp) design.
For narrow band you can use a single-ended PA but you have to add a
resonance circuit. For broad band you should use a push-pull PA and have
to build a suitable output transformer. I opted for broad band design
because it is usable for LF and MF without changes at the PA. With this
design and sin-driving I reach a total harmonic distortion of about 5 %
at 700 W RF on a pure resistive dummy load. With the usual narrow,
narrow band antennas on LF and MF you don´t need additional filters!

Concerning the tubes: If you take the common TX tubes with plate
voltages of several kV all output circuits have rather high impedances,
that means large coils for the resonance circuits resp. large
transformer windings and very high voltages - potentially a construction
problem. This led me to the choice of 2x 4x PL519 in push-pull, a rugged
colour TV line output tube with low plate voltage and high plate
current. In this way I came down to a plate-to-plate resistance of about
1 kOhm at 600 V DC plate voltage, where you easily can build a ferrite
broad band output transformer down to 50 Ohms. A disadvantage of this
concept is that you have to give individual bias to each tube, that
means for the first start-up you have to align 8 potentiometers
carefully to nearly equal cathode currents for all the tubes. But
according to my experience this alignment remains stable over a long
time. I have inserted 1-Ohm-resistors in each cathode line and have
brought the voltage drops to 8 cinch connectors, where I can monitor the
DC component (with external filtering) as well as the real time current.
With 4 tubes in parallel per branch of course you have to take care for
self oscillations. The extensive use of bypass capacitors, ferrite beads
and parasitic chokes in the plate lines is mandatory as well as good
grounding concepts are. The tubes don´t pull control grid current (this
would even be true in class C!) but you need 3 or 4 W RF input power due
to all the ohmic loads at the tube´s control grids caused by the
individual bias paths. On the other hand this certainly helps to avoid
oscillations. You can see some pictures of this PA at https://www.qrz.com/db/DK1IS

By the way: why not to try these tubes at class D? With DC plate
voltages of perhaps 1200 V you should get a nice QRO-PA ...

Wolf, you are right: building such a PA from scratch is a time consuming
enterprise. I didn´t count the working hours but according to my lab log
the whole project took about 9 months - an adequate time for a new baby!
It was a great experience anyway.

Good luck and 73,
Tom, DK1IS

2x 4x PL519 Push-Pull

TX, power supply, RX, exciter

As 'day one' for U.S. amateurs gaining their new LF / MF bands draws closer, many are seriously thinking about building or modifying rigs to get on the air. I'm sure the use of switching MOSFET amplifiers will still reign supreme when it comes to decision-making time but with some feeling more comfortable with tube technology, especially when used for power amplifiers, I suspect we will see some interesting tube transmitters being employed as well. I'm sure that some will even seek to modify older tube rigs, knowing that a pair of 6146's can muster enough power to have plenty of fun on the new band as well.

DK1IS has provided an inspiring example of what can be done using vacuum tubes ... they certainly should not be discounted as a viable method of generating your hard-earned LF / MF ERP.
Steve McDonald, VE7SL, is a regular contributor to AmateurRadio.com and writes from British Columbia, Canada. Contact him at [email protected].

Stories you’ll find in our April, 2017 issue:

Going Portable with Amateur Radio Satellites
By Keith Baker KB1SF/VA3KSF

Low solar cycle productivity and plummeting propagation got you down? Keith tells us that the sky’s the limit when you start operating the amateur radio satellites. It’s a low-power mode that is also versatile. But, don’t worry, working the AMSATs doesn’t require a lot of room at home, can be fairly inexpensive to set up and you can take this part of the hobby on the road too!

Community Emergency Response Teams: Are You Ready for a Disaster?
By Alan M. Vigeant N6HPO

Community Emergency Response Team (CERT) is a nationwide volunteer program supported by FEMA that educates people about disaster preparedness for natural and man-made hazards that may impact their lives when least expected. CERT classes are offered free to the public and go hand in hand with amateur radio training. Alan tells us how effective CERT is and how we can all help.

