Part 15, Low Power, AM Transmitter, with an Audio Pre-Amp
I’m using this schematic from user “35Z5” at the ARF.
Many thanks to user “Norm Leal” (for the original 6888 Transmitter), Bill Hamre (Parts Kit), and 35Z5 (6GY6 Version) and many more who know more about tubes than I’ve learned since I was 12 years old.
I decided to use a Blonder-Tongue BTC-99 UHF Converter as a cabinet. Somewhere on the ARF forums, someone is weeping, I know, but I needed an enclosure and it was unlikely to ever be used again in it’s original incarnation.
Besides, the lil’ fellah will live on in a new project. Everything will be used, except the tube (6AF4) and the transformer, which will be saved for future projects.
Some additional prep work was done cleaning the chassis up (thank you, steel wool!), and recycling for reuse the original switch, 7-pin tube socket, and terminal strips. The original transformer was only 10 watts — too small for the new load, so it will be saved for some future project.
I stripped the chassis the evening of the final Analog to Digital TV conversion: June 12, 2009.
I think chassis prep, whether it’s recycling an old project or re-purposing an unusual container for an enclosure (like a mint-tin, or an old UHF converter) takes about 50% of the project and maybe 60% of the physical work (metal work, cleaning), but when done correctly will result in something worth looking at.
- An additional 7-pin socket is fitted.
- A fuse is added, retaining the original cord, but clipping the ‘neutral’ side to assure one-way insertion.
- The longer rear terminal is removed and saved and replaced with a piece of perf-board. The crystal oscillator and R3 will be mounted here.
- Holes are drilled and grommetted for the new transformer leads.
The HV Supply is complete (blue electrolytics), as well as the filament supply (pins 3 and 4, both tubes). Also the crystal oscillator is wired and supplied via the 5.1V Zener (top-most).
End of the first evening, about 1 1/2 hours of actual build-out, excluding cabinet prep. Note the marking, in red of tube placement. I changed this later to place the 6AB4 in back, closest to where the input jack would be to keep audio signal paths short.
Some testing of the Audio Output from the 6AB4 – nice! Input signal on bottom\; output of Pin 1, after C4. About an 8:1 amplification. Also tested Saw and Square. Square Wave photo is fuzzy. Testing is done with a Heathkit IG-1271 Function Generator.
One more evening’s work and we’re complete. Did some preliminary testing to make sure there were no High Voltage wiring disasters awaiting, and also some spot-checks to make sure the audio and RF wiring is correct. A final check with a DVM *and* a VTVM (belt and suspenders approach) to make sure there’s no large DC or AC on the audio input path, or with reference to ground and only at this stage are we willing to risk connecting a $135 iPod nano!
At this point the only technically negative thing I’ve found is that my recycled transformer seems to output
a higher than 6.3 VAC for the tube filaments. For testing, I’ve kept the Iso-Variac at a low voltage output (about 114 VAC), but when I do the final checking at full house current (here, about 124 VAC), I’ll include a 2 ohm, 5-watt resistor in series with the filaments in order to drop the voltage from about 7.4 VAC to just above 6.3 VAC. This will make the tubes have a much longer life.
Finally Complete, and it looks as well as it sounds.
Transmitted audio is fantastic. It lacks the compressor of the SSTran solid-state Part 15 transmitter, so there’s a small amount of unevenness in volume. ‘Big Band’, or older mono music that was mixed originally for AM Radio sounds great, certainly, but I’m quite impressed with the bandwidth – more modern music sounds swell. Very good coverage of the highs. I did a quick test and found that the transmitted audio covers at least 8500 hz (that I can hear) and that would mean a bandwidth of 17 khz – quite a bit more than a standard (US) AM Broadcast channel spacing of 10 khz. See the excellent discussion of AM Broadcasting bandwidth on Wikipedia.
Distance: Testing with the “SSTran-style Antenna” yields about 75 feet (23 meters) distance from the little ham-shack / office / la-BOR-atory to a receiver in the front room, which is using a tuned Terk AM Advantage antenna, and shows full-scale and good audio.
The tuning knob is non-functional, but I may move the output filter’s VC1 to a larger capacitor and mount it there. The “On” know works!
Very fun to build and this one is turning out to have the best transmitted audio so far.
Accumulated Kit Comments / Suggestions
- J1 (supplied, not listed) was a 3.5mm (1/8″) mono – suggest a stereo jack; schematic indicates a rudimentary mixer and most folks have some
sort of stereo source (iPod, computer speakers, etc.)
- R1 and R2 – 10K supplied as 1/2 Watt. Suggest 1/4 watt – they’re cheaper and fit the holes in the audio input jack better
Build Comments / Suggestions
- I used some inexpensive RG-174 coax on both the audio paths and the signal paths. Only one end of each shield was attached to chassis ground. This seemed to remove any trace of hum incursion from signal paths passing by AC lines (Filament, transformer). Segments shielded with RG-174: AFTER the 6AB4 to the 6GY4, the crystal oscillator TO the 6GY4 and the RF after the PI network to the antenna terminals.
- I used a take-out (recycled) transformer from an old Heathkit which had dual secondaries (150 / 6.3), but two transformers such as the PC-12-800.with a 12 V (CT) secondary may be used, as per Jon’s Electronics And More two-tube write up (see: Alternate Power Supply).
- The Power Cord – make sure it can be inserted only ONE WAY, and that one way assures that the ‘HOT’ leg does NOT wind up on the chassis. I identify the ‘neutral’ side, and clip a bit out of the middle out of the prong and widen it a bit, making sure it fits in the LONG side (at least in the USA) of a power socket. If your house is older and has pre-war or non-standard wiring, use an isolation transformer.
- As Always when working with house-current / mains voltages, please use extra care and caution. Use an isolation transformer if you have one available. Test before touching. Use a ‘one-hand’ rule – don’t get the HV (or any AC voltage) across your heart. Generally – do not get yourself between ground-reference high-voltage (your house current) and ground.
- December 2009 – Since I used a 6AB4 instead of a 6C4, I changed R5 from 100K down to 82K – this improved the plate voltage to 92V and increased volume.
- August 2010 – Finally cracked how to put an air variable capacitor in the blank spot behind the big dial. The original tuning mechanism rotated with a Bakelite shaft. I was able to cut that shaft and use a shaft coupler to put an air variable tuning capaciter there. This basically replaces the little trimmer capacitor VC1, and gives some tank tuning to handle differences in antenna. With the original VC1 trimmer, this could only be tuned with the cabinet removed and was fixed.
- January 2011 – Replaced the Crystal Oscillator power supply with a regulator – replaced: D2 (1N200X), D3 (Zener), and R12 with: D2 – 1N5817 (Schottky), D3/R12 – LM2931 LDO 5V Regulator.
The purpose is to provide better regulation and slightly more current in order to use a PLL Oscillator plug-in.
Measured B+ is 177.5 VDC.