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Thinking back to the Pride DX300 input circuit. The control-grid's capacitance prevents the 10-meter input from being resistive, even with swamping resistors as shown. The Pride deals with this by putting a toroid inductor across the grid. The inductance is chosen to 'cancel' the grid's capacitance and become broadly resonant across the 10-meter band. Lower bands require an added capacitor to resonate those bands' grid coils. So long as you have a broadly-resonant parallel tuned circuit across the grid-swamping resistors, the input impedance remains largely resistive.

That amplifier uses an untuned 3-to-1 voltage step-up transformer wound trifilar on a toroid core. Never have discovered the ferrite type they used, just bought them from RF parts as needed over the years.

Takes very close to 1 uH to get the input SWR down on 10 meters with a 250B tube. I'm feeling too lazy to look up the 300's grid capacitance just now, but you get the idea.

I know we tried using a transmission-line style input transformer on a 250B tube decades ago. What I remember was that it required a trimmer cap from the its output to ground. It was sufficiently long ago there are probably no bench notes about the setup.

I'll admit to being a bit puzzled over the diagrams. Only one end each of the red, blue and yellow wire is show. I would ground the 'start' end of the blue, connect 'finish' end of the blue to the start end of the red wire. This will be the 50-ohm input. The finish end of the red goes to the start end of the yellow. The finish end of the yellow will be your 450-ohm output to the tube grid and swamping resistor. This is how the trifilar step-up toroid in the DX300 is built.

73

<blockquote data-quote="nomadradio" data-source="post: 818568" data-attributes="member: 22">Thinking back to the Pride DX300 input circuit. The control-grid&#039;s capacitance prevents the 10-meter input from being resistive, even with swamping resistors as shown. The Pride deals with this by putting a toroid inductor across the grid. The inductance is chosen to &#039;cancel&#039; the grid&#039;s capacitance and become broadly resonant across the 10-meter band. Lower bands require an added capacitor to resonate those bands&#039; grid coils. So long as you have a broadly-resonant parallel tuned circuit across the grid-swamping resistors, the input impedance remains largely resistive.That amplifier uses an untuned 3-to-1 voltage step-up transformer wound trifilar on a toroid core. Never have discovered the ferrite type they used, just bought them from RF parts as needed over the years.Takes very close to 1 uH to get the input SWR down on 10 meters with a 250B tube. I&#039;m feeling too lazy to look up the 300&#039;s grid capacitance just now, but you get the idea.I know we tried using a transmission-line style input transformer on a 250B tube decades ago. What I remember was that it required a trimmer cap from the its output to ground. It was sufficiently long ago there are probably no bench notes about the setup.I&#039;ll admit to being a bit puzzled over the diagrams. Only one end each of the red, blue and yellow wire is show. I would ground the &#039;start&#039; end of the blue, connect &#039;finish&#039; end of the blue to the start end of the red wire. This will be the 50-ohm input. The finish end of the red goes to the start end of the yellow. The finish end of the yellow will be your 450-ohm output to the tube grid and swamping resistor. This is how the trifilar step-up toroid in the DX300 is built.73</blockquote>

[QUOTE="nomadradio, post: 818568, member: 22"] Thinking back to the Pride DX300 input circuit. The control-grid's capacitance prevents the 10-meter input from being resistive, even with swamping resistors as shown. The Pride deals with this by putting a toroid inductor across the grid. The inductance is chosen to 'cancel' the grid's capacitance and become broadly resonant across the 10-meter band. Lower bands require an added capacitor to resonate those bands' grid coils. So long as you have a broadly-resonant parallel tuned circuit across the grid-swamping resistors, the input impedance remains largely resistive. That amplifier uses an untuned 3-to-1 voltage step-up transformer wound trifilar on a toroid core. Never have discovered the ferrite type they used, just bought them from RF parts as needed over the years. Takes very close to 1 uH to get the input SWR down on 10 meters with a 250B tube. I'm feeling too lazy to look up the 300's grid capacitance just now, but you get the idea. I know we tried using a transmission-line style input transformer on a 250B tube decades ago. What I remember was that it required a trimmer cap from the its output to ground. It was sufficiently long ago there are probably no bench notes about the setup. I'll admit to being a bit puzzled over the diagrams. Only one end each of the red, blue and yellow wire is show. I would ground the 'start' end of the blue, connect 'finish' end of the blue to the start end of the red wire. This will be the 50-ohm input. The finish end of the red goes to the start end of the yellow. The finish end of the yellow will be your 450-ohm output to the tube grid and swamping resistor. This is how the trifilar step-up toroid in the DX300 is built. 73 [/QUOTE]

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