DX100 improvements

[John K5PRO]

Dave
Thanks for the question, no I haven't checked the optimal Modulator load Z after adding NFB. I hope it isn't different by much. Its too hard to get back in there and hook the modulator back to my load bank, since it was all hacked together and hanging in air above the TX which was lying on its side.

Last night I cleaned up all the air circuitry under the deck, and shortened the modulator plate leads from the tranny to the sockets of the EL34.

I have removed that little copper plate that shields the audio from the rest about 20 times now. Gotten real good doing it. Getting it back on is a chore trying to get a nutdriver back under there. While I have it open right now, I am trying to get all the filament circuitry wired so that the 12.6VCT winding is balanced with full load on each side. With the EL34s I maxed out the filament power. I noted that in my DX100, the audio filaments (both 12AX7 and 12BY7) are wired only to the "X" filament string, not X and Y as shown in the schematic. I wonder if this was due to the builders discretion or some change Heath made in the design? I would think that running off both sides X and Y would have given lower hum in the mic preamp. When I turn up the gain, there is noticible background hum. I measure about 50 dB s/n with my line input circuit- which comes in on the cathode of the second 12AX7 section, along with the NFB connection.

Another thing, since I am already slightly over 4 amps with the filament current overall, I started looking at ways to reduce it. That LV transformer runs warm, but I don't know what the filament winding is doing inside. I found that someone had used a #47 pilot lamp on the VFO and a #44 inside the meter. The 47 draws 150 ma and the 44 draws 1/4 amp. I got a 14 volt bulb to try, which will be quite dim inside the meter, but drawing only about 50 mA instead. If i add the small Stancor 6.3@0.3 transformer that I plan to do, that will relieve the overload on the transformer. It will power the 6AU6 all the time, like the Apache.

I plan to take some pictures of the various mods in place, and put all of the reports into one article, either online or in ER. Since a lot of stuff for DX100 has been discussed before, its really regurgitation. But I did do a lot of measurements to back up what I tried, and some of the improvements didn't do much towards making measurable differences. For instance, tying the screen voltage of the 6146B to both modulated B+ through a dropping R and to the fixed B+, to eliminate that last bit of nonlinearity. Or using a 15 Hy reactor for self modulation of screen. The benefits of these were lost in the overall performance of the modulator when coupled to the 6146.

I use the following equipment for the work:
Harris AM90 mod monitor
Homebrew voltage divider off the coax
Bird 600 watt load
Tek 7603 scope with dual plug in
Tek AA501/SG505 THD/IMD test set
Tek 7L5 Spectrum analyzer
Werlatone directional coupler
Mini circuits splitter
CBS Volumax 4300A for asymmetric audio tests

[w3jn]

John, that's what's sorely lacking in many articles either in ER or elsewhere on the net.  Lotsa info on what resistor to clip, etc., but not backed up by solid data using quality test gear as to the results of said mods.

Hats off to you for documenting this.  I suspect this will become very popular.

73 John

[W2VW]

John, that's what's sorely lacking in many articles either in ER or elsewhere on the net.  Lotsa info on what resistor to clip, etc., but not backed up by solid data using quality test gear as to the results of said mods.

Hats off to you for documenting this.  I suspect this will become very popular.

73 John

Yeah, what he said. Big problem with mods is the users like to pick and choose and leave certain things out. Your before and after measurements really show why and not just how. Great job.
The thing about feedback is that if your mod lowers the optimum load impedance then it would be useful to someone who has tetrode or pentode modulators and only has (or desires the use of, for positive peak capability)  modulation iron with a low numeric ratio.

[John K5PRO]

Dave, I hear what you are saying. My experience with the DX100 using an outboard multitap SNC transformer was, that the optimal Z to the pentodes is not black and white, like the data books for the EL34 state (11K for AB1 at 100 watts or so with 750 volts of Ep). As I got lower and lower, towards 6K, the peak positive cability got higher and higher, and the THD rose slowly. I suppose that 11K would have had the lowest THD, but also lower power and it was clipping (plate swing limited) at 100%, no headroom. For an audio amplifier, this would be perfect, as you would know you have a 100 watt amplifier, no more than that for clean sound. But for needing headroom for peak modulation (as long as the negative side can be limited with audio processor or a negative limiter at high level) there is a compromise to get more audio voltage, while living with a little more THD.

I certainly saw that, and after modest feedback was applied (not 10-20 dB as some have suggested) it has reasonably low amounts of distortion (mostly odd order from the tetrode/pentodes with push pull). I remember doing a proof of performance on a Gates BC1G in the 1970s. It was not much different from the DX100 final results, although it had a LOT of feedback inside, and could not run 7500 Hz sinewaves at 90% mod without tripping off, into a shunt fed BC antenna.

Anyway, about your idea that NFB might lower the optimal modulator load:
The point where I picked up the feedback is the familiar 500 ohm tap on the secondary of the mod tranny. This is after the two EL34s, which are plate to plate connected to the primary. My intuition (from feedback theory) would suggest that it lowers the reflected impedance that the RF side sees, when looking back into the secondary. But that Z isn't important, as the RF side actually sets the Z, i.e., 3050 ohms for 800V/0.27A, the ratio I am using in my DX100. This then is transformed back to the EL34s.

