50w plexi vs 2204 (I’m confused)

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Explorerman
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50w plexi vs 2204 (I’m confused)

Post by Explorerman » Mon Jan 14, 2019 6:31 pm

Hi, just a few questions regarding the differences between the two. I’m looking at building a 2204 (is this a JMP 2204 or JCM clone?? I’m confused) or the 50w plexi - the Valvestorm storm site says to use the 50w plexi instructions for the 2204, but the layout of the 2204 is different to the 50w layout - namely the 2204 has 2 x input jacks whereas the 50w has 4, however the component checklist for the 2204 is the same as the 50w (except for the face plate and number of jacks).

Also, i would like a MV - I see the 2204 has a master volume, do I still need a PPIMV for the 2204 and the 50w? The Valvestorm 2204 page shows a layout with a PPIMV.

I’m torn between the two at the moment - what are the differences in clean tones and crunch between the two?

I’ve read there are noticeable differences between the iron used in the JTM45 (Heyboer seems to be favourable over some of the others) is this also the same with the 2204 and 50w? What make of OT and PT do you recommend?

Cheers

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Re: 50w plexi vs 2204 (I’m confused)

Post by danman » Mon Jan 14, 2019 7:08 pm

Plexi is a term used to describe a few certain years of the 4 input, non master volume amplifiers in the late 60's. I believe that the plexi face plates ended in 68 but folks continue to use the term for the later non master volume amps. Around 1976, the circuit changed to a two input, cascaded preamp design known as 2203 (100 watt) and 2204 (50 watt). These cascaded preamps produce more gain and also have a master volume placed before the phase inverter. Later in the early 80's, the amps got renamed with the JCM 800 moniker but they were still the same two input, master volume amps as the 2203 and 2204. The original 4 hole, non master volume circuit (1959 superlead 100 watt and 1987 lead 50 watt) were still being made also during this entire period. So basically the 4 input amps are non master volume with standard preamp and the two input amps are master volume with cascaded preamp. All of these circuits changed gradually throughout the years as people wanted tighter, brighter amps with more gain.
The Valvestorm instructions tell you to use the same instructions for the regular 50 watt lead and the 2204 because both amps are almost identical except for a couple different components on v1 and the input jack wiring. It will really help you if you teach yourself to read schematics and layout drawings because different wiring of the input jacks can be confusing. There are plenty of 2204 layouts online if you need help. Keep in mind that they all differ slightly depending on which year and circuit revision you intend to build. If you want more info on the different Marshall circuit, this page gives good info...https://reverb.com/news/a-history-of-ma ... mps-part-i

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Re: 50w plexi vs 2204 (I’m confused)

Post by danman » Mon Jan 14, 2019 7:11 pm

Oh, you can add a ppimv to a master volume amp but I prefer the pre MV that the amp already has. The non master volume circuit can benefit from a ppimv but many people prefer to use an attenuator as the ppimv does change the sound as you dial the volume down. When the ppimv is at 10, the amp sounds just like it would without it installed but as you turn down the volume, it does alter the amp's sound.

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Re: 50w plexi vs 2204 (I’m confused)

Post by danman » Mon Jan 14, 2019 7:27 pm

The brand of iron you choose is totally up to you. I have had good reults with Classictone and Marstran but in the end, you will need to do lots of reading to decide on the best choice for you. Also don't forget that the 4 hole, non master volume amps are extremely loud when trying to get good crunchy tones. This is where the MV amps do best but even they need to breathe a bit to sound good. I have built both but in the end, they are just too loud for my situation of playing in the home.

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Re: 50w plexi vs 2204 (I’m confused)

Post by Explorerman » Mon Jan 14, 2019 8:27 pm

Awesome Danman, thanks for the detailed info that clears things for me. I have have an electrical trade so I’m all over the layouts and schematics, I might just need to make sure I order the right components as I don’t want to miss anything since I’m in Australia.

The loudness was also something I’m a little concerned about as this will mainly be for home use, with the 2204 pre inv master volume, how does this compare to a 50 with a post inv Mv mod compare a home volume levels?

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Re: 50w plexi vs 2204 (I’m confused)

Post by danman » Tue Jan 15, 2019 7:25 pm

Reading schematics will be a big help in the build so it's great that you are familiar with them. When plugged into the hi input, the 2204 runs the signal through one half of v1 and then routes it back through the lo input jack where it then travels to the other half of v1. This cascaded design increases the signal gain before heading to the v2 tube. The 1987 circuit uses one half of v1 for the bright channel and the other half for the normal channel and then mixes before v2. It has one less gain stage than the 2204 so it will be very loud before any overdrive sets in. Adding the ppimv does help to get overdriven tones because it allows you to wind up the preamp gain and then cuts the signal just before the power section. You do notice a tone change when turning the ppimv way down but many people are very happy with them. The 2204 is better at achieving overdrive at lower volumes because of the cascaded preamp and the pre phase inverter MV but they do sound kinda thin until you really open up the volume a bit. Both of mine were just too loud to play comfortably at home so I ended up building a 1974 clone (18 watter). It's still loud as hell but I can get dirty tones at a more reasonable volume.

