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Fixing a Fender Twin Silverface Amp

Discussion in 'Amp Tech Center' started by karateinthenow, Jan 2, 2014.

  1. jazzguitar

    jazzguitar Tele-Afflicted

    Mar 17, 2003
    Hi Allen,

    I would not mess with that, the hum balance is much better than those two 100 Ohm resistors. Turn up the volumes and treble with a patch cord from NORMAL Input 1 to VIBRATO Input 1, and adjust for minimal hum.

    I would just ignore those, turn up the master, don't pull it, and it is practically out of the circuit.

    It is not entirely clear in the picture whether that tube is bad and redplating deposited metal vapor on the glass making it dark, or whether the tube has two additional getter flashes on the side as some tubes do (are there metal rings mounted inside below the dark spots?) You need to test that tube.

    Anyway that larger bottle Sylvania is the only US made tube, the other power tubes are Russian (even if they are labeled Sylvania).

    I think your amp was made about 1977 (assuming the transformers are original) and in this era all electrolytics were of mediocre quality, in particular the small white plastic Mallory's. Replace all electrolytics, most important the power supply caps under the metal cover. Keep the blue blobs (non-electrolytic) those do not have a good name but in fact are excellent.

    The correct schematic for your amp is here:[​IMG]
    (click top bar to get full size)

    Before you change anything get the amp serviced properly and then decide if it needs anything!

    In particular, don't rip out the bias balance control which is a very neat feature allowing you to match unmatched power tubes, simply adjust for lowest hum with all volumes turned fully down. You can replace the 15k resistor on that bias balance pot with a 20k trimpot and have adjustable bias too!

    Three more things: 5881 tubes (real ones, not the Russian tubes only labeled 5881) have lower ratings than 6L6GC tubes, I advise against using 5881 in that amp.
    Also, some Silverfaces run their rever driver tube very hot, best replace the 470 Ohm cathode resistor on the reverb driver 12AT7 with 820 Ohms or 1k.
    Finally, some changes in your amp show that somebody already messed with the circuit.

    I hope this helped, good luck!
  2. Ike286

    Ike286 Tele-Holic

    Sep 13, 2011
    I guess my suggestions were based on my own experience with my 79 twin. It had the hum removed before I got it, and I liked it better that way, but mine was really quiet, also had good matched tubes. Probably would have been better off with it if it hadn't had such well matched tubes...

    The master and boost were horrible to me, maybe I got a dud. It was great afterward though. I also rebiased the reverb driver to 820 I think... I really liked that amp.

    Jazz is right though, get your amp serviced so it works correctly, then decide whether it needs anything then.
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