Bare Knuckle Pickups Forum
Forum Ringside => Guitars, Amps and Effects => Topic started by: Twinfan on September 16, 2011, 11:07:42 AM
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For those folks like me who enjoy techie stuff - here's the accurate #34 JCM800 mod as used by Slash on numerous GnR and solo recordings:
www.mediafire.com/?bbnxtoc0fvfk8h7
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I've had that schematic Santiago posted for a while - think he put it up on the Marshall forum.
Makes the amp brighter (ouch), but also a certain snarl - you really need to crank the amp with these mods to get it sounding right.
I reckon the softer AII Duncans Slash uses counter this to some extent.
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Thankyou!
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I'm with you HTH. I'm really loving #34 mode on my AFD at the moment - use the power scaling and crank the gain is the way to go.
I hated #34 to start with and I prefer it to AFD now! :lol:
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Makes the amp brighter (ouch), but also a certain snarl - you really need to crank the amp with these mods to get it sounding right.
I reckon the softer AII Duncans Slash uses counter this to some extent.
Well that 500pf output coupler (instead of the 22nf = 22000pf which is more than 40fold) cuts out so much bass that the overall result must be a lot brighter. This is not fully compensated by replacing C4 with the tenfold value. IMHO it is a good idea to reduce bass in the 2203 circuit as I find it turns to mush rather quickly with the gain control turned up.
Also that .1uf cathode cap on the second gain stage will make things very bright and add more gain.
The cathode bypass cap on the third stage is so large that it adds only gain but no frequency shaping.
I second the comments on the AII Duncans. The #39 circuit also sounds very bright and benefits from a smoother pickup.
Cheers Stephan
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#69 HTH JCM800 mod :lol:
http://www.youtube.com/watch?v=5AiShyTy0xY
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Nice, any idea if it works on a 50w model? Also, great playing HTH
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Nice, any idea if it works on a 50w model? Also, great playing HTH
cheers man - was a bit ropey on that video, only really learnt it this afternoon AND I gashed my right hand open yesterday, so it was a bit sore trying to play :(
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one thing i don't reeeeally get in that document, is attaching the sheild of a wire to the plate/anode of a valve. I've read about it briefly but never understood the practice.
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one thing i don't reeeeally get in that document, is attaching the sheild of a wire to the plate/anode of a valve. I've read about it briefly but never understood the practice.
it creates a capacitance between the shield and core (afaik), however it's never sat easy with me and you can get the same effect by putting a small cap between the plate and cathode or grid and cathode.
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it always seemed dodgy to me to have a shield with plate voltage on it.
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it always seemed dodgy to me to have a shield with plate voltage on it.
same here - I always changed over the 2203/2204 amps I've had to ground the shield and never noticed a difference.
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cool. i was going to ask if it ever made a difference. seems like one of those myth things. I've never read about anyone arguing that its better to do it that way but that doesn't mean its never happened.
The schematic doesn't really make me want to try it out im afraid. cool video by the way.
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I've always thought that attaching the shield to the anode was a strange idea.
Yes it increases the capacitance from the anode to the grid, which will be multiplied by the gain of the following stage +1. This can be employed to prevent oscillation or reduce top end response by limiting high frequency bandwidth.
However, it seems a bad idea as:
1) the extra capacitance will be poorly defined; you would be better off using a small capacitor of known value for reproducibility.
2) The shield will now be virtually ineffective as it now has a high impedance path to ground.
3) It's probably NOT a good idea have a high voltage on the shield as it may inadvertently end up somewhere it shouldn't.
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Hello all!!!
I actually penned and posted these #34 mod instructions. To comment on the shielded wire technique, I never actually tried using a cap instead of the shielded wire. For the most part, I used it merely because it was utilized in the #39 AND #34 mods by Tim and Frank. One thing to consider when modifying your amp; it is the sum total of all of the mods that creates the end resulting sound. I'm going to guess that since using a single cap instead of the shield wire was an option when these mods were created, and that there is some benefit in using the shielded wire over the cap.
