Fender Champion 600 squeal

[Electron Tornado]

I repaired and modified a Champ 600 discussed in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=116348.0

Here is the schematic: http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Champion-600-Schematic.pdf


I added bass, mid, and treble tone controls and a switch to switch the negative feedback in or out (R7). I also changed the OT.

As part of the repairs, I replaced the 6V6 cathode bias resistor, R10. It was the incorrect value and was blown (open circuit).

Now, when I turn the volume up past 5, the hum level increases until the knob is on 11 (the volume goes to 12), where it begins to get scratchy and then a high pitched squeal begins. Continuing to turn the volume up, the squeal goes away and it gets scratchy again until it is on 12. When it's on full at 12, the hum level goes back down to where it was with the volume at 5. However, the amp's tone is very blatty when playing with anything other than a very light touch. Increasing or decreasing the treble will increase or decrease the squeal, respectively.

At first, I thought the scratchyness was a bad volume pot, so that's already been changed. At lower volumes the amp seems to be OK - no blatty tones.

Am I on the right track thinking that when R10 was damaged that the cap in parallel with it, C4, may have also been damaged and needs to be replaced?

[thermionix]

Sounds like you have DC on your volume pot, along with other issues.  Can you draw up a schematic of how you have the amp wired now?  Post pics?

Yeah if R10 got fried open you should replace C4.  But not with C4.

[PRR]

There's a lot of gain from tone to 6V6 plate. Those wires to new pots must be VERY carefully routed, maybe shielded. Prove it by taking it to squeal and moving the wires with a dry chopstick-- the squeal changes.

[amptramp]

The negative feedback resistor (R7) from the output transformer secondary back to the second audio stage should have a low-value capacitor across it for high-frequency stability.  Size it for a rolloff at 10 KHz to 15 KHz.

In a vintage piece like this, check that your electrolytic decoupling the triode plates has not gone to low capacitance or high ESR (equivalent series resistance).

If R10 got fried, check that the coupling capacitor to the 6V6 grid is not leaky.

As Paul said, wire routing is critical and shielding is necessary for the inputs from the jacks and the output from the resistor array to the amp and from the volume control, both the output of the first stage going to the top of the control and the slider going to the second stage.

Check that the 5-pin connector from the amp to the inputs and volume control is making good contact.

[thermionix]

In a vintage piece like this

This is a modern amp, they just revived the old "Champion 600" name.

[PRR]

> The negative feedback resistor

There's hardly-any NFB in this rig. Not enough for the stability issues of high-NFB hi-fi amps.

Counting thumbs: gain is 50 * 10 * 1/30 or like 16 at nominal load. 2200:47 limits gain to 46. On nominal load there is insignificant NFB; at speaker bass rise there is a touch (and that's why the NFB is used here).

[Electron Tornado]

There's a lot of gain from tone to 6V6 plate. Those wires to new pots must be VERY carefully routed, maybe shielded. Prove it by taking it to squeal and moving the wires with a dry chopstick-- the squeal changes.

Yes, the squeal changes. Can you elaborate on "very carefully routed"?

[Electron Tornado]

Finally got some shielded wire and rewired the tone pots. Also replaced C4. Amp works great now.