sanrico

I am having a simple modification done to my JCM head by a local tech. It's a really simple mod, but I'm not crazy enough to stick my grubby mitts in that amp and end up meeting Jesus much earlier than I was planning to.

The tech was cool enough to tell me I can save money by ordering the parts myself through Tubes and More. Sounds easy enough, but when I got to the site, I'm finding myself confused by all the choices. Silver mica capacitors versus ceramic, different wattage resistors, etc...

Is there anyone here who might be able to help me with this? Here is the part list:

Resistors - 220K, 56k, 10k, 100k (what wattages do I use?)

capacitors - 100pF and 250pF (silver mica versus ceramic?)

Here is the full mod text, if this helps:

From Liam on Marshall Forum:
http://www.blamepro.com/CGI-BIN/cute...m=31&thread=76

Grev put up some stuff to help people with biasing, so I thought I'd
whack my mods up here to, for future reference.
Firstly, the standard warning:

ALWAYS discharge the caps in your amp BEFORE doing ANY work inside it.
If you don't, you might not live to hear your modded amp!

If you don't know what you're doing, DON'T DO IT! Get someone who does
to.

Ok, this mod is very simple, cheap, and best of all, it's easy to undo
if you don't like it.

It gives the amp more gain, bass and mids, and gets rid of a lot of
that ear piercing treble and brightness the dual reverbs are notorious
for. It also seems to smooth the grittiness of the distortion, and
make the amp sound more full and punchy.

The following is a list of parts to change, and what they do.

Just a note: The PCB's in these amps don't take to kindly to rough
handling. The copper etching comes off very easily. So take care when
desoldering components.

R10 (100K) - change to 220K or 470K
gain boost (higher resistor value = more gain) I ended up using a
220K.

C8 (470 pF) - change to 100 pF
Stability

R37 (22K) - change to 10K
Gain boost

R7 (4.7K) - change to 10K
Pre-distortion mid boost.

R55 (100K) - change to 220K
Reduced loading, more effective gain

R15 (100K) - change to 220K, in parallel with that, run a 100K
resistor in series with a .005 uF (.0047) cap.

This gives a post-distortion bass boost. You can change it to suit
your liking. The 220k resistor sets the amount of boost (higher
value=more boost) and the cap sets the frequency that is boosted. I
originally used a 470k resistor, and the bass was really flabby, and
boomy. So to cut the boost back, I used a 220k.

You'll have to add the series resistor/cap combination external to the
PCB.

Something like this

|----|100k|--|.005uF|---|
|---------|R15|---------|

Extras:

You can also change your tone slope resistor (R61) from 33k to 56k,
and the treble cap (C37) from 470pF to 250pF. I had already done this
to my amp before I did the rest of the mods. It also helped to cut
back highs, and add mids.

You can also change around the values of R9 and C7 on the clean
channel. I haven't done this, so I don't know what it sounds like. If
anyone has, how did it go?

That's pretty much it.

I hope I haven't left anything out.

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MartyStrat54

How's this for a concept? Let your tech order the parts. Most tech's I know do not want the customer bringing in their own parts.

You state JCM amp, but no model. For you to get your information, another forum member would have to find the correct schematic and look up each part for you...for free.

Like I said, let your tech order the parts. He has a schematic and will know if he needs 1/4W, 1/2W, or bigger resistors and the voltage values on the caps. He will also know what type of resistor he needs.

However, in case someone is willing to help you, you first need to be very specific about the amp you have and post that info.

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MARTY

2009 Gibson Explorer-Hell Raising Machine
1989 Fender Strat Plus USA
2008 Fender Tele USA
2004 Schecter Elite w/DiMarzio's

TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers.
TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends."
DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker.

Tubes: Let's just say they are NOS that kick ass.

sanrico

Well, here's a concept for you: As my original post states, the TECH suggested I order the parts.

JCM 900 4500. And thanks for your help.

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MartyStrat54

On the 4500, there are two different boards used, so the date of the amp must be included in your info.

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MARTY

2009 Gibson Explorer-Hell Raising Machine
1989 Fender Strat Plus USA
2008 Fender Tele USA
2004 Schecter Elite w/DiMarzio's

TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers.
TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends."
DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker.

Tubes: Let's just say they are NOS that kick ass.

Riffraff

I can understand doing that if you are talking about tubes or some other high dollar part but these are super low cost parts. Maybe a few bucks total for everything. I hate to say it but Marty is right, it's a little odd that the tech would suggest you get them. Especially given the fact that you will need to read the schematic and understand it to know what to get. He may be assuming you understand what to get since you are bringing the mod to him. I would just tell the tech you don't know what exact values and types he will need so you are leaving that up to him.

Jesstaa

As I'm guessing there's 2 different boards used on the 4100 (I think I've heard there was 2 boards used before anyway), I must ask, what's the difference? Which is better, and what date did they introduce the second board?

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