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Modifying the Dentron Super Tuner

Antenna Tuner

 I’ve owned quite a number of antenna tuners, both commercial and home-brew throughout my 50 plus years of ham radio.  One of the most practical and useful tuners I ever owned was the one made by the Dentron Company back in the 1970s and early 80s.  The one they called the “Super Tuner” is small but will match a wide range of loads.  It was 10 1/4” wide, 5 1/2” high and 9 3/4” deep and had a nice black finish. 

The tuner was the popular circuit with two variable capacitors in series with the input and output coaxial sockets and a tapped inductor between the junction of the two variables and ground.

 

I’ve seen various power ratings assigned to this unit but I think it was typically advertised as a KW tuner.  I would not hesitate to run a 1200 watt amplifier to it and perhaps even higher if the tuner was not required to deal with an extreme SWR.  The box sold for $99.99 back during its day and I’ve seen them sell for up around the $150 range on ebay, even in this day and age.

When I owned my ham store in Broken Arrow, Derrick Electronics, I sold many of these tuners and everyone always seemed happy with their purchases.  The tuner has insulators in the rear apron to attach a single wire, a coax output, or a balanced feedline via the internal 4:1 balun. 

 

To use these various outputs, one must disconnect the others, i.e., to use the coax output, the feedline to the balanced line must be disconnected or to load up a single wire antenna, the coax must be removed from the output coax socket.

 

I use my Super Tuner when camping in my trailer and I’m never sure which type of antenna or feedline I might be using so I use them all, however, I thought it would be much easier to use a switch, accessible from the front panel, to switch the tuner circuit to the desired output appendage and remove the connection to the other, unused ones.  While I was at it, I decided to add an SWR Bridge, a low and high scale wattmeter, and a dummy load.  That prompted a desire for an additional feature of a hang-time peak-reading on the wattmeter.

 

This is the revised schematic for my Dentron Super Tuner 160-10.
The switch I used is a 3 wafer ceramic 6 position switch but I’m only using 5 of the 6 positions.  This allowed me to select the 5 positions of: 1 – Wire antenna 2 – Balanced antenna (using 450 ohm ladder-line, etc.) 3 – Tuner to output to coaxial Antenna socket 4 – Bypass tuner – Input connects directly to Output 5 – Dummy load

 

The dummy load I used is the same non-inductive resistor used in the original Heathkit HM-31 Cantenna.  As a matter of fact, I bought this one sometime in the 1970s from Heath Company as a replacement part for the Cantenna.  Dentron also used the same resistor in their MT-3000A tuner as a built-in dummy load.  Obviously, these sources are no longer available but you could add an additional SO-239 coax socket to the back apron and wire it as shown in the schematic.  If you have a Heath Cantenna, a Dentron “Big Dummy,” an MFJ Dummy Load or any other good 50 ohm non-inductive load, you could plug it, via a piece of coax, into this added socket and accomplish the same thing.

I also added an SWR / Wattmeter circuit to allow easier tune-ups without have additional external pieces and extra coax jumpers.  The wattmeter has two ranges:  0-100 watts and 0-1000 watts.  The front panel also has a switch to allow for peak reading of the power.  Since the meter movement cannot respond as quickly as the human voice, the peak reading feature lets you see a bit more accurately how high your output power is peaking.

 

The SWR bridge even has an extra RCA phono socket on the back to allow the user to plug in an additional meter to read reflected power while the panel meter is reading the forward power.  I always intended to replace the front panel meter with a dual-needle meter so I could watch both forward and reflected power simultaneously but I just never quite got around to changing out the meters.  The extra plug-in reflected meter accomplishes the same goal.

 

The small meter used on the front panel comes from one of the cheap CB SWR bridges sold for many years.  The pick-up circuit I use is more accurate than the one used in the cheaper CB bridge but the meter comes already printed with a calibrated SWR and wattmeter meter scale so that gave it a nicer appearance.  I picked up my cheap CB bridge at a garage sale some years ago for, I believe, a dollar.

 

The original tuner unit is well built on a plated steel chassis.  The built-in 4:1 balun is heavy duty and is the same one Dentron used in their MT-3000A tuner that was rated at 2000 watts.  The pictures below show the Dentron Super Tuner as it was originally, prior to my modifications.

 

 

 

Here’s what my modified Super Tuner looks like, looking at the front panel, the insides, and the rear part.

 

The basic schematic for the Wattmeter/SWR Bridge is shown below:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The new schematic for the tuner is shown above when the additional pieces are added, the complete schematic for the wattmeter and tuner looks like this:

 

If you prefer to wire your tuner with the dual-needle meter you can use this schematic but it only shows it wired as an SWR bridge without the wattmeter calibration components:

The wattmeter is made using an Amidon T-68-2 core.  This is defined as .68 inch OD and the –2 designates the frequency range for this core.  The –2 core will be painted red.  I wound 45 turns of #20 enameled wire around the core and it is center-tapped.  The best way to do this task is to get a long piece of the wire in hand, find the center and scrape off the enamel for an inch or so.  Twist that into a loop and wind the turns on the core.  I would have used a bit smaller gauge wire if it had been available.

 

 

The diodes used were silicon 1N914, which are just general-purpose units from my junk box.  If I were buying new parts, and particularly if I expected to be using lower power transmitters, I would try to find a matched pair of germanium diode units like 1N34s.  The germanium diodes have only a .2 volt junction drop whereas the silicon units have a .7 volt drop.

 

 

The potentiometer that sets the forward and reverse SET position can be other values but should always be a dual-pot where both pots are the same value and move together. Since the Low/High calibrations are in series, adjust the Low power pot first, then the pot for High power.

 

The capacitor used for the peak-reading feature is a tantalum type capacitor.  There is a lesser possibility of the capacitor changing its value over time with this type.  I used a 33 mfd/10 volt cap because I had that in my junk box and that value seemed to give me about the right hold time on peaks.  I just noticed that my schematic shows this as a 33 pfd but that is incorrect and should be mfd.

 

 

The switch used to direct the tuner to the various outputs was also a junk-box special and is held in place by the screws which hold the wafers together.  Originally the switch had 4 wafers so the screws are too long after one wafer was removed.  The extra length of the screws is noticeable in the rear view of my tuner as they stick out for an extra inch or so.  I always planned to trim them down to something neater and, perhaps, some day I will.

If you were buying a new switch you would want a 3 pole-5 position non-shorting type and I prefer ceramic wafers since it’s carrying RF voltages.  Many switches like this are actually 12 position for full rotation and if you want it to turn fewer positions, you can place a mechanical stop to limit the rotation to 5 or whatever you desire.

 

 My tuner now does not look much like the original Dentron Super Tuner but the added features have made it much more useful and practical for my use.  It will match the proverbial bedspring and, in the process, show you your SWR and power output.

Created January 13, 2009   

 

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Last updated 04/07/2009 12:22:25 PM

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