New amp project...One Watt JCM 800

[Greg_L]

Because I can...this will be an attempt at a ONE WATT Marshall JCM 800.

As we all know the JCM 800 2203/2204 (100w/50w) amps are among the greatest and most copied amps ever made. They utilize a cascaded gain stage preamp and Plexi/Super Lead style power section for what is the best of all worlds. EL34 or 6550 output tubes for massive punch and volume.

But this aint that. I'm gonna build the preamp totally accurate. The tone stack...totally accurate. The phase inverter...accurate. The output tubes...doh! This is where I'm gonna get stupid and use both halves of a single 12AU7 for a micro push/pull output stage that will make approximately one watt.

Why? Why not?

I found this on Craigslist for 20 bucks. The guy was like "hey you wanna try it out, it sounds great!" I said "nope I don't care if it works or not it's about to die and become something else". He seemed confused and I didn't even explain my plans. I handed him a 20 dollar bill and walked away with it. The donor chassis....


I chose this little shitstain amp for three reasons...

It's already equipped with a fused IEC plug.


And a perfect little mini power switch.


And the bare chassis has the appropriate number of holes for controls and jacks.


Although I may make a new faceplate. Not sure. Probably will. This will also be a head so the combo box and speaker are going bye-bye. I just want the chassis with the fused IEC plug and power switch.

Next up...transformers have been ordered and lots of mock ups for transformers and tube socket placements.

@WhiskeyJack ....consider this thread your tutorial.

[Lt. Bob]

interesting for sure.
So the pre is gonna be a stock pre and will sound like the Marshall pre .... got it.

But you always make a point that power tube distortion is a big part of the sound you like.
Obviously you'll be able to drive this poower tube into distortion .... but I have to think that it'll be it's own sound since big power tubes all have their own sound.
How close do you think it'll sound like a Marshall power section?

[Greg_L]

interesting for sure.
So the pre is gonna be a stock pre and will sound like the Marshall pre .... got it.

But you always make a point that power tube distortion is a big part of the sound you like.
Obviously you'll be able to drive this poower tube into distortion .... but I have to think that it'll be it's own sound since big power tubes all have their own sound.
How close do you think it'll sound like a Marshall power section?

Power tube distortion isn't really super critical with a JCM 800 type preamp. The cascaded preamp was developed to use a master volume so you could get that dirty Marshall sound at lower volumes. In most practical applications with a JCM 800 2203 or 2204 you're barely able to get the power section working much before everyone is dying from ear damage. So for something like this I'm not too concerned because the preamp section will be doing most of the work and the single 12AU7 output tube will most likely stay fairly clean. A 12AU7 is a robust but low gain dual-triode.

But we shall see. This is just for fun and to have a micro 800 just for dicking around. I bet it'll sound pretty good through a proper 4x12 cab and plenty loud in it's own right.

[vomitHatSteve]

Well this sounds fun!

[rayc]

COOL...sort of Watch Mr Wizard and Bob Ross in Amp Land.
I enjoy these threads enormously.

[CrowsofFritz]

Greg, do you have one of those desktop tripods that can hold a phone? You should record these!

[Greg_L]

Greg, do you have one of those desktop tripods that can hold a phone? You should record these!

I do. I did some vids of one of my last builds. It's a pain in the ass.

[Lt. Bob]

interesting for sure.
So the pre is gonna be a stock pre and will sound like the Marshall pre .... got it.

But you always make a point that power tube distortion is a big part of the sound you like.
Obviously you'll be able to drive this poower tube into distortion .... but I have to think that it'll be it's own sound since big power tubes all have their own sound.
How close do you think it'll sound like a Marshall power section?

Power tube distortion isn't really super critical with a JCM 800 type preamp. The cascaded preamp was developed to use a master volume so you could get that dirty Marshall sound at lower volumes. In most practical applications with a JCM 800 2203 or 2204 you're barely able to get the power section working much before everyone is dying from ear damage. So for something like this I'm not too concerned because the preamp section will be doing most of the work and the single 12AU7 output tube will most likely stay fairly clean. A 12AU7 is a robust but low gain dual-triode.

