Marshall Superlead Normal Channel

[matt1969]

Hey Folks I believe this is my first post!

So I have a 1974 Superlead that I been using regularly since around the mid 90s. Over the years I learned to modify, repair and maintain this amp. I know the inside very well. Now it's pretty much a stock circuit.

What I noticed is that the normal channel has become very bassy / boomy and I don't know if it's my hearing or what but I'd like to make the normal channel less bassy like I remember it always sounding. I like to jumper the channels (low treble to high normal).

So what are my easy options to start with?
- a bright cap on normal channel's volume pot?
- change the 330uF cap to a lower value?

Thanks!

[bepone]

- smaller cathode capacitor (0.68-2uF)
- smaller coupling capacitor (2-4.7nF)
- when used normal ch., rotate "treble volume" pot above 50%
- bright cap 47pF on the normal volume

doing this one by one, and see what works best

[pdf64]

Note that the setting of the bright channel volume control determines how muffled the normal channel is.
With it set to minimum CCW, there’s a 1k5Hz low pass filter in play.

[matt1969]

Thanks for the suggestions! I will try when I get a chance and report back.

[matt1969]

Note that the setting of the bright channel volume control determines how muffled the normal channel is.
With it set to minimum CCW, there’s a 1k5Hz low pass filter in play.

That's interesting. I'll have to check that out.

[Littlewyan]

Do you want to modify it so you can play the normal channel by itself? As it's always going to be a bit muffled with a 470pF bypassing the bright channel's mixer resistor. You can lower the bass of the normal channel to help, but you won't be able to do anything to the high end without affecting the bright channel unfortunately.

[matt1969]

Do you want to modify it so you can play the normal channel by itself? As it's always going to be a bit muffled with a 470pF bypassing the bright channel's mixer resistor. You can lower the bass of the normal channel to help, but you won't be able to do anything to the high end without affecting the bright channel unfortunately.

No. I always use the High treble and jumper into the Normal channel but the Normal Channel adds so much bass, even jumpered with the bright channel one turned up.

[matt1969]

Note that the setting of the bright channel volume control determines how muffled the normal channel is.
With it set to minimum CCW, there’s a 1k5Hz low pass filter in play.

I tried that last night. I plugged into the normal channel only and no matter where I turned the bright channel's volume, the sound stayed exactly the same it did not get less bassy. It did get more distorted though.

[bepone]

you need to remove jumper for that test.. only normal.. then rotating the bright channel , will give you more highs on normal

[bepone]

doesn't matter, do some work, go for the others

[pdf64]

Note that the setting of the bright channel volume control determines how muffled the normal channel is.
With it set to minimum CCW, there’s a 1k5Hz low pass filter in play.

I tried that last night. I plugged into the normal channel only and no matter where I turned the bright channel's volume, the sound stayed exactly the same it did not get less bassy…

I use ‘muffled’ to mean that a low pass filter LPF was in effect, ie high frequencies are rolled off.
Bass should be little affected by that filter.
If the LPF with the bright channel volume at CCW isn’t apparent, then perhaps your circuit isn’t in full compliance with the 1959 schematic, or it’s not working properly?
Using a distortion / fuzz pedal can help to bring the LPF effect (eg as the bright channel volume is twizzled from mid way to full CCW) to the fore.

[matt1969]

you need to remove jumper for that test.. only normal.. then rotating the bright channel , will give you more highs on normal

yeah I did.

[Roe]

- smaller cathode capacitor (0.68-2uF)
- smaller coupling capacitor (2-4.7nF)
- when used normal ch., rotate "treble volume" pot above 50%
- bright cap 47pF on the normal volume

doing this one by one, and see what works best

good advice. even a 220pf bright cap may work

[WhopperPlate]

What I noticed is that the normal channel has become very bassy / boomy and I don't know if it's my hearing or what but I'd like to make the normal channel less bassy like I remember it always sounding.

Parts wear out after 30 years. Tubes are a given consideration.

Consider swapping electrolytic bypass cap on normal channel with a new one, same brand for less variability . Maybe even swap the preamp filter caps . Less punch and clarity can become a noticeable factor with time.

Ime deviating from stock values isn’t necessary if the parts vibe together . Eventually you gotta change the tires in your old car , doesn’t necessarily mean I need to make them bigger or smaller

[JPK64]

Do you want it back to how it was or want to modify it to sound different? I'd want to fix what's broken before modifying it. Otherwise you're just modifying a poor foundation.