Outernet Update: Free, One-Way Internet Now Via L-Band Satellite
By Kenneth Barbi

Outernet is the low cost, one-way Internet service that delivers radio and news feeds to nearly the entire planet for free via L-band satellite. Designed to meet the minimal needs of those without modern infrastructure (grid power and Internet access), all you need to start taking advantage of this service is a $79 kit that includes antenna, LNA, SDR radio and CHIP single-board computer. Kenneth Barbi has the latest update on this evolving service.

Computers and Ham Radio: Part 010
By Cory GB Sickles WA3UVV

From the Radio Shack TRS-80 to the Apple II, to IBM’s first PC and the inevitable first PC clone, personal computing started out as an expensive device with very limited capability. From its initially slow start in the 1970s, Cory takes us through the fits and starts of the second phase of this fledgling industry that would change the radio hobby forever.

Motorized Ku-Band Satellite Reception
By Mike Kohl

There are dozens of Ku-band satellites in the skies over every part of the world. And, while Free-to-Air satellite TV systems are very inexpensive, you miss most of the action by not being able to steer your dish from satellite to satellite. But, not all small dish motors are the same. Mike shows us the best way to motorize your own FTA satellite system.

Scanning America
By Dan Veeneman
Marin County (CA), North Dakota and Radio Shack Bankruptcy (Again)

Federal Wavelenghts
By Chris Parris
Federal Aviation Administration

Utility Planet
By Hugh Stegman
New Life Evolves on MARS

Shortwave Utility Logs
By Hugh Stegman and Mike Chace-Ortiz

Digitally Speaking
By Cory GB Sickles WA3UVV
The Ground We Talk On

VHF and Above
By Joe Lynch N6CL
New Radios and Antennas for VHF and Above

Amateur Radio Insights
By Kirk Kleinschmidt NT0Z
Morse Code: The Original Digital Mode is Still Hanging On

Radio 101
By Ken Reitz KS4ZR
Baseball on the Radio: 1921 and 2017

Radio Propagation
By Tomas Hood NW7US
An Ice Age and Disappearing Sunspots?

World of Shortwave Listening
By Rob Wagner VK3VBW
Radio Australia: A Possible Return?

The Shortwave Listener
By Fred Waterer
VOA at 75; WBCQ, RRI, and RNZI

Maritime Monitoring
By Ron Walsh VE3GO
A System for a System

The Longwave Zone
By Kevin O’Hern Carey WB2QMY
Turning the Tables: Reverse Beacon Network

Adventures in Radio Restoration
By Rich Post KB8TAD
Philco 46-350: The “Tambour Door” Portable

Antenna Connections
By Dan Farber AC0LW
HF Antenna “Comfort Zone”

The Spectrum Monitor is available in PDF format which can be read on any desktop, laptop, iPad®, Kindle® Fire, or other device capable of opening a PDF file. Annual subscription is $24. Individual monthly issues are available for $3 each.

Ken Reitz, KS4ZR, is publisher and managing editor of The Spectrum Monitor. Contact him at [email protected].

In this episode, Martin M1MRB / W9ICQ is joined by Edmund Spicer M0MNG and Chris Howard M0TCH to discuss the latest Amateur / Ham Radio news. Colin M6BOY rounds up the news in brief, and this episode’s features is Portable Power Distribution

We would like to thank William Hester N0LAJ and our monthly and annual donors for keeping the podcast advert free. To donate, please visit - http://www.icqpodcast.com/donate

 

• New Ham Radio Regulations in Netherlands
• IRTS Ham Radio Publications Archive
• 21 New US Hams
• Pocket Prefix for Android
• Students in Belize Introduced to Amateur Radio
• ARRL Wants Exclusion from California Law
• 5000 Australian Spectrum Intruders
• Map of Italian Ham Radio Repeaters​​​​​​​

Colin Butler, M6BOY, is the host of the ICQ Podcast, a weekly radio show about Amateur Radio. Contact him at [email protected].

