Troubleshooting a Champion 600

[Elias Graves]

Two year old Champion 600 went out. No sound of any kind. I knew the volume pot was going out so I did a mod there. Does this look right?



The only pot I had on hand was a 250k no load. I still have no sound from the inputs but I do at least get a pop now when I jump to the no load click.

Both tubes light up but upon poking around, R10 looks burned.



My meter reads 0 on checking it.

Is a simple resistor change all I need here? Anything else to check out?

[Elias Graves]

[cooldude666]

Yeah, if R10 is reading open you've got a problem. Definitely replace that guy. Do you have a schematic? If not here's one. http://support.fender.com/schematics..._schematic.pdf R10 is your cathode resistor.

[Elias Graves]

Yeah, if R10 is reading open you've got a problem. Definitely replace that guy. Do you have a schematic? If not here's one. http://support.fender.com/schematics..._schematic.pdf R10 is your cathode resistor.

For that to go, a tube problem is the culprit?
Might it have taken anything else out with it?

[Steve A.]

My meter reads 0 on checking it.

My Fluke meter reads "0.L" for open and "0.0" for full continuity. What exactly does your meter read and what is the exact range and setting of the meter?

Steve Ahola

P.S. To read the exact value of a resistor in a circuit you usually do need to desolder one of the leads.

[cooldude666]

only if you don't know whether or not something else in parallel with it. this guy should be fine to read in the board

[Elias Graves]

My Fluke meter reads "0.L" for open and "0.0" for full continuity. What exactly does your meter read and what is the exact range and setting of the meter?

Steve Ahola

P.S. To read the exact value of a resistor in a circuit you usually do need to desolder one of the leads.

[Steve A.]

...

Yep- it sure is open.

only if you don't know whether or not something else in parallel with it. this guy should be fine to read in the board

I posted that before I downloaded the schematic. I did say "usually," one of my favorite wiggle words. In any case with only a rare theoretical exception*** if the reading of a resistor in a circuit is "0.L" it is safe to assume that the resistor is open without having to remove it from the circuit.

Steve Ahola

*** I have gotten strange ohm readings when there is a capacitor or other voltage source so I can't discard this possibility (unless someone can convince me that it just ain't gonna happen.)

[Steve A.]

...

Yep- it sure is open.

only if you don't know whether or not something else in parallel with it. this guy should be fine to read in the board

I posted that before I downloaded the schematic. I did say "usually," one of my favorite wiggle words. In any case with only a rare theoretical exception*** if the reading of a resistor in a circuit is "0.L" it is safe to assume that the resistor is open without having to remove it from the circuit.

Steve Ahola

*** I have gotten strange ohm readings when there is a capacitor or other voltage source so I can't discard this possibility (unless someone can convince me that it just ain't gonna happen.)

[Elias Graves]

According to the schematic, this is a 2 watt 470 ohm resistor. I read a few places that its recommended to replace with a 5 watt. Is that a good idea or internet mythology?

[Jazz P Bass]

According to the schematic, this is a 2 watt 470 ohm resistor. I read a few places that its recommended to replace with a 5 watt. Is that a good idea or internet mythology?

The resistor did not fail because it is a 2 watt device.
Something caused more than 2 watts to flow through it.
So in failing open, the resistor just may have saved the copper traces on the board. (among other things)
You can put a 25 watt device in there if the space allows.
The resistor may then never fail.
You are simply moving the 'fuseable point'.

[corliss1]

What Jazz P Bass is saying is that the resistor likely isn't the issue, but something else causing it to draw more current than it should.

[eschertron]

What Jazz P Bass is saying is that the resistor likely isn't the issue, but something else causing it to draw more current than it should.

Elias, I'll chime in for no other reason than I have one of these little beasties, and have been around inside of it more than once. In my experience, I have not had any components fail nor have I replaced the cathode resistor for any reason. I have swapped out some caps (including the cathode bypass) for tuning the low end "fartiness". So far so good. I have, however, taken the EH tubes from the amp and safely stowed them for the end of the world. I'm not saying that the stock tubes in your amp have taken a s*** but it's always good to have a spare

The 6V6 is the next thing in the circuit that includes the bad cathode resistor. It might be reasonable to pull the 6V6 and power the amp up. since the tube is out, it matters not if the cathode resistor is replaced before you do this. Voltages in and around the preamp (including B+) should be real close to those on the diagram. On my amp, the actually readings were a few volts higher, but not by much (probably attributable to line voltage). Those tests may help identify or eliminate trouble spots.

BTW, if the resistor reads open and the 6V6 is pulled, you will be able to verify the cathode bypass cap value while in the circuit.

[eschertron]

The only pot I had on hand was a 250k no load. I still have no sound from the inputs but I do at least get a pop now when I jump to the no load click.

If I understand the design of a no load pot, you disconnect the grid's self-bias reference when you 'click' it. Probably not a good thing. The grid of V2-B needs to be referenced to ground for it to work. Notice how R1 references the grid of V2-A. All tubes need the voltage relationship between the grid and cathode well-defined for them to work. Most 12A.7 types use a 1M resistor. Until you replace the pot with a proper type, please DON'T click it!

[Steve A.]

There was a recent thread here on output tube cathode resistors and I believe that the gentleman was using a 510 ohm resistor and was even considering 650 ohms. The higher value cathode resistors allow the output tube to run cooler. So unless you are looking for mondo distortion when cranked to 10 it might be a good idea to use a 510 ohm resistor. I usually use the 5W cement ("bathtub") resistors but Jazz P Bass brought up a good point about the 2W resistor acting as a fuse. And I agree with eschertron that it might have been a cheap EH 6V6GT that caused the damage.

A question for the experts: when a 2W cathode resistor starts to overheat does it change in value, and if so does it go higher or lower. Perhaps something like that caused a snowball effect. One other possibility is that a poor factory solder joint caused it to overheat.

I'd put in a good part and a good tube and see how it runs. And maybe take a look at the resistor after a month to see if it has been overheating.

Steve Ahola