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Modifying the Tut 80 for Varactor Tuning and More Power

✇Dave Richards AA7EE
By: AA7EE
1 March 2024 at 16:36

Back in 2009/2010 the Arizona QRP Scorpions released a little kit designed by Dan N7VE, called the Fort Tuthill 80. It was a QRP CW transceiver, with a direct conversion receiver and a transmitter capable of putting out about 3W. It caught my imagination, and I just had to build it. I have never had a great antenna for 80, and don’t operate on that band a lot, but one of my successes with the Tut 80, was a QSO with the T32C DXpedition team in Kiritimati. I was running 3W from the Tut80 into a 40M coax-fed dipole! This made the QSO feel like a great victory.

The first run of Tut 80 kits was over fairly swiftly. This run was, I think, either 100 or, at the most, 200 kits. There was talk of another run, but it never materialized. Doug Hendricks began selling kits of the design for 160M and 15M through his QRP Kits website. Ownership of QRP Kits has now changed, and both Fort Tuthill kits have been retired. In other words, like a lot of QRP kit rigs of that era, if you have one, you are sitting on a limited edition classic!

I have never used my Tut 80 a lot, though every now and again it comes down off the shelf for a jaunt around the lower end of 80. There used to be a Yahoo Group for Tut 80 builders. Sadly, that wonderful repository of information on so many subjects was all lost several years ago when the plug was pulled on Yahoo Groups. One of those valuable bits of info was a post by noted QRPer Cam Hartford N6GA (now sadly, SK). He had modified his Tut 80 for varactor tuning, which struck me as a very desirable mod. The stock polyvaricon, though serviceable, didn’t make for the best user experience, in my opinion. The polyvaricon is just a little “spongy” in feel, making precise spotting a bit tricky at times. Also, I had added the suggested toggle switch to separate the tuning into two bands. It switched in an extra NPO capacitor for the lower band. Although it worked, I didn’t love the idea of using a toggle switch in a frequency determining circuit. Cam’s varactor mod had been on my mental list of things to do for years.

Several times over the years, I have come close to performing this mod, but never quite found the necessary mojo. Then, very recently, inspiration struck, the rig came down from the shelf, the Bournes 10-turn 10K pot that had been stashed in my parts drawer for a rainy day was retrieved from the parts drawer, and I got to work.

A side-benefit of making the decision to do this was that, in the process, I could tidy up the wiring a bit. At the time I assembled the Tut 80, my soldering was already good, but I had not yet become quite as focused on making the wiring to the boards in my projects quite as tidy. This photo of the Tut 80 before the modifications makes my point. Not a huge mess, but it could use some improvement. This picture was taken in 2010, when I had just finished building my Tut 80 –

The next few pictures were taken recently, just before the mods were performed. This wiring needs to be tidied up! –

The “Hi Lo” toggle switch in the above picture will be removed, as there will be no need for it. No idea what I will do with the hole.

The rig on my desk, with the polyvaricon and 10-turn pot getting ready to swap places. The 10-turn pot is made by Bourns. They are not cheap, but they are quality pieces. The part # is 3590S-2-103L –

Here is the schematic of Cam’s mod, as drawn by him, and posted to the Tut 80 Yahoo Group –

And here is what he wrote about it –

Sorry about the pencil/paper rendering. All of the parts came from my inventory. The MVAM108 was left over from another project, probably a 2NXX. The ten-turn pot came from a swap meet. It’s a Bourns 100K unit, but the value is not critical. A 50K or 10K would probably do as well. For starters, I did this more than ten minutes ago so short-term memory vacancy has taken it’s toll. I reverse-engineered the schematic but can’t really remember the why of some of the decisions. Lots of trial and error, no real science involved. It works, however. The radio covers the bottom 100 KHz of 80 meters almost exactly. As far as linearity is concerned, the first turn at the bottom of the band yields 6 KHz, this increases to about 13 KHz in the middle of the band, then drops back down to 6 KHz at the top end. So it tunes faster mid-band but is still vastly better than 100 KHz in one turn. I added a digital dial so the linearity really isn’t an issue. And it retains the stability of the original design, which is to say, like a rock. I’m thinking that the on-board regulated 5 volts didn’t give me enough band spread so I went to a little 3-terminal 78L09 9 volt regulator. I mounted this Manhattan-style on a little scrap piece of pc board and mounted it in an available corner of the box. The rest of the pieces soldered directly to the terminals of the 10-turn pot. The lead from the varactor soldered to the hole in the board that was vacated by the polyvaricon. In looking at the board now I see a couple other caps that I added to fine tune the frequency range. I tack-soldered a 33 pF NPO cap across C73, as I’ve noted on the hand-drawn schematic. This was probably the cap that was supplied with the kit for location C83. The notation on the original schematic says “C83 optional, use if needed.” In that location I used a 47pF NPO from my collection. The changes I made were necessary to accommodate the parts I had on hand, so the usual disclaimers apply. Caveat emptor, YMMV etc. It’s a fun bit to play with, IMHO.

