My latest creation

[W8ACR]

Since my original 160 meter RF deck ended up being a 75/40 meter RF deck, I figured I better get something with a little scrote for 160 before the winter season is upon us. This is a 160 meter RF deck that will use the same power supply and modulator as the other RF deck. Just finished her tonight. It has a single RCA 8000 tube which will run at 1900V@225mA or so. Pi network output/balanced input with grid neutralization. I put the tube in front because I'm gonna put a window in the front panel - love to watch those tubes glow. Will get her hooked up tomorrow and see if she works!
Ron W8ACR

[W8ACR]

Two more pics

[W7TFO]

Sweet looking job, Ron.

Gotta love HB TX's!

73DG

[WA1GFZ]

The tank coil windings a bit close spaced. This will cause a low Q and higher harmonic output.
that plate blocker looks a bit under sizer....big smile

[WA3VJB]

Sweet job, Ron, can't wait to hear you on the air with it.

You using a slice of tempered glass in your observation window? 

I once saw someone's HB rig using the kind of glass sandwiched over wire-mesh, like you'd see in a exterior door at school. 

I thought it was great -- it would contain the explosion nicely.

[W8ACR]

Yeah, I know, the blocking cap looks a bit awkward. I'll probably change it out for an HT57 ceramic doorknob when I can lay my hands on one. Used what I had available in the junkbox.  
Ron

[KL7OF]

See you on 160 Ron......Looks good

[W8ACR]

A couple of comments/questions for Frank (and anyone else who wants to chime in). Regarding coil Q, I know there are multiple factors affecting the Q of a coil, but I thought one of the most important was the coil diameter to length ratio. More specifically, a long narrow coil will have a lower Q than a short wide coil. The coil pictured has a dia/length ratio of 1.4 (I believe there are 19 turns on a 3.5 inch form with a total coil length of 2.5 inches). When choosing a coil to use for something like a tuning coil for a vertical antenna, I know it's very important to use as high a Q coil as possible.
When designing a pi network tank circuit, I thought the important thing was the Q of the whole tank circuit, which is determined by L/C ratios and plate load. I have done a lot of reading on the subject of designing tank circuits, and I don't recall that coil Q was emphsized as an important design parameter. I can certainly rewind the tank coil to use the entire ceramic form if the consensus is that would be better.
Ron

[WA1GFZ]

Ron I learned through testing that you want the turns spaced at least 1 wire radius or the harmonics will increase. Close spacing adds a shunt C in parallel with the inductor making it less effective. This shows up as decreased attenuation of harmonics. I found this out checking out a linear and noticed I had reflected power from a reference dipole. When I spread the turns the reflected power went down.

[w3jn]

When designing a pi network tank circuit, I thought the important thing was the Q of the whole tank circuit, which is determined by L/C ratios and plate load. I have done a lot of reading on the subject of designing tank circuits, and I don't recall that coil Q was emphsized as an important design parameter. I can certainly rewind the tank coil to use the entire ceramic form if the consensus is that would be better.
Ron

True, but what they're obviously discussing is loaded Q.   In order for the tank to be as effective at its job as possible (efficient impedance transformation and low pass filtering) the components themselves need to be as close to ideal as possible; hence, high unloaded Q.  In other words you want as little distributed capacitance and inductance as possible.  You could wind a small coil from nichrome wire which would meet your inductance target but obviously the unloaded Q would be horrible as would its performance.

The material in the coil form also has a big effect on unloaded Q.  Variable caps with iffy bearings or dirty contacts will negatively affect their Q and cause unexpected problems.

[W8ACR]

Did some initial testing of the 160 meter RF deck with promising results. Grid circuit resonated properly, giving me good grid current and voltage numbers with modest drive from the Viking I. Final amp neutralized properly and without difficulty. Applied plate voltage without drive - no arcs or plate current. Applied drive power and plate voltage and got good RF output at 1945 kHz. Modulation looked exceptionally nice on the SB-610. One minor issue - the plate tank resonated at minimum capacitance of the tuning cap, so I'll either have to reduce the inductance in the tank coil, or reduce the capcitance value of the doorknob padder on the tuning cap. Tank coil inductance is currently about 27uH (by calculation - I don't have an inductance meter). I think 27uH is a good value for a tank Q of about 12 at plate load of 4750 ohms. So I'll probably just reduce the padder value.

Actually, I'm astounded that it worked as well on the first try as it did. No smoke, no impressive light shows, not even a blown fuse or circuit breaker. I must  be getting the hang of this homebrew thing!

Ron

[WA1GFZ]

It is critical that your tank resonate at the right plate C. This will assure that you are running the right Q. So even though you don't have an inductance meter you can determine where the caps land. Running low Q will limit your output power