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1975 Silver Face Champ- snap, crackle and pop!

Discussion in 'Amp Tech Center' started by SanAnTex, Nov 18, 2017.

  1. SanAnTex

    SanAnTex TDPRI Member

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    Hello TDPRI!

    I read these forums all the time but have never posted. I figured some of you in this forum with more experience with tube amps might be able to help me out with a diagnosis. I recently acquired a 1975 SF champ, it looks to have a new large cap and I replaced the tubes with rebranded RCAs. When I first got the amp, I noticed when turned to 10 ( no guitar plugged into input) it crackles or sizzles so I flipped the amp over speaker down to wiggle the 6v6 to make sure it was snug. Before I could touch the tube, the amp made a pop noise and the 6v6 seemed to “arc” or flash and then return to its regular glow. After this pop, the amp sounded great! A week later, the sizzle has returned. Has any body ever had an experience like this?
     
  2. Slim Chance

    Slim Chance Tele-Afflicted

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    Disclaimer: I'm probably the least qualified to answer your question, but I do own a Vibro Champ.

    If wiggling the tube made a difference, I'd try spraying a cleaner like DeoxIt D5 in the tube socket and carefully re-insert and remove the tube a few times using a gentle circular motion. Make sure that the amp if off when you do this.

    If that doesn't work, read below. :>)

    It's a great amp so be careful. There are good amp "tex" in Texas, some of which have been know to haunt this forum.

    Oh, and welcome to TDPRI.
     
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  3. tweedman2001

    tweedman2001 Tele-Afflicted

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    ^^^ This and you might need to tighten the tube socket pins too. If the tube goes in and out easily then that would be the problem.
     
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  4. keithb7

    keithb7 Poster Extraordinaire

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    I'll guess, it's the cathode cap and resistor for the power tube. And the 6V6 power tube. Get 'em swapped out and up the wattage of the resistor, as well as the voltage rating of the cap.
    Be sure to only run a vintage 6V6 in this amp. It runs at insanely high voltages. A modern tube won't likely last a few months from regular daily playing.
    A vintage tube is good for what? 40 years?
     
    Last edited: Nov 18, 2017
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  5. keithb7

    keithb7 Poster Extraordinaire

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    @Wally is somewhere in Texas. If you need help PM him. He's one of the good guys teaching all us rookies here how not to screw these amps up when we take a soldering iron to them.
     
  6. aerhed

    aerhed Friend of Leo's

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    A hunk of something conductive may be in the chassis bouncing around. Or, a wire sagged when upside down and touched something. You need to investigate (drain your caps first).
     
  7. Darkness

    Darkness Tele-Afflicted

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    Pics of chassis interior please.
     
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  8. Dacious

    Dacious Poster Extraordinaire

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    Take a close look at the power and rectifier tube sockets. They can develop carbon traces around the pin clinches and arc. Very nasty. If you discharge the caps you can tighten them with a jeweller's fine screwdriver. That'll fix if they're not really burnt. If there's blackening of the sockets, replace for a few bucks. Job's good for another 40 years.
     
  9. muchxs

    muchxs Doctor of Teleocity

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    The supply voltage in '60s and '70s Champs is too high, usually in excess of 420v.

    RCA 6V6GTs are prone to arcing in SF Champs under certain conditions. It's usually not a carbon trail on the socket. The arc internally 'cuz the supply voltage is too high. As I've mentioned about 11,000 times, 420v is about 100v more than I've seen on any 6V6 tube data sheet.

    '60s and '70s Champs share a common wiring error. The 470 ohm bias resistor is usually in contact with the adjacent 25uf /25v bypass capacitor. If you cook the resistor (common) you also cook the capacitor (also common). Failure mode is the high voltage cooks the 6V6 which shorts and cooks the bias network. Neophytes simply replace the 6V6 tube and assume it's all good. It ain't.
     
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  10. JD0x0

    JD0x0 Poster Extraordinaire

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    Those fiberboards can get conductive and cause those issues. Bacon frying sound is a good indicator of a possible bad plate resistor.
     
  11. Darkness

    Darkness Tele-Afflicted

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    This is why I add a dropping resistor from the rectifier to the board. A 10 watt 470ohm wire wound lowers the voltage to 350-360v, which is in line with BF specs. The heat dissipation is nominal in this application and there are isn’t any tone suck from it. My 2 cents.
     
  12. muchxs

    muchxs Doctor of Teleocity

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    O.P.'s '75 is from the era when Fender attempted to remedy the conductive circuit board issue. Everything on the board is heavily goobered with paraffin. I cleaned up a '77 the other day. It's probably easier to clean up after Mardi Gras.

