Footswitch issues

Discussion in 'Amp Central Station' started by Ken Wilson, Dec 31, 2011.

  1. Ken Wilson

    Ken Wilson Former Member

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    My amp is a blackfaced 70 Twin Reverb. I purchased a reproduction footswitch for it which worked fine for about three years.

    I've been chasing noise issues in the amp over the few months (tube swaps) and in the process noticed that the amp's significantly noisier with the reproduction footswitch plugged in, and gets noisiest when the reverb is turned on at the footswitch. I just plugged the original footswitch in and the noise is reduced, so I assume the new footswitch is the problem, and not the amp.

    What do I do? Is there anything useful I can do to diagnose this? I have a multimeter but have no idea if these footswitches can be pulled apart, and if so, how do I go about troubleshooting?

    http://i396.photobucket.com/albums/pp43/spellcasterguitars/AMPS/IMGP3202.jpg
     
  2. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Ken,

    You're not crazy - here's the issue: Reproduction footswitches do not typically use genuine US made Carling brand switches. They use those cheap Chinese made reproductions. (Besides the slave labor issue) Sino-switches are cheaper because they do not feature palladium coating on the contactor tang inside the switch. They use tin. These tin coated contactor tangs are subject to arcing under load, which severely reduces their service life. After some months or a few years of use, the arcing on those non-palladium contacts creates a contact irregularity - which literally turns into a homemade diode. This diode effect causes noise in the form of a few millivolts.

    Here's how that diode noise gets in your amp. The reverb footswitch wire you plug into the back of your amp is literally directly connected to pin two of V4 in your Twin. Pin two is the grid of your reverb recovery tube - and the noise you are hearing is the diode voltage sourcing from that cheap footswitch - which feeds right into the grid of that 7025. The 7025 dutifully takes this signal, amplifies it, and injects it into your amp for further amplification. You hear that as noise.

    Solution - buy a Genuine US made Carling brand SPST switch. They cost about 16 dollars, as compared to the Chinese imitations...and they're worth every penny.
     
  3. megafiddle

    megafiddle Former Member

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    The switch contact is open when the reverb is on, I believe. The switch turns
    off the reverb by shorting the reverb signal to ground.

    If noise is worse with reverb on, switch would be out of the circuit.

    I am guessing the noise you hear is is 60 cycle hum? If so it sounds like a bad
    shield connection in the footswitch cable.
     
  4. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Megafiddle's right. I missed the line about "worse when the reverb's on". My bad.

    But the cheap switch issue is very real. I've seen numerous failures in these economy switches due to arcing. The shielding in the cables on these repro switches is also suspect, which is what Megafiddle's suggesting as a noise source.

    bottom line - crackly irregular frying noises are diode effect noises - humming noises are bad shielding.
     
  5. Ken Wilson

    Ken Wilson Former Member

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    Thank you both for your responses.

    I guess the first thing to do is use a meter to check the shield side of the cable. Do these footswitches generally come apart for service? I'm wondering if it's a clamshell that'll split if I try taking off the vinyl ring.

    The point about the cheap switches is a good one. Whether the shield ground is the issue or not right now, I'd still like to replace the switches to circumvent future problems. Can you point me at a source for those Carling switches?
     
  6. megafiddle

    megafiddle Former Member

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    CoolBlueGlow -
    You're also right about contacts.

    When we started using solid state industrial logic controls, we had problems with the older
    type switches trying to switch low voltage (5V DC) logic signals. The silver cadmium oxide
    contacts worked great with 120VAC, but just a little oxidation made them unusable at low
    voltage.
     
  7. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Very interesting. I wonder why they did that? I wonder, was there some inherent self-healing of the surface at 120vac with the arc of the switch that wasn't present at 5vdc...perhaps due to the higher currents? since those were not present at low voltages, no arc to reform the surface?

    Better minds than mine would know...ah, switch metallurgy, another lost art.

    Oh, sorry Ken - "REAL" Carling made switches - Google it - Should be easy to find. SPST Carling footswitch. You might try Mouser.
     
  8. Ken Wilson

    Ken Wilson Former Member

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    Thanks guys!
     
  9. megafiddle

    megafiddle Former Member

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    That's what I would figure. I've re-dressed a lot of those contacts, and they seem to
    get pitted and worn, but there always seems to be good solid metal at the surface. At
    least as long as there was no overload current. Perfectly good silver contacts often
    measure many ohms. This is probably due to a very thin oxide layer. Low voltage can't
    break down the layer, but higher voltage like 120VAC can. Or perhaps the arcing that
    occurs when trying to make contact, "cleans" away the oxide.

    Actually many contacts are convex, and if you look at the actual contact area, it can
    be extremely small. So I guess the contacts only need to find themselves the tiniest bit
    of area for contact. Many contacts also have a slight wiping action when they close.

    With the right type of metal, it might very well form a diode.
     
  10. jbmando

    jbmando Poster Extraordinaire Gold Supporter

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    I have a different footswitch issue and I hope someone can give me advice. I have a Peavey Bandit 112 Transtube and the channel switching and boost are footswitchable. Booth functions work with the generic LED footswitch I bought on eBay, but the boost is on when the light is out. Can I reverse anything to make the light be on when the boost is on, or should I either find another pedal or live with it?
     
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