SimSmith Design for Hi Z Tube Tank CIrcuit

Peak the GRID   -   Increase the LOAD   -   Dip the PLATE

1. How many thousands of times have you done that on older Tube Transmitters? - Many.
2. Did you know what you were really doing? - No, just following the documented procedure.
3. Is the adjustment order important? - Yes, maybe, but did you know why.
4. When building Transmitters, Did you know why a particular part value was used in a Tank Circuit? - No, just followed other previous designs.

The last few days I have been working with the new SimSmith program, another program by Ward - AE6TY, see previous blog.

With SimSmith I can experiment with the a Transmitters Output Tank Circuit, to see and understand the importance of each component and the effects of each.

I have always had questions about the magic of a Tank Circuit.

For example;

• How important is the "Low Q Harmonic Suppression Inductor" on the resulting Harmonic Suppression. 
• What if I had (or could) NOT use the exact value as suggested in a build instructions?
• How important is the Tank Values on Harmonic Suppression?
• Does the value of the DC blocking Capacitor effect the suggested component values for the rest of the Tank Circuit?
• How can a match be achieved, for all bands with only the Plate Cap, the Loading Cap, and a few Standard Values for the Band Select Inductor?
• What range of antenna miss-match can the Tank Circuit correct?
• What is the loss when there is a miss match?
• How important is the Antenna to coax match?
• Does the length of the coax really matter?

With SimSmith, I now know most of the answers, and can experiment to learn, and ask more.

The following table of numbers are for a "Tube Tank Circuit" and were obtained by using SimSmith within about 30 minute. I have not checked these results against standard published circuits, but I think they are close, or at least a starting point for further experiments.

The circuit was modeled after a standard Tube Style Output Circuit, 10K ohm Impedance at the plate  matched to a 50 ohm load.

The Tank Circuit Schematic

The Elements in order left-to-right are:

Load - Coax - Ant Load Cap - Ind - Plate Tuning Cap - Bulk Ind - DC Blocking Cap - Harmonic Sup - Src

Note: on SimSmith the layout of components are with power flowing right-to-left ( << ), and impedance is defined left-to-right ( >>, it is a standard Smith Chart and Engineering thing).

The table include the older standard Bands and 30 meters (my favorite band). The other newer bands could have been easily included.

Source (Tube)	Test Frequency	Low 10Q Harmonic  Suppression Inductor	Coupling Cap	Bulk Ind	Plate Tune Var Cap	Band Select Ind	Antenna Loading Var Cap	Test Antenna Load	Calculated Second Harmonic Suppression
10KR 100X	1.9MHz	100uH	.01uF	2.8mH	43pF	180uH	450pF	50+j0	19.6db
10KR  100X	3.75MHz	100uH	.01uF	450uH	45.2pF	43uH	523pF	50+j0	25.0db
10KR  100X	7.25MHz	100uH	.01uF	450uH	12.1pF	20uH	142.5pF	50+j0	25.7db
10KR  100X	10.14MHz	100uH	.01uF	120uH	13.1pF	20uH	43.7pF	50+j0	30.5db
10KR  100X	14.25MHz	100uH	.01uF	120uH	7.45pF	18uH	62.7pF	50+j0	28.3db
10KR  100X	21.4MHz	100uH	.01uF	0uH	10.6pF	5.6uH	212.1pF	50+j0	38.8db
10KR  100X	28.25MHz	100uH	.01uF	0uH	10.9pF	3.0uH	359.1pF	50+j0	46.7db

Do not design a project with these numbers, check the values for yourself

Note: With additional effort, a slightly better matches can be achieved, for this experimental purpose I was looking for anything better than a 1.2 to 1 ratio.

Working with SimSmith allows for quick and easy adjustments of component values for  "what if" games to be played. The following SWR Plot shows the results, similar to that expected or obtained while tuning a rig with meters.

30 Meter, 10.140MHz SWR Plot
SWR Value, and Power Ratio (In/Out, Red Dotted line) on the Right

With Black Dotted Cursor placed at the Second Harmonic Freq

The following Smith Chart Plot shows how each component contributes to the match and it circular impedance significants. Without a Smith Chart Plot component interaction is difficult to understand.

Smith Chart, Showing Match at Center 50 ohms
Hi Resistance Values are on the left, Low Values are on the Right
With 50 ohms in the Center

Colors are the same as corresponding schematic elements above

The Transmitting Tube High Impedance (source) is represented by the "dot" on the left, the 50 ohm load is represented by the "X" in the center of the Smith Chart Plot. It is easy to see the significants of the Plate Tuning Cap (the long light blue arc). All reactive components plot as an arced line parallel to the arc lines of Smith Chart Grid. Knowing that component reactance values change along an arc, makes adjustments easy to understand and predict.

I plan to repeat this experiment with a modern Transistor or FET output circuit, where source impedances are much lower (typically less the 20 ohms).

In the future, SimSmith with be used when I build any transmitting or impedance matching circuits.

It is easy, no longer do you have to let a complex Multi Frequency Smith Chart scare you off (as seen below). In this view, the point of interest is that of the last schematic element (line colored blue, same as schematic) which runs through the center for a 50 ohm match.

This Multi Frequency Chart is Interesting

But not necessary to understand normal usage of the Software

SimSmith is very interactive. Give it a try, see if you agree, you will learn a lot about your Tanks.

Thanks, Ward for providing the Software.

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UPDATE
If you would like to try the 10.14mHz configuration, cut and save between the lines, and then load the file into SimSmith.

--Cut------------------HiZ-to-50ohm---10.14mHz-----------
LOAD R:50.0 X:0.0 load:
SERIES_TLINE deg:554.56 @MHz:10.0 ft:100 vf:0.66 Zo:50.0 /100f:0.5 @frq:10.0
SHUNT_CAP Fd:43p Q:1.0K @MHz:0.0
SERIES_IND Hn:20u Q:1.0K @MHz:0.0
SHUNT_CAP Fd:13.167p Q:1.0K @MHz:0.0
SERIES_IND Hn:120u Q:1.0K @MHz:0.0
SERIES_CAP Fd:.01u Q:1.0K @MHz:0.0
SERIES_IND Hn:100u Q:10 @MHz:0.0
GENERATOR MHz:10.14 R:10000 X:100.0
CONTROLLER lower__Scan__Freq:5 upper__Scan__Freq:25 #__of__Segments:500.0 SWR__R:10000 SWR__X:0.0 SWR:1.2
---End----------

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