I use to have a Luxman CD player with a tube audio stage. It sounded so sweet compared to the rest of them.
TheLegacy you are the only person I've ever heard refer to the Luxman brand.
I have a solid-state Luxman R-404 AM/FM stereo tuner with built in power amp (20 Watts (?))
It's wonderful, and I forgot I had it. I'll have to fire it up and give it a spin.
Oh Carl before I lost everything but the clothes on my back that Luxman was a pride of mine. I got it at Gemstone Audio in Lansing, Michigan. I also got my Rotel tuner from Classic Stereo in Grand Rapids. Believe me I'd travel all around to find these great gems. Yamaha made awesome surround sound receivers too that would make a lot of Today's system sound like a Fisher Price toy sound system. Many people don't know that Alpine car stereo's were made from the same company that made Luxman audio systems for the home. I don't know how I'd ever replace my Luxman stuff now evern if I get money and can afford stuff like a normal human being. Its the price you pay when you have ungrateful kids who want more more more and blindside you with bills bills bills. And then think your Internet enjoyment is not important and lie about making sure that the net was paid for while you tend to your Wife who has medical issues.
We are packing up our house
to move - - - so I will comment
more later.
Having your skills and experience is
something I can only dream about.
Brooce, WLP
Neil, I've read the details about your transmitter for the 5th time, trying to guess at the function of each tube.
I am guessing the smaller tube is a 12AX7 serving as oscillator, and the larger tubes are modulator and final RF amplifier.
Time for a 6th look.
I'm back.
I think the PLL circuit serves as the "front-end" exciter/oscillator and the 12AX7 is a buffer amplifier.
What else do I wonder about?
7th look.
I don't see an audio input jack.
Brooce, WLP
Is typically used as an audio tube.
Carl,
The 12AX7 in the back amplifies the audio from the line input which comes in via a 1/8 inch phone connector on the back panel. Only one half of this tube is needed so there is a spare in case another feature is added.
The big tube on the front left is the modulator which is just an audio power amplifier which raises the audio signal to about 120 volts peak to peak. This voltage is applied to the RF amplifier which is the tube on the right. The circuitry is similar to that used in the KnightKit broadcaster but several component changes have been made.
Rather than setting the volume between the preamp and modulator as is done in the KK the audio control pot is at the input of the preamplifier. This is an internal adjustment and is set to give the proper modulation according to the VU meter in my audio chain. All of my transmitters are set the same way so they can simulcast without a bunch of separate pots to adjust.
The PLL circuit produces a 5 volts sine wave which is lowpass filtered and drives the grid of the final amplifier. The grid requires about 6 volts P-P for proper drive and filtering out the harmonics of a square wave leaves only the fundamental which is just the right voltage here. The second half of the 12AX7 could have been used as a buffer but it wasn't needed.
I still haven't done any range checks other than with a portable around the yard but it does cover the yard (half an acre) nicely.
Neil
No self respecting Part 15er would put a transmitter on the air without doing a range check so the obligatory check was done.
The first check was with the transmitter in my basement with a 3 meter wire antenna which extended vertical from my desk to the ceiling and thence 2 meters horizontal. The range with my car radio was 300 feet listenable and 500 feet detectable.
The transmitter was then moved to the outdoor location of my regular ground mounted over radials antenna and it was connected to the radial ground and the antenna. It would not tune possibly due to the relative large diameter of the antenna so the thin wire antenna was installed next to the permanent antenna and it tuned nicely. The range was 300 feet listenable and 1000 feet detectable. Essentially there is no practical difference in the range compared to the basement setup.
The DC input power is 60 mW but the efficiency to drive a wire antenna is unknown and apparently very low. Though the transmitter was built as old tube geek eye candy it would have been nice to have better range though it is perfectly adequate for house casting.
Bench tests showed that the transmitter can be link coupled to a simulated 30 ohm 30 pF dummy load antenna with an efficiency of 60 to 70 percent or so but implementing this is not worth the effort since I have a good working transmitter and antenna already.
This is not a negative against tube transmitters since if properly matched to the antenna system they will perform well.
Neil
EDIT to Correct: The load mentioned above did not use the 30 pF capacitor, it was just a 30 ohm resistor which simulated the load presented by a resonant base coil loaded antenna. A 30 ohm resistor in series with a 30 pF cap would simulate a simple non loaded 3 meter antenna.
Not to rare at all! Ebay has a whole page of them!
Very easy to replace.
Mark
I found a schematic for modified Heising that might apply to your transmitter Neil, modulating a tetrode plate and screen, with negative feedback.
http://www.w8ji.com/Heising%20modulation.htm
I wonder if other constant current sources could be used in place of the choke, like active ones using a tube?
Didn't Globe use Heising modulation?
Nate,
Thanks for posting that link. I am using the output tube as a triode and cannot apply the author's ideas about screen grid modulation.
Regarding your comment about a constant current source rather than an inductor, I think that is a sensible idea. It occurred to me that this is used in the Wenzel designed AM transmitter except that the current sink is used to apply the audio. The down side is that there will be a quiescent current and voltage across the current source which will consume power. With an inductor, neglecting the Rcoil, there is no wasted power.
My transmitter was built mostly from parts in my junk box. Usually, small transmitters like this use an output transformer as the inductor but I didn't have one and they are now going for $17 so I looked for an alternative. I found a salvaged power transformer with a current rating exceeding the maximum plate current for the modulator so saturation should not be an issue. As I faced the negative modulation limit problem it occurred to me that I could use the secondary to boost the audio voltage applied to the RF tube plate by wiring the transformer as an auto-transformer. B+ to the top of the 120V winding, modulator plate to the bottom of the 120V winding and the 24V secondary in series with the connection from the modulator plate to the final plate phased to boost the voltage. This gives the ability to pull the RF plate voltage to zero and below. It also permits the modulator to operate without swinging the plate down to the cathode voltage. Reducing the plate voltage swing also reduces audio distortion.
A rough scope measurement shows the negative modulation to be greater than 90% and no clipping on the positive peaks. I think the negative limit I see is due to a virtual cathode forming in the final and maybe using the screen grid to aid modulation would help but I am satisfied with what I have and probably will not pursue this.
It remains to measure the linearity, probably with a trapezoidal pattern, but until this is done all I can claim is that the signal sounds really good.
Thanks again for your interest and suggestions for this project.
Neil
Neil, your presenting this excellent Part 15 tube transmitter is more exciting than most exciting things, at least to some of us who grew up with tubes as brothers until the tran sisters came along.
Yet if it were mine, I could not do without a buffer stage.
Flash back with me now to those halcyon days in 1959 when, mop handle in hand, we would project into the large metal cabinet to force those slamming relays to do what a button couldn't make them do.
That big fat transmitter occupied several cabinets and a rack.
The exciter was in a rack, next to it was a cabinet larger than an industrial refrigerator containing the "driver" (the buffer), and the final big cabinet was the "final," the last RF stage.
We would turn on the driver section first, followed by the final.
Neil, you could even make a small mop handle to turn on your transmitter.
I am a dumb ass. I admit it. I should have read the article. Right there in plain sight was reference to Globe. Sheesh.