Fisher 500-C and J&J 7591 problem

Discussion in 'Fisher' started by RS Steve, Apr 4, 2018.

  1. RS Steve

    RS Steve Tube Junkie Subscriber

    Jupiter, Florida
    This 500-C is the first Fisher I actually have tried to fix for someone else. He heard I have restored many Fisher's and coaxed me do the repair by offering to make a Bimini top for my boat. First thing I noticed was a J&J 7591 has gone nuclear, ever seen a J&J with copper side getters? :eek:

    This tubes shows a short on my tester, and looks like it had a melt down. The two J&J tubes were paired with two new Mullard's that normally run very hot, and there was no bias adjustment added to adjust things down. Needless to say, whoever restored this unit didn't do it much justice. They replaced the 1.2K 7 watt (R-143) with a much lower wattage resistor, this resistor burnt out and shows no resistance now, probably saved the OPT's. Hope to get the new resistor soon and get this unit back up and running, owner has agreed to have me install sampling, and stability resistors along with bias adjustment. This JJ tube can be a nice souvenir!



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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Glen Burnie Md.
    One of the big reasons we like EH and T-S 7591's here. JJ's in stock form FISHER's have a AWFUL tendency to S.I.D.S.(Sudden Infant Death Syndrome) and in units that have all the upgrades can't hack 70% dissipation in most cases. They're cheap (compared to EH's and T-S) and some guys swear by them. But I've seen more than my share of "Chernobyl JJ 7591's". I'll stick with the EH's and T-S's. Only had one T-S go bad and that was back in 2014 when they 1st came out. Even with the Screen stab, and all the other upgrades, the screen and the grid tried to make the beast with 2 backs. Lots of fireworks and it went Chernobyl. Jim replaced it and verified the problem. Manufacturing error. They've since got that problem fixed. Probably took Boris and Natasha out back and gave them a 9mm lobotomy! :naughty:

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