1970 Marshall Super PA Mod Suggestions

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itsgoodnow

Well-known member
Hey all,

I just bought a Marshall 1970 Super PA 100. Original condition essentially. One output jack was swapped for a master (no drilling). I’m debating leaving the amp stock, but also curious if anyone has any mod suggestions for it. I am debating doing one set of inputs as a super lead spec and one as a 2203 but am open to hearing any and all ideas. Thanks
 
Let’s back up a second, because the answer to this question drives the steps we suggest from here.

What did you see in the amp when you bought it? What was your vision or purpose for it? What is your intent? What do you want from it to make you want to play it and never put it down?

When I bought my broken and poorly converted 74 2203, it needed a lot of work. But I had a vision for it all along.
 
glpg80 said:
Let’s back up a second, because the answer to this question drives the steps we suggest from here.

What did you see in the amp when you bought it? What was your vision or purpose for it? What is your intent? What do you want from it to make you want to play it and never put it down?

When I bought my broken and poorly converted 74 2203, it needed a lot of work. But I had a vision for it all along.
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This is a solid question. I have always loved the 2203 circuit and a lot of amps I’ve owned and still owned are really variations of a hot rodded 2203. I, oddly, don’t own a 2203 now (but have in the past). The super PA was a way for me to get vintage iron and a platform to potentially stick 2 different circuits in (including a 2203). So one I think I want a stock 2203. The plexi was another option simply because it might be easy to convert a channel. Alternatively, I could see having one circuit for really tight thrashier riffs with more gain and negative feedback to help shape the signal.
 
I personally find the caps in vintage Marshall’s have a smear to them that I personally don’t like. I’m more of a fan of MKP WIMA, ceramics, and orange drops. Even though my amp is a 74 chassis and iron, the board is 80’s 2203 WIMA loaded.

If your goal is vintage iron and 2203 or 1959, I’d suggest picking 1959 or 2203 and capping it accordingly. If you pick 2203, I’d suggest to expect to recap the preamp board et-al. Not impossible just more work.

The thing with 2203 and vintage iron is that different (higher) plate voltages can actually reduce stock gain normally expected, so some more fine tuning of either plate resistors or screen dropping resistors will be needed based on taste.

I’d personally leave the other side disconnected and/or save the holes for mods on the 2203 circuit.

Just what I’d do. What do you think?
 
glpg80 said:
I personally find the caps in vintage Marshall’s have a smear to them that I personally don’t like. I’m more of a fan of MKP WIMA, ceramics, and orange drops. Even though my amp is a 74 chassis and iron, the board is 80’s 2203 WIMA loaded.

If your goal is vintage iron and 2203 or 1959, I’d suggest picking 1959 or 2203 and capping it accordingly. If you pick 2203, I’d suggest to expect to recap the preamp board et-al. Not impossible just more work.

The thing with 2203 and vintage iron is that different (higher) plate voltages can actually reduce stock gain normally expected, so some more fine tuning of either plate resistors or screen dropping resistors will be needed based on taste.

I’d personally leave the other side disconnected and/or save the holes for mods on the 2203 circuit.

Just what I’d do. What do you think?
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That’s definitely a smart play. Since my taste is really for the 2203 rather than the plexi it may be better to disconnect the inputs like you mentioned to accommodate mods and voicings. For a tighter thrash sound are there any particular mods you might recommend. I also have always loved the Aldrich mods but wasn’t sure if that sound was one that can be tweaked with voicing to tighten up
 
Have you tried begging @dawnofdreamx97 to mod it? I heard he makes a killer Aldrich mod 😂

Tightening up is stupid easy to do and there’s many different ways to do it. I prefer to tighten up in the preamp dumping gain between voltage dividers. Others like to use the negative feedback circuit. Sometimes a mix of both are needed.

I do think what you’re wanting out of the amp would require a complete gut and rebuild. It’s not a small task and could leave you with a lot of money invested with not a lot of promise if you don’t know how to tune an amp or have a circuit in mind.

As far as mods, a depth mod is a must.
 
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glpg80 said:
Have you tried begging @dawnofdreamx97 to mod it? I heard he makes a killer Aldrich mod 😂

Tightening up is stupid easy to do and there’s many different ways to do it. I prefer to tighten up in the preamp dumping gain between voltage dividers. Others like to use the negative feedback circuit. Sometimes a mix of both are needed.

I do think what you’re wanting out of the amp would require a complete gut and rebuild. It’s not a small task and could leave you with a lot of money invested with not a lot of promise if you don’t know how to tune an amp or have a circuit in mind.

As far as mods, a depth mod is a must.
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I’ve heard from a reliable source they’re good mods!
 
glpg80 said:
Have you tried begging @dawnofdreamx97 to mod it? I heard he makes a killer Aldrich mod 😂

Tightening up is stupid easy to do and there’s many different ways to do it. I prefer to tighten up in the preamp dumping gain between voltage dividers. Others like to use the negative feedback circuit. Sometimes a mix of both are needed.

