JCM 800 4104 - