Fizzy noise around 50% gain on Thor

[sebarb]

Hi everyone,
I recently built a Thor from runoffgroove but without EQ/cab simulation and it sound incredible in all the gain range with the 470p bright cap engage.

Troubles start when I disengage the bright cap  with around 30 to 80% on the gain knob, start to sound fizzy, like additional background of high frequency distortion, zzzzzzzz and a lot more amplified when linked with simple TMB marshall tone stack.
I checked and rechecked all solder joints, links, switch, pots...

Surely these symptoms are very significant, but I'm running out of knowledge, any ideas or certainties will be welcome.

[PRR]

When a high-gain amplifier squeals, mostly when the gain control is half-way, suspect that output is sneaking/leaking in at the pot wiper wire. Move it, shield it, do something.

[pinkjimiphoton]

try disengaging the OTHER side of the bright cap, sounds to me like its being disconnected from the "active" side so its becoming an antennae. the other side of the cap connect to the switch, and basically reverse it and i bet the issue goes away.

probably wrong, but worth a shot. if you add a cap to pretty much any audio input that isn't terminated in some way, stuff can get very very dicey.

[sebarb]

Thank for your replies guys,

1. Ok PRR, I dont have any shieldwire yet, so I disconnected the gain pot and simulat 50% gain with two realy short resistors. And same noisy effect

2. pinkjimiphoton, I have disengaged all the bright cap option, cap, switch and wire. And same badness

[BetterOffShred]

Mine acted weird till I rebiased the FETs..  just an idea

[pinkjimiphoton]

Mine acted weird till I rebiased the FETs..  just an idea

+1

weird biasing can definitely lead to noises like that. some classes of operation <which the fet may be sweeping thru as you turn the pot> can be much more rfi sensitive, even act like a theremin.

[sebarb]

Yes, "sometime"  it sound gated, crakle, like miss biased bipolar!
I initially keeped Q1 around 6.40v and Q2 4.5v with the best result as rog recommendation but to figure out the noisy effect I tried to rebias Q1 and Q2 but without affecting this noise.

For now the only thing make the noise disappear is engage the 470p bright cap or one 470k impedance resistor at the Q2 gate or gain knob at 100%.

[pinkjimiphoton]

is this still on the breadboard, or is it boxed?
on the breadboard, it may just be rfi that will dissapear once its encaged in metal?

i would try a bigger cap in parallel with that 470k resistor going to q2, maybe 1 or 2n instead?

the two caps in series there may be the problem too,  caps in series act like resistors in parallel so two 470p caps becomes the equiv of one 220p cap i think. that's kinda puny imho

brain dead..... of COURSE going to 220p would make it brighter.

could the pot be damaged?  sometimes enough current can draw thru 'em where the wiper smokes. the pot still works, kinda, but will "freak out" in the damaged area.

or try one of my weird snubber tone controls, wire it right between the  input and output of q2 as a snubber in the feedback loop maybe...

take say, a 47p cap and a 2.2n cap and solder them to lugs 1 and 3 of say, a 250k pot or so. tie the free ends together, and wire this between gate and source <i hope i got that right, i don't work much with fets and can never remember which pin is which,.. in the pic on runoffgroove's site, that would be the left and top of q2> with one connection being the ends of the caps, and the other the wiper of the pot. this will let ya fake a variable cap, and sweep between very little treble cut and a noticeable one, but won't lose all the gain of the circuit like a standard tone control.

it should work on almost any circuit that's gotta noise or hiss problem, as we all know, the bigger the snubber cap, the less high end noise... this lets ya sweep between the two caps and not only helps reduce the noise but is a fairly decent treble cut control.

outside of that, i got nothin bro, sorry

[sebarb]

Thank guys for all your ideas!
The gain pot is new.. but i tryed another one! Not that
I retryed bias exploration and.. Not that

I played with small caps on Q1 and Q2 gates and tada!
A small 47p cap at Q2 gate resolved all the noisy effect!

[sebarb]

Thank guys for all your ideas!
The gain pot is new.. but i tryed another one! Not that
I retryed bias exploration and.. Not that

I played with small caps on Q1 and Q2 gates and tada!
A small 47p cap at Q2 gate resolved all the noisy effect!
That also smooth the breakup a bit, in good way!