Negative Feedback Loop? Weird alien noises when OFF.

[JohnnyCrash]

Between Ceriatone's layout, the 18watt.com Layout, and the 18watt Schematic there are three different values for the Negative Feedback Circuit!

1. Ritchie's layout shows a 47k resistor across the switch.
2. Ritchie's schematic (rev 5) shows a 27k.
3. Ceriatone's layout shows a 100k across the switch (with the 47k on the board).

I followed Ritchie's 4.5.06 layout with the 47k across the switch and am having problems when the NF switch is in the Off position (weird alien noises).

A little background: I had to reverse the PI's plate's wires to be able to the Master without squeal.

[JohnnyCrash]

I'm going to try a 27k in place of the 47k in the meantime.

If this works, at least folks with similar problems will have a thread they can search/find

[JohnnyCrash]

So...

Anybody got any thoughts on why I'm tuning into Martian mind control transmissions when my Negative Feedback is switched off?

Sounds like some sort of 80's computerized sound effect.

[kd]

Run a wire from the ground on your output jack to the chassis and see if that helps.

Ken

[clobo]

Good evening,

I went through the same problem with my early prototype, had to lift speaker ground and reverse OT primary (brown/blue) to clear the problem.

Now my present version has the feedback switch, so I had to re-address this noise issue. This time I followed a tip from Ritchie, one star ground dedicated for the normal channel, and a separate bus ground for the TMB. OT secondary is grounded right at the chassis, OT primary is on the normal side (blue/brown), and the amp is now dead quiet at all settings.

Hope this helps

Claude

[JohnnyCrash]

Got it all figured out...

I was used to hearing motorboating and high pitched squeal when it came to oscillation, not Alf channeling the homeplanet.

I had swapped the OT wires previously, but that was before I "fixed" something else.

I swapped the OT wires and got it hammered out now. I dig the sounds of both no NF and NF on.

I like the NF on the 36w, it tightens up the bass considerabley. Off is nice and raw.

[clobo]

I like FB and also the tone defeat options.

I've been tweaking my prototype chassis for almost a year, but was not satisfied with the previous tMB designs. Now I think I reached my goal with rev 5, will build my final version around it. Will be a head, and a 4x10 slant cab.

Claude