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Re: 50w plexi vs 2204 (I’m confused)

Post by neikeel » Thu Jan 17, 2019 5:15 pm

Why not build both into the same amp with a push-pull switch to go between versions :wink:
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Re: 50w plexi vs 2204 (I’m confused)

Post by danman » Thu Jan 17, 2019 7:12 pm

Neileel has the right idea...build both circuits into one amp. I was trying to find your 2 n 1 layout Neikeel, but I can't remember where I saw it last.

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Re: 50w plexi vs 2204 (I’m confused)

Post by Explorerman » Thu Jan 17, 2019 11:59 pm

Awesome idea, what advantage would having the two circuits give me? I’m think I’m after the higher gain of the 2204, I’m also after nice cleans, are ther noticeable differences in cleans between the two?

How many contacts would I need on the pots to do the switching between the circuits?

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Re: 50w plexi vs 2204 (I’m confused)

Post by neikeel » Fri Jan 18, 2019 2:28 am

Image
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Re: 50w plexi vs 2204 (I’m confused)

Post by Explorerman » Sat Jan 19, 2019 9:08 am

Great diagram, cheers.

Also, is the Valvestorm 2204 instructions/kit a JMP 2204 or JCM800 2204? You’re right there are differing layouts and schematics available, some have the high and low channel inputs both go in through 68k resistors similar to the 50w lead circuit, then some have only one channel with the 68k and the other through a different configuration like on the Valvestorm layout.

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Re: 50w plexi vs 2204 (I’m confused)

Post by neikeel » Sat Jan 19, 2019 10:18 am

To my knowledge the instructions are the Metro ones for a 1987.

If you follow the instructions on the Valvestorm/Metro kit but follow the layout above (put the gain pot in the Channel 2 high jack hole) you will get easily switchable 2204/1987. It works very well. Ideally for maximum flexibility you should use a 100pF ceramic disc on the plexi volume pot.

Iif you go to SDMs website the picture above is in high resolution PDF and you can blow it right up. The diagram you see is for a 100w with dual voltage and PPIMV as well but you can ignore that.
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Re: 50w plexi vs 2204 (I’m confused)

Post by danman » Sat Jan 19, 2019 7:07 pm

Yep...the Valvestorm instructions are for a 1987 but building a 2204 would follow the exact same steps as the amps are very similar. The only real differences are a few items around the inputs and the wiring and components for v1. If you follow the layout closely, you should have no trouble building. Good solder joints and clean wiring lead dress are very important for building a quiet and dependable amp. Look at gut shots of original Marshalls to get an idea of how the wiring should be arranged (dressed) for a quiet amp and invest in a good soldering station with several size tips.

When looking at layouts online, you will find quite a few variations between them. Marshalls were constantly changing so it's good to know what year circuit you would like to build. You will also see differences in layouts because whoever designed the layout (and a kit) may have chosen a different grounding scheme than originals, different chassis and turret board size and standup or laydown style power transformer. Marshall also moved the filter cap locations over the years so different layouts will show them in different spots. For your first build, it's important to use the layout that comes with the kit you decide on and stick with it. Trying to deviate from it on the first build will make the amp very hard to troubleshoot if it does not work properly when you fire it up the first time.

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Re: 50w plexi vs 2204 (I’m confused)

Post by Explorerman » Sun Jan 20, 2019 4:50 pm

Sorry for all the questions, but just with the above switchable 2204 and 50w, I might build a stock circuit first then add something like this would be great. Which circuit would be the easiest to modify layer ie. do I start with a stock 2204 or 50w?

And will a PPIMV and effects loop (kit from Metroamp) work ok with a switchable circuit like this? Cheers

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Re: 50w plexi vs 2204 (I’m confused)

Post by danman » Sun Jan 20, 2019 6:54 pm

Either circuit will be easily changeable. Marshall actually used the same boards for the 1987 and 2204 circuit for many years as their are only a couple different components between them. Most of the difference lies in the wiring of the input jacks because the 2204 routes the hi channel back through the lo input after it goes through the first half of v1. It then further amplifies the signal in the second half of v1 before heading to v2. Neils layout looks like you would need the four hole chassis (1987) to follow his layout so I would probably build the 1987 first. The loop and ppimv will work fine with it also if you decide to add one later.

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