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Is the Cap type important in the input tank section?

Discussion in 'Home Brew' started by ElectronTubesRule, Sep 18, 2011.

  1. ElectronTubesRule

    ElectronTubesRule Active Member

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    I have noticed that in recent years you almost always see Door Knob tuning/loading section. I am thinking this a matter of "Monkey See Monkey Do"???? I have seen some older Amps in old ARRL Hand/Guide Books and in Old Commercial and CB tubes amp that have orange disk cap's all over the place in the tuning circuit in the rectifier circuit etc.....



    So does the cap make up matter or is it just the pf rating that is important.

    For instance could I use high voltage Silver Mica caps that I can get a t a great price???? What about non-variable air cap's.

    It is just a matter not wanting to spend any more then I must on cap.'s!! If I can make due with what I can a good deal on that would be great. I am guessing that PIO caps would not work well in that area seeing how I have yet to see one in any design at all.

    Also would love to her why I should or should not include balancing resistors across caps and diode's out side of bleeding the unit down. I have read where some people claim that balancing resistors are obsolete and some still use them.

    With regard to air variables and vacuum variables am I right in understanding that you base the peak voltage that it can handle on the amount of RF energy going through the tank not the AC voltage going to the tube? I ask because I cam e across a nice air variable that if it is the RF energy will work perfect giving me the best overalls Q for 10/12m and enough adjustment for the other HF bands as well. They are also smaller then most , multi-ganged, with off set shaft. The price is right and the size will make for a compact chassis layout. I am only looking at 1000watt's of RF energy in the system but will have 2500V in the system. The unit is rated for 2000V 500 volts under the max voltage in the system

    Given my power output of 1000watts as the target and the fact I will prob. seldom run it at that level of output is a vacuum relay needed? I was going to use a heavy duty open style relay design for all the relay needs.

    I am not trying to build the most expensive money no object amp. My goal is for simple, powerful, durable and clean sounding amp. If I really need something I will get it but I hate to spend more then I have to. Not saying I am cheap either but I do not want to buy something mostly to show off if something cheaper will do the same job.I hate the idea of spending $150-$350 on a vacuum variable if a $40-$75 air variable will get the job done. I could see it if size was an issue for easy transport but this is not my situation.

    For instance I am using Teflon 450V caps not because I wanted to show off but because I got a case of them for about $19 if not for that deal I would not be using them. So I try to look for parts I need that I can get a good deal on. So understanding what is a "must have" and what is " that would be nice to have" helps me to know what deals I can say yes to and which ones I should pass on.
     

  2. wavrider

    wavrider W9WDX Amateur Radio Club Member

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  3. BJ radionut

    BJ radionut Supporting Member and 6m addict

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  4. ElectronTubesRule

    ElectronTubesRule Active Member

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    Thanks guys for the links. Their are so many out their often one has to spend hours wading through non-informative sites to find the ones with solid info.....


    I am fantastic at coping other's but I want to make sure I do not do things just because someone else did it that way. I also know that their is always "Best Practice". Even reading the ARRL Guide for 2008 it really does not explain why they use what they use in the AMP sections. I know every type of capacitor has it's place and does one thing better then the other's. So knowing what each type is good at would help a lot. I cannot imagine that a doorknob is any good at filtering yet I have seen some amps built with doorknob's in the power supply section. I know small mica caps are often used to adjust tuning in small sweep tube amps. The only universal thing I have seen is no one tends to use electrostatics outside the power supply area's.
     

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