72, Cam
N6GA

I had an MVAM108, though it was an unmarked part, and I didn’t completely trust the source. I could have tested it, but I had some MVAM109’s from Kits and Parts, so decided to use one of those instead. Here is my version of Cam’s mod. My values are a little different, and I have added one part –

The 78L09 regulator was mounted at the back of the board, next to the existing 5V regulator IC. The input and ground legs of the part were poked through the holes in the board for 12V power and ground respectively. The 9v regulated output leg was bent up away from the board. The next two pictures may help to make it a bit clearer. The existing 5V regulator, the 78L05, is the one furthest away from the back of the enclosure (i.e. closest to the CA3086 IC). Also, in this picture, you will see that I moved the 12V DC input power connector higher up the rear of the enclosure. This was to avoid fouling the board, which had been moved. A small piece of PCB material blocks the old hole, and is held in place by the new connector –

In the next picture, you can see how the cable (lavalier mic cable, incidentally) that carries the regulated 9V supply to the rest of the tuning circuit is soldered to the bent-up output leg of the 78L09. You can also see the little 0.1µF bypass cap that connects from the output of the 78L05 to ground –

The MVAM109 varactor was mounted on the board, into the two holes marked “Tune” that had previously been used to connect to the main tuning polyvaricon. It is circled in red in this picture –

With the MVAM109, I found that a 68pF NPO cap across C73 gave me coverage from 3487 to 3615 KHz. In retrospect, I am not entirely sure why I used a 68pF cap at that point. The shorter and much less convoluted version of the story is that, by the time I had soldered that cap to the bottom of the board, I was past the point of wanting to fine tune the coverage. I figured that having that extra cap in parallel with C73 could only help the long-term stability, by spreading any heating effect over two capacitors instead of one, so I left it there. The reason I didn’t adjust trimcap C82 (which is on the main Tut 80 board and not shown in any schematic here) to bring the coverage higher, is that it was already at minimum capacitance. Perhaps one day I will further fiddle with the capacitance values to bring the coverage more in line with my ideal of ~3495-3585 KHz but tuning with the 10-turn pot is already a massive improvement over the half-turn with the polyvaricon, so I’m not bothered. The 10-turn pot has a much lower friction and smoother feel, in addition to the fact that it covers the tuning range in 10 whole turns.

The 47µF capacitor between the slider of the 10K tuning pot and ground was added to remove the slight “whizzing” sound that occurs in the headphones when a wirewound pot is used for varactor tuning. If your pot has a carbon track, you don’t need this part. In fact, the whizzing sound is so faint that 80M band noise usually covers it, but I like to add that capacitor anyway.

Another mod that piqued my curiosity was one posted by Dan N7VE. He had modeled some changes to the PA to make it more efficient and put out more power. Although at the time, he hadn’t tried it, he felt sure it would work. I found mention of it online by someone who did the mod and found that, as a result, his Tut 80 put out 4.5W. Here is the description of N7VE’s PA mod, in his own words, as posted to the now-defunct Yahoo Group –

Tut 80 PA Mod for Class D

The more efficient output network that I have simulated involves:

1) Changing L4 from 24 turns to 19 turns
2) Shorting out C58 so that C57 and C59 together provide 2000 pF of C at
that point rather than the current 1500 pf.
3) Replacing L6 (a FT37-43 core) with a T37-2 core with 22 turns. There
is no tap on this new coil. The coil end points go to pad 1 and pad 2.
Pad 3, where the tap used to go, is shorted to pad 2.

