NEED A VACUUM TUBE KNOWLAGE PERSION TO CHECK VT CIRCUIT

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Craig Kendrick Sellen
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NEED A VACUUM TUBE KNOWLAGE PERSION TO CHECK VT CIRCUIT

Post by Craig Kendrick Sellen » Mon Jul 02, 2007 6:14 pm

I NEED A PERSION THAT HAS KNOWAGE OF VACUUM TUBES OR USE TO WORK WITH THEM. TO CHECK OUT A VT CIRCUIT.

HOW THE ELECTRONIC CANARY WORKS & the THEORY of it

WHEN TURNED ON, THE BIRD WHISTLES DOWN-SCALE FOR A FEW SECONDS, BREAKS INTO A WARBLE, AND AFTER SEVERAL SECONDS, SHUTS OFF-ONLY TO START AGAIN AUTOMATICALLY IN A FEW MORE SECONDS.
tHE BIRD'S CIRCUIT, SHOWN SCHEMATICALLY IN fIG. 1, IS DECEPTIVELY SIMPLE IN APPEARANCE. tHE MOST IMMEDIATELY OBYIOUS FEATURE IS AN ASTABLE MULTIVIBRATOR MADE UP OF V1a AND V1b, AND THEIR ASSOCIATED GRID TIMING CIRCUITS. NOT SO OBVIOUS IS THE BLOCKING OSCILLATOR WHOSE PRINCIPAL COMPONENTS ARE V1a, AND V1b, AND T2. THE LATTER PRODUCES THE WARBLE.
WHEN POWER IS TURNED ON, THE BIAS CIRCUITS CAUSE V1a TO BE TURNED ON AND V1b OFF. CAPACITOR C1 IS INITIALLY DISCHARGED: BUT, AS IT BEGINS TO CHARGE UP THROUGH R3, V1b BECOMES FORWARD BIASED BY THE CURRENT THROUGH T1 AND R4. EVENTUALLY, THE POINT IS REACHED WHERE V1b ACTS AS A BLOCKING OSCILLATOR, AND V1a FOLLOWS IT BECAUSE OF COUPLING THROUGH C5.
DURING THIS OSCILLATION, C1 IS CHARGED IN A NEGATIVE DIRECTION AS A RESULT OF THE HALF-WAVE RECTIFICATION PROVIDED BY V1b (WHICH IS REVERSE BIASED DURING NEGATIVE HALF CYCLES). SINCE THE CHARGING CURRENT IS HEAVY AT FIRST AND TAPERS OFF AS C1 CHARGES, THE INDUCTANCE OF T1 GOES FROM A LOW VALUE TO A HIGH ONE BECAUSE OF THE DECREASING CORE SATURATION. THE OUTPUT TONE THEN DECREASES IN PITCH.
CAPACITOR C1 GOES RAPIDLY NEGATIVE WITH REPECT TO GROUND, AND ITS EFFECT IN THE BIASING OF V1b IS REPLACED BY THE ACTION OF THE OSCILLATOR ITSELF DURING THE POSITIVE HALF CYCLES AND THE CHARGING CURRENT THROUGH C4 DURING THE NEGATIVE HALF CYCLES. EVENTUALLY, C4 CHARGES TO THE POINT WHERE ITS CONTRIBUTION TO THE V1b BIASING IS SMALL AND THE GRID/CATHODE JUNCTION OF V1bIS REVERSE BIASED DURING A LARGER PORTION OF EACH SUCCEEDING CYCLE. AT SOME POINT, THE BLOCKING OSCILLATOR STARTS TO "SQUEGG" THAT IS, THE CHARGE ACCUMULATED ON C2 DURING NEGATIVE HALF CYCLES IS LARGE ENOUGH THAT V1b IS TURNED OFF COMPLETELY UNTIL THE CHARGE LEAKS OFF. ONCE THE CHARGE HAS LEAKED OFF, V1b CAN OSCILLATE ONCE MORE. THIS CHARGE-DISCHARGE CYCLE HAPPENS SO RAPIDLY THAT THE AUDIO TONE GENERATED SOUNDS LIKE A WARBLING BIRD. FINALLY, C4 DISCHARGES TO THE POINT WHERE IT MAKES NO CONTRUBUTION TO THE BIASING OF V1b AND THE LATTER TURNS OFF. WITH V1b OFF, V1a TURNS ON AND THE CYCLE REPEATS.
Conponent values are inportant. The circuit will work with the 5% tolerance of resistors and 10-to-20% tolerance of capacitors, but variations of any larger degree will throw it off. Every componet determines some type of time constant. So if you change the value of one conponent be preared to change the rest. Since the output stage V1b works directly into the speaker without any transfering any change to the speaker loading will be reflected back into the oscillator circuit. Resistor R8 is included as an limiter to isolate partly the speaker from the circuit. :cool: :smile:
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dyarker
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Post by dyarker » Tue Jul 03, 2007 4:58 am

e-mailed a rotated and cleaned-up copy of the schematic to your hotmail address.

Multiple things going on in there simultaneously, is there a specific question?
Dale Y

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