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So while prepping my B-15N for sale and the 6L6GC - 387STRs outputs that had worked fine last spring when I last fired the beast up were "dead." And by that I mean no detectable emission that I could discern - absolutely no voltage drop across the 1 ohm cathode current sensing resistors I had put in place. Now it took me a while to determine the tubes were to blame cuz I just couldn't believe that both had simultaneously expired - wasted a while checking voltages, resistances, resoldering. Finally stuck a couple of SED test 6L6s in and instant great sound. "What the hell" I exclaimed - must be corrosion or something. So I retensioned the sockets, polished the pins until I noticed that I hadn't shaved that morning, and remelted the solder on each pin. Still as dead as our president's morals!
This one really confuses me! The heaters light up and draw sufficient current and since there is no current flow through the cathode with 500V on the plate and screen while varying the G1 bias from - 25 to 50 VDC - the problem isn't screen nor plate but related to both - the cathode. OK, I'm familiar with "sleeping sickness" but this is caused by long idle at cathode current cutoff with plate, etc. voltages. This amp was simply "stored" - warm, dry, foldly stroked (well, maybe not <grin>) and not one but both of the output tubes - from the same manufacturing batch according to the screened Sylvania codes - went dead while the two other identical tubes in box storage (see "salvaging" post) immediately worked well (and matched within 3 mA).
My cows are in a circle ("beef-huddled"). After I finish going over the V-4 I'm also selling I may try upping the heater dissipation for a little while to see if that restores cathode emission - I'll let you know - but I would be interested in any other stories as strange as this.
Rob
So while prepping my B-15N for sale and the 6L6GC - 387STRs outputs that had worked fine last spring when I last fired the beast up were "dead." And by that I mean no detectable emission that I could discern - absolutely no voltage drop across the 1 ohm cathode current sensing resistors I had put in place. Now it took me a while to determine the tubes were to blame cuz I just couldn't believe that both had simultaneously expired - wasted a while checking voltages, resistances, resoldering. Finally stuck a couple of SED test 6L6s in and instant great sound. "What the hell" I exclaimed - must be corrosion or something. So I retensioned the sockets, polished the pins until I noticed that I hadn't shaved that morning, and remelted the solder on each pin. Still as dead as our president's morals!
This one really confuses me! The heaters light up and draw sufficient current and since there is no current flow through the cathode with 500V on the plate and screen while varying the G1 bias from - 25 to 50 VDC - the problem isn't screen nor plate but related to both - the cathode. OK, I'm familiar with "sleeping sickness" but this is caused by long idle at cathode current cutoff with plate, etc. voltages. This amp was simply "stored" - warm, dry, foldly stroked (well, maybe not <grin>) and not one but both of the output tubes - from the same manufacturing batch according to the screened Sylvania codes - went dead while the two other identical tubes in box storage (see "salvaging" post) immediately worked well (and matched within 3 mA).
My cows are in a circle ("beef-huddled"). After I finish going over the V-4 I'm also selling I may try upping the heater dissipation for a little while to see if that restores cathode emission - I'll let you know - but I would be interested in any other stories as strange as this.
Rob