The AM Forum

I've heard stories about ops zorching components while frantically trying to adjust antenna tuners and tank circuits to get the PA current down from "pinned"  but some rigs lack a reduced power "Tune" mode. I have a Johnson Ranger and Valiant and have been very careful about establishing the proper settings before keying up for the frequencies I often use, but what about frequency/band hoppers who like to chase DX?

I hope I have an answer for you! The simple Tune Up Load circuit shown below connects the transmitter to a dummy load and lets a small amount of RF get through for antenna tuning. This lets the operator tune the PA then turn attention to the antenna. The greatly attenuated signal reduces the detuning effect of the untuned load while also reducing QRM. I've noticed that my Johnson Matchbox requires very little drive to establish a match.

Once the transmitter and tuner are adjusted, the DPDT switch can be placed in the Operate position, placing the full output to the tuner for final tweaks.

The value of R depends on what power level you need for antenna tuning and can be easily calculated. The resistance should be high enough so that there is sufficient buffering.

Three connectors (such as SO-239), a properly rated DPDT switch, and a resistor are all that's needed and they can be mounted in a small aluminum box.

Many ways to skin the proverbial cat. KISS ;D

The smaller number of things between TX/RX & antenna the better, fewer parts & pieces to go wrong. The setup at this QTH has the standard TX, a RX, TX/RX relay, and tunna. Using the gray matter between the ears and the ears; with the receiver connected to the antenna, adjust the tunna for the loudest signal. It is OK to cheat and look at the S meter or carrier level meter.

This should get one very close to a 50 ohm resistive load for the TX. Key the TX; dip & load. Then using a #2 pencil & paper write the settings down for bands/frequencies. Do this once and you are done!

YMMV
Craig

Most all exciters had drive controls, just reduce drive while making adjustments, and raise/adjust as you approach desired OP level.

A few Amp makers had "tune" provisions, such as Gonset's GSB-201, where they put a 100W 4K resistor in the B- lead.  
(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/GSB%20201/G2011_zps1381932f.jpg) (http://s670.photobucket.com/user/mikeinkcmo/library/Radios/GSB%20201?sort=3&page=1)

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/GSB%20201/TuneTransResistor_zpsceoevgb5.jpg)

I think he is referring to stand-alone transmitters. Transmitters like the Valiant or DX-100 had no Tune/Operate switch or a "tune" position. Johnson Ranger does have a tune position. Heathkit Apache and the B&W 5100 series had a tune/operate switch. I'm sure there were others.

True no "tune" position, but, they all had drive level controls.   Even sophisticated transmitters like the Harvey Wells had them. ;)

When first setting up a new antenna tuner or using a tuner with a new antenna I use a MFJ antenna analyzer to adjust the tuner and then log the settings.   These make short work of setting the tuner.  Afterwards, returning to the logged settings are easy and a slight mismatch certainly won't bother vintage gear.

Like Mike states just turn the drive control down to avoid excessive final current during initial tuning although again using logged settings gets you close and is easier on the gear.  I used to contest a lot with a Drake C line and SB-220 and switched bands frequently, often many times per hour.  The Drake control markings were close enough to get you really close to final settings and I used a couple of index cards resting on the SB-220 Tune and Load shafts marked for bands so band changes were very fast.  I don't contest much anymore and I use a modern no tune rig and the homebrew amp I built has turns counters for the roller inductor and vacuum plate capacitor, the load control uses a conventional capacitor with a large calibrated dial.  With its large reserve dissipation and low drive requirement the amplifier logged settings are fine for making quick changes without further tuneup.

I use a military surplus ME-165 for tune-up.  It has the switch that N1BCG recommends, plus the dummy load, plus a sensitive meter for SWR.  They are fairly easy to find on the surplus market; Fair Radio used to (still have?) have them at a reasonable price.

I use a ME-165 also. I love it. It does not seem to have the same function as the OP device but could be changed.

Here are some of those nice 'cone' ceramic heaters that can be put in series with plate transformer primaries.

50-200W
http://www.zoro.com/g/Edison%20Screw-In%20Infrared%20Heater%20Bulbs/00061931/

400-600W
http://www.processheating.com/index.php?page=Coneheater2

slim/flat chassis-mount 20-1600W
http://www.tempco.com/Strip%20Heaters/strip_heaters_hub.htm

My intention was to design a circuit that would be simple, useful, and easy to build. A new development is the use of an incadescent bulb as "R". This allows the user to select a bulb to suit their antenna tuner drive requirements. A chassis mount socket or ceramic base could be used.

As an example using a Viking Valiant (about 130 watts) and a Johnson Matchbox, a 60 watt bulb provides just enough RF to allow the SWR CAL level to reach full scale. Disconnecting the antenna or shorting the terminals did not significantly affect plate current yet allowed enough RF through to accurately tune the Matchbox.

You may notice that an SPDT switch could be used, but the DPDT design prevents the RF from flowing through any more of the circuit than just the switch. This becomes more important as frequency increases.

Can't beat a light bulb for flexibility and economy. As R it would be a nice way to limit the current and give consistent adjustability under different power conditions.

Most valuable tool in my shack. Never lies. Feed with coax and have good connections. Feedline only as long as need be. Solder connections.  I use it with a coax switch. Tune on lamp switch to ant to transmit. Usually without retrimming.
Watching FS meter in shack radiated RF is minimal when load is being used. Usually about 1/10th of antenna measured about 4 feet from transmitter. If total shielding is desired mount inverted inside a steel paint can and drill a few vent holes so you can see and so heat wont build up. A very handy practical device. Does not respond to signal that isn't where it should be. for those who believe this is a good antenna a challenge. Go ahead work me. I know you cant. Tested and approved.
don

Found one of these and I like it. Changed out the N connectors and put it in line.
See pictures.

Fair still has them but they're up to $175 plus shipping... not sure if that is considered "reasonable" or not.

I made my own with ten 470 ohm "sand" wirewound power resistors mounted in a circle of copper strip with a muffin fan blowing through it, an air variable in parallel to tune out the inductance, and an RF ammeter on the panel. No detectable SWR on 80 and 40, and the cap can be tuned for the higher HF bands.  8)

$115 Shipped for my ME165/g

Unlike today's plug n' play hams the old buzzards know what they're doing and do it well. I aught to know, I am one.

"Can't beat a light bulb for flexibility and economy."
Don't forget they make great antennas for local chit-chat and you can watch them blink with the modulation. If you're running a full gallon I guess you'd need one from an airport beacon. Eh, scratch that, I hooked one up to the 230V mains and when I threw the switch it not only blinded me but I nearly got zorched from the searing radiant heat!

I have a 500 watt lamp for use as a dummy load for the high power stuff.

350W carrier, 1400W pep that's allot of RF exposure in the shack. I guess you could put the bulb in a can like a previous reply stated. Don't forget the heat vent holes. 😊
For the cost of a good bulb, socket you can by a Cantenna at a hamfest for $20-25 bucks. But then if you need a night light or tanning bulb go with the light option. Hi hi. ☀️

My Five Hundred has a "Tune" position at reduced power. Just peak the plate & load, unkey & switch to operate & you're in the ballpark.

The Valiant doesn't have this circuit but I have a Excel spreadsheet that I put all my settings on for the boat anchors including the tuner. A quick glance, turn a couple knobs and you're there.