Great AM Transmitters with high level Modulation

[n8fvj]

Most of you do not need this post, but newcomers to AM may find it useful.
The best transmitter IMO is not including commercial units or any transmitters over 80lbs. The Johnson Valiant is 83 lbs. The Collins 32V-3 is 110lbs. As for the Johnson Valiant it has some design issues and cannot fully modulate the output so the extra 50 watts is not worth it IMO. The Collins 32V-3 is a good transmitter with PI-L tank coil upgrade vs 32V-1 and 31V-2.

For a lot of us older hams 40lbs is about the limit to handle. The other light weight transmitter with high level modulation is the Multi Elmac A-54, AF-67 and AF-68. These transmitters require an expensive and rare power supply and the output network needs modifications for a 50 ohm antenna. So, I did not include the Multi Elmac.

The Globe Scout 65, 680 and Globe Scout Deluxe use a choke for Heising high level modulation. The design is simple vs a PP audio modulator. These transmitters are reliable and inexpensive. Note- the model 65 and 680 need a 160 to 220pF 1000 volt mica cap on the load capacitor to tune correctly on 80 & 40 meters. Also, W8JI Scout mods on the internet search details audio mods for 100% modulation however keep the 22K or 25K screen resistor and remove 10uF screen capacitor unlike W8JI mods. The 6L6 tube is too highly stressed after mods, use a Sovtek 6L6WXT+ heavy duty 6L6 or use a 7027A tube.

Three transmitters stand out. The Johnson Viking II, Johnson Viking Ranger or Ranger II and the Globe Scouts.

The Globe Scouts produce a 25 watt carrier and modulate to 100 watts output- perfect for a linear amplifier. The weight under 35lbs and are reliable. Many sell for under $100 on eBay.

The 70 Lb Johnson Viking II is a great 100 watt carrier design however requires an external VFO. The Wireless Girl stated likely the best AM transmitter ever built. View the few mods she listed on the internet. Nothing is wrong with this transmitter. The 5 Lb modulation transformer will produce 150Hz audio and it is 25% larger than a Heathkit DX-100 mod transformer. Some replace the audio driver transformer with a Hammond 124B. The stock 6AU6 audio tubes are good and stereo amps used the 6AU6. Use polypropylene .05uF for C2 cap. Increase the 6AU6 10uF cathode caps to 25uF. If using non amplified D-104 increase R1 to 3.3 meg ohm. RCA 6AU6 are not microphonic like some other 6AU6 brands. 1950s RCA 6AU6 black plate are good, Mullard 6AU6 are best sounding and also non-microphonic.
Use the 5R4s with the large base extending up the side of tube. Chatham is a good 5R4 manufacture. Regular 5R4 can arc, Chatham will not. Arcing is due to the large HV choke discharging at going from xmit to receive. I would install 13 1N5359A transorbs (zeners) in series at $1 each for 312 regulated volts at the 807 modulator tube screens. Adjust R13 to about 335 volts and then connect the transorbs. Finally replace the paper and electrolytic capacitors. This transmitter will provide service for many years and the tubes are commonly found.

The Johnson Ranger and Ranger II produce a 40 watt carrier. The transmitters are very convenient but do have some issues in stock form. The transmitters has a built-in VFO and are a light 40lbs weight. Although many state this transmitter sound great the tiny modulation transformer is reported to only provide 400Hz low frequency audio at 95% modulation. I addressed this issue in AMfone transmitter section titled Johnson Ranger Audio Mod. The VFO has a 2 watt resistor R3 that should be 5 watts. The Ranger is known to fail more than the Viking II and the Globe Scout though.

The recommended transmitters can be found in good cosmetic condition on eBay for as low as $300 ($100 for Globe Scout) and most are untested. $600 is becoming more common and that price is usually from Hams. eBay transmitters are usually from an estate or yard sale untested. If you have no tech skills rebuilt cost more at $700 to $1K. Buying from a Ham vs eBay is better IMO, but few Hams are selling either transmitter now.

Note- replace all electrolytic capacitors in these transmitters unless alredy rebuilt.

[K1JJ]

Yes, the Ranger is a very FB transmitter.  It's my favorite AM commercial transmitter.

Two years ago I tried to decide between buying a clean Ranger and modifying it heavily, or building my own version from stratch.

I ended up building my own "Commando," a two-6146 plate modulated transmitter good for about 90 watts carrier out. The modulator is a Chinese 140 Watt solid state module driving the biggest Hammond mod transformer available. All self contained including power supply and modulator. Below is the link to the entire project on AMFone.

It is one of my best efforts for a smaller rig and it works flawlessly, 150% positive peaks and 20 - 15KHz audio.  The project gets very interesting later in the thread.

Maybe it will give you some ideas. Good luck with your Ranger endeavers!

Tom, K1JJ


Commando:

http://amfone.net/Amforum/index.php?topic=47350.0

[n8fvj]

Yes, the Ranger is a very FB transmitter.  It's my favorite AM commercial transmitter.

Two years ago I tried to decide between buying a clean Ranger and modifying it heavily, or building my own version from stratch.

I ended up building my own "Commando" a two-6146 plate modulated transmitter good for about 90 watts carrier out.  All self contained including power supply and modulator. Below is the link to the entire project on AMFone.

It is one of my best efforts for a snaller rig and it works flawlessly, 150% positive peaks and 20 - 15KHz audio. The project gets very interesting later in the thread. Maybe it will give you some ideas. Good luck with your Ranger endeavers!
Tom, K1JJ
Commando: http://amfone.net/Amforum/index.php?topic=47350.0

A lot of work.

[W2JBL]

Built a new moblle transmitter this spring on an 80 meter ARC-5 chassis. Used the ARC-5 VFO, a 12BY7 buffer/doubler and a 6LF6 in the PA. Pi network output tank. Easliy makes a 125 watt carier 150% modulated. Bandswitched 80 and 40. Exsisting modulator
is Class B triode connected 6146's. I actually had to throttle the PA screen voltage back because my inverter power supply started running out of steam as the power out soared to 150 watts. This thing works so well I'm going to build anonther one for the ARC-5 base station system at my camp. A lot of work? Yes it was but the results were worth the effort. The 6LF6 mad ea super compact 100+ watt PA.

[KD6VXI]

Now find an M2057 and put 12 volts on the filament of the final.

The M2057 is the real RF tube put out by GE, based upon the 8950.  Mo Pdiss and MO Gain!  And, they run into 6 meters pretty well!

Great job on your tx though.  Sounds like a winner.  The 6LF6 is an amazing tube!

--Shane
WP2ASS / ex KD6VXI

[DMOD]

Another tube consideration is the octal socket version of the above, the 6LW6.

It was one of the later high power Sweep tubes developed for large screen color televisions using the NTSC format.

I have three of them and they have high power sensitivities, meaning very little RF drive is needed for high output power in Class C operation.

I tend to think of the 6LW6 as a 6DQ5 on steroids.