I do think what you’re wanting out of the amp would require a complete gut and rebuild. It’s not a small task and could leave you with a lot of money invested with not a lot of promise if you don’t know how to tune an amp or have a circuit in mind.

As far as mods, a depth mod is a must.
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Out of curiosity, since I know nothing about technical aspects of amps, why do you prefer to tighten up by dumping gain between the voltage dividers rather than the negative feedback? I’m just curious how that impacts the tone.
 
itsgoodnow said:
Out of curiosity, since I know nothing about technical aspects of amps, why do you prefer to tighten up by dumping gain between the voltage dividers rather than the negative feedback? I’m just curious how that impacts the tone.
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Too much gain added by voltage dividers adds sustain, but add too much, and low end gets flubby regardless of the tone shaping done on the top of the divider since the bottom of the divider is the ground reference for the grid of the next stage which is cathode biased and is dependent upon that grid voltage. Tone shaping doesn’t make sense in word form - some of the things we do to stages is counter intuitive to wanting gain, but it’s a sum of all parts that make the amp work and sound good. In this scenario you recoup tube gain in a hot next stage to make up for what you dumped.

In general you don’t want to cascade a ton of sustain like gain over and over as it can cause oscillations and instability. I’m more impressed by amps that play easy than play tight because IME those are the hardest to get right.

If Nick won’t touch it, I’d grab a ceriatone kit to learn on first before diving into a 1970 PA. That’s just me though if I were in your shoes.
 
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Do you know how much gain you normally play through? 3 stages? 4? Possibly with boosts?

It will help knowing what to recommend.

One option is to get the depth mod in the 2203 conversion, recap the amp and save the original good stuff in a bag, and then get the hot mod plugin to turn the amp into a fire breather. Just something to think about.
 
glpg80 said:
Do you know how much gain you normally play through? 3 stages? 4? Possibly with boosts?

It will help knowing what to recommend.

One option is to get the depth mod in the 2203 conversion, recap the amp and save the original good stuff in a bag, and then get the hot mod plugin to turn the amp into a fire breather. Just something to think about.
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Generally I play 4 stages with a boost in front.

That’s also a thought I was having as well—the hot mod. Just since it’s easy etc. I haven’t used one before but it seems worth a shot for the ease and the simplicity.
 
I have to investigate a little because I've never worked on a super pa before, but I do know that I've heard of marshall majors blowing up when doing high gain mods to them. Output transformer related issues I believe. Obviously you'd want to avoid that! Lol let me look into it a little more, thats a really cool amp though not sure i would mod it , maybe get a mojotone kit and do that instead ....
 
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As Nick stated id leave it alone for high gain, if its not completely original but mostly there do the super lead/ bass its only few changes. If its original i wouldnt touch it, original marshalls are getting hard to find. Like old harleys with original paint.
 
I wouldn’t mod it out of principal but that’s me, if you were to this is my thought process.

1: pull the board and use new components on a fresh turret board. It’s really hard to put old component’s back after they’ve been removed without looking like shit.
2: Build or have the amp you desire built and use the transformers from the PA. Although I haven’t done it, the switchable 1959/2203 mod exists and would probably scratch that itch.
Personally I’d build or buy a copy and mod it to taste. They’re something very freeing about being able to beat the shit out of an amp build with readily available parts, drill holes for extra tube sockets, effects loops,etc.
 
Chubbs said:
I wouldn’t mod it out of principal but that’s me, if you were to this is my thought process.

1: pull the board and use new components on a fresh turret board. It’s really hard to put old component’s back after they’ve been removed without looking like shit.
2: Build or have the amp you desire built and use the transformers from the PA. Although I haven’t done it, the switchable 1959/2203 mod exists and would probably scratch that itch.
Personally I’d build or buy a copy and mod it to taste. They’re something very freeing about being able to beat the shit out of an amp build with readily available parts, drill holes for extra tube sockets, effects loops,etc.
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this is interesting. If I were to pull the transformers to put into a new build would it nuke the value of the super PA if they were put back in? Likewise with pulling the board and putting a new one in and transplanting the original board back in for the future? Is putting a bright cap or something on going to do the same? These are not a deterrent necessarily but just curious if anyone is aware of that.
 
PatF said:
If done well, it could look like a factory issued Jose. 4 preamp tubes.
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That's true, and I wouldn't have to do any drilling if that were the case either. The question for me is, how would it sound, how reversible is it etc. As pointed out removing certain things could make the original board look bad if I wanted to re-convert. The additional holes in the chassis (and the cool look), the 4th preamp tube, and old iron was sort of the draw for this amp for me. Plus it being so similar to a super/lead super bass minus a couple of values etc.
 
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