Champ 5F1: Snap!...Pop!...then...nothing...Help!

Discussion in 'Amp Tech Center' started by TomK, Jan 25, 2020.

  1. TomK

    TomK Tele-Holic

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    My '57 CS Champ started make occasional loud snaps and pops . . . but then it went 'pop' and silent. Nothing. No sound at all.

    But the light stays on, so I'm guessing it's not a fuse.

    I swapped out each tube one at a time, and with each swap, the same thing would happen: the amp would work, then within a few minutes make a few loud snaps and pops followed by silence.

    Okay, so it's not a tube.

    So what do I check next?
     
  2. FenderLover

    FenderLover Poster Extraordinaire

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    Snaps and pops from where? Are the tubes arcing? From inside the chassis? Pull the back panel, any damage inside? Can you leave in just the rectifier tube and check voltages?
     
  3. hdvades

    hdvades Tele-Holic Silver Supporter

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    Does it have a fuse somewhere? I'd try checking fwiw.
     
  4. TomK

    TomK Tele-Holic

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    Snaps and pops are coming through the speaker.

    I pulled the back--nothing looks burnt and all connections/components seem firmly in place (I poked around at all of them after discharging the caps).

    There are four fuses (one in the usual-for-Fender fuse holder and three on the board). I'll check all of them next--but the pilot light stays lit even after the sound dies, so there's that.
     
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  5. ce24

    ce24 Poster Extraordinaire

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    Were the swapped tubes new and known to be good?
     
  6. muchxs

    muchxs Doctor of Teleocity

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    Regardless, it's probably a tube. It's usually the 6V6 although 12AX7s from the Czech Republic tend to go gassy.
     
  7. Whatizitman

    Whatizitman Friend of Leo's

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    Some indicator lights are 120ac, and can be pre-fuse. Like a standard light bulb. Just sayin’. Someone with more knowledge about tweed circuits than I have (pretty much everyone on the planet) could verify if that’s the case for the original circuit. But the actual amp could be different, too.!
     
  8. Wally

    Wally Telefied Ad Free Member

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    The lamp is on the heater filament....as in the vintage amps. So, the heater filaments are good....otherwise it probably would not exhibit the problems it is exhibiting because the tubes would not be creating some sort of signal.
    Tech time.
     
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  9. Jon Snell

    Jon Snell Tele-Meister

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    The 5F1 has three valves; 5Y3 rectifier, 6v6 output and 12AX7 pre-amp and driver stage. As you have tried new valves,
    I would monitor the HT on the anodes of the 12AX7. You may find one of the feed resistors is failing when warm. It may even be the 22k HT feed resistor.
    Mind your fingers as there are lethal voltages on these points!
     
  10. Dacious

    Dacious Poster Extraordinaire

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    The CS 5F1 if it's like my EC Vibrochamp has a 305 glass fuse on the circuit board. You have to just about disassemble the amp to get to it.

    Blown fuse on mine was caused by power tube socket clinches spreading and arcing. Easy to fix by levering with jewellers screwdriver.

    The 6.3v pilot and tube filaments will light. But HT is out and no power is flowing through the rectifier.
     
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  11. Jon Snell

    Jon Snell Tele-Meister

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    champ_5f1-layout-1023x882.png
    This may help.
     
  12. Chunkocaster

    Chunkocaster Poster Extraordinaire

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  13. Jon Snell

    Jon Snell Tele-Meister

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    That is because it runs in class A. Not Ab.
     
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  14. Whatizitman

    Whatizitman Friend of Leo's

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    In which case the fuse is doing exactly what it's supposed to do. Probably not good to keep turning it on and trying with new tubes. OP said he can see more than one fuse, so arching on the socket sounds like it could be a culprit as much as anything else.

    Being a noob myself, I'm sincerely asking here, for my own learning. Someone please correct me if I have this wrong.

    Given that it's a newer reissue amp, that does play into how and where fuses are in the circuit, right? According to the 5F1 layout above, the original only had one fuse, pre PT, and was two prong. If the fuse blew on an original, there would be no light, right? The only amps I've worked on that have lamps were already grounded three pronged style. The multivox in my avatar was two prong, but had no indicator lamp. I didn't have to take that into account when wiring it for three prong/ground.

    OP is right to open it up and check ALL of the fuses before doing anything else.
     
  15. Wally

    Wally Telefied Ad Free Member

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    interesting video. That is a good reveal on Russian rectifiers. I always am skeptical of then and take note of their voltage drop. I also like that he found that the PT had been weird to yield higher voltage than would have been achieved with the 240VAC tap.
    Regarding the current draw/plate dissipation in that amp vis-a-vis tube wear, it is commonly understood among amp techs that Champs exhibit high numbers....they always have. I have observed one SF Champ in particular that drew even more current draw than was observed in this video....just over 20ma. (8^0... it sounded glorious. It was a 1971 with all original tubes, and I owned it more than 40 years after its build. It is still running as far as I know. I did an experiment with that particular amp just to observe what happened with different levels of current draw. I increased the resistance for the bias resistor in steps....took measurements and listened. At just over 16 watts of plate dissipati9n, which would be around 114% of max plate dissipation, the amp still had what I would want to hear...very musical, very responsive. With an 820ohm resistor which set the plate dissipati9n at 14.7 watts....just over 100% of the max....that amp was sterile. It sounded very much like a small solid state amp in comparison; to what it had sounded like. I put the original 470ohm resistor, which was still measuring to spec, back in and the glory returned to the Sonics. one can bias to any point that pleases one, but I like the 470ohm that Leo chose.
    Fwiw, one can bias the amp with the power tube. With that same SF Champ, I tested the few handfuls of NOS RCA 6V6 tubes I have...trying to match some pairs. Those tubes yielded various plate dissipation numbers from the low 80% range to just over 150%, and no two tubes were close enough to be paired with each other. They are good for s8ngle-ended application only.



    Because I am easily confused, I have to note that “HT” is misused in the statement above. HT is an abbreviation for High Tension, which is a term used for the high voltage AC secondary on the PT that feeds the rectifier. The rectifier takes that HT AC voltage and rectifies it...changes it to DC for the feed to the power supply rail. Some note the nodes of the power rail with B+,B1,B2, etc. Fender was in the habit of using letters to denote those nodes....A,B, C, etc. fender schematics do not use numbers or letters for the small ?Champ schematics. If they did, that preamp voltage would be fed from the C node. Rather than see HT used there, it would be more fitting imho to refer to the voltage on the anodes of the 12AX7 as plate voltages.
    Please understand that I am not making this observation to be pedantic but rather I can still remember trying to understand this stuff back when I was getting into it, and the use of HT here would be confusing to a beginner...because...well, it is not the HT voltage.
     
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  16. Wally

    Wally Telefied Ad Free Member

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  17. Dacious

    Dacious Poster Extraordinaire

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    Wally is right - I wrote HT without thinking but it's the B+ off the rectifier before it hits the first smoothing filter cap IIRC. That lead goes to the fuse then through the filter array. The chassis interior is cramped due to the switches. The fuse holder is rivetted to the board edge IIRC.
     
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  18. Chunkocaster

    Chunkocaster Poster Extraordinaire

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    I watched a few of these videos today as I wanted to have a better understanding of the circuit. In another test the guy mentioned how the voltage has increased at the mains since the champ was put into production which is the reason for the higher than spec readings. I have a beginners level of knowledge on this stuff but the champ circuit seems like a great way to learn.
     
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  19. Wally

    Wally Telefied Ad Free Member

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    it is the ideal starting point, imho, Chunk.
     
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  20. TomK

    TomK Tele-Holic

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