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The coil we use on one 250B grid is 1 uH or slightly less. Rewinding the original 11 turn factory toroid to 13 turns moves the resonant frequency down to around 28 MHz. Close enough for 10 and 11 meters both. Years ago I wondered about the input impedance of the Pride. Hooked the antennal analyzer to one and keyed it. Found that lowest SWR with the factory circuit was almost 30 MHz. Odd. Adding more inductance brings down the resonant frequency, but now the input SWR was high, but resistive.

Decided that the original grid swamping resistance of 750 ohms didn't make sense to me, so ever since I have used four 470 ohm resistors in series/parallel to put 470 ohms in parallel with the grid. A 3-to-one step up ratio in the input transformer should provide a nine-to-one impedance ratio, closer to 470 than 750.

If we feed RF into the grid without the coil with just swamping resistors in parallel with the grid circuit, the impedance will reflect the grid's parasitic capacitance in parallel with the resistors. Raises the input SWR and makes it reactive. Cancelling this capacitance with a parallel inductance makes the input resistive again.

My strongest suspicion about the factory using too much resistance and too little inductance in the grid circuit was meant to increase the RF drive voltage to the tube grid, and make the wattmeter as happy as possible.

45 years later, even a wimpy radio puts out half-again more than the 1970s radios it was meant to be used with. Even with the setup we favor the tube starts to show excess drive symptoms around the 18 or 20 Watt peak range. Not worried about cutting down the drive sensitivity.

<blockquote data-quote="nomadradio" data-source="post: 840589" data-attributes="member: 22">The coil we use on one 250B grid is 1 uH or slightly less. Rewinding the original 11 turn factory toroid to 13 turns moves the resonant frequency down to around 28 MHz. Close enough for 10 and 11 meters both. Years ago I wondered about the input impedance of the Pride. Hooked the antennal analyzer to one and keyed it. Found that lowest SWR with the factory circuit was almost 30 MHz. Odd. Adding more inductance brings down the resonant frequency, but now the input SWR was high, but resistive. Decided that the original grid swamping resistance of 750 ohms didn&#039;t make sense to me, so ever since I have used four 470 ohm resistors in series/parallel to put 470 ohms in parallel with the grid. A 3-to-one step up ratio in the input transformer should provide a nine-to-one impedance ratio, closer to 470 than 750.If we feed RF into the grid without the coil with just swamping resistors in parallel with the grid circuit, the impedance will reflect the grid&#039;s parasitic capacitance in parallel with the resistors. Raises the input SWR and makes it reactive. Cancelling this capacitance with a parallel inductance makes the input resistive again. My strongest suspicion about the factory using too much resistance and too little inductance in the grid circuit was meant to increase the RF drive voltage to the tube grid, and make the wattmeter as happy as possible. 45 years later, even a wimpy radio puts out half-again more than the 1970s radios it was meant to be used with. Even with the setup we favor the tube starts to show excess drive symptoms around the 18 or 20 Watt peak range. Not worried about cutting down the drive sensitivity.</blockquote>

[QUOTE="nomadradio, post: 840589, member: 22"] The coil we use on one 250B grid is 1 uH or slightly less. Rewinding the original 11 turn factory toroid to 13 turns moves the resonant frequency down to around 28 MHz. Close enough for 10 and 11 meters both. Years ago I wondered about the input impedance of the Pride. Hooked the antennal analyzer to one and keyed it. Found that lowest SWR with the factory circuit was almost 30 MHz. Odd. Adding more inductance brings down the resonant frequency, but now the input SWR was high, but resistive. Decided that the original grid swamping resistance of 750 ohms didn't make sense to me, so ever since I have used four 470 ohm resistors in series/parallel to put 470 ohms in parallel with the grid. A 3-to-one step up ratio in the input transformer should provide a nine-to-one impedance ratio, closer to 470 than 750. If we feed RF into the grid without the coil with just swamping resistors in parallel with the grid circuit, the impedance will reflect the grid's parasitic capacitance in parallel with the resistors. Raises the input SWR and makes it reactive. Cancelling this capacitance with a parallel inductance makes the input resistive again. My strongest suspicion about the factory using too much resistance and too little inductance in the grid circuit was meant to increase the RF drive voltage to the tube grid, and make the wattmeter as happy as possible. 45 years later, even a wimpy radio puts out half-again more than the 1970s radios it was meant to be used with. Even with the setup we favor the tube starts to show excess drive symptoms around the 18 or 20 Watt peak range. Not worried about cutting down the drive sensitivity. [/QUOTE]

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