1974 Marshall Superlead - Tuning Presence

glpg80

Well-known member
I’ve been spending this week tuning the presence circuit on my project after the resonance mod from last year. The presence circuit that was in it had a 25pF cap in for what I assume to be an easy gain mod, but it basically made the presence circuit open and not very reactive to knob turning especially after the resonance mod shifting frequency responses around to even the amplifier out a bit to be less top end heavy all the time.

I went up to a 0.02uF and liked what I was hearing, so I went one step further to 0.68uF ala old Marshall plexis and it was magical. Gave the amp some D chord bark and made it sound less flat overall. The top end of the notes on solos have a very distinct Marshall smear to them which I personally love. The amp seems to get tight down low and very smear like up top - I’d say it was a successful week :)

Here’s a quick video clip. I dialed in a darker tone to tune the presence circuit. Having it dialed in for a brighter tone made it more challenging to tell the difference between different values as I went up.

I should mention all cathode resistors are stock in this video - it’s mainly the SD-1 doing the work.

 
Sweet! I have a 1959 1974 Superlead as well! my presence is set up with a .1uF cap at the pot and a 47K feedback resistor connected to the speaker jack, not the impedance selector.
 

SpiderWars

Well-known member
It is kind of funny that it would appear Marshall had a crap ton of 0.68uF caps and just started using them there. But it works great. It almost turns the Presence into a NFB control but the bass stays tighter.

For MV amps, especially ones with some other sort of NFB control, I generally like the 0.1uF. But for the plexi circuit the 0.68uF works great.

That amp sounds fat! :rock:
 

glpg80

Well-known member
It can do rip your head off as well, it is a Marshall still at heart 😂

I have the treble on 2, mids on 2.5, bass on 10, resonance all the way up. It definitely fattens it up - I plan to pull some of that resonance back and try dialing in more mids with the same SD-1 settings and see how it sounds. If it sounds alright I’ll repost a clip.

I’m also going to see how far down I can get the cold clipper resistor without causing the low input side to get into clipping since it’s my poor mans clean. That’s my next gameplan with the new presence cap.

Totally odd they did that but you’re right it totally works too. I don’t think I’ll put in a 25k pot - I don’t think it needs that much swing. The cap seems to work well in this amplifier but at heart the amp itself still doesn’t have a lot of gain. The pedal makes it seem like it though.
 

psychodave

Well-known member
You can add some mids in by adding another resistor with the depth cap. It will focus the frequency of the cap. Something like 15k-82k. Gotta test a bunch .... or slap a 50-100k pot and start turning until you like the mids. Then replace with a resistor.
 

glpg80

Well-known member
You can add some mids in by adding another resistor with the depth cap. It will focus the frequency of the cap. Something like 15k-82k. Gotta test a bunch .... or slap a 50-100k pot and start turning until you like the mids. Then replace with a resistor.
I actually like where the mids are right now to be honest. It shifted them to low-mid type of voicing which helps with aggressiveness. The amp is just mean as hell in person and the mods are subtle changes for personal preference but really don’t add much gain at all on their own.

I’ve been doing some more slight modifications to take away some harsh top end and add some stability. I’ve pretty much got it buttoned up and where I like it now I believe - I’ll be posting another clip this weekend.
 
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LESPAULY69

Member
I’ve been spending this week tuning the presence circuit on my project after the resonance mod from last year. The presence circuit that was in it had a 25pF cap in for what I assume to be an easy gain mod, but it basically made the presence circuit open and not very reactive to knob turning especially after the resonance mod shifting frequency responses around to even the amplifier out a bit to be less top end heavy all the time.

I went up to a 0.02uF and liked what I was hearing, so I went one step further to 0.68uF ala old Marshall plexis and it was magical. Gave the amp some D chord bark and made it sound less flat overall. The top end of the notes on solos have a very distinct Marshall smear to them which I personally love. The amp seems to get tight down low and very smear like up top - I’d say it was a successful week :)

Here’s a quick video clip. I dialed in a darker tone to tune the presence circuit. Having it dialed in for a brighter tone made it more challenging to tell the difference between different values as I went up.

I should mention all cathode resistors are stock in this video - it’s mainly the SD-1 doing the work.

Sounds like a perfectly healthy amp to me!!!
 

glpg80

Well-known member
I actually found out the power tubes weren’t biased correctly, i had an old meter loading the circuit when biasing causing the tubes to not read correctly. I found the problem after ordering higher quality meters. Stay tuned! I’ll have it biased correctly soon and updated clips to share. It’s sounding real healthy :rock:
 

RedPlated

Well-known member
I prefer the .1 presence caps typically. But .68 is a good way to tweak the sound if that's what your going for. The presence basically becomes a high mid control.
 

glpg80

Well-known member
Thanks dude!

This is what it sounds like now. It’s a ton of fun to play and still cleans up great. It’s got more of a low mid thing going on now but sounds more full and balanced.

 
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