How do you install a 68k grid stopper resistor on the input?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by Trotter, Jan 3, 2011.

  1. Trotter

    Trotter Supporting Member

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    I have a new VHT Special 6 combo amp... Its great little practice amp but it really picks up the radio signals (at least in my home).

    I noticed a guy on the Telecaster Forum has the same problem but was easily able to fix it with a simple mod (68k grid stopper resistor on the input).

    BTW... The amp has 2 inputs (High and Low).

    Can someone please walk me through how to do this mod? It sounds easy, but I need a step-by-step outline for dummies so to speak! A pic would really be great!

    Oh, and yes... I will drain the filter caps first (I do know how to do that)!

    Thanks in advance!
     
  2. TweeDLX

    TweeDLX Member

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    Trotter,
    With two inputs, they should already be in place, but you can always mount one directly to the 2nd pin of the first pre-amp tube. There will be a wire (possibly shielded) from the inputs to the first tube. It may go to the board first, so follow it to the tube from there. Disconnect the wire from the pin and solder your 68K resistor where the wire was. Solder the other end of the resistor to the wire and cover with heat shrink or electrical tape. Done.
     
  3. Trotter

    Trotter Supporting Member

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    Thanks TweeDLX! I will give it a shot my friend. Wish me luck;)
     
  4. TweeDLX

    TweeDLX Member

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    One thing to remember. If it IS a shielded wire, make sure your resistor doesn't contact the shielding. Good luck!
     
  5. CaptainJake

    CaptainJake Member

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    If there are already resistors on the input 10k will be a plenty high enough value, even then you don't really ever need to go higher than 33k
     
  6. Trotter

    Trotter Supporting Member

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    Good to know. Thanks! I'll try starting with a smaller value first then.
     
  7. momomo

    momomo Member

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    hello,
    i'm new to the gear page. i just bought a vht special combo & when I returned to my home I noticed extensive radio noise / signals. I tried various different cables, but no luck.

    so my question here:

    did this fix work?

    can you post a picture, of where to change what? I have no experience with this kind of modding..
    thank you in advance!
     
  8. TweeDLX

    TweeDLX Member

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    Like this:
    [​IMG]
     
  9. momomo

    momomo Member

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    Thank you TweeDLX.
    I think I will bring my amp together with this schematic to some amp repair shop & ask them to do it.
     
  10. Billm

    Billm Member

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    If you study the schematic, you'll find that the signal flows through two 68K resistors, in parallel. The resulting 33K is enough to kill most spurious radio signals. If you look at the construction:

    [​IMG]
    you can see that the wire from the jack down to the 12AX7 is shielded. About the only other thing you can do inside the amp is to build additional shielding around the plastic jacks, a little cage inside the amp.

    Does the amp pick up radio with nothing plugged in? With just a cord? With a cord and guitar? Plugged into the high input? Into the low input?

    Dirty contacts, including the shorting switch inside the jack, can cause a "diode effect" that detects radio signals. So can a crystallized solder joint or even a dirty tube pin.

    How's your electrical ground? Do you have one of those $10 outlet checkers that tell you if you have a proper ground and if the outlet is wired correctly?

    The amp needs an overall health check before you mod it. Of course, if you live under a big ol' radio tower, all bets are off.
     
  11. guitarcapo

    guitarcapo Member

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    An extra resistor before your guitar signal hits the first preamp tube might make your tone muddy too. It's not like your signal from the guitar is amplified any yet so basically some of your guitar signal is just winding up heating that little resistor you installed instead of getting amplified.
     
  12. momomo

    momomo Member

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    thank you all for your help!
    the guy in the amp shop installed the resistor & now the radio problem is solved!
     
    Last edited: Jan 18, 2012

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