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My amp chokes on the high notes??

Discussion in 'Amp Central Station' started by Josey, Sep 29, 2017.

  1. Josey

    Josey Tele-Meister

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    Anyone know what may be causing this? It's an original Fender '65 DR. Until it has a chance to warm up real good the amp seems to choke out when I hit a real high treble note. After I've been playing for a while it doesn't have a problem and works with out issue. Is this a tube or some sort of capacitor?
     
  2. Mart the Hat

    Mart the Hat Tele-Holic

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    It's hard to tell what sound you mean by "choking out" but I once had a speaker with a voice coil rub that sounded most noticeable on higher notes; a kind of nasty distortion on the tail of the notes. Maybe try it plugged into another speaker before you look any deeper. Though an amp of that age may be due some tech attention anyway.
     
  3. Dacious

    Dacious Poster Extraordinaire

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    How old? Original 90's? Be due for power tubes, chassis service - caps, jacks, general tighten and spruce up.
     
    Last edited: Sep 29, 2017
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  4. Wally

    Wally Telefied Ad Free Member

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    Vintage BFDR....what is the service history of this amp?
     
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  5. Josey

    Josey Tele-Meister

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    Filter Caps and Electrolytics (All Sprague Atoms) have been changed prior to me but the work is clean and appears professional. Tubes are an assortment of NOS glass: Mullard GZ34 and smoked glass RCA 6v6's matched within a couple ma's. The pre amp tubes are a couple of telefunken 12at7's, couple of mullard 12ax7's and a couple of RCA 7025's. I've cleaned all my contacts with De-Oxit and and everything checked out visually. Speaker is a new Tone Tubby Red Alnico without an issue so I know I can rule out bad voice coil. I did notice where the bright cap has been clipped on the Vibrato channel, which is the channel I use. Could that be causing an issue? Like I said once everything is good and warm the problem goes away.
     
  6. Wally

    Wally Telefied Ad Free Member

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    Is this problem present in both channels? Have you done a voltage chart with measurements done during and after the problem is being exhibited? Something makes me suspect a tube in the preamp...or the PI. Maybe because it is a simple thing to check. whether or not the problem is limited to one channel is a big clue.
     
  7. JD0x0

    JD0x0 Poster Extraordinaire

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    No.

    Sounds like tubes a solder join or potentially a conductive circuit board. When the amp gets hot it may be increasing resistance in the conductive area enough to eliminate the issue, or it may be evaporating moisture that got into the board, enough to fix the issue at temp.. Meter the circuit board near a few eyelets. One probe to ground one probe to the circuit board (test many different areas)
    I believe oscillations can cause notes to cut out.
     
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