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You're not setting voltage, really. Yes, it will be in the ballpark of 6/10th of a Volt. The exact voltage is affected by temperature, power-supply voltage and the manufacturing variation of one transistor to the next.

Your target is the zero-signal DC current through the RF transistors with NO DRIVE.

This usually involves keying the relay with a gator-clip lead, dummy load attached. A DC Ammeter in line with the amplifier's power lead will show the current drawn by the relay coil. front-panel LEDs and the bias base-current source all added together. With the bias pot set BELOW the setting where the RF transistors start to draw current, note the meter reading. This is your reference or "virtual" zero current. Advance the bias trimmer until the current reading begins to rise. Pretty sure that type transistor should have either 80 mA or 100 mA zero-signal idle current. Setting the trimpot for the original reference reading PLUS 150 to 200 mA should clean up the sound of sideband transmit. Setting it higher just gets the RF transistors hotter for no good reason. And you already know what setting it too low will sound like on the air. It's not an exact adjustment, just needs to be in the Goldilocks zone between too hot and too cold.

Once this is done, feel free to measure the exact base-bias voltage that was needed for this set of transistors. That exact voltage will be different from one batch of transistors to the next. That's the reason to make it adjustable. It's not the voltage that's important. What matters is the RF transistors' collector current.

73

<blockquote data-quote="nomadradio" data-source="post: 855416" data-attributes="member: 22">You&#039;re not setting voltage, really. Yes, it will be in the ballpark of 6/10th of a Volt. The exact voltage is affected by temperature, power-supply voltage and the manufacturing variation of one transistor to the next.Your target is the zero-signal DC current through the RF transistors with NO DRIVE. This usually involves keying the relay with a gator-clip lead, dummy load attached. A DC Ammeter in line with the amplifier&#039;s power lead will show the current drawn by the relay coil. front-panel LEDs and the bias base-current source all added together. With the bias pot set BELOW the setting where the RF transistors start to draw current, note the meter reading. This is your reference or &quot;virtual&quot; zero current. Advance the bias trimmer until the current reading begins to rise. Pretty sure that type transistor should have either 80 mA or 100 mA zero-signal idle current. Setting the trimpot for the original reference reading PLUS 150 to 200 mA should clean up the sound of sideband transmit. Setting it higher just gets the RF transistors hotter for no good reason. And you already know what setting it too low will sound like on the air. It&#039;s not an exact adjustment, just needs to be in the Goldilocks zone between too hot and too cold.Once this is done, feel free to measure the exact base-bias voltage that was needed for this set of transistors. That exact voltage will be different from one batch of transistors to the next. That&#039;s the reason to make it adjustable. It&#039;s not the voltage that&#039;s important. What matters is the RF transistors&#039; collector current. 73</blockquote>

[QUOTE="nomadradio, post: 855416, member: 22"] You're not setting voltage, really. Yes, it will be in the ballpark of 6/10th of a Volt. The exact voltage is affected by temperature, power-supply voltage and the manufacturing variation of one transistor to the next. Your target is the zero-signal DC current through the RF transistors with NO DRIVE. This usually involves keying the relay with a gator-clip lead, dummy load attached. A DC Ammeter in line with the amplifier's power lead will show the current drawn by the relay coil. front-panel LEDs and the bias base-current source all added together. With the bias pot set BELOW the setting where the RF transistors start to draw current, note the meter reading. This is your reference or "virtual" zero current. Advance the bias trimmer until the current reading begins to rise. Pretty sure that type transistor should have either 80 mA or 100 mA zero-signal idle current. Setting the trimpot for the original reference reading PLUS 150 to 200 mA should clean up the sound of sideband transmit. Setting it higher just gets the RF transistors hotter for no good reason. And you already know what setting it too low will sound like on the air. It's not an exact adjustment, just needs to be in the Goldilocks zone between too hot and too cold. Once this is done, feel free to measure the exact base-bias voltage that was needed for this set of transistors. That exact voltage will be different from one batch of transistors to the next. That's the reason to make it adjustable. It's not the voltage that's important. What matters is the RF transistors' collector current. 73 [/QUOTE]

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