Another new 5F11

Discussion in 'Amp Central Station' started by Bob Arbogast, Jun 26, 2019.

  1. Bob Arbogast

    Bob Arbogast Tele-Afflicted

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    I just completed the chassis of new 5F11 (tweed Vibrolux) amp. This one will function as a spare for, or a pair with, the 5F11 I built in May. I don't have the cabinet yet. It will be brown tolex with wheat grille cloth (or close).

    Here's what the chassis looks like. I had to replace the fiberboard-mounted 68k input resistors with a flying trio because of "tweed disease." In other words, there was a few mV DC on the input jacks, which makes the guitar volume pot scratchy. Now everything is AOk.

    So far the first 5F11 has been working for most of my gigs. And for an old geezer like me, the good news is that, even with tiltback legs installed, it only weighs 26 lbs!


    [​IMG]

    [​IMG]
     
  2. jhundt

    jhundt Doctor of Teleocity

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    is that voltage on the input resistors related to the fibreboard mounting somehow?
     
  3. Bob Arbogast

    Bob Arbogast Tele-Afflicted

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    Yes. Some fiberboards are somewhat conductive. On this one, for example, there’s about .5 V DC on the board itself near those 68k resistors. On the board near the filter caps, there’s about 1 V. It’s not a problem, except for the fraction of the .5 V that winds up on the guitar’s volume pot—or on the footswitches of the last true bypass pedals. That leads to scratchiness—or popping.

    Floating the 68k resistors is a simple fix—and standard on BF and SF Fenders.
     
  4. jhundt

    jhundt Doctor of Teleocity

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    thanks Bob; I had never heard of this problem, though I had noticed it in my own 5f11 amp. Next time I'm in there I might try this, though the scratch doesn't really bother me much.
     
  5. Bob Arbogast

    Bob Arbogast Tele-Afflicted

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    Yeah, we all have different sensitivity levels. I tend to have some OCD tendencies about some things, like scratchy pots! (My wife wishes those tendencies extended to some other things, like the yard.)

    Actually, at a gig last weekend I noticed the popping as I switched pedals on and off. I thought it was an issue with the pedal(s), but I have since realized that it had to do with the voltage on the amp input. That was the 5F11 I built last month. Earlier today, I rewired the 68ks on that amp, too.
     
  6. FiddlinJim

    FiddlinJim Tele-Holic

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    Hmmmm. I built a 5e3 that exhibits that kind of scratchiness with some instruments. I know the input resistors are fiberboard mounted... maybe...

    Thanks


    Sent from my iPhone using Tapatalk
     
  7. Bob Arbogast

    Bob Arbogast Tele-Afflicted

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    Could be. Plug a cable into an input, then measure the DC voltage on the plug at the other end of the cable, from tip to sleeve. It should be zero. It probably isn't.
     
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