This simulates well to give a non-critical, class E type output, even
though it is being driven with a less than optimum sine wave input. The
output would be somewhere in the 4.5w range.

I want to test this when I get back, but someone could try this if they
wanted to see how it does. I think this would take unwanted heat and
change it into extra output power, allowing the finals to run cooler
than they currently do.

– Dan, N7VE

I made the above mods to the PA, and my Tut 80 now puts out 4W instead of 3W. Score!

The final task to accomplish was to tidy up the wiring a bit. This was accomplished by connecting all the wiring to the underside of the board, and running it underneath the board. Shielded cable was used to connect the AF gain pot to the board. Shielded cable wasn’t really necessary, but the stuff that is intended for lavalier mics is thin and flexible, and works well for this purpose. I use Mogami W2697. The twisted wires for the key and headphone sockets were scavenged from a multi-conductor cable. The feed from the board to the KD1JV Digital Dial is RG174/U. You can’t see it in the pictures, as it is hidden quite well. I think you’ll agree that it looks tidier inside the enclosure now –

Tut 80 with completed mods

My little Tut 80 now had nicer tuning and more power out, but what about the stability of the VFO? I ran a series of tests, from a few hours long, to one that was 36 hours in total. All the tests gave very similar results. At 3560KHz, from a cold start, it drifted steadily downwards about 260Hz in the first 30 mins, and a total of about 300Hz in the first hour. Thereafter, it didn’t drift more than ±30Hz for the next 10 hours. Most of that time, it didn’t drift more than 10Hz in an hour. In the one very long test, that lasted for 36 hours, after 10 hours of holding relatively steady, it began a new steady downward drift, reaching a maximum excursion of an extra 270Hz from the frequency it was at 1 hour after the cold start. Nearly all of this extra drift was steady though, and for most of this time, drift was no more than 10 or 20 Hz in an hour. This level of drift is good enough for the type of operating I do with my Tut 80.

Well, I rather wish I’d had a brand new exciting build to share with you, but that’s not how things have been going here at the AA7EE radio ranch recently. I figured that there might be a few Tut 80 owners who would be interested in these mods and, as the Tut 80 Yahoo Group no longer exists, decided to share them here. It it helps one person, my mission has been accomplished. FYI, the manuals, including schematics, for the 160 and 15M versions of this rig are hosted in the “Retired Kits” section of this site. For anyone who needs a copy of the assembly manual for the original Tut 80, drop me a line. My email address is good on QRZ.

Cam N6GA and I exchanged friendly emails some years ago. He had family in Oakland, and talked of visiting when he was next up here. I do wish he were still around, so I could say – hey Cam. I tried your Tut 80 varactor mod, and it works like a champ!

High power Tayloe (a.k.a. Wheatstone) absorptive bridge for VSWR indication and rig protection.

✇KA7OEI's blog
By: Unknown
28 February 2022 at 05:35

Figure 1:  The completed absorptive VSWR bridge.
Last year, I was "car camping" with a bunch of friends - all of which happened to be amateur radio operators.  Being in the middle of nowhere where mobile phone coverage was not even available, we couldn't resist putting together a "portable" 100 watt HF station.  While the usual antenna tuner+VSWR meter would work fine, I decided to build a different piece of equipment that would facilitate matching the antenna and protecting the radio - but more on this in a moment.

A bit about the Wheatstone bridge:

The Wheatsone bridge is one of the oldest-known types of electrical circuits, first having been originated around 1833 - but popularized about a decade later by Mr. Wheatstone itself.  Used for detecting electrical balance between the halves of the circuit, it is useful for indirectly measuring all three components represented by Ohm's law - resistance, current and voltage.

Figure 2:  Wheatstone bridge (Wikipedia)
It makes sense, then, that an adaptation of this circuit - its use popularized by Dan Tayloe (N7VE) - can be used for detecting when an antenna is matched to its load.  To be fair, this circuit has been used many decades for RF measurement in instrumentation - and variations of it are represented in telephony - but  some of its properties that are not directly related to its use for measurement that make it doubly useful - more on that shortly.

