To: |
"Chuck Curran" <ccurran@wi.rr.com>,"'STEVEN & NANCY FRAASCH'" <sjfraasch@embarqmail.com>,"'Robert Bonner'" <rbonner@qro.com> |
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Subject: |
Re: [Amps] Grid Dipping the Pi network in a new amp? |

From: |
"Tom W8JI" <w8ji@w8ji.com> |

Date: |
Wed, 18 Jul 2007 06:02:55 -0400 |

List-post: |
<mailto:amps@contesting.com> |

All of the questions, I hope to answer with this description. I have a brand new National R-175A, as the plate choke, feeding into a pair of 500 pf doorknob caps. A 300 pf vacuum variable tune cap into the B&W 852 tank coil, then a 1300 pf vacuum variable with a padding cap to get up to 1800 pf on 80 meters. I have checked the tank coil using two impedance bridges, getting 6.9 and 7.1 uH against a B&W spec of 7.0 uH for the 80 meter inductance. All was checked for shorts to ground. Each cap was also checked, and they can be varied from minimum to maximum capacitance without any shorts or other negative items being detected. >> Chuck, I will have 3,150 VDC no load, coming from a 5 Kw distribution transformer that was unwound down to 2150 VAC -- lots of fun. (That's another story) I am figuring about 2900 VDC under load, at 800 ma. So the resulting plate impedance, using the formula Rp = Ep/2*Ip gives around 1800 ohms. The specific B&W 852 tank coil was selected, since it was designed to work with a low voltage high current system, which I believe I have. Do the calculations yourself and you will find that this results in a design Q of 12, the good ol' magic number. I fully agree with the observations made by one of the poster, this tank coil would be bad news with a 4500 VDC plate voltage and low current. However, I feel it has a high probability of working , within it's intended work envelope. We will see soon, I hope!>> You may agree with that post, but if you do you are agreeing with bad advice. Even if you are off by a factor of four in loaded Q the end result, if you have reasonable size and Q components, is negligable. As such "bad news" is a highly dramatic exaggeration. Even the Q number "12" so many people worship is a number pulled from someone's rear pocket, and in actual fact the formula given for both plate impedance and tank Q with that loadline are approximations. The minimum Q you can use and have the network behave like a pi is some number slightly over the square root of the impedance ratios. Say 1800/50=36 and sqrt 36=6. Any Q over SIX would transform impedances fine in this case. Below that and the tank would "quit working". The high end of Q, before the tank "quits working", is the Q where circulating currents get so high something in the tank overheats, or where the bandwidth is so narrow you can't tolerate the tank selectivity. Other than that there will be almost no change you would ever notice. The only real problem you face with the B&W, other than it might not make some people have a warm fuzzy feeling and be in their happy place, is you might have to use a little more C than normal. After all that nice looking work, I'd do the minimal amount possible to check for something **in the resonant path** that is totally screwed up. The blocking cap cannot possibly be an issue at this point, and neither should the choke. As a matter of fact the first thing I would have done is isolate the tank to ONLY the vacuum variables and the inductor and then checked it. At this stage anything else you did without doing that is wasting time and energy. Read what Jim Brown wrote. You are being given good advice on building a complex sound proof room, but the real problem is a pot hole in the path between those three components or you are doing something wrong with the grid dip meter. 73 Tom _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps |

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