strange treble bleed behaviour

[mat]

I went back to my first dumble (50W nonHRM skyline based on Normsters bassman layout) style build and noticed a weird thing when I disconnect the treble bleed circuit from ground the sound becomes much darker than with the TB on the circuit ?
After V2b plate cap the signal goes to the TB 1n cap and to 500k pot to ground. I dont have the 150k resistor going to OD volume pot. The signal goes straight from the .0047 to OD volume pot.

At reherseals the OD channel was not working right. Too 'narrow' middly sound. So I suspected that I had the treble bleed wired in a wrong way or loose wire somewhere. Everything seems to be ok but was wondering about the TB's strange behaviour.

So, any idea what the sound comes darker when disconnecting the TB ?

[ChrisM]

Install that 150K resistor, it is needed.

[mat]

Install that 150K resistor, it is needed.

Ok, will do. Thanks,

[llemtt]

Install that 150K resistor, it is needed.

I can't understand how that relates to mat's problem, can you explain?

[ayan]

Install that 150K resistor, it is needed.

I can't understand how that relates to mat's problem, can you explain?

Something must be going on that Mat's not been able to spot, otherwise I don't have an explanation.

Gil

[ChrisM]

I see that 150K resistor doing two things.

One it add resistance in series with the OD Level, so even when the OD Level is dimed there is still some signal being dropped across that resistor. Smoothness anyone?

Two that 150K resistor acts along with the 1nF TB cap as a lowpass filter. The 500K pot is in series with the cap going to ground, so the filter is kinda variable I guess? The 150K/1nF cap form the lowpass filter and the 500K pot just controls it's range/effectiveness.

No idea why the amp is darker with the TB removed. Might be time to go over the entire circuit carefully.

[llemtt]

Two that 150K resistor acts along with the 1nF TB cap as a lowpass filter. The 500K pot is in series with the cap going to ground, so the filter is kinda variable I guess? The 150K/1nF cap form the lowpass filter and the 500K pot just controls it's range/effectiveness.

+1

I agree with you it's a matter of effectiveness, the way mat implemented the bleed let it work "against" the V2b source resistance only (40k-50k) instead of source+150k=200k, so the low-pass rolloff freq is much higher

but getting darker while disconnecting the ground leg let me think something else isn't wired as we are used to

cheers
teo

[mat]

Think I found the cause. I disconnected again the TB ground point. There was a low volume high pitch whining from the speaker. I then removed the long wire going from 1n (TB) cap to the pot (at the back panel). The high pitch noise went away. I think that long wire somehow caused the amp to not sound right

The OD sound is now very good.

I'll try the amp at reherseals to see if I need at all the TB.