Channel Switching JCM800 Design? + BIAS pot question

[ViperDoc]

Three guys have come in on 18watt.com in the last couple of weeks, all with untamable squealing on 18W TMB kits. The layout and the drawing is a complete mess. These clowns shouldn't be in business. I bought cabinets from them once. The tolex was peeling in under a month. I bought a 5F8A chassis with poor finish work and sent that back. Lesson learned. Now ViperDoc is getting the wrong parts and a layout full of errors. STEER CLEAR

Thanks for the info. Where would a green but motivated amp guy go to from here?

Dude, you are off and running and have a great amp build under your belt. You don't need to be buying kits from these tools. They are in it for the money, cutting corners wherever possible. You can source all of the parts, select your own transformers (Heyboer, ClassicTone, Mercury), draw up your layout, and run everything by your friends here. Friends don't let friends get taken. Life is too short for bad tone and needless troubleshooting.

I don't doubt that for a second. What is immediately obvious in a forum like this is a depth of knowledge and experience held by most members. Obviously, that comes with time at the bench. Being in the "paint-by-numbers" gear at the moment, it's a bit daunting to know how to proceed. Schematics for me make some sense, but they don't show me how to build up the amp like a layout does. Being in a bit of a trial by fire, then, I wonder who has reliable 800/2204 layouts out in the ether?

[Colossal]

I don't doubt that for a second. What is immediately obvious in a forum like this is a depth of knowledge and experience held by most members. Obviously, that comes with time at the bench. Being in the "paint-by-numbers" gear at the moment, it's a bit daunting to know how to proceed. Schematics for me make some sense, but they don't show me how to build up the amp like a layout does. Being in a bit of a trial by fire, then, I wonder who has reliable 800/2204 layouts out in the ether?

Don't sweat it. Acquiring amp knowledge is fun and part of the hobby/obsession. It takes time, but you have nothing but time. Learning to read and understand a schematic is a great skill. Drink from the well, my friend. This place is great. We all start at the beginning and everybody puts their pants on one leg at a time.

I tried to attach the MetroAmp forum step-by-step JCM800 build guide but it is too large. Check out Valvestorm for parts. Those guys are serious about their Marshalls and every detail matters to them; no corner cutting. Shoot me your email in a PM and I'll send you a bunch of stuff to large to attach here. Between me and the rest of the guys here, we've got every layout and schematic known to man.

The only problem you have, like the rest of us, is how to work less, and build amps and jam more.

[ViperDoc]

I don't doubt that for a second. What is immediately obvious in a forum like this is a depth of knowledge and experience held by most members. Obviously, that comes with time at the bench. Being in the "paint-by-numbers" gear at the moment, it's a bit daunting to know how to proceed. Schematics for me make some sense, but they don't show me how to build up the amp like a layout does. Being in a bit of a trial by fire, then, I wonder who has reliable 800/2204 layouts out in the ether?

Don't sweat it. Acquiring amp knowledge is fun and part of the hobby/obsession. It takes time, but you have nothing but time. Learning to read and understand a schematic is a great skill. Drink from the well, my friend. This place is great. We all start at the beginning and everybody puts their pants on one leg at a time.

I tried to attach the MetroAmp forum step-by-step JCM800 build guide but it is too large. Check out Valvestorm for parts. Those guys are serious about their Marshalls and every detail matters to them; no corner cutting. Shoot me your email in a PM and I'll send you a bunch of stuff to large to attach here. Between me and the rest of the guys here, we've got every layout and schematic known to man.

The only problem you have, like the rest of us, is how to work less, and build amps and jam more.

Yeah, buddy. Just sent you a PM.

I noticed something on the mojo vs. the El34world schematic that pdf64 sent me the link to. Not sure if this makes a difference. The mojo layout has the V3 plate wires hitting the 47pF cap and the 82K/100K resistors and the 22 nF caps simultaneously, whereas on the Marshall 2204 schematic, the C18 47 pF cap is wired AFTER the C14 and C16 22 nF caps. This could very well make no difference, I still need to buff up on my equations.

[sluckey]

It makes no difference. It's more common to see that 47pF directly across the PI plates just like mojo shows. Some people even mount the cap directly on the tube socket. The original plexi amp schematics show the cap just like mojo.

[ViperDoc]

It makes no difference. It's more common to see that 47pF directly across the PI plates just like mojo shows. Some people even mount the cap directly on the tube socket. The original plexi amp schematics show the cap just like mojo.

Right on, Sluckey! Thanks, man!

[Colossal]

It makes no difference. It's more common to see that 47pF directly across the PI plates just like mojo shows. Some people even mount the cap directly on the tube socket. The original plexi amp schematics show the cap just like mojo.

Right on, Sluckey! Thanks, man!

They got that part of the layout right

[ViperDoc]

I see another error on the mojo layout. There is a yellow wire labeled "S" that connects a 10K resistor on the board back to the screen node cap can. DO NOT INSTALL THIS WIRE! Doing so will short out that 10K dropping resistor. I can't believe that mojo has released such crappy docs.

