Hi all, maybe that 3-500Z is gassy, and shorts out in the presence of HV, OR maybe there is a VHF parasitic at work here. Check out AG6K's work on the topic. See link, and excerpt below.
Jim An excerpt from Richard L. Measures, AG6K: http://www.somis.org/Oct88qst.html "I set the unruly amplifier aside for a week and discussed the problem with some of my amplifier-builder friends. After some enlightening technical discussions and a suggestion to have the amplifier exorcised , I was ready to proceed. In every HF amplifier design, there is an unavoidable VHF tuned circuit formed by the anode to ground capacitance and the total inductance of the wires or straps between the anode and the output tuning capacitor. The resonant frequency of this anode-circuit can be varied only slightly by adjusting the output tuning capacitor. I measured the anode-circuit's self-resonant frequency in the unruly amplifier, with a dip-meter coupled to the wire between the HV blocking capacitor, and the anode-choke. I found a very sharp, high-Q dip at 130MHz. Next, I checked the self-resonance of the center-conductor of the coax that delivers the input signal to the cathodes. The input circuit self-resonated near the same frequency. This was not good. Much of the inductance that formed the resonance in the anode-circuit appeared to be in the 50mm [2 inches] of "U"-shaped #12 copper wire that connected the HV blocking capacitor to the top of the anode RF-choke. This innocent looking #12 wire has about 39nH of inductance. At 130MHz this inductance has a reactance of +j32. I soldered a 5.1 ohm non-inductive MOF resistor, with "zero" lead-length, across the "U"-shaped #12 wire to damp the Q of the tuned circuit. I "fired up" the amplifier on the 14MHz band and applied drive power. As usual, I saw fire and I heard a familiar bang. The fuse-resistor exploded again as did the added 5.1 ohm MOF Q damping resistor ! Thanks to the fuse resistor, the 3-500Zs remained undamaged and unshorted after this, fifth, full-blown parasitic-oscillation.. The 5.1 ohm Q-damping resistor's demise was amazing because it was virtually shorted-out by less than 0.0003 DC ohms of #12 copper wire when it went kaput ! This resistor had an overload rating of 20W for 5 seconds and it had been destroyed in milliseconds. The only thing that could have so quickly blown away a tough, essentially DC and HF shorted resistor like that was VHF current in the multi-ampere range. I concluded that the anode-circuit self-resonance of 130MHz was probably the culprit due to the 3-500Z's 110MHz+ rating and the fact that the input resonance was tuned to almost the same frequency. If I could increase the self-resonant frequency of the anode-circuit to a higher frequency, where the 3-500Z's excellent amplifying ability was waning, I suspected that it might reduce the chance for a parasitic-oscillation. I also decided that, because of the extremely sharp dip at 130MHz, the high Q of the anode-circuit was probably another contributing factor. This problem seemed to be exacerbated by the fact that high VHF-Q silver-plated strap had been used for the combination anode-suppressors/anode-leads. It did not seem logical to use the highest Q material to build a circuit that obviously requires a low-Q to prevent the creation of a transient- induced VHF seed-voltage that could start a parasitic-oscillation." --- John Lawson <[EMAIL PROTECTED]> wrote: > > > On Wed, 23 Nov 2005, Patrick Jankowiak wrote: > > > Probably a bad tube, any 3-500Z should be fine in > there. Sometimes, an abused > > one will have a deformed grid from overdriving, > and it's too close to the > > plate, or one of the grid wires is broken and > sticking out towards the plate, > > and it can arc. Or the tube could have some gas in > it, causing an arc. Had > > this happen with a few old used tubes before. I > even blew a fuse on a bias > > supply with a 304TH that had a grid-filament short > once. > > > That is likely what is happening - it was from > the estate of a silent > key by way of a friend of mine - I saw it a his > place right after he'd > unloaded it - I made him an offer he couldn't refuse > (traded it for a > small debt) - but it was as-is... I've checked > the tube for gross > shorts, none - the fils work brightly so I don't > thing the tube's very > gassy, if it is. But as sson as the B+ hits the > plate it shorts > internally. > > > Right now I'm scouting for a new 3-500Z - NIB, > NOS, or Known Good Pull. > > > > > > If you are worried, you may try next time to put > perhaps a 2 to 5K/100 to 200 > > watt resistor in series with the plate of the tube > and the plate cap of the > > amplifier, just to check, for a test only. You can > lay the resistor on a > > piece of wood or other insulating material, and > use 10KV test probe wire, or > > just route the leads so they are away from other > things. This is very > > dangerous and must be done with great caution of > course, and only for a few > > moments. -but then the arc in a bad tube will only > be pretty, and not > > destructive (hopefully). -and you can just shut > off the amp if bad things > > happen. > > > > Maybe if I also run a small set of Jacob's Lddder > wires up from the > tube, when it shorts I can shut the lights off and > laugh evilly whilst > exclaiming "It's ALIVE!!! It's ALIIIIVE!!!" > > > Damn electronics anyway. O well - need to get the > Ranger and the Valiant > working - hopefully this weekend. > > > Cheers, thanks, and Happy Thanksgiving! > > > John KB6SCO > > ______________________________________________________________ > AMRadio mailing list > Home: > http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami >
