.15 uF cap on the modulator input to the 6 FET TX

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.15 uF cap on the modulator input to the 6 FET TX

Postby k3xf » Thu Mar 13, 2003 9:26 am

I encountered a "fading" output problem with my recently constructe 80M 6 FET TX. The modulated DC input has a .15uF cap specified to ground. This cap apparently has a significant amount of RF on it and if one uses a typical disc ceramic the power will decrease as this cap heats up. I replaced it with a .005uF 3KV transmitting cap and the power output is steady at 150W (30VDC input test level). Also, the small 5KV 500pF shunt caps really need to be RF rated . . . they get hot if you try to get by with less capable caps. . .for testing several 1000V micas in parallel to get the 1000pF will work. . .
-ap
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Postby blaine » Thu Mar 13, 2003 11:47 am

yea the ceramic disk caps will not work for the modulator bypass, i used one of those large orange caps like K1KBW has in his pictures.
i think they are spragues
never had issues with the shunt caps, but make sure you use large doorknobs for this purpose.
These are very valid observations Art, and well worth noting for people building these transmitters.
used oversized if you can, and use good quality parts.
*Just Say No to JS" :P
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so I installed Linux." --- LinuxNewbie.org
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.15uF RF bypass caps

Postby steve_qix » Thu Mar 13, 2003 2:36 pm

Hi Art,

Glad to hear things are coming along!

The .15uF capacitor needs to be around .15uF for sure. That is a high current, very low impedance point, and the capacitor MUST have a very low reactance. A .15uF capacitor will have between 1/2 and 1/3 ohms of capacitive reactance on the 80 meter band. The RF impedance of the high current primary is only a few ohms, so having a low reactance for the bypass is very important.

Any very good capacitor (or combination of capacitors in parallel - even better!) will work. I use orange drops or silver mica capacitors here.

Talk later and Regards,

Steve
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tnx. . .

Postby k3xf » Fri Mar 14, 2003 9:16 pm

I found a couple of .1uF black beauties and installed em. . .worked great. . .'will get the proper cap this weekend . . .
Thanks for the help. . .
-ap
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Pi filter here?

Postby Dan Brown » Sat May 24, 2003 6:32 am

Hi:

I am in the testing stage of my 6 FET 75 meter amp. I put an oscilloscope here and there is still some RF at this point. I may play with a PI filter here (like they do in the AM rigs like my Viking 2) to get better RF filtering without cutting into the audio.

73
Dan
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.15 uF

Postby k3xf » Sat May 24, 2003 6:37 am

Righto Dan. . .
I found the final tuning was impacted by changing the .15uF cap. I settled on a pair of .1uF black beauties that seemed to work best. . .very solid performers. . .
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