Fender Champ Build

Discussion in 'Building the Classics' started by Chris-in-LA, Jul 6, 2019.

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  1. Chris-in-LA

    Chris-in-LA Well-Known Member

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    This Champ was one of my first ever builds. I initially had a few problems with cold solder joints that were causing some crackling but that is all worked out. Since then I noticed that, on occasion, the volume seems to fluctuate. I changed out V1 a couple of times and that would fix it.

    I checked the B+ voltages today after the problem returned. The voltage at the first dropping resistor is 360 DC, 358 after the first, 356 after the second, 354 at the V1 plate resistors and about 345 at the plates. It’s almost as if none of the resistors are doing much. The high voltage at the rectifier is about 325 AC. Everything is fine until it hits the dropping resistors.

    I believe that I checked all components before I built the amp. I assume I need to go back in and verify the values on the resistors. I did once mix up the 100k dropping resistors with the 100 ohm resistors for the heater’s virtual center tap on another build.

    Any other suggestions?
     
  2. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Here’s a pic of the board.

    A7609076-2388-4B89-BEF4-95346A7FA161.jpeg
     
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Could you give us some wider shots of the board, and get the rest of the insides?
     
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  4. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Yeah, I need to take one that will be small enough to post. In the meantime I check the resistors with my Fluke and they all showed as working correctly. The line from the rectifier to the board only shows as having continuity to the first dropping resistor, there does not seem to be any other direct connections to the board, no shorts.

    A2CF89E5-A08A-4C0F-A355-F186B7D04118.jpeg

    286C16E9-0C7C-453C-A94C-25CFCD040072.jpeg
     
  5. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The green wire on the far right is the ground for all of the filter caps, I did not split the power section and pre-amp. The yellow wire below the green is the ground for the V1a cathode. Some diagrams suggest that the green wire should be grounded to the volume pot and the yellow wire grounded to the input jack. That’s my plan for tomorrow, to redo the grounding scheme.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    It's hard to see because of the limited field of the photos. Take more pictures show more details.
     
  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I started swapping in different V1 tubes and the voltage at the plate resistors seems to change with every tube. When the voltages dropped to where they were supposed to be, the amp worked but the sound began to cut in and out and seemed to related to a loose speaker connection. I tightened the outside leaves a bit to tighten it up and the amp stopped working completely.

    I cut the jack off and connected the wire with jumpers, still no sound. But, touching v1p6 would still give me static from the speaker.

    One v1 Tube caused the plate voltage to drop to 125v. The amp played but had less gain than usual. Pulling that tube and putting in a brand new tube caused the voltage to go back up to 275 at v1p6 and v1p1. Swapping back to the previous tube that had just been working and voltage at v1p6 was no longer 125v, it jumped up to 275 for that tube at v1p6 and the amp no longer worked.

    The only tube I didn’t swap out was the 6v6. That’s my next plan.

    No sign of burning or arcing at v1. Is this a bad socket? Did I burn out my OT?
     
  8. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Any specific area that you want to see?
     
  9. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The 5Y3 rectifier was arcing but was still working in another amp but will need to be replaced. Now in the process of testing the other 12ax7 tubes in another amp just to see if they are good.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    The whole inside, with clear views of the controls and tube socket connections.
    The wiring is not entirely visible...

    "...The 5Y3 rectifier was arcing..."

    don't operate the amp till you find out why.
     
  11. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The 5y3 arced once while in the Champ, but I was not sure that I saw it until I put that tube into my 5f11 Vibrolux. Then I saw it arc twice very clearly at the end of the glass away from the pins. It is going to be tossed out. I did use the 5y3 from the 5f11 in the Champ without incident but it is now back in the Vibrolux and working perfectly. I also tried all of the 12ax7’s in the Vibrolux and they worked perfectly as well, they were all hit or miss in the Champ.

    BBFB0141-053B-4F1D-9C96-FBEE8501B4D6.jpeg

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    Last edited: Jul 7, 2019
  12. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I also need to add that the intermittent volume loss problem has been ongoing since I built this amp. I did fix two cold solder joints and I replaced the V1 socket months ago. I’ve also done some tube swapping. Each time I did some work, the amp would run fine for a few months. The problem persists obviously and is not due to the tubes as they have all been swapped out. Earlier today all the voltages were correct and the amp began working but the volume began to fluctuate almost immediately. I have no clue. OT maybe?
     
  13. Micky

    Micky Well-Known Member VIP Member

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    First swap the 6V6. Then you need to get in and use a chopstick to wiggle things around. Could be anything at this point, until you narrow things down a bit more...
     
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  14. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Yeah, I’ve got new tubes on order. And wiggling the speaker jack duplicated the original volume problem I was having. It seemed a little loose so I tightened it but got no sound after that. I’ve used a chopstick on everything else in there, that’s how I found the loose solder joints some months ago. We’ll see how things go after the new tubes get here, the 5y3 was definitely bad.
     
  15. Timbo

    Timbo Active Member

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    Hey mate, A couple of suggestions.....
    Re-route the filter cap ground to one of the PT bolts
    Replace the two 100ohm artificial CT resistors with metal oxide (or carbon film) and connect these to the HT CT on the PT bolt, a short length of wire will be needed the get you there, heat shrink.
    Replace the 22k dropping resistor with a metal oxide.
    Replace the two 100k plate resistors with metal oxide.
    Check the cloth covered wire for breaks in the length of the wire and at socket pins and eyelets.
     
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  16. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Thanks for the advice but I’ll have to pick up some of those parts first. I did change some of the grounding scheme per a diagram that Robinette has. Filters for the power section are grounded to a PT lug with the HT CT. The preamp section is now grounded to the volume pot and input jack. I left the heater CT where it is for now but shortened the wires, I may move the CT to V1 and ground the resistors there to two lugs connected to the V1 screws. That makes for a pretty clean installation.
    ABB79155-E594-4883-A379-EDC1374CFFFF.jpeg
    I also removed that cheap RCA-style output jack as I believe my original volume issues were due to a loose connection. I was able to duplicate the volume issue by jiggling the plug for the speaker, it was loose. I did put in a regular 1/4” jack but I need somebody to verify that I soldered it up correctly. The NFB wire and OT secondary is soldered to the positive lug. My OT secondary is grounded at the OT frame so I grounded the negative lug of the jack to the frame, to the screw on V1.

    122111EB-936E-4F48-8C28-C8DF77F9379F.jpeg

    601C85C9-44DB-4C28-BDE6-CF556831F9A1.jpeg
     
    Last edited: Jul 8, 2019
  17. CraigP

    CraigP Well-Known Member

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    Just curious but I was wondering about the RCA jack from the earlier photo. What is/was it for? I am not familiar with the Champ. Thanks
     
  18. Chris-in-LA

    Chris-in-LA Well-Known Member

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    That’s the way Fender did the output jacks on some of the tweed amps. I’ve seen them on the Bassman, too.
     
  19. CraigP

    CraigP Well-Known Member

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    Ah, I did not know this, thanks!
     
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  20. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Ok, the initial problem of volume fluctuation was the jack that the speaker was plugged into. I also believe that the voltage fluctuations were due to a bad rectifier tube or loose speaker jack. Putting in a new 1/4 jack and a rectifier tube seems to have fixed everything. It’s even possible that the whole thing could have been fixed with Deoxit on the old jack. The amp has been running for two hours this evening without any problems. Thanks to all for your patience and suggestions.
     
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