Amp based on the SLO Nakid -- clean/crunch/overdrive voices and effects loop restored

[Snfoilhat]

Chopped off the hat brim from the top, finished the front slant, did a bunch of sanding and the roundover and more sanding. First coat of Danish oil.

I think I'll make a Fender-type front/baffle with the same grill cloth as the 2X12 cab

 

[Snfoilhat]

Testing the fit of the baffle (a little too tight -- leave room for the fabric!)

Nothing left to do but the wires and stuff

 

[robrob]

Love your woodworking and you've got a great eye for design.

 

[kleydejong]

Beautiful work!

 

[JPKmusicman]

Sik Skillz !!!

 

[Mr Ridesglide]

beauty!

 

[Snfoilhat]

Thank you, @robrob @kleydejong @JPKmusicman @Mr Ridesglide that means a lot to me

I picked this project back up and got to here:

I worked through the startup procedure, and it runs but there's something going on when the overdrive switch is on.

The bad part: with the overdrive switch engaged, the amp makes a whooshing sound (controlable by the master volume, thankfully) that sounds like a restroom hot air hand dryer.

The good part: the overdrive signal path passes guitar signal and all of the controls are working, so you can hear guitar on top of the whoosh -- passing by where I was working and playing the first few notes through this thing, my girlfriend said "that sounds metal."

So almost a total success

 

[JPKmusicman]

Can you post a more high res pic? I'm looking for bad solder joints but I can't zoom in enough. Do you have a list of expected idle voltages? I would check that after close examination of all solder joints. Use something non-conductive to poke around. I guess before that I'd print out a layout/wiring diagram and check all components and wires one at a time, highlighting those you've checked. The above should shake something out. Most importanly lethal voltages are present so proceed with caution.

 

[Snfoilhat]

What I've tracked down is that several of the 6 leads that connect to the DPDT overdrive on/off switch are microphonic, and depending on how the leads are arranged, the microphony can show up in the tubes or in the chassis or in some other leads. I think this kind of moving microphonics doesn't mean bad components but instead means parasitic oscillation.

It's one of those really interesting (?? ) instances of oscillation where just putting the tip of a chopstick near a particular wire can increase or decrease the noise. Like you're disrupting a field.

I can mush a grid lead down against the chassis and the noise will drop for a few seconds, then slowly build back up even if the lead is pinned down, like moving it breaks the positive feedback loop temporarily but then it ramps back up.

I may try shielding the grid leads around V2 and V3 next.

 

[dan40]

Does disconnecting the nfb wire from the output jack or reversing the plate wires help to alleviate the noise? Positive feedback from the nfb circuit can cause all kinds of weird noises. Lifting the nfb wire from the jack is a quick and easy way to test for it.

 

[JPKmusicman]

^
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[Snfoilhat]

I really appreciate the help. Unfortunately this doesn’t have a global negative feedback wired in

 

[dan40]

I'm surprised that the schematic does not include nfb. I just finished building an SLO kit last month and the original SLO does use nfb.

 

[Snfoilhat]

Both of the original sources I used include NFB, but I've left it out until the rest of the amp works properly.

I started by looking at robrob's SLO nakid, which is pretty stripped down from the SLO 100, and then I started adding back some of the features the nakid left out: the buffered effects loops and all three switchable preamp gain levels.

One issue I've pinned down: when I first drew my schematic, I made some changes to the SLO 100 overdrive DPDT that I thought were just making the drawing look neater, but really had electrical consequences -- oops! So now I've fixed that on the schematic and in the amp's wiring.

If I ignore the noise, all three gain settings work, but all three seem to have way too much gain. The clean gets a preamp drive sound when turned just half way up, the crunch sounds awesome on top of the noise, and the overdrive setting is unusable -- feedback and too much distortion as soon as the potentiometer cracks open.

Plugging into the effects return kills the noise. That's as far as I've gotten. One grid lead in V2 is still microphonic. So there's still a parasitic oscillation and I think there's a grounding or signal attenuation mistake between V1 and V3.