Figure 2 shows the classic implementation of a Wheatstone bridge.  In this circuit, balance of the two legs (R1/R2 and R3/Rx) results in zero voltage across the center, represented by "Vg" which can only occur when the ratio between R1 and R2 is the same as the ratio between R3 and Rx.  For operation, that actual values of these resistors is not particularly important as long as the ratios are preserved.

If you think of this is a pair of voltage dividers (R1/R2 and R3/Rx) its operation makes sense - particularly  if you consider the simplest case where all four values are equal.  In this case, the voltage between the negative lead (point "C") and point "D" and points "C" and "B" will be half that of the battery voltage - which means the voltage between points "D" and "B" will be zero since they must be at the same voltage.

Putting it in an RF circuit:

Useful at DC, there's no reason why it couldn't be used at AC - or RF - as well.  What, for example, would happen if we made R1, R2, and R3 the same value (let's say, 50 ohms), instead of using a battery, substituted a transmitter - and for the "unknown" value (Rx) connected our antenna?

Figure 3:  The bridge, used in an antenna circuit.

This describes a typical RF bridge - known when placed between the transmitter and antenna as the "Tayloe" bridge, the simplified diagram of which being represented in Figure 3.

Clearly, if we used, as a stand-in for our antenna, a 50 ohm load, the RF Sensor will detect nothing at all as the bridge would be balanced, so it would make sense that a perfectly-matched 50 ohm antenna would be indistinguishable from a 50 ohm load.  If the "antenna" were open or shorted, voltage would appear across the RF sensor and be detected - so you would be correct in presuming that this circuit could be used to tell when the antenna itself is matched.  Further extending this idea, if your "Unknown antenna" were to include an antenna tuner, looking for the output of the RF sensor to go to zero would indicate that the antenna itself was properly matched.

At this point it's worth noting that this simple circuit cannot directly indicate the magnitude of mismatch (e.g. VSWR - but it can tell you when the antenna is matched:  It is possible to do this with additional circuitry (as is done with many antenna analyzers) but for this simplest case, all we really care about is finding when our antenna is matched.  (A somewhat similar circuit to that depicted in Figure 3 has been at the heart of many antenna analyzers for decades.)

Antenna match indication and radio protection:

An examination of the circuit of Figure 3 also reveals another interesting property of this circuit used in this manner:  The transmitter itself can never see an infinite VSWR.  For example, if the antenna is very low resistance, we will present about 33 ohms to the transmitter (e.g. the two 50 ohm resistors on the left side will be in parallel with the 50 ohm resistor on the right side) - which represents a VSWR of about 1.5:1.  If you were to forget to connect an antenna at all, we end up with only the two resistors on the left being in series (100 ohms) so our worst-case VSWR would, in theory, be 2:1.

In context, any modern, well-designed transmitter will be able to tolerate even a 2.5:1 VSWR (probably higher) so this means that no matter what happens on the "antenna" side, the rig will never see a really high VSWR.

If modern rigs are supposed to have built-in VSWR protection, why does this matter?

One of the first places that the implementation of the "Tayloe" bridge was popularized was in the QRP (low power) community where transmitters have traditionally been very simple and lightweight - but that also means that they may lack any sophisticated protection circuit.  Building a simple circuit like this into a small antenna tuner handily solves three problems:  Tuning the antenna, being able to tell when the antenna is matched, and protecting the transmitter from high VSWR during the tuning process.

Even in a more modern radio with SWR protection there is good reason to do this.  While one is supposed to turn down the transmitter's power when tuning an antenna, if you have an external, wide-range tuner and are quickly setting things up in the field, it would be easy to forget to do so.  The way that most modern transmitter's SWR protection circuits work is by detecting the reflected power, and when it exceeds a certain value, it reduced the output power - but this measurement is not instantaneous:  By the time you detect excess reflected power, the transmitter has already been exposed - if only for a fraction of a second - to a high VSWR, and it may be that that brief instant was enough to damage an output transistor.

