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RF PA grid current vs. modulation capability for 4-400's




 
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Author Topic: RF PA grid current vs. modulation capability for 4-400's  (Read 4480 times)
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VE7 Kilohertz
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« on: December 30, 2004, 01:34:09 PM »

Another question for operational parameters of your broadcast rig or other rig that uses 4-400's in the final of a high level modulated rig. What do you see for grid current in the finals and how do you find it affects the over 100% modulation capability of the TX? I am seeing about 18-20mA in my Bauer 707 and am getting over 100% peaks with the crystal and am seeing about 15-16mA grid drive with the VFO and am only seeing about 80-90% positive peaks, as shown on the TFT 923 Mod monitor. Not sure why the difference as the manual says normal current is about 12-15mA.

Anyway, look forward to your replies.

Thanks for all the help.

Cheers
Paul
VE7KHz
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WA1HZK
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« Reply #1 on: December 30, 2004, 02:33:31 PM »

Book says 10 mils per tube in Class C. Probably 20 or a scoatch more for a pair. Assuming the Plate & screen are running at normal levels you should see about 720 watts per bottle. I found I need to run high grid current to get high peaks. I can vary the controls & watch the results in most transmitters. Lo drive = Low peaks generally.
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« Reply #2 on: January 02, 2005, 09:12:32 PM »

For what it's worth I just played with my Johnson 500 & set the rig for 330 mils & 12 Mils grid current. Modulation was possible to 100%. Dropping it to 10 mils, then 8 mils then 6 mils made little difference in the modulation. Dropping it under 6 mils began to drop the positive peaks noticeably. So I guess with a good tube, volts & iron the 12 mil point works for me. The 4-400 did not act like a triode & demand a lot of grid drive to make a decent signal. This rig only makes 350 watts of carrier & runs very low plate voltage for this tube.
Keith
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AM is Not A Hobby - It's a "Way of Life"!
Timmy, Sometime in 2007 on a Mountain Far Away..
www.criticalradio.com
www.criticalbattery.com
www.criticaltowers.com
www.criticalresponder.com
Official Registered "Old Buzzard"
VE7 Kilohertz
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« Reply #3 on: January 04, 2005, 05:41:17 PM »

Thanks Keith.

I found by winding a little step up torroid xfmr that I was able to get about 20V p-p out of the HP generator and able to get about 20mA of grid drive into the pair of 4-400's and now I have well over 125% + on the Bauer 707.

Cheers

Paul
VE7KHz
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