The Beamus

"Beam Me Up, Scotty"

AM Transmitter

 

First Prototype

 

 

I had always wanted to do 'something' with the 6ME8 so what we have here is a 'dollar days' special AM transmitter using the 6ME8 and 6CM8. To the right is the local variable frequency oscillator with a preamp on the left. RF goes to the deflection plates with audio on g1. g1 linearity is improved with preamp negative feedback.

Frequency response is stellar, down 1 dB at 18 KHz, which means it's using more than the officially allotted bandwidth. That was measured at max mod so there is no slew rate limitation either, which one wouldn't expect the 6ME8 to have anyway. Bandwidth is determined by the triode.

Plate power is currently more than officially 'allowed' at roughly 180 mW, based on 165 V plate and 2.2 total mA, half on each plate. However, the amount of power actually generated, plate swing times current, is significantly less than 100 mW.

Sensitivity is more than needed for a PC output and fine with notebooks too. It's a hair low for cell phones, but workable. On my ancient Audiovox SMT5600 adding 25% preamp boost in TCMP brings it up to snuff. More gain can he had by increasing R14 but I don't think it's necessary and distortion is best the lower it is. If you're using a PC for audio, or any other 'standard' 2 V device, it might be worth lowering R14 because max mod occurs way down around 15% volume at that level.

 

 

 

 

Audio Thru Radio

(1 Meg MP3 Download)

Audio was recorded by a cell phone headset microphone free air suspended 10 inches from the table radio speaker running normal volume.

 

The rewind of a UHF Coil turned out to not be efficient: low plate signal amplitude. While range is nevertheless fairly decent, going to a 250 uH ferrite antenna gives 80 Vpp plate at idle. That puts generated power, now, around 35 mW. Range is much improved and the ferrite alone is good enough for a 'room' transmitter. Max mod is also improved and virtually 100%. This is a winning combination.

For testing that means two 365 pF caps and dropping the antenna padder. Note that AES also carries a 250 uH ferrite antenna for $3 so that can ship along with the P-C70-OSC.

As a side note, the 'Gort' modulation indicator will work with this transmitter as well, with the addition of an RF filter on the input divider and adjusting for amplitude. R4 is the sense point.

The revised testing schematic with two 365 pF tuning caps is as follows.

 

 

(Probe is x10 so V/div is 20)

 

The 'Arguably' 100 mW Beamus Broadcaster

"Arguably" because it depends on how one interprets the "RF Final" and here its interpreted as the one operative plate on the 6EM8 conducting half the cathode current.

The osc 365 pF has been padded with 100 pF and the slug coil adjusted to 190 uH so as to avoid the irritation of being able to set a frequency the antenna can't be tuned to. Frequency range is then narrowed to roughly 580 KHz to 1150 KHz. Note that, if desired, the upper end could be extended with a different tank but the minimum inductance of the existing ferrite antenna is 250 uH.

An antenna tuning indicator has been added off the 'external antenna' winding on the ferrite antenna. The current ferrite antenna already had one but it can be added if not there (approx 9 turns). It does slightly load the output so a disconnect switch is included. The LEDs are off till right at peaking and then fine peaking is done according to brightness. The LEDs used are ultra brights from Niktronix and the same as in the 'Gort' modulation indicator.

An adjustment for RF amplitude to the deflectors has also been added and it, along with the deflector bias pot, is set for maximum peak to peak during 90% modulation.

Negative feedback was increased and gain reduced to standard line level.

 

A shot of the breadboard showing the LED antenna tuning indicator (upper right) glowing 'tuned'. Note that only the 365 pF section of the tuning caps is being used.

 

 

 

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