18Watt.com

this can not be the schematic of that amp. Your amp has 2 preamp tubes.. this schematic shows 3 preamp tubes

You are right, I had not noticed anyway in the brightness channel the 680nf capacitor gives a cutoff frequency of 86 Hz, perhaps modifying it?

I have asked for the correct scheme

Thank you

cisatracurio wrote: ↑

Mon 04/26/21 9:09 am

You are right, I had not noticed anyway in the brightness channel the 680nf capacitor gives a cutoff frequency of 86 Hz, perhaps modifying it?

I have asked for the correct scheme

Thank you

If you're referring to the 0.68uf cathode bypass cap... that's already a low value and likely not the problem. It's typical marshall value, intended to prevent the preamp from having too much low end boost. To be sure you need to untie the wire from pin 3 or 8 on V1 ( the one connected to the .68uf cap ) and put a 2.2k resistor in series with that wire, with the other end of that resistor to ground. That will tame the low end boost...but will change the character of the overdrive. But I doubt that is the problem.
What is the value of that electrolytic cap on the far left on the board? Looking at the amp from the front. If that is a 220uf cap like geoff implied that is going to be the possible bass pusher. You could temporarily disconnect that other cathode wire and put a lower value cap to ground, parallelled with a 1.5k resistor. Try a 50uf 50V cap first. Go even lower when needed.