JCM-800/2204 Build

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ampgeek
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JCM-800/2204 Build

Post by ampgeek »

A childhood friend of mine offered to sell me his early 80's Gibson Explorer. However, he also needs a new amp. The rest...you can guess! :wink:

I have most everything that I need lying around in spares/in various experimental builds so I have a great start. Will probably splurge on a full set of MC iron though just to keep it legit.

I am using Mark Huss's JCM-800/2204 schematic as the basis but am planning on numerous additional features including VVR, PPIMV, switchable cathode bypass cap(s), DC on pre-amp filaments, etc..

I would like to put the Low/High gain inputs on a relay and use a single input jack. I have a few of AmpDocs boards that will service my needs perfectly.

Soliciting input on whether or not I could/should throw an additional Vol control at the added gain stage when switched to high-gain mode.

I am also considering a pre-PI MV like I recently did on an 18W plan but don't want to over do/overly complicate the volume/gain control philosophy.

Any input/thoughts on those items, and any others that might come to mind, would be greatly appreciated!

TIA,
Dave O.
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Reeltarded
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Re: JCM-800/2204 Build

Post by Reeltarded »

Awesome!

After so many modded versions of the stock circuit I have come to really like these features:

All 2k7/.68u stages (3)

Switchable 500p/1000p/2200p peaker at the entrance of the usual 3rd stage

100p bright cap over gain

CF master (Jose mod) with clipping circuit I like transistors and 1n914s in pairs. I add a 25k or so pot to ground on that control to vary the diode dump. That is VERY cool. Red LED and 20v zener also very good with jist a fattening action without so much dump.

Simple 800 style master as you said above (also)

56k CF

You could wire up a three pos switch to bypass second stage for very plexi, normal 2204, and added stage.

1M/.0047u resonance

Bunches of other stuff.. :) these amps are very forgiving as far as the dress is concerned. I recently started converting my mod amps to rear input to get the input right on the tube.

As far as switching input from hi to lo.. that is kind of the same thing as the two stage setting on that staging switch I was talking abooot.

VARIABLE SLOPE! 33k-83k! Yes!! I'm excited. Let's hurry.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
ampgeek
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Re: JCM-800/2204 Build

Post by ampgeek »

Nice Miles! Great grist for the mill. I am still developing the "specification" with my friend (he can be a kind of anal at times) and I will need to do a bit of research on some of those items. Hopefully you won't mind if a pick'yer brain during the process.

A couple of quick ones for clarification:

Variable slope: On a pot (with a tail resistor) or via switch? I will admit that I have never played around (neither via math or with actual parts) with the tone stack slope resistor. How, in practice, does it effect the tone controls? Is it a set-and-forget option or one that would be adjusted OTF between songs to extend (or shorten) the range of the tone pots?

Third stage cathode by-pass cap: Any utility in putting that on a relay to provide an OTF volume "boost"?

Thanks for mentioning the clipping circuit options. I have always wanted to play around with that. Looks like the perfect opportunity here!!

Thanks a million mate!!
Dave O.
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Re: JCM-800/2204 Build

Post by Reeltarded »

I do slope with a 33k off a 50kl pot. If you use 33k+25kl it gets you sweep between JMP and JTM stock slope values, but the extension up to 83k gets you sparkling highs that take it into Fender territory. Using 500p treble cap and the common resonance thing gives you the ability to go almost anywhere with any guitar and sound awesome. The slope starts rolling off bass around 65k so it is nice to add a little more resonance.

Look at Duncan's TSC changing the slope value from 33-83. Big scoop as you get up there. My tone controls are set for whatever guitar I am playing then I use the slope as "Thee Tone Control" I could live with a 50% fixed value TMB tone stack and only a slope control.

With the amp set at 2 stages, the controls all straight up, and the slope maxed at 83k sounds like a Bandmaster right at break up. Stonesy kind of sound. With the gain cranked and the slope at 33k you get EVH or whatever if you like....

Then there is adding the middle stage ala-800. Oh man. Thumpy. Then you can add another stage and make a Marshall run like a Dumble, sort of. So many tones with a couple simple clicks.