However, there is the effect of damping, which is used to advantage in audio hifi amplifiers by adding NFB. This helps the amplifier to handle speaker loads with 'tighter' compliance from the amplifier. I would think there is a similar effect from modulators towards the RF side of the transformer, but I don't know what it is called or how it is manifested.

I wish I had more time to play with all this. But spring is here, and I still don't have an AM BA TX on my antenna. Static is rising already. The XYL has me working on the garden some, but my pale crouched body prefers the smell of resin and burnt Heathkit wire insulation.

My next project is a Continental 314R1, PDM 1 kW, needing to move up to 3880 from 1230. It is already hifi, sitting next to the DX100 wondering why I have abandoned it for months. That might  be used next fall when the Heathkit cannot cut the QRN/QRM.

[John K5PRO]

I got off on a tangent, which consumed weeks of evenings, with that radio. I rewired all of the filament power to equally load both sides of the existing LV power transfomer (4 Amps on each side, 12.6 VCT). But in doing so, I looked at the 6AU6 VFO, with plans to install a small transformer to keep it powered all the time, like the Apache and Marauder did. I needed to trace those black filament X wires that went through grommets into the VFO and popped out on top for the dial lamp - as I didn't want it powered all the time when the AC switch was off. Curiousity got the best of me first, and I finally opened up that copper box to see what was hiding inside. Wiring was messy, with a rats nest due to the HV, screen and filament wires being about a foot long stuffed in there. I noted that a lot of the RF conductors from the tube to the LC networks were made with tiny bus wire with very little solder in places. This prompted me to try and do better.

Instead of powering the 6AU6 filaments, I removed the tube, socket, and associated components, except for the LC networks. I buit a small vector board and made a JFET VFO similar to what has been in the handbooks. It was still a Clapp (series Colpitts) configuration, with a source follower second stage. After a disasterous attempt to use a current feedback opamp - AD829 - as a gain stage (4x), I built a small board with the priceless (really!) 2N5109 buffer stage. It had a 200 ohm to 50 ohm wideband output toroid, and I ran RG174 coax up to the VHF/XTAL switch and over through the spring shield thing to the 12BY7 buffer. There I installed a terminal strip and a 1:9 ferrite wideband transformer to drive a 450 ohm load and get some voltage there. Through a coupling cap it paralleled the 100K grid resistor for the 12BY7. With about 15 volts Peak to peak, I closed it all up and fired up the DX100, using a pair of 9 volt batteries for the VFO and buffer temporarily (voltage regulators dropping the voltage to 15 and 8 inside the VFO box).

Classic problems cropped up, including too much pulling when tuning the finals, some shift in frequency when the HV was switched on, and a bit of chirp. On the bench the VFO looked and sounded great, but next to the driver/finals in the DX100, it got too much RF feedback. Part of the problem was that the level was too low at the buffer grid. On 10 meters I couldn't get enough drive for >2 mA of grid current. On 80 meters the transmitter was regenerative and ran about 150 watts output, could not reduce the output loading enough!  Given the amount of time spent, and the abysmal performance, I punted the idea and went back to the glow bottle.

Last night I aborted the solid state VFO design and rebuilt the original 6AU6 VFO. I tested it carefully with both 6AU6 and 6AH6, and will post the final verdict here in the future. I am trying to do the right things for stability for thermal drift and line voltage as well as fix that pesky problem of having too much drive on 160 and 80 and barely enough on 15 and 10. If I run up there, and leave the drive pot where it is, then switch back to the low bands,as i retune the driver tuning the meter goes slapping against the peg at >10 mA, which is hard on 6146 grids.

Heath used a 26 uH plate choke for the VFO, which is probably tuned with the stray capacitance to enhance the output. However, all it does is creates a very ugly waveform for the 1.8-2 MHz output, loaded with third harmonics. If I raised it to 510 uH, the low band was sinusoidal, with about 120 volts peak-peak unloaded output and about 40 on the high band. I will look into what can be done to resonate the new layout on the  7 MHz output, and run it nonresonant for 2 MHz.

The moral of this story, if it works, don't mess it up. I continue to write off my time on this project as a learning exercise.

[Steve - WB3HUZ]

And we're learning along with you John. Great stuff. Thanks for posting.

[Steve - WB3HUZ]

And we're learning along with you John. Great stuff. Thanks for posting.

[Steve - WB3HUZ]

And we're learning along with you John. Great stuff. Thanks for posting.

[wa1knx]

Ah dx100's are my favorite.  i have 2 stations in maine running them,
2 in vermont and one here in az!    one in maine is opened up but
stock. the other one runs 4 1625's push pull parallel, driven by
a bridged couple of 14 dip audio amps. with a multitap mod xfmr
i set the ratio to give me 130-40% peaks

      One in VT is opened up but stock, the other has 2x813's in the final (they fit!) and 2 811's as modulators.  mod xfmr and audio drive is external. it'll put out 300watts+!   i switched the hv supply to capacitor input, replaced the choke with another dx100 plate xfmr to run in parallel
to give it the grunt!

       The one here is a work in progress. I've replaced the 1625's with
a pair of 811's. i'll be making a separate recfier supply off the plate
supply to give me around 1100 volts for the 811's.  i'll drive them with
a power audio chip. this should sound and talk up pretty nice, i hope!

deano!