Marshall Origin Headfirst mod questions

ledvedder

ledvedder

Well-known member
I posted this on Headfirst Amplification's Facebook page, but figured I'd also post here. I finished converting my Origin 20 back to the Headfirst 800 mod from its previous Headfirst plexi mod. I omitted the boost switch since I was having issues with it, so I'm just leaving the 0.68 cap on v1b permanently. I'm also using a 4k7 cathode resistor on v1b instead of 2k7 for a bit less gain. I tried a 10k with no cap, but I think I like 4k7/680 better. 1nF bright cap. I also put the previous dropping resistors back, instead of the 10k ones from the Plexi mod. But I do have some questions. It seems more bright than it was before. Not enough oomph. I'm wondering if that could be caused by any of these things, which I left from the previous plexi mod.

A. 1M gain pot with the 470k/470pF into the input, instead of the previous 100k gain pot.

B. 1M PI grid leak resistors instead of the previous 470k resistors.

C. 47pF fizz cap instead of the previous 100pF.

D. Classic presence circuit.

These were changes I made when doing the Plexi mod, but I left them when going back to the 800 mod.

Any insights would be greatly appreciated. Thanks!
 
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The Origin amps are closer related to the JTM amps than the JMP it appears at least that's what I've read on the Marshall forum. I wish I was smart enough to know about this kind of stuff but nope .
 
Will need a schematic of your exact amp in its current state in order to actually get an idea for what you're talking about.
 
A and B won't make the amp seem brighter. Especially a 1M pot vs 100k, which will leave a lot more gain in the circuit and boost lows. If you mean that you added a 470k/470pf peaker with this change, that will add some highs. But I doubt more than the perceived highs of a 100k pot based on how much gain it will dump and lows will be lost.

C will cause a huge increase in brightness. Try 68pf instead.

I don't understand what you mean by "classic" presence circuit. But that could potentially cause more brightness depending on how it differs from what was there before.

Also, when you say "4k7" bypass cap, I'm guessing you mean a 4.7k resistor in parallel over the 10k. 4k7 refers to resistance, not capacitance. Just FYI.
 
RedPlated said:
A and B won't make the amp seem brighter. Especially a 1M pot vs 100k, which will leave a lot more gain in the circuit and boost lows. If you mean that you added a 470k/470pf peaker with this change, that will add some highs. But I doubt more than the perceived highs of a 100k pot based on how much gain it will dump and lows will be lost.

C will cause a huge increase in brightness. Try 68pf instead.

I don't understand what you mean by "classic" presence circuit. But that could potentially cause more brightness depending on how it differs from what was there before.

Also, when you say "4k7" bypass cap, I'm guessing you mean a 4.7k resistor in parallel over the 10k. 4k7 refers to resistance, not capacitance. Just FYI.
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Sorry, I just realized I worded it wrong. It's a 4k7 cathode resistor with a 0.68uF bypass cap.
 
burger said:
1nF bright cap on the gain pot, and the Tilt pot function?
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I tried it without the bright cap, and it still seems like too much high end. The tilt pot is 100k linear, correct? I'm going to go through the circuit again just to be certain I didn't miss anything while converting it back to 800 spec. It sounded really good at the Plexi spec, but since I already have another plexi, I decided to take the Origin back to an 800. Maybe I should have left it alone.
 
RedPlated said:
A and B won't make the amp seem brighter. Especially a 1M pot vs 100k, which will leave a lot more gain in the circuit and boost lows. If you mean that you added a 470k/470pf peaker with this change, that will add some highs. But I doubt more than the perceived highs of a 100k pot based on how much gain it will dump and lows will be lost.

C will cause a huge increase in brightness. Try 68pf instead.
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Yes, I added a 470k/470pF peaker going into the 1M gain pot.

I didn't realize that PI fizz cap would make that much of a difference. Maybe I should put it back to the original 100pF value?
 
ledvedder said:
Yes, I added a 470k/470pF peaker going into the 1M gain pot.

I didn't realize that PI fizz cap would make that much of a difference. Maybe I should put it back to the original 100pF value?
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It will cut a lot of highs at 100pf. Definitely try that.

You can also try putting a 100 to 500pf cap over the v1 plate resistor.

The other thing that I'd try is bumping your peaker from 470pf to 2200 pf. It'll cut some highs.
 
ledvedder said:
I tried it without the bright cap, and it still seems like too much high end. The tilt pot is 100k linear, correct? I'm going to go through the circuit again just to be certain I didn't miss anything while converting it back to 800 spec. It sounded really good at the Plexi spec, but since I already have another plexi, I decided to take the Origin back to an 800. Maybe I should have left it alone.
Click to expand...

Yes B100k in the O20. If you have no bright cap, but the Tilt pot restored to stock, and you dial it back AND it's still too bright? Maybe take that bypass cap off the 2nd cathode, you might prefer it.
 
