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121. Re: [Amps] grid fuses (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Thu, 20 Jul 2006 19:26:36 -0400
It's really pretty simple Will. I'll try to explain it one last time. There are two main catagories of excessive grid current. One is from excessive RF drive. This is caused by any number of sceneri
/archives//html/Amps/2006-07/msg00701.html (12,271 bytes)

122. Re: [Amps] grid fuses (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Thu, 20 Jul 2006 21:57:01 -0400
So the grid instantly cools, all leakage currents or arcs to the anode vanish, and the grid remains at zero volts at the instant of grid opening. Is that what you are proposing?. Never mind, I proba
/archives//html/Amps/2006-07/msg00708.html (9,760 bytes)

123. Re: [Amps] grid fuses (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Thu, 20 Jul 2006 22:00:04 -0400
Yes. Through the filament choke, input circuit, and/or cathode return path. If the cathode had no "ground path" to the negative rail or a high frequency ground path through the input circuit and exc
/archives//html/Amps/2006-07/msg00709.html (9,294 bytes)

124. [Amps] Runaway grid (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 07:03:10 -0400
That's the same experience anyone who has built audio amps and played with grid resistors runs into. I had a pair of 6146's in a bass guitar amp and in an effort to get better bass used very high gr
/archives//html/Amps/2006-07/msg00723.html (7,991 bytes)

125. Re: [Amps] Grid fuses (was: Life and gain of 3-500Z) (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 07:21:30 -0400
That sounds factual on the surface but the 50 amps actually comes from anode current at 10kV anode voltage in pulsed applications with a 3CPX1500A7 pulse rated tube with hotter cathode and driven wi
/archives//html/Amps/2006-07/msg00726.html (10,469 bytes)

126. Re: [Amps] grid fuses (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 07:33:55 -0400
It is already arcing. That is what makes the resistor fail. You can doubt it, but if you start an arc with 50 volts the lead can pull back 1/4 inch and still maintain the arc through plasma. With 30
/archives//html/Amps/2006-07/msg00727.html (10,934 bytes)

127. Re: [Amps] Grid fuses (was: Life and gain of 3-500Z) (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 09:28:54 -0400
Will, What you imagine happens runs contrary to how the systems actually behave in real life. 1.) The rated dissipation is determined by the point of secondary emission by heat. So if we trip the gr
/archives//html/Amps/2006-07/msg00753.html (11,478 bytes)

128. [Amps] Gassy tubes (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 13:58:36 -0400
That's absolutely incorrect for large power grid tubes! Large power grid tubes often have noticeable seal leakage. It comes from the less than perfect bond between the leads and the envelope. A perf
/archives//html/Amps/2006-07/msg00781.html (11,858 bytes)

129. Re: [Amps] Grid fuses (was: Life and gain of 3-500Z) (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 14:05:21 -0400
No Chris. If you open the grid path, the anode now dumps energy to the cathode via the grid. This isn't a guess, wild speculation, or something that happens once in a while. This is something that o
/archives//html/Amps/2006-07/msg00782.html (11,089 bytes)

130. Re: [Amps] Grid fuses (was: Life and gain of 3-500Z) (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 14:17:41 -0400
No, but that is not what we are talking about. In order to have a fault the grid is under operation and subject to excessive dissipation or to a direct arc (which means plasma). The real question is
/archives//html/Amps/2006-07/msg00784.html (11,782 bytes)

131. Re: [Amps] Grid fuses (was: Life and gain of 3-500Z) (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 14:23:26 -0400
I think we're talking about two different problems here: 1. Excessive grid current, without internal arcing. 2. Internal arcing. A fuse, resistor or solid state protective circuit is not going to be
/archives//html/Amps/2006-07/msg00786.html (10,665 bytes)

132. Re: [Amps] Grid fuses (was: Life and gain of 3-500Z) (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 15:36:08 -0400
That's a slow boat to China! First it has to dump all the energy of the supply through the cathode, then it has to ramp primary current up to fuse meleting levels while all that energy is duming in
/archives//html/Amps/2006-07/msg00794.html (11,603 bytes)

133. Re: [Amps] grid fuses (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 16:38:59 -0400
That's how a solid strap works. RF current pushes out to the edges. The amount of inductance is directly related to the flux surrounding the conductor, and since the flux is spread out the inductanc
/archives//html/Amps/2006-07/msg00799.html (8,123 bytes)

134. Re: [Amps] grid fuses (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 22:13:30 -0400
Thanks Peter, I was sure anyone with experience in manufacturing new PA's with power grid tubes would know this, but maybe some do not. I guess it is good to repeat it. If I tossed back 3CX3000A7's
/archives//html/Amps/2006-07/msg00826.html (9,529 bytes)

135. Re: [Amps] Grid fuses (was: Life and gain of 3-500Z) (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 22:29:16 -0400
Nonsense about manufacturers being in a conspiricy to make money really is just a last ditch attempt to appeal to emotions rather than common sense and good engineering. Common sense and good engine
/archives//html/Amps/2006-07/msg00827.html (12,168 bytes)

136. [Amps] Why Heathkit and others floated the grids (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 23:33:32 -0400
Being involved in the design process for Heath and others I can tell you exactly why Heath and others floated the grids. They did so because Bill Orr placed huge amounts of pressure on manufacturers
/archives//html/Amps/2006-07/msg00833.html (12,770 bytes)

137. Re: [Amps] SB-220 rebuild advice (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Sat, 22 Jul 2006 12:04:29 -0400
That's fine if you want a project, but in truth there is almost nothing to be gained perfromance wise assuming some components have not gone bad. In other words, if you want a project to keep busy c
/archives//html/Amps/2006-07/msg00868.html (11,717 bytes)

138. Re: [Amps] QRO membership revoked (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Sat, 22 Jul 2006 12:08:42 -0400
The Stew Perry 160 meter contest now gives a QRP multiplier to station copying the weak signal. I used a similar argument with the rules committee and it worked. Now the guy who digs the p-ants out
/archives//html/Amps/2006-07/msg00870.html (9,250 bytes)

139. Re: [Amps] grid fuses (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Sat, 22 Jul 2006 12:17:19 -0400
People like to misapply what happens in a 12AX7 dual triode and other thumb-sized tubes operating at 50-300 volts P-G voltage to fist-sized tubes running at 3kv and more. Contact bias generally does
/archives//html/Amps/2006-07/msg00872.html (8,629 bytes)

140. Re: [Amps] grid fuses (score: 1)
Author: "Tom W8JI" <w8ji@w8ji.com>
Date: Sat, 22 Jul 2006 16:25:01 -0400
Actually both of you are mixing applications and very different systems. Contact bias or space charge biasing is NOT the same as grid leak biasing. Grid leak biasing is dependent on grid current to
/archives//html/Amps/2006-07/msg00894.html (10,320 bytes)

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