But we shall see. This is just for fun and to have a micro 800 just for dicking around. I bet it'll sound pretty good through a proper 4x12 cab and plenty loud in it's own right.

oh it'll be awesome whatever happens.

So that erfa of Marshalls get their sound in the pre ..... like Mesa's do.
And my 6101 too ..... I'm seriously looking forward to seeing this build.
You gonna do whatever switching is part of that preamp or is it a single channel?

[CrowsofFritz]

Greg, do you have one of those desktop tripods that can hold a phone? You should record these!

I do. I did some vids of one of my last builds. It's a pain in the ass.

If you want to send me the files, I can edit it.

[SweetDan]

[Greg_L]

oh it'll be awesome whatever happens.

So that erfa of Marshalls get their sound in the pre ..... like Mesa's do.
And my 6101 too ..... I'm seriously looking forward to seeing this build.
You gonna do whatever switching is part of that preamp or is it a single channel?

The master vol JMPs and JCMs do get most of their sound from the preamp section. By the time the power section really kicks in it's so loud that it's unusable anywhere but the studio and stadiums. Technically it's the second gain stage that makes the magic of these amps. The "cold clipper" stage.

It's a single channel. There's no switching.

[JD01]

I'm really interested in this one, this is just the sort of Marshall that I'd find useful at home.

[Greg_L]

Besides just having a new project to work on...I'm curious how close it'll get to the sound of the real thing.

[einstein magoo]

Maybe stupid question...does 1 watt give you enough power to get 4 x 12s moving?

[Greg_L]

Maybe stupid question...does 1 watt give you enough power to get 4 x 12s moving?

No but it'll make sound just the same as turning a big amp down to bedroom levels. Speakers don't turn off from a lack of power feeding them. Most cabs are way overrated for the amps feeding them anyway. The modern stock standard Marshall 1960 4x12 cab consists of four 75w speakers for a rating of 300 watts. There's not a guitar amp out there that comes close to 300 watts of output. Nor could anyone even use 300 watts of output. The cabs still sound great though.

[Greg_L]

Waiting on some parts....did the board design layout.



This is version 1. I might do another. I see a few things I can make better maybe.


Also, these guys showed up. Tiny Hammond transformers for a tiny amp.

The output transformer is a generic project 5w push/pull. The choke is a generic 5h choke. The power transformer though is basically a Fender Princeton, Princeton Reverb, Bronco, Vibro Champ type PT. It has a winding for a tube rectifier. My plan is solid state rectification but I might experiment with both.



So now it's time to start making a plan for how I'm gonna lay out the hardware and cutting holes.

[Lt. Bob]

[WhiskeyJack]

@WhiskeyJack ....consider this thread your tutorial.

subscribed.

I'm stoked on this. Currently sourcing some importanter parts right now. Might have a lead on a little garbage practice amp too but for the sake of knowing myself, i may just opt to buy the parts outright rather than scrounge for them. I know i will just pull a total Shan™ "hmm i better keep this old cab speaker and bits and pieces for a rainy day" move on myself and i am in the process of trying to undo 10 years of doing that.

and i know @Armistice will chime in asking if i got the baseboards and casing done" no i haven't as i have run out of time for favorable weather to do do the wet finishing in my unheated shop. So currently trying to find a bro with a heated garage that will let me borrow that space to get that taken care of.

My version of greg 1 watt amp build is a kind of a "over the xmas holidays / new year 2025" thing for me. I also have to pace myself with buying stuff. xmas is coming and i gotta keep the CC balance manageable.

[JD01]

I'm stoked on this. Currently sourcing some importanter parts right now.

Have you finished your telecaster yet?

[Greg_L]

Shanathan has moved on to better projects now.