I figured I'd put a 100k gain pot back in, like the original spec, in place of the 1M today. Now it sounds much fuller. I don't know if it's my ears, the weather, or if it actually made a difference 😂
 
Pretty surprising that it's fuller. More signal going to ground with a 100k. Sometimes tighter feels "fuller" so that may be what happened here.
 
ledvedder said:
I posted this on Headfirst Amplification's Facebook page, but figured I'd also post here. I finished converting my Origin 20 back to the Headfirst 800 mod from its previous Headfirst plexi mod. I omitted the boost switch since I was having issues with it, so I'm just leaving the 0.68 cap on v1b permanently. I'm also using a 4k7 cathode resistor on v1b instead of 2k7 for a bit less gain. I tried a 10k with no cap, but I think I like 4k7/680 better. 1nF bright cap. I also put the previous dropping resistors back, instead of the 10k ones from the Plexi mod. But I do have some questions. It seems more bright than it was before. Not enough oomph. I'm wondering if that could be caused by any of these things, which I left from the previous plexi mod.

A. 1M gain pot with the 470k/470pF into the input, instead of the previous 100k gain pot.

B. 1M PI grid leak resistors instead of the previous 470k resistors.

C. 47pF fizz cap instead of the previous 100pF.

D. Classic presence circuit.

These were changes I made when doing the Plexi mod, but I left them when going back to the 800 mod.

Any insights would be greatly appreciated. Thanks!
Click to expand...
Hey sorry for the thread revival but I saw you’ve got experience with both the Plexi and JCM800 Headfirst mods.

I have a versatile DSL40CR (w Creamback) that I like very much for crunch tones and higher gain stuff.

So I was considering a Plexi mod Origin 50combo (I can get it cheap for like 450$). I can’t afford a 2-3k plexi setup tho and thought this could be good. Correct me if I’m wrong. The combo at 40lb and cheap price is alluring if it works great. I could try to find a 20w head but they are still like 400$ and then I’d need the cab to go with it so price gets up there. If the 50w combo is foolish for some at home (but does shows once in a while) let me know.

Questions about the Plexi mod. Mainly…does it still sound good at lower volume?? I know the real plexi needs some wild volume to get that beautiful distorted tone but I can’t do that or afford a 300-700$ attenuator to accompany it. Jason at headfirst indicated it doesn’t need to be turned up to sound good (my 40w DSL sounds good at low volume).

What has your experience been? Any advise?
 
svinyard said:
Hey sorry for the thread revival but I saw you’ve got experience with both the Plexi and JCM800 Headfirst mods.

I have a versatile DSL40CR (w Creamback) that I like very much for crunch tones and higher gain stuff.

So I was considering a Plexi mod Origin 50combo (I can get it cheap for like 450$). I can’t afford a 2-3k plexi setup tho and thought this could be good. Correct me if I’m wrong. The combo at 40lb and cheap price is alluring if it works great. I could try to find a 20w head but they are still like 400$ and then I’d need the cab to go with it so price gets up there. If the 50w combo is foolish for some at home (but does shows once in a while) let me know.

Questions about the Plexi mod. Mainly…does it still sound good at lower volume?? I know the real plexi needs some wild volume to get that beautiful distorted tone but I can’t do that or afford a 300-700$ attenuator to accompany it. Jason at headfirst indicated it doesn’t need to be turned up to sound good (my 40w DSL sounds good at low volume).

What has your experience been? Any advise?
Click to expand...
My buddy has a Marshall Origin. I tried my little Lead 12 head against it and preferred it all day long.

Shocked me. I had a little Lead 12 and sold it and then had to pick one back up! At $150, they are insane and louder than you would ever expect. Sounds killer. (coming from a tube amp guy)
 
svinyard said:
Hey sorry for the thread revival but I saw you’ve got experience with both the Plexi and JCM800 Headfirst mods.

I have a versatile DSL40CR (w Creamback) that I like very much for crunch tones and higher gain stuff.

So I was considering a Plexi mod Origin 50combo (I can get it cheap for like 450$). I can’t afford a 2-3k plexi setup tho and thought this could be good. Correct me if I’m wrong. The combo at 40lb and cheap price is alluring if it works great. I could try to find a 20w head but they are still like 400$ and then I’d need the cab to go with it so price gets up there. If the 50w combo is foolish for some at home (but does shows once in a while) let me know.

Questions about the Plexi mod. Mainly…does it still sound good at lower volume?? I know the real plexi needs some wild volume to get that beautiful distorted tone but I can’t do that or afford a 300-700$ attenuator to accompany it. Jason at headfirst indicated it doesn’t need to be turned up to sound good (my 40w DSL sounds good at low volume).

What has your experience been? Any advise?
Click to expand...
That plexi mod has a PPIMV which will give you gain down to low volume levels
 
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