Someday when I’m feeling adventurous, I’ll swap the shield wires for some caps, and we’ll see what happens. 8)
Till then, it sounds so spot on that I hate to touch it!
Cheers!!!
Jay
BMW FREQ
jay@jjlcustoms.com
I've always thought that attaching the shield to the anode was a strange idea.
Yes it increases the capacitance from the anode to the grid, which will be multiplied by the gain of the following stage +1. This can be employed to prevent oscillation or reduce top end response by limiting high frequency bandwidth.
However, it seems a bad idea as:
1) the extra capacitance will be poorly defined; you would be better off using a small capacitor of known value for reproducibility.
2) The shield will now be virtually ineffective as it now has a high impedance path to ground.
3) It's probably NOT a good idea have a high voltage on the shield as it may inadvertently end up somewhere it shouldn't.
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I'm going to guess that since using a single cap instead of the shield wire was an option when these mods were created, and that there is some benefit in using the shielded wire over the cap.
I wouldn't be so quick to assume that. The likelihood is that the Marshall guys thought, hmm.... connect the shield to the plate, or add a capacitor that costs money. Catch my drift?
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Well, if it were just Marshall we were talking about, and the cost of thousands of capacitors, I may agree with you, but; we're also talking about Tim and Frank, who HAD the option to use a small, single and safe capacitor. Instead they used the hot-shield method, which (and I'm guessing here) MUST have provided something to the overall tone that a single capacitor did/could not.
One of these days I am going to swap the hot-shields for caps, and we'll see what happens... It sounds so damn good that I hate to even mess with it. :lol:
For anyone that is interested in doing this mod based on my instructions, and you're wondering why Frank Levi's #34 mod is so different than mine, it's because in order to acheive the #34 tone with Franks mod, you had to crank the amp. The Master had to be set at 8, and Pre-amp had to be set at 3. This mod enables you to get the #34 tone at bedroom levels with the Master Volume set anywhere from 1 to 3.
The settings I use for this mod are: P10 B0 M3~5 T3~5 M1~3 PA 7~8.
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Well, if it were just Marshall we were talking about, and the cost of thousands of capacitors, I may agree with you, but; we're also talking about Tim and Frank, who HAD the option to use a small, single and safe capacitor. Instead they used the hot-shield method, which (and I'm guessing here) MUST have provided something to the overall tone that a single capacitor did/could not.
One of these days I am going to swap the hot-shields for caps, and we'll see what happens... It sounds so damn good that I hate to even mess with it. :lol:
For anyone that is interested in doing this mod based on my instructions, and you're wondering why Frank Levi's #34 mod is so different than mine, it's because in order to acheive the #34 tone with Franks mod, you had to crank the amp. The Master had to be set at 8, and Pre-amp had to be set at 3. This mod enables you to get the #34 tone at bedroom levels with the Master Volume set anywhere from 1 to 3.
The settings I use for this mod are: P10 B0 M3~5 T3~5 M1~3 PA 7~8.
you could argue one way or the other all day - I really couldn't give a fu*ck, I know what I like and thats good enough for me, people seem to like my amps too, so thats good enough for me too.
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Well, if it were just Marshall we were talking about, and the cost of thousands of capacitors, I may agree with you, but; we're also talking about Tim and Frank, who HAD the option to use a small, single and safe capacitor. Instead they used the hot-shield method, which (and I'm guessing here) MUST have provided something to the overall tone that a single capacitor did/could not.
One of these days I am going to swap the hot-shields for caps, and we'll see what happens... It sounds so damn good that I hate to even mess with it. :lol:
For anyone that is interested in doing this mod based on my instructions, and you're wondering why Frank Levi's #34 mod is so different than mine, it's because in order to acheive the #34 tone with Franks mod, you had to crank the amp. The Master had to be set at 8, and Pre-amp had to be set at 3. This mod enables you to get the #34 tone at bedroom levels with the Master Volume set anywhere from 1 to 3.
The settings I use for this mod are: P10 B0 M3~5 T3~5 M1~3 PA 7~8.
I would love to get you mod as i have wanted to get the #34 tone at bedroom levels. For a long time.
if you can help me id love it
thanks