    Even with the majority of the wax removed, the board is still saturated. Float the amp down the Buffalo Bayou, the board will be the least of your worries.

    I got a Musicmaster Bass Amp I call "Texas Flood" after Hurricane Harvey. It looked rough. The seller neglected the part where it had been submerged long enough to soak most of the tolex loose. Knowing that, I expected it to be packed with mud. It's pristine inside. Still doesn't work.

    I've gone 'round and 'round with the "drop the voltage" thing. It's like adding an isolation transformer to a transformerless amp. You know they won't.

    Instead, I'd remove the stock carbon comp 6V6 cathode resistor and stick that 470 ohm 10 watter in there. I miss Radio Shack. They used to have a whole bin of Nichicon capacitors. Replace the cooked white Mallory with a Nichicon. Then just make it take it except leave that RCA 6V6GT in your sock drawer. They arc. Can't tell you why just that they do.

    A grey plate GE should work just fine as long as the old white Mallory cathode bypass cap isn't torched. A wafer base 6V6GTA is probably what the amp came with. Fender used wafer base 6V6GTAs in everything in the '70s. A Musicmaster Bass amp is a good place to harvest a pair. It's the only '70s Fender amp with low, low, plate voltage. With 290v on the plates, the 6V6s in the typical MMB are virtually unused.
     
  13. JD0x0

    JD0x0 Poster Extraordinaire

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    My 'wax saturated board' was reading 45V in one spot, not even near an eyelet, because the board was conducting so bad. If the wax gets dirty it can become conductive. Sometimes cleaning the board doesn't even help. I had to ditch my board after attempting to fix it 3 or 4 times with cleaning. It now has a thick fiberglass board in it, which doesn't flex or conduct like those flimsy fiber boards.

    Problems w/ the board included, but not limited to...
    -Random pops, cracks and sizzles amd occasionally whirring.
    -Scratchy tone controls
    - Tremolo tick (YES. Even with the cap across the roach which is supposed to kill 'thump')

    Not only did I attempt to clean the board multiple times, I also isolated, and lifted the board off the 'insulation board' which can also sometimes be conductive. I tried everything I could before pulling those fiber boards out, after realizing attempting to continually fix this problem isn't working. I could get the voltages down to .2-.4 V on the board, but after a few days the board would (I assume) absorb water and the voltage would creep up again, along with the issues I was having.

    Just my $.02
     
    Last edited: Nov 19, 2017
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  14. SanAnTex

    SanAnTex TDPRI Member

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    09B4E383-5617-43B9-9B58-EC785C6E55C8.jpeg F5106CC3-8E8B-49BE-B5AF-FFD90CAF49E9.jpeg D1FA828B-224F-43D6-9878-D7D5DB1A63FD.jpeg A0DB08AE-DCE5-447A-9557-01A5D373B2FF.jpeg 048D474D-61A3-42B1-9711-AC831486E274.jpeg
     

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  15. SanAnTex

    SanAnTex TDPRI Member

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    Wow! Thanks for the replies everyone.. you all are very helpful. I have attached some pictures to show the guts of the chassis.. it seems like there is some sloppy solder work done on the large cap.. I will try to retention/ clean the tube sockets as some of you have suggested. Should I change the board? Resistors? Caps? Whole amp? Just kidding!
     
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  16. keithb7

    keithb7 Poster Extraordinaire

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    Any idea who serviced this amp?
     
  17. SanAnTex

    SanAnTex TDPRI Member

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    Also, these pots don’t look original.. maybe this amp has been gone through and taken apart in the past.
     

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  18. SanAnTex

    SanAnTex TDPRI Member

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    No idea who serviced this amp. It’s a craigslist purchase and I have no contact with the seller.
     
  19. keithb7

    keithb7 Poster Extraordinaire

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    Ok good. I did not want to hurt any feelings here.

    The person who went in there needs a hot soldering iron. The Harbor Freight one used to solder the cap can in ain't cutting it.

    What is the reason for leaving a death cap in place? The 6V6 cathode cap snuggled right up to 2W resistor is like a high school student, teacher affair. Bad news.
     
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  20. BobbyZ

    BobbyZ Doctor of Teleocity

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    Is the speaker 4 ohms impedance?
    Years ago I had a VC hooked to an 8 ohm speakers untill I noticed lighting bolts inside the 6v6.
    Apparently that old line about vintage Fenders taking a 100% mismatch doesn't apply to all vintage Fenders.
    Then again I found that amp in a hillbilly junkyard between a rusty Chevrolet and a dead calf. . . .
     
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