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Re: Values of the screen grid resistors influence sound
Posted: Sat 05/15/21 2:53 pm
by musicheals
Have you tried pointing the mic off-center from the speaker's dust cap? That usually helps tame the treble; although I thought the clip sounded pretty darn fine on my laptop speakers
Yes, of course I tried that. In this case the mic is off center and with an angle of about 50 degrees...
Re: Values of the screen grid resistors influence sound
Posted: Sat 05/15/21 3:19 pm
by musicheals
zaphod_phil wrote: ↑Sat 05/15/21 12:32 pm
musicheals wrote: ↑Sat 05/15/21 9:40 am
Yes, I will give it a try without negative feedback....
Then you will get the real deal Marshall 18W sound. NFB tends to make the tone dull, so you lose detail
Maybe this will lead to even more highs
Re: Values of the screen grid resistors influence sound
Posted: Sun 05/16/21 3:28 am
by Bieworm
musicheals wrote: ↑Sat 05/15/21 3:19 pm
zaphod_phil wrote: ↑Sat 05/15/21 12:32 pm
musicheals wrote: ↑Sat 05/15/21 9:40 am
Yes, I will give it a try without negative feedback....
Then you will get the real deal Marshall 18W sound. NFB tends to make the tone dull, so you lose detail
Maybe this will lead to even more highs
Get rid of the NFB loop. It's not only eliminating treble..
If you still think the amp is too trebly you can tweak that in many other ways.
- You can put a bigger bright cap, but there's already a 500p in there. Try a .001uf and see what that brings. ( try parallelling another 500p)
- try a snubber cap across the plates of the PI. 100p is a good value to start with. ( don't try to alligator clip on your tube pins!!! Do this where the wires between pin 1 and 6 are soldered to a turret)
Right now alligator clips are your friend.
Maybe try these things first...
Re: Values of the screen grid resistors influence sound
Posted: Sun 05/16/21 9:43 am
by musicheals
I cut the NFB and have to say, that the tone gets indeed more complex and the breakup is more evenly. Great!
To get less treble, I tried a 220K in front of the second tube's grid ( right after the volume control). This worked quite well, but I got mor hiss then. So instead of the resistor I used a 220pF on that grid. The added hiss went away. But I'm not happy 100% with this. Maybe there might be a better place in the preamp? A 47p snubber between the PI anodes I had already...
Thanks a lot
Re: Values of the screen grid resistors influence sound
Posted: Sun 05/16/21 10:35 am
by Bieworm
Re: Values of the screen grid resistors influence sound
Posted: Mon 05/17/21 8:22 am
by musicheals
the treble problem ist fixed. No more NFB and the presence control is now a cut control a la AC30. It sound amazing
Thanks a lot!