Bought a “modded” Marshall…what is this??

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SLOgriff

SLOgriff

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I just bought, what I thought, was a stock “used” Marshall 1987x. Inside the chassis it says modified by “Fusionbear - Carl Esparza”….to Early 1986 circuit”

From my research seems like he’s a pretty respected and knowledgeable guy!

It actually sounds great, with a lowered C11 bright cap and KT88’s.

My question is what’s the mod to R27 in the photo? Is that wire bypassing that resistor??

Thanks
Bryan

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stock amp has 2 dropping resistors in series (for more resistance). Looks like Carl bypassed one of the 2 series Rs to lower the resistance (increase voltages at PI node).
 
V2a said:
stock amp has 2 dropping resistors in series (for more resistance). Looks like Carl bypassed one of the 2 series Rs to lower the resistance (increase voltages at PI node).
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What does that do exactly? Increase gain, or for the KT88’s that were installed?
 
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It makes the amp less saggy and more punchy. It raises voltages at the PI, and consequently, everywhere else, all the way down to V1.

It’ll technically reduce gain at each stage. Higher plate voltages will have more clarity and less compression. Has nothing to do with the power tubes.
 
RedPlated said:
It makes the amp less saggy and more punchy. It raises voltages at the PI, and consequently, everywhere else, all the way down to V1.

It’ll technically reduce gain at each stage. Higher plate voltages will have more clarity and less compression. Has nothing to do with the power tubes.
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Learning more every day on this forum.
Thanks for that break down.
 
RedPlated said:
It makes the amp less saggy and more punchy. It raises voltages at the PI, and consequently, everywhere else, all the way down to V1.

It’ll technically reduce gain at each stage. Higher plate voltages will have more clarity and less compression. Has nothing to do with the power tubes.
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I’ll also add one more thing. It makes the preamp tubes more susceptible to damage/premature wear if you wall voltages are too high. There’s less B+ drop going to the preamp tubes and if your wall voltage is 123-125, your preamp tubes could be seeing 75V or more than intended by design. Just keep that in mind.
 
glpg80 said:
I’ll also add one more thing. It makes the preamp tubes more susceptible to damage/premature wear if you wall voltages are too high. There’s less B+ drop going to the preamp tubes and if your wall voltage is 123-125, your preamp tubes could be seeing 75V or more than intended by design. Just keep that in mind.
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Good to know! I always use a Brown Box with my Marshall’s…down to around 115V
 
And you can always split the difference if you want lower voltages, but not as low as stock. 20K total = stock. 10K is where it's at now. You can always throw some other value across that "modified" resistor to split the difference. 2K, 5K, whatever. Personally, I like 10K (where it is now). I like a punchier, stiffer feel. You can always add gain and grit in other areas, rather than creating distortion by lowering voltages and compromising punch/response.
 
FourT6and2 said:
And you can always split the difference if you want lower voltages, but not as low as stock. 20K total = stock. 10K is where it's at now. You can always throw some other value across that "modified" resistor to split the difference. 2K, 5K, whatever. Personally, I like 10K (where it is now). I like a punchier, stiffer feel. You can always add gain and grit in other areas, rather than creating distortion by lowering voltages and compromising punch/response.
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I like “punchier & stiffer”…. Although I would like to have a little more “gain” from the amp.
 
SLOgriff said:
I like “punchier & stiffer”…. Although I would like to have a little more “gain” from the amp.
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Ok yeah, keep the voltages higher (where they are with the single 10K dropper). If you want more gain, you'll have to play with the three gain stages a bit. You can bias any or all of them hotter or colder to suit your preferences by using various combinations of cathode bypass caps and resistors. You can also alter the first stage's plate resistor value. Increasing that one will reduce voltage to that stage (usually V1a), which will increase gain. Stock is 100K. If the amp has been modified already, it might have been changed. But you can try 200K - 330K. Somewhere in there.

Tons of other ways to skin the cat. NFB is a good way to go about it as well. What's in the amp now?

Also I'm sure Carl will pop in at some point.
 
Yeah. You can universally bump the preamp voltage but also raise or lower individual stages by adjusting the plate resistors.

You can’t really use that 10k as an arbitrary good value. You need to measure the preamp voltage and see where it’s at. Could be anywhere depending on the B+ supply from the PT. I’m “guessing” it’s in the 350-370vdc range if it’s a healthy 450 or so B+ supply. I think 1987x’s are around there. But every amp is different. If you have an active loop for example, it’ll pull the voltage down quite a bit. Stock Marshall loops don’t pull a lot of voltage, but a Metro etc…will.

Use a voltage target, not a resistor value target. And really even further than that, use your ears as a target. Numbers really mean nothing if it doesn’t sound good.

Once you get that set, play with the voltage at V1 plate. Try 140vdc-160vdc and see if that gets you some better feel and compression. You’ll likely be around 180-220k value on the plate resistor.
 
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FourT6and2 said:
Ok yeah, keep the voltages higher (where they are with the single 10K dropper). If you want more gain, you'll have to play with the three gain stages a bit. You can bias any or all of them hotter or colder to suit your preferences by using various combinations of cathode bypass caps and resistors. You can also alter the first stage's plate resistor value. Increasing that one will reduce voltage to that stage (usually V1a), which will increase gain. Stock is 100K. If the amp has been modified already, it might have been changed. But you can try 200K - 330K. Somewhere in there.

Tons of other ways to skin the cat. NFB is a good way to go about it as well. What's in the amp now?

Also I'm sure Carl will pop in at some point.
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NFB is 100k on the 8ohm tap. Bright cap is 500pf. The amp “looks” stock inside…Carl is an artist for sure!
 

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SLOgriff said:
NFB is 100k on the 8ohm tap. Bright cap is 500pf. The amp “looks” stock inside…Carl is an artist for sure!
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Looks like 56k slope resistor? Or am I looking at the wrong one? If that's the slope, you could try lowering it to give you more of a mid-range bark. 33K is the most common Marshall value, but they used 56k in some amps depending on other parts of the circuit.

You could try swapping the NFB over to the 4 ohm tap, which would give you a more wild/raunchy sound/feel. Might be a bit too much though.
 
It’s a great amp! I’ve been playing it non stop since Sunday! It’s got that early AC/DC punch and sizzle! I’m still pleasantly surprised with this amp…but curious about the choice for KT88’s.

I’d like to chat with Carl (Fusionbear) to see if he remembers this amp and what he was going for.
 
SLOgriff said:
It’s a great amp! I’ve been playing it non stop since Sunday! It’s got that early AC/DC punch and sizzle! I’m still pleasantly surprised with this amp…but curious about the choice for KT88’s.

I’d like to chat with Carl (Fusionbear) to see if he remembers this amp and what he was going for.
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Howdy!

Yes, I remember that amp. I modified it for a buddy who plays in a Deep Purple/Rainbow Tribute band. He was having trouble with it being too middy and mushing out too quickly. So, I converted it to Bass spec with the higher voltages in the preamp to get closer to the Blackmore preamp voltages in the Marshall Major amps. I used a jumper on the 10k dropper resistor so that if he didn't like it, I could easily remove it to make the preamp voltages stock. I'm not sure who added the KT88's. I think it came stock with 5881's. I know he got into some financial struggles and sold most of his gear. I offered to restore the amp to stock for free, but he had already sold the amp. I'm no longer working on amps, but it would be easy for a competent tech to restore. I pulled the board out to change the treble cap and slope resistor, and PI feed coupling caps to make it look clean and professional.

It's a loud amp!

Hope this helps...
 
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