In the "old" days of manual antenna tuners with variable capacitors and roller inductors, this may have not been as big a deal:  In this case, the VSWR seen by the transmitter might not be able to change too quickly (assuming that the inductor and capacitors didn't have intermittent connections) but consider a modern, automatic antenna tuner full of relays:  Each time the internal tuner configuration is changed to determine the match, these "hot-switched" relays will inevitably "glitch" the VSWR seen by the radio, and with modern tuners, this can occur many times a second - far faster than the internal VSWR protection can occur meaning that it can go from being low, with the transmitter at high power, to suddenly high VSWR before the power can be reduced, something that is potentially damaging to a radio's final amplifier.

While this may seem to be an unlikely situation, it's one that I have personally experienced in a moment of carelessness - and it put an abrupt end to the remote operation using that radio - but fortunately, another rig was at hand.

A high-power Tayloe bridge:

It can be argued that these days, the world is lousy with Tayloe bridges as they are seemingly found everywhere - particularly in the QRP world, but there are fewer of them that are intended to be used with a typical 100 watt mobile radio - but one such example may be seen below:

Figure 4:  As-built high-power Tayloe bridge with a more sensible bypass switch arrangement!  This diagram was updated to include a second LED to visually indicate extreme mismatches and provide another clue as to when one is approaching a match.

Figure 4 shows a variation of the circuit in Figure 2, but it includes two other features:  An RF detector, in the form of an LED (with RF rectifier) and a "bypass" switch, so that it would not need to be manually removed from the coax cable connection from the radio.

In this case, the 50 ohm resistors are thick-film, 50 watt units (about $3 each) which means that between the three of them, they are capable of handling the full power of the radio for at least a brief period.  Suitable resistors may be found at the usual suppliers (Digi-Key, Mouser Electronics) and the devices that I used were Johanson P/N RHXH2Q050R0F4 (A link to the Mouser Electronics page is here) - but there is nothing special about these particular devices:  Any 50-100 watt, TO-220 package, 50 ohm thick-film resistor with a tolerance of 5% or better could have been used, provided that its tab is insulated from the internal resistor itself (most are). 

How it works:

Knowing the general theory behind the Wheatstone bridge, the main point of interest is the indicator, which is, in this case, an LED circuit placed across the middle of the bridge in lieu of the meter shown in  Figure 1.  Because RF is present across these two points - and because neither side of this indicator is ground-referenced, this circuit must "float" with respect to ground.

If we presume that there will be 25 volts across the circuit - which would be in the ballpark of 25 watts into a 2:1 VSWR - we see that the current through 2k could not exceed 25 mA - a reasonable current to light an LED.  To rectify it, a 1N4148 diode - which is both cheap and suitably fast to rectify RF (a garden-variety 1N4000 series diodes is not recommended) along with a capacitor across the LED.  An extra 2k LED is present to reduce the magnitude of the reverse voltage across the diode:  Probably not necessary, bit I used it, anyway.  QRP versions of this circuit often include a transformer to step up the low RF voltage to a level that is high enough to reliably drive the LED, but with 5-10 watts, minimum, this is simply not an issue.

Because the voltage across the bridge goes to zero when the source and load impedance are matched (or the switch is set to "bypass" mode) there is no need to switch the detector out of circuit but note that the LED and associated components are "hot" at RF when in "Measure" position which means that you should keep the leads for this circuit quite short and avoid the temptation to run long wires from one end of a large enclosure (like an antenna tuner) to the other as excess stray reactance can affect the operation of the circuit. 

Note:  See the end of this article for an updated/modified version with a second LED .

A more sensible bypass switch configuration:

While there are many examples of this sort of circuit - all of them with DPDT switches to bypass the circuit - every one that I saw wired the switch in such a way that if one were to be inadvertently transmitting while the switch was operated, there would be a brief instant when the transmitter was disconnected (presuming that the switch itself is a typical "break-before-make" - and almost all of them are!) that could expose the transmitter to a brief high VSWR transient.  In Figure 3, this switch is wired differently:

  • When in "Bypass" mode, the "top" 50 ohm resistor is shorted out and the "ground" side of the circuit is lifted.
  • When in "Measure" mode, the switch across the "top" 50 ohm resistor is un-bridged and the bottom side of the circuit is grounded.

