---------------------------------------------------------------------- Message: 1 Date: Thu, 27 May 2010 12:36:24 -0400 (EDT) From: "Larry Szendrei" <n...@securespeed.us> Subject: Re: [AMRadio] Heathkit DX-60 grid current drops To: "Discussion of AM Radio in the Amateur Service" <amradio@mailman.qth.net> Message-ID: <61478.24.97.227.120.1274978184.squir...@mail.securespeed.us> Content-Type: text/plain;charset=iso-8859-1 As Jim says, you may be losing drive to the final grid because the drive signal is going away. But that's not all that can go wrong. Make sure the final grid bias resistor hasn't drifted off-tolerance or changing value as it heats up. If it is, replace it. I have also had bad final tubes (pair of 6146s in a DX-100) create the symptoms you describe. Originally I thought for sure the problem was with the drive signal, but that wasn't what it turned out to be. 73 es GL, -Larry/NE1S Reply by Jim, WD5JKO: Adding to what Larry says, it could be that the 6146 is running too hot. By hot I mean the filament voltage is too high, and a fragment of the cathode oxide has detached and landed on the control grid. If this is the case, with RF excitation, the fragment gets hot and becomes an electron emitter which will cause the grid current to gradually drop, and then reverse. This happens frequently with the 4D32. With the 4D32, lowering the filament voltage from 7'ish to around 6 volts usually makes the tube behave better. I've not seen this with the 6146, but it could happen, especially when running a vintage BA at today's higher AC line voltage. Jim WD5JKO ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: AMRadio@mailman.qth.net To unsubscribe, send an email to amradio-requ...@mailman.qth.net with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html