ROG Thunderchief Gain pot changing the tone

[Atodovax]

Hi. Does anyone know why the gain knob on the ROG Thunderchief cuts lows when decreased? Is there any way to make this effect less pronounced?

[antonis]

(It actually is a Volume pot but let it be..)

Probably, due to 2nd J201 elevated Gate leakage current..
It sets a multiple of 470k resistor in parallel with wiper thus making 1M pot resistanace, as seen from 22nF viewpoint, variable..

(or maybe it's too late here..)

[idy]

I was looking at the volume knob too. Maybe Atodovax mean the gain. That has a treble bypass cap that brightens up the sound when you turn it down...

[antonis]

I was dealing with the pot named "Gain" which actually controls 2^nd stage incoming signal amplitude (volume)..

[Atodovax]

I was dealing with the pot named "Gain" which actually controls 2^nd stage incoming signal amplitude (volume)..

Thanks ! Yeah! I know, a classic voltage divider setup. Still... I have removed the treble bleed capacitor but the control remains cutting lows when turned down. Do you know any workaround to stop the rolloff?

[anotherjim]

Your gain pot could be miswired, that is the 22nF is feeding the pot middle lug (wiper) instead of the top lug 3. Miswired this way it will appear to work, but bass cut with the 22nF coupling cap is inevitable.

[Atodovax]

Your gain pot could be miswired, that is the 22nF is feeding the pot middle lug (wiper) instead of the top lug 3. Miswired this way it will appear to work, but bass cut with the 22nF coupling cap is inevitable.

Thanks! Its actually wired as the schematic, 22nf to lug 3... Could it be that lowering the gain is lowering the gate resistor to the point that the fet´s missbias is sounding more trebly? I will replace the 470k from lug2 to Gate and see if that helps..

[antonis]

Could it be that lowering the gain is lowering the gate resistor to the point that the fet´s missbias is sounding more trebly?

Only for considerable Gate leakage current..

[Atodovax]

Could it be that lowering the gain is lowering the gate resistor to the point that the fet´s missbias is sounding more trebly?

Only for considerable Gate leakage current..

Great! So how do mortals measure gate leakage current xD?

[anotherjim]

As I don't believe in hunting microbugs until everything else is exhausted. I mean, if the output impedance of the preceding stage isn't huge (shouldn't be far off 20k is it? Measure drain trimmer) and the pot really is 1M and the cap is 22nF and the 470k to the next stage really is 470k then it's hard to see how tone changes as described no matter what may be wrong with the next FET...
...except human loudness response. Bass always seems too quiet when the volume is low.
Anyway, I await an outcome with interest. I might learn something!

[Vivek]

For ease in reference:

[Vivek]

It appears to my dull brain that gate of 2nd from left FET will have extremely high impedance

Hence the 470p which is in parallel with 470k will do nothing at all.

Does that sound right ?

[antonis]

It's a "simulation" of Marshall Tube Lead amp..



https://robrobinette.com/How_the_Marshall_JCM800_Works.htm

[anotherjim]

I'd have thought if you want a bright cap it goes between the pot feed and wiper. The 470k in series with the gate does nothing except add noise, at least to my poor brain.   I suppose it's meant to make FET input Millar capacitance equal a triode tube.

[PRR]

> extremely high impedance
> Hence the 470p which is in parallel with 470k will do nothing at all.

Q1 is very able to drive the gate of Q2 to conduction, low-Z. Only on the positive side of the wave. Keep it up, and the 22nF cap will charge-down, drive Q2 into grid-blocking. The 470k is a good value (with the 1meg) to balance this, but is enough with Q2 C(gate) to dull the sound; 470pF passes treble around.

[Atodovax]

Hi, thanks all for the replies. I have everything stock but still, when i lower the GAIN, the pedal starts loosing low end. Not much but its definetly loosing some. This is more evident when im stacking an overdrive pedal before the Thunderchief. If im using both pedals stacked and i play a chord and start lowering the GAIN knob on the Thunderchief, i can hear kind of a Low Pass filtering effect.
I still haven't tried replacing Q2 but everything is well biased and sounding pretty good actually. Its just that annoying tone effect in the GAIN knob that's worring me

[GibsonGM]

Maybe you can pinpoint where this occurs with an audio probe?

[Atodovax]

Maybe you can pinpoint where this occurs with an audio probe?

I will try that tomorrow. Right now i have both a pedal finished and a second circuit on a breadboard and both behave the same... Again, the pedal is working fine and it sounds really good. Im just curious on how to eliminate that effect on the gain knob. I increased both Bypass capacitors and the coupling cap before the second stage and the pedal now has more bass but still, it gets thinner at lower gain settings and fuller when the gain is maxed