If you use both the CF master and simple master off the treble control you have this other way of setting up the gain structure. There are bunches of sweet spots. ;)

The cathode bypass doesn't really do much boost at any medium gain setting or higher.. once the PI is driven at least. I removed all my bypass switching. I used to switch between 250u-Null-.68u. Now I just use a fixed value. .68-10u depending on how everything else works.

The clipping circuit off the CF master gets you insane dirty at ipod volume if you want. You can play a 100w amp at television volume from about 4" out and get the amp crying into feedback. I think the clipper sounds best when it is lifted from ground by 10k. Put that on a 25kl and it becomes squash control. A dirty compressor.

This ^^^ all makes my favorite amp, ever!! Even though I have raved on this long I can't begin to explain how the interactions of all the controls work. It will make you giggle. :)

I'm excited. Let's build an amp!! heh
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Bob S
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Re: JCM-800/2204 Build

Post by Bob S »

+++ What Miles said.
Not sure where this is gonna lead - but it's fun.
Big chassis with lots of tubes.
I's excited.
Why Aye Man
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Reeltarded
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Re: JCM-800/2204 Build

Post by Reeltarded »

++++1 on what Bob said about what I said.

Me too!

Ok. Looks unanimous! :)

The above is all but a 270k plate resistor on V1 from being the dirty side of the channel switcher I am building. Still haven't found the fully engineered switching rig for that. Damn.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
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cbass
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Re: JCM-800/2204 Build

Post by cbass »

Ok i gotta build one of these Marshall thingies.As soon as I'm over my rocket kick
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M Fowler
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Re: JCM-800/2204 Build

Post by M Fowler »

Reeltarded wrote:Awesome!

After so many modded versions of the stock circuit I have come to really like these features:

All 2k7/.68u stages (3)

Switchable 500p/1000p/2200p peaker at the entrance of the usual 3rd stage

100p bright cap over gain

CF master (Jose mod) with clipping circuit I like transistors and 1n914s in pairs. I add a 25k or so pot to ground on that control to vary the diode dump. That is VERY cool. Red LED and 20v zener also very good with jist a fattening action without so much dump.

Simple 800 style master as you said above (also)

56k CF

You could wire up a three pos switch to bypass second stage for very plexi, normal 2204, and added stage.

1M/.0047u resonance

Bunches of other stuff.. :) these amps are very forgiving as far as the dress is concerned. I recently started converting my mod amps to rear input to get the input right on the tube.

As far as switching input from hi to lo.. that is kind of the same thing as the two stage setting on that staging switch I was talking abooot.

VARIABLE SLOPE! 33k-83k! Yes!! I'm excited. Let's hurry.
Need a schematic with all these changes on it, thanks Miles. :)
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Reeltarded
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Re: JCM-800/2204 Build

Post by Reeltarded »

A first for me. I will see what I can do. :)
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
ampgeek
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Re: JCM-800/2204 Build

Post by ampgeek »

Still soaking it all in....but....I am going to guess that after all is said and done I am probably going to wish that I didn't have to trade it for a guitar!!

Thank you again and again.

Onward!

Dave O.
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gktamps
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Re: JCM-800/2204 Build

Post by gktamps »

Yes, Miles, PLEASE - this needs a schemo!
Cheers,
Greg
Chris G
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Re: JCM-800/2204 Build

Post by Chris G »

Sounds like a Friedman BE-100 to me....
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Reeltarded
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Re: JCM-800/2204 Build

Post by Reeltarded »

Mmmm.. nope.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
ampgeek
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Re: JCM-800/2204 Build

Post by ampgeek »

Decisions are slow in coming unfortunately.

At least I know what type of chassis I can start designing around.
I have built a couple of amps in these over the last year, this one being a recent 18W mutation.

As much as I love wood and tolex....these are a total breeze to work with.
Will be starting with a 17" X 10" X 3" variant just to see how it initially lays out.

Hear..hear on a schematic/sketch! I was skinny-dipping in a deep Google pool of "Jose mods" to Marshall's last night and think that I drowned!! Even a quickie hand-sketch of the cathode follower MV with clipper would be a great help just to provide some focus.

Thanks as always,
Dave O.
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Reeltarded
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Re: JCM-800/2204 Build

Post by Reeltarded »

You can't build a Marshall in there. It's backwards!

:)
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
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