KB7OCY 304TL Modulator Deck Tests

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modulator schematic
Modulator Schematics PDF
4.768 MegaBytes
304tl modulator deck-test setup.jpg
Test Setup
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304tl modulator deck-test setup-02.jpg
Test Setup - view 2
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304tl modulator deck-test setup-03.jpg
Test Setup - view 3
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304tl modulator test-02.jpg
Modulator Test Bench
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304tl modulator test-03.jpg
Test Bench - front
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304tl modulator test-max rms sig.jpg
Test at max rms sig
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304tl test-scope shot-350 w rms.jpg
Scope Shot-350 w rms
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KB7OCY writes:

Did some load testing and a little bit of tweaking to the bias circuit and here’s what I found:

(Clark, I think you hit the nail on the head with 500 watt class)

First of all, the power supply is a choke input, so unloaded was 2900 volts; w/bias only-2100 volts; w/full signal-2000 volts. With that in mind  and a 1.73:1 ratio on the modulation xfmr, I chose a 3200 ohm / 225 watt resistor for the test.

Measured across load resistor: 330ma / 3.2K / 1050vrms = 350 watts rms, input signal ranged from 65~500hz, anything over 320ma on the 304 plates would cause distortion visible on the scope. Scope shot photo is at full output. Load resistor got smokin’ hot, so I know AC power was being pushed though the xfmr. Voice peaks gave about 800 watts.

Now just to polish up the case and drop it all in.

Need to start making a decision on the RF deck specs. Maybe run the 4-400’s at 2500 volts, tetrode screen mode with swamping resistors on the input (no tune and very low drive needed) might match up somewhat OK. Sound OK?

Regards and 73’s,


304tl grid connection.JPG
304tl grid connection
1.62 MB
chassis complete-02.JPG
chassis complete-02
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filament ballast res and bias.JPG
filament ballast res and bias
1.69 MB
filament glow-side view.JPG
filament glow-side view
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filament glow-top.JPG
filament glow-top
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Tesla Coil Test
833 Tesla Coil Test
304tl line amps-full load 117VAC.jpg
line amps-full load 117VAC
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I just landed the last wires to the 304tl modulator reconstruct. Added a 1 ohm ballast resistor to the filament xfmr for inrush and to correct for 115 volt primary winding. Ran the usual audio test of the driver with the filaments on (50A @ 5V), and did not notice any additional hum or buzz coming from the test output xfmr and speaker. Hope everything keeps going in the right direction.

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