C2C King Nothing - great oscillator, terrible pedal

[drdn0]

Howdy,

I built a King Nothing. Everything works fine, voltages are spot on (254v on the plate resistors, and 29v heading into the J201), but there's significant amounts of oscillation that decreases as the gain is turned up. Low switch has no impact on the noise, but the left position of the mode decreases the range where it oscillates from about 1-9 to 1-3 (assuming each tick is about 30 degrees of rotation).

Weirdly enough, it oscillates MORE when it's in bypass - when it's in bypass, grounding the input stops the noise (even though the switch PCB is meant to ground the input when it's off), when the pedal is on grounding the input changes the frequency of noise by a few hundred hz but doesn't stop it. It oscillates at 2500-3200hz, with increases in tone increasing the frequency, and increases in gain decreasing the frequency/volume.

I've tried multiple power supplies, from isolated DC bricks, to cheap SMPS to an old unregulated power supply I've got around (all capable of +500ma), and no difference. 9/12v are identical; no difference in frequency of volume of oscillation. Tried multiple guitars, multiple leads, and multiple amps with no difference.

The pedal is built as per the schematic (except for using a IRFB17N50 instead of IRF740; I have used these on other builds with no problems), and I have verified all components. I have reflowed the entire board, scoped all joints and also cleaned the PCB's off with an ultrasonic IPA bath in case there was any schmoo on there. Photos of the PCB below are pre-bath.

Tube was a brand new Shuguang 7021 (confirmed to be good in other pedals), J201 is a onSemi MMBFJ201 on a SOT23>T092 adapter board.

Since originally building it, I have:
*verified all components on the board and polarities
*swapped the J201 and adaptor for a TO92 2N5458 (no difference)
*swapped to other tubes, both other 7021s and EHX 12AX7 (no difference)
*reduced the plate resistor from 220k to 100k (no difference)
*paralleled RF suppression caps at the input (no difference)
*added Miller cap on the JFET (no difference)
*moved input/output wires around, tapped components, poked stuff (zero difference with anything)
*used a disconnected lead on the input to see if I could isolate the oscillation (the frequency increases from ~2600hz to 3100hz if I'm poking the tube, but that's it - no increase/decrease in volume).

There are probably a dozen successful builds I have read of, but I've also read of a few people having the same oscillation issues that were never solved.

Thoughts?

[drdn0]

Ok, so just for shits and giggles I wrapped the entire tube/socket in aluminium foil and earthed it directly on the power jack.

It probably lead to a 40% reduction in oscillation - weirdly enough now it oscillates from somewhere between 2 and 8 gain - at either max or min it is quiet. Still oscillates more on bypass than when the pedal is on, and still utterly unusable; the oscillation is significantly louder than my guitar tone coming through.

I also remembered aluminium is a phenomenal conductor for B+ after a little slip 

I also saw R9 (triode 1's grid stopper) is a mile away from the grid, so I removed it, ran a shielded jumper from there to the tube directly (with R9 right against the tube) - no change either.

[Chillums]

Did you use a UF4007?  I'm pretty sure it is a must have in the power supply.

[GibsonGM]

Did you use a UF4007?  I'm pretty sure it is a must have in the power supply.

It is, or an equivalent high speed diode.

[drdn0]

Did you use a UF4007?  I'm pretty sure it is a must have in the power supply.

Yes. As I wrote, every other component except for the MOSFET/FET is as per the schematic.

I have built probably half a dozen power supplies based off a 555, none had any switching noise whatsoever.

[duck_arse]

scope the output pin of the 555. what frequency? can you scope the noise in the audio section as well?

[R.G.]

DA's right - first find out if the oscillator is emitting/leaking into the audio path.

More generally, the board seems to have the (very) high voltage oscillator section close to the J201. The J201 would be more than happy to pick up high frequency spikes.

More generally still, grounding and routing will be critical on a layout like this, especially on the exact manner of grounding the 555 (which pulls sub-microsecond amp-size spikes by itself) and its MOSFET and inductor/rectifier setup so that ground spikes don't leak back into the grounding of the J201.

[PRR]

at either max or min it is quiet.

The receiving antenna is probably the wire from the pot wiper.

[drdn0]

scope the output pin of the 555. what frequency? can you scope the noise in the audio section as well?

About ~40khz. Ran out of time to scope the rest of the audio path, but it doesn't change when I can manipulate the oscillation frequency with knobs so I'm not sure its directly the issue (although that's not saying that it isn't getting modulated anywhere else in the circuit).

DA's right - first find out if the oscillator is emitting/leaking into the audio path.

More generally, the board seems to have the (very) high voltage oscillator section close to the J201. The J201 would be more than happy to pick up high frequency spikes.

Are you looking at the switching transistor for the PSU, not the FET? FET and the HV section are on opposite sides of the board, and there's not many traces from anywhere pre-tube that goes near HV from a quick squiz.

at either max or min it is quiet.

The receiving antenna is probably the wire from the pot wiper.

That's what I was thinking re: the grid stopper routing being right up in the guts of the PSU, but moving it offboard didn't make a difference either. I didn't remove the R9 trace from being connected to the rest of the circuit so could still be causing grief I guess.