Troubleshooting a Fender Champ 600 Tube Amp

Discussion in 'The Projects Forum' started by Sledge, Oct 16, 2013.

  1. Sledge

    Thread Starter New Member

    Oct 16, 2013
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    Greetings Forum Members.

    This is my first post in this forum so please excuse me if I accidentally break any of the rules.

    Here is the problem: I have a fender champ 600 tube amp that produces a sharp twacking like sound when I play notes within a certain frequency range. There is no sustain to the sound and the volume is very low compared to what the expected output should be for the volume setting on the amp. Also, there is no apparent pitch to the sound, just a dead, soft, twack when I strum any of the higher pitched strings on the mid to upper frets.

    I know that tube & speaker failures are common with this type of amp and at this point I have replaced both of the two tubes as well as the speaker with no improvement in the overall situation. I have also checked both the fuses and they appear to be intact. The power light comes on when the power switch is on and both of the tubes appear to glow.

    I have visually inspected each of the capacitors and they appear to be in good condition. I have double checked the guitar and cable with another system and they are working fine.

    Can anyone give me some advice as to what to check next, and possibly a good methodology for doing so? I am aware of basic amp safety precautions such as high voltage, working with 1 hand only, grounding out caps etc... Any help would be much appreciated.

    Sledge
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Does the amp work ok on low notes?

    While playing the guitar through the amp, starting with the lowest note E on the 6th string, work your way up the scale, and tell us when you start losing the note, volume and clarity, i.e on which string and fret, or which note.

    Also can you post a clear, well focused photo of the amp showing the components?
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
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  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Cap failure will give that symptom, and (unfortunately) a shorted turn on the speaker transformer can also kill all high freq output.

    I would replace the caps, there are not many and it's good practice to replace all the caps every few years anyway.

    If new tubes and new caps doesn't fix it my next guess would be that output transformer. They are available from specialty suppliers, but can run expensive.
     
  6. #12

    Expert

    Nov 30, 2010
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  7. Sledge

    Thread Starter New Member

    Oct 16, 2013
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    Wow, thanks for all the great tips!

    @MrChips: I am sorry to report that my initial testing was not very precise. When I tested the amp again I found that I could reproduce the twack at any frequency if I allowed the amp to wait for 5-15 seconds. In fact on the second pass through the lower frequencies produced the most output. This might suggest a potential capacitor problem.

    @tubeguy, @shteii01: This amp was bought in 2011 so the modern schematic is correct.

    @#12, @THE_RB: I think this is where I need to go next, assuming the problem is not the transformer. Also thanks for the vendor info.

    In summary, it seems like this is likely a capacitor issue or/and output transformer problem. I should say that when it failed I was playing this lil guy all the way on 12 while also driving it with a Boss RC_20 looper so this is probably not a normal wear and tear type issue.

    Also, here is a short video of the kinds of sounds I am getting out of the amp at the moment with the hope that a more experienced person might be able to ID my problem. I did this in one take and the commentary is pretty bad - sorry!

    http://youtu.be/THT4xxHZJQg
     
  8. #12

    Expert

    Nov 30, 2010
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    Check the speaker for ripped paper.
     
  9. Sledge

    Thread Starter New Member

    Oct 16, 2013
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    I had the same problem with the original speaker as I had with the one that I replaced today which is what is on the vid. I took it straight out of the box and put it in the amp. Also, I tested the old speaker with another amp and got enough sound out to make me think that it is not the problem.
     
  10. #12

    Expert

    Nov 30, 2010
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    Some days the magic works and some days it doesn't.
     
  11. tindel

    Active Member

    Sep 16, 2012
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    I suggest replacing all of the parts in your amp...

    Kidding...

    Have you tried different cables and guitar, just to eliminate them as the source? Do you have a volt meter? If you do, you could check to make sure the rail voltages are good. If they aren't, it's a good clue that your caps are bad.

    Be careful in there... many guitar amp caps can store enough energy to kill you, though I didn't look at this one specifically. Try to keep one hand in your pocket.

    Sounds to me like it's a filter cap problem.
     
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  12. Sledge

    Thread Starter New Member

    Oct 16, 2013
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    I have a follow up question regarding the Fender Champ 600 circuit schematic. I see a number of test points and corresponding AC & DC voltage ratings on the schematic. I assume that the voltage values shown here are what I should typically expect to see under "normal" operating circumstances.

    What I am wondering is how do I define "normal" in this situation. For the DC test voltages, I expect them to be consistent regardless of the input signal. However I would expect the AC test voltages to vary somewhat depending how the amp is driven. Can anyone suggest how I should drive the amp when testing these voltages? Also, should I have the volume set at a particular level while doing so? Thanks!

    link to schematic:
    http://support.fender.com/schematics/guitar_amplifiers/Champion_600_schematic.pdf

    edit: @Tindel, I didn't see your reply until after I made this post. Thanks for the tips! I think that I might end up replacing all of the caps after all assuming it does not break the bank
     
  13. #12

    Expert

    Nov 30, 2010
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    Start with the highest voltage, +366V. Assume the power line voltage is not exactly right and use this for the bench mark for the plate circuits. Check the plate voltages and make sure they are proportionally less as you move toward the input stages. In this amp there are only 3 measurements, B+, down 6 and down about 26 volts. That ends the high voltage portion.

    The DC voltages of 1 or 2 or 22 volts indicate the tube bias conditions. If they are off by more than 20% look for why. The output tube cathode is very important. Too much voltage here means the tube is in danger. Look up the plate power and make sure it is not being exceeded, even if you have to add more resistance!

    The power line voltages have been increasing bit by bit for 40 years. You are not going to get exactly what the labels said.

    The AC voltages assume you can input 25 mv p-p and check the gain of each stage.

    Sorry about the quickie. Got a customer at the door.

    Edit: For DC checks, no signal, volume turned all the way down.
    For AC, not sure about this. It seems you need the volume turned all the way up, but just in case, measure for 1.18 VAC on the plate (pin 1) of the 12AX7 as you are turning the knob.
     
    Last edited: Oct 22, 2013
  14. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Typically for the AC signal test you drive the amp with a 1khz sine wave from a signal generator at the specified input level (25mv pp).

    You can calculate the expected DC voltages at the plates of the tubes based on the specified voltages at the cathodes as follows:

    For 12ax7 (V2) pin 1:
    1.7vdc/1500 ohms (R2) = .0011 amps (1.1 milliamps)
    100,000 ohms (R8) X .0011 amps ~ 113 volts across plate resistor.
    Then subtract this voltage from the supply voltage on the 100k resistor.
     
    Last edited: Oct 22, 2013
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