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Author Topic: 6T9 AM transmitter problem  (Read 1765 times)
N6AF
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Posts: 54




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« on: October 11, 2017, 06:44:20 AM »

This question is directed to anyone familiar with the 6T9 based transmitter featured in the 1970 ARRL handbook. The audio waveform at pin 2 of V3A looks great until I connect it with a known good coupling cap  to pin 8 of V3B.  When connected to pin 8 the waveform at that node becomes horribly distorted and non-symmetrical. Pin 8 is a grid so I cannot fathom how this could be loading the plate output from V3A pin 2. I checked the input impedance and it's simply the grid leak resistor value. Changing V3 tube didn't help. V3B bias is sitting nicely at about -8 volts.  Anyone seen this? I hate it when I run into a bug like this but eventually I'll get it corrected with your help. Thanks! 73 N6AF Chuck
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WA2ISE
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Posts: 1051




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« Reply #1 on: October 12, 2017, 12:06:09 PM »

How big is your input waveform?  Maybe you're driving the grid positive for part of the waveform, which will cause distortion.  The positive grid will act like a diode. 
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N7EKU
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Posts: 703




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« Reply #2 on: October 25, 2017, 12:27:38 PM »

Hi,

Your reasoning all seems good to me, so it's hard to figure out.  Is the rest of the V3B circuit constructed and working?  How is the filament voltage of the 6T9?  I've heard they draw a lot and the tube doesn't work well at low filament voltage.  Plate supply voltage is staying up?  That's the only circuit things I can think of.

Perhaps your scope is loading the node?

Sorry not any specialized help...  This is the 160M CW/AM rig?

73,


Mark
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Mark -- N7EKU/VE3
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