For the plate voltage to find its way back to the exciter with an RF choke grounding the grids, there would have to be added, multiple, unlikely failures, in addition to the grid choke failing open. For example, in the L4B, should there be a plate to grid short, the 0.82 ohm resistor in the power supply would have to fail shorted; I've never seen this happen since all the amps I've repaired had the resistor do something akin to an explosion, long before the grid choke(s) opened (which I have never seen in the amps I have repaired). Then, the 1 ohm wirewound resistor in the metering circuit would have to fail open. Then, the input tuned circuit would have to have the coil fail shorted or maintain integrity, and not flash over the low voltage capacitors from the coils to ground. Then the center tap on the filament transformer would have to fail open. And finally, the grid coupling capacitor in series with the filament would have to pass the DC (something capacitors don't do very well) or fail in a shorted mode. And all this must occur with the linear in transmit, not standby. While I can offer no scientific proof about these multiple, counter-intuitive failures, that must occur simultaneously in order for a plate to grid short to put HV on the transciever, anything is possible. One must follow one's own convictions...
73, Evan -----Original Message----- From: Jim Shorney <[email protected]> To: [email protected] <[email protected]> Sent: Sat, Apr 2, 2011 12:05 am Subject: Re: [Drakelist] L-7 Grid Choke value On Fri, 01 Apr 2011 23:21:01 -0400, K9sqg wrote: >I do a compromise. When I repair or upgrade the L4, L4B, or L7 linears, I typically remove the grid grounding chokes, put a 33 ohm 1/2 watt carbon resistor to ground from each grid pin, and leave all of the bypass caps in place. I don't see a problem with that, although W8JI's assertion is that having that grounded grid in between the plate and the cathode offers an extra line of defense against having plate voltage find it's way to the exciter. I think the idea is, which would you rather replace as a result of such an event, a tube or your radio? Considering what a grid is made of, I don't know that it would offer a whole lot of protection. The bottom line for me is that choke/cap is a parallel resonant circuit at SOME frequency, very low in the spectrum. Floating a grid above ground at any frequency where the tube has some gain just seems like a bad idea. Perhaps I'll try the resistor idea in my L-4B. 73 -Jim -- Ham Radio NU0C Lincoln, Nebraska, U.S.S.A. TR7/RV7/R7A/L7, TR6/RV6, T4XC/R4C/L4B, NCL2000, SB104A, R390A, GT550A/RV550A, HyGain 3750, IBM PS/2 - all vintage, all the time! "Give a man a URL, and he will learn for an hour; teach him to Google, and he will learn for a lifetime." HyGain 3750 User's Group - http://groups.yahoo.com/group/HyGain_3750/ http://incolor.inetnebr.com/jshorney http://www.nebraskaghosts.org _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
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