JM/Wonderland Contour Pot

[Blues Hound]

Starting on a JM/Wonderland 100 watt amp and have seen some posts about the Contour pot and wanted to see if 250KA or 250KL is better for the Contour pot.

Thanks,
Steve

[rccolgan]

I think the sweep is pretty decent with Audio taper however if you want the amp to look brighter, you can go with a linear taper. For example, audio taper at noon position is standard/darker than linear taper at noon position. Adjustments from there.

[Blues Hound]

Ryan,

Thanks for the reply, great information, will start with the linear pot.

Steve

[Blues Hound]

Wanted to see if anyone else has had an issue with the contour pots...

I currently have is a 250KA pot, tried the 250KB and both had the same issue, when I turn it past 3, volume drops and sounds gets fizzy.

Thanks,
Steve

[WhopperPlate]

Wanted to see if anyone else has had an issue with the contour pots...

I currently have is a 250KA pot, tried the 250KB and both had the same issue, when I turn it past 3, volume drops and sounds gets fizzy.

Thanks,
Steve

Sounds like you got bigger problems. Contour knob affects negative feedback response , so I assume there is some related oscillation occurring with that part of the circuit . Double check wiring, lead dress , solder joints, ect.

[dorrisant]

Without looking... Isn't it similar to a Vox style Cut control?

[Blues Hound]

Wanted to see if anyone else has had an issue with the contour pots...

I currently have is a 250KA pot, tried the 250KB and both had the same issue, when I turn it past 3, volume drops and sounds gets fizzy.

Thanks,
Steve

Sounds like you got bigger problems. Contour knob affects negative feedback response , so I assume there is some related oscillation occurring with that part of the circuit . Double check wiring, lead dress , solder joints, ect.

I have checked wiring, it is correct, reflowed the solder joints, lead dress is good, will post pictures tomorrow.

When I use a Dumblelator, there is no issue with the amp.

[Blues Hound]

Without looking... Isn't it similar to a Vox style Cut control?

I believe so.

[Blues Hound]

Here are some pictures.

Took apart and reassembled the .1 caps, 221k resistors and .005 cap and re soldered, still the same fizziness without the Dumblelator, with the Dumblelator in the send/return, Contour works as it should.

IMG_0665.JPG

IMG_0664.JPG

IMG_0663.JPG

IMG_0662.JPG

IMG_0661.JPG

[WhopperPlate]

Come to think of it , I remember having a similar issue with a JM wonderland when I utilized an electrolytic cap in the negative feedback loop. I just used what I had lying around . I ended up swapping for a film cap and the issue was heavily diminished. I just assumed it was a bad old cap.

[Blues Hound]

Come to think of it , I remember having a similar issue with a JM wonderland when I utilized an electrolytic cap in the negative feedback loop. I just used what I had lying around . I ended up swapping for a film cap and the issue was heavily diminished. I just assumed it was a bad old cap.

Thaks for the reply, I will try that today...