In this episode, Martin M1MRB / W9ICQ is joined by Edmund Spicer M0MNG and Chris Howard M0TCH to discuss the latest Amateur / Ham Radio news. Colin M6BOY rounds up the news in brief, and this episode’s features is Portable Power Distribution

We would like to thank William Hester N0LAJ and our monthly and annual donors for keeping the podcast advert free. To donate, please visit - http://www.icqpodcast.com/donate

 

• New Ham Radio Regulations in Netherlands
• IRTS Ham Radio Publications Archive
• 21 New US Hams
• Pocket Prefix for Android
• Students in Belize Introduced to Amateur Radio
• ARRL Wants Exclusion from California Law
• 5000 Australian Spectrum Intruders
• Map of Italian Ham Radio Repeaters​​​​​​​

Colin Butler, M6BOY, is the host of the ICQ Podcast, a weekly radio show about Amateur Radio. Contact him at [email protected].

Results from a study published by University of California Berkeley in The Journal of Psychology found that amateur radio operators are 45% more likely than the general public to believe fake news articles or material with misleading or outright wrong claims.  Researchers weren’t specifically studying amateur radio operators, but discovered the correlation by accident.  The report noted:

“We tested over 15,000 adult subjects and recorded profiles of each person… the common attributes like age, race, ethnicity, etc. and also their likes, dislikes, hobbies, diets, habits, etc.  We discovered a strong and unexpected correlation with those claiming to be amateur or “ham” radio operators after running all of the data through an analysis using a recently developed Reverse Polish Bayesian algorithm.  The unexpected data correlation continues to have the research team puzzled and further study is warranted.”

There is currently no proven explanation for the correlation, however one plausible hypothesis has emerged, that exposure to high levels of radio frequency energy combined with very fatty diets and long periods of television viewing, with political and news programming exposure, causing the frontal cortex of the brain to rapidly atrophy.  Researchers are struggling to reproduce the characteristics as lab mice either die or become very disinterested when exposed to the combination of factors, especially when frequencies in the 3.5 Mhz range are used in experiments.

This article originally appeared in Radio Artisan.

Anthony, K3NG, is a regular contributor to AmateurRadio.com.

Amateur Radio Weekly is curated by Cale Mooth K4HCK. Sign up free to receive ham radio's most relevant news, projects, technology and events by e-mail each week at http://www.hamweekly.com.

5. Antenna height will be limited:

... we will require that the antennas used to transmit in these bands not exceed 60 meters in height above ground level (AGL), as ARRL proposed.

6. Regarding transmission modes, no bandwidths have been specified in order to encourage experimentation:

Consistent with our proposal in the WRC-12 NPRM,  and with the existing rules in Section 97.305 for the frequency bands below 30 MHz, we authorize amateur stations to transmit the following emission types throughout the new amateur bands: CW (international Morse code telegraphy), RTTY (narrow-band direct-printing telegraphy), data, phone, and image emissions.   These emission types provide amateur operators with maximum flexibility, and we find that additional restrictions would needlessly hinder experimentation.

7. Experimental stations appear to 'still be in business' but are encouraged to transition to the 'amateur' service:

Finally, we decline to permit previously licensed experimental stations – some of which have been authorized with significantly more radiated power than the adopted EIRP limits for these new amateur service bands – to communicate with amateur stations operating in these bands.  Amateur operations in these bands currently authorized under experimental licenses should transition their operations in accordance with the adopted rules and not circumvent such rules by use of experimental licenses.

My understanding of the R&O document is that participating parties may still file a 'Petition For Reconsideration' notification within 30 days of the R&O's publication in the Federal Register. Once these (if any) are dealt with, there are no other roadblocks preventing immediate implementation.

The document contains additional details not discussed here and makes fascinating reading for amateurs that might be looking forward to the new allocations.

This is the news that many U.S. amateurs have been waiting many years to hear! It is also good news for Canadian's operating on these bands to know that they may soon see a large increase in activity south of the border. Let's hope things continue to transpire favorably and that we will finally see the new bands become a reality.

Get those soldering irons out guys and gals!


Steve McDonald, VE7SL, is a regular contributor to AmateurRadio.com and writes from British Columbia, Canada. Contact him at [email protected].