'Twin Triode' AM Transmitter

 

Current state of the breadboard

With three active elements (2 in the one twin triode bottle) this AM transmitter is more complex than the "One Tube Wonder" but linearity and maximum modulation depth is significantly better.

The design uses a 7AU7 which is electrically equivalent to, but much less expensive than, the 12AU7. In short, V2A is a series modulator that controls how much current flows through the V2B oscillator and, hence, the amount of RF power generated. Audio to V2A's grid varies it's current and, so, we have modulated RF. Idle current is approximately 2mA with 50V across the oscillator giving a plate power of 100mW, the FCC maximum allowed.

R9 provides a trickle current to reduce carrier cutoff on deep modulation.

The preamp is a 6AB4 (equivalent to half a 12AT7) which, at 150mA, has a smaller heater requirement than the more commonly thought of 6AT6. Note that gain and distortion can be slightly improved by substituting the pin compatible, but more difficult to find, 6DR4 (equivalent to half a 12AX7) although I doubt anyone could tell the difference.

The two 100k input resistors are a L+R stereo mixer and summing junction for negative feedback from the modulator cathode via R6.

Gain is sufficient for at or near maximum volume on .5V devices, such as single cell MP3 players, to drive the transmitter up to 80% modulation, assuming proper normalization of the MP3 being played. However, if the MP3 is 'quiet' then it's likely the transmitter output will be too because there is little or no additional volume gain to compensate. The same applies to my AT&T Touch Pro 2 cell phone. On the other hand, the typical PC sound card has more than sufficient output to grossly over drive the transmitter and a volume level around 25% is a good place to start.

 

Anticipated casing

The "One Tube Wonder" went into a Blonder Tongue "Ultraverter" but the Mallory, and the many virtually identical clone of, UHF converter boxes are also excellent for small tube projects like AM Transmitters. In addition to the obvious case and chassis they have two 7 pin sockets and a transformer good for roughly 525mA of 6.3VAC filament power (more than the Blonder Tongue and the reason it was chosen for this project) and 150-180VDC B+, depending on load and filtering. Note that U.S. line voltage these days is higher than when the converters were manufactured so a series filament resistor is usually required to bring it down to 6.3VAC.

(A list of some common UHF converters with their tube compliment and heater power can be found here.)

That huge dial begs for something to do so a second project was inspired: the

Gort Modulation Indicator

 

First build of transmitter section into the Mallory converter

 

The original terminal block for the antenna connection was defective so one was moved from the right position, leaving a hole, but I had a box of dual RCA jacks so those were 'Dremel modified' to fit in the two right slots instead of using 1/8" stereo jacks. The two pair are paralleled to allow daisy chaining the source on through to, say, amplified speakers for local audio. Note that is not an "AM monitor" as it bypasses the transmitter.

As a side note, the hand drill hack job around the power switch was done by the factory, as were the 'free floating' and uncovered AC connections. That will be fixed when the 3 wire power cord is fitted.

 

As built Schematic (BOM)

 

Completed "Gort" Twin Triode Transmitter with Modulation Indicator

The 3-wire power cord has been fitted, the original Inductuner cover remounted, chassis 'finger' holes covered with wood blocks and the Gort Modulation Indicator is installed.

Left knob is On/Off. Right hand knob (originally tuning) rotates but does nothing.

Transmitter frequency, approximately 1300 Khz to 1700 kHz, is adjusted by the coil slug on the rear left.

 

Don't forget to visit the

Gort Modulation Indicator

 

I built two Gort transmitters as Christmas presents and, in addition to schematics, decided to include a controls layout document and 50's style 'advertisment' located here.

 

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