This from Mojo regarding the S wire: “No, it doesn't create a short. It's your screen supply. It has its own dedicated filtering that also supplies plate voltage for the preamp. The filtering runs in parallel, not series.”

Thoughts?

[sluckey]

Hahaha! Bullshit!

Edit... Your pictures indicate you have wired the "S" points together. Measure the resistance across that 10K/5W resistor. What do you get?

[ViperDoc]

Hahaha! Bullshit!

Edit... Your pictures indicate you have wired the "S" points together. Measure the resistance across that 10K/5W resistor. What do you get?

I know, I'd get "0", that's clear. I took it back to Mojo because it didn't make sense at all. I got this:

"Hi (ViperDoc), after looking over this, you are correct. There should not be a jumper on the cap for the S connection. Thanks for pointing that out."

I really appreciate your attention on that. The wire shall be removed!

[ViperDoc]

I see another error on the mojo layout. There is a yellow wire labeled "S" that connects a 10K resistor on the board back to the screen node cap can. DO NOT INSTALL THIS WIRE! Doing so will short out that 10K dropping resistor. I can't believe that mojo has released such crappy docs.

Three guys have come in on 18watt.com in the last couple of weeks, all with untamable squealing on 18W TMB kits. The layout and the drawing is a complete mess. These clowns shouldn't be in business. I bought cabinets from them once. The tolex was peeling in under a month. I bought a 5F8A chassis with poor finish work and sent that back. Lesson learned. Now ViperDoc is getting the wrong parts and a layout full of errors. STEER CLEAR

I just plugged my Mojo 18 Watt TMB combo in to test it out, it has super high-pitched squealing on the 800 channel with the master up and, to boot, it smells like something is overheating through the screen. Those tubes are bright, man! What a drag. The low gain channel sounds great, but I can't turn it up to test it. Kids in bed. What to do?

[martin manning]

Looking at the Mojo "800" layout a little more closely I see they have deviated from the JCM 2204 power supply arrangement, putting 100u on the screen, 50u on the PI and letting V2 and V1 share the remaining 50u (saving one 10k resistor). That is the way it is shown on their schematic too. In the original, it's 50u on each: screen, PI, V2 and V1. A marked up layout to get back to the original is below.

[ViperDoc]

Looking at the Mojo "800" layout a little more closely I see they have deviated from the JCM 2204 power supply arrangement, putting 100u on the screen, 50u on the PI and letting V2 and V1 share the remaining 50u (saving one 10k resistor). That is the way it is shown on their schematic too. In the original, it's 50u on each: screen, PI, V2 and V1. A marked up layout to get back to the original is below.

Much appreciated, Martin. Looks like I have some work to do!

Just out of curiosity, the way the output jacks are wired looks unlike other arrangements I've seen. I usually see the Impedence Selector out put connected to the outer lugs all the way though both jacks, and the OT ground wire through the lugs closer to the chassis, like rails through both sets. Do the output jacks look OK to you as wired?

I don't want to b!tch, but are these differences intending to deviate from the original design for legal reasons, or am I dealing with ineptitude? I have no idea, I'm just a noob!

[sluckey]

Looking at the Mojo "800" layout a little more closely I see they have deviated from the JCM 2204 power supply arrangement, putting 100u on the screen, 50u on the PI and letting V2 and V1 share the remaining 50u (saving one 10k resistor). That is the way it is shown on their schematic too. In the original, it's 50u on each: screen, PI, V2 and V1. A marked up layout to get back to the original is below.

Hmm. I think it depends on which schematic you look at. This looks like the one mojo may have been using. The screens are 100µF and there are only two 10K dropping resistors. This is the one that I usually refer to...

https://el34world.com/charts/Schematics ... w_2204.pdf

EDIT... I just went searching for pics of the 2204 and I did not find any that had the screens running 100µF (parallel can). Guess I'll start using the other schematic for reference. Sorry for the side track.

[ViperDoc]

Looking at the Mojo "800" layout a little more closely I see they have deviated from the JCM 2204 power supply arrangement, putting 100u on the screen, 50u on the PI and letting V2 and V1 share the remaining 50u (saving one 10k resistor). That is the way it is shown on their schematic too. In the original, it's 50u on each: screen, PI, V2 and V1. A marked up layout to get back to the original is below.

Hi Martin,

Which schematic did you refer to as the original? The el34world 2204 schematic that's been posted a few times regarding this circuit shows the preamp 50 + 50 uF can cap connected across its + terminals with the 10K resistor, just like you added back in your modded mojo diagram. The 2nd and third 50 + 50 uF can caps on the el34world 2204 diagram have both of their + terminal jumped, or so it appears to me--the one after the R29 10K dropping (?) resistor and then the one connected to the HT fuse. If we leave the S wire intact, then it would be as if each half of the 50/50 uF can cap would be connected to either side of the dropping resistor, right? Thanks for your help.

[sluckey]

If we leave the S wire intact, then it would be as if each half of the 50/50 uF can cap would be connected to either side of the dropping resistor, right?

Argh!