Figure 5:  Inside the bridge, before the 2nd LED was added
Wired this way, there is no possible configuration during the operation of the switch where the transmitter will be exposed to an extraordinarily high VSWR - except, of course, if the antenna itself is has an extreme mismatch - which would happen no matter what if you were to switch to "bypass" mode.

An as-built example:

I built my circuit into a small die-cast aluminum box as shown in Figure 5.  Inside the box, the 50 ohm resistors are bolted to the box itself using countersunk screws and heat-sink paste for thermal transfer.  To accommodate the small size of the box, single-hole UHF connectors were used and the circuit itself was point-to-point wired within the box.

For the "bypass" switch (see Figure 6) I rescued a 120/240 volt DPDT switch from an old PC power supply, choosing it because it has a flat profile with a recessed handle with a slot:  By filing a bevel around the square hole (which, itself was produced using the "drill-then-file" method) one may use a fingernail to switch the position.  I chose the "flush handle" type of switch to reduce the probability of it accidentally being switched, but also to prevent the switch itself from being broken when it inevitably ends at the bottom of a box of other gear.
Figure 6:  The "switch" side of the bridge.

 
On the other side of the box (Figure 7) the LED is nearly flush-mounted, secured initially with cyanoacrylate (e.g. "Super") glue - but later bolstered with some epoxy on the inside of the box.
 
It's worth noting that even though the resistors are rated for 50 watts, it's unlikely that even this much power will be output by the radio will approach that in the worst-case condition - but even if it does, the circuit is perfectly capable of handling 100 watts for a few seconds.  The die-cast box itself, being quite small, has rather limited power dissipation on its own (10-15 watts continuous, at most) but it is perfectly capable of withstanding an "oops" or two if one forgets to turn down the power when tuning and dumps full power into it.  It will, of course, not withstand 100 watts for very long - but you'll probably smell it before anything is too-badly damaged!
 
Operation:

As on might posit from the description, the operation of this bridge is as follows:

  • Place this device between the radio and the external tuner.
  • Turn the power of the radio down to 10-15 watts and select FM mode.  You may also use AM as that should be limited to 20-25 watts of carrier when no audio is present.
  • Disable the radio's built-in tuner, if it has one.
  • If using a manual tuner, do an initial "rough" tuning to peak the receive noise, if possible.
  • Switch the unit to "Bridge" (e.g. "Measure") mode.
  • Key the transmitter.
  • If you are using an automatic tuner, start its auto-tune cycle.  There should be enough power coming through the bridge for it to operate (most will work reliably down to at about 5 watts - which means that you'll need the 10-15 watts from the radio for this.) 
  • If you are using a manual tuner, look at both its SWR meter (if it has one) and the LED brightness and adjust for minimum brightness/reflected power.  A perfect match will result in the LED being completely extinguished.
  • After tuning is complete, switch to "Bypass" mode and commence normal operation.
 * * *
 
Modification/enhancement
 
More recently (July, 2023) I made a slight modification to this bridge by adding a second LED driven by the opposite swing of the RF waveform so that it would not have any effect on the first - this LED designed to illuminate only under highly-mismatched conditions at higher power levels.
Figure 7:  The "enhanced" version with TWO LEDs.
 
As seen in the Figure 7 (above) the "original" LED is now designated as being yellow (the different color allowing easy differentiation) - but the second LED - which indicates a worse condition - is red and placed with a series 6.8 volt Zener diode (I used a 1N754A).  The idea here is that if the VSWR is REALLY bad and the power is high enough, BOTH LEDs will illuminate - but the "new" (red) LED will go out first as you get "close-ish" to the match.
 
Figure 8:  It has two LEDs now!

In testing with an open or short on the output and in "measure" mode the red LED illuminated only above about 15 watts, so this second LED isn't really too helpful for QRP unless the value of the 2k, 1 watt resistor is reduced.  Again, this isn't really to indicate the SWR, but having this second, less-sensitive LED helps with the situation when using a manual tuner in which the match is so bad that it's difficult to spot subtle variations in the brightness of +the more sensitive (yellow) LED - particularly at higher power levels.
 
 
This page stolen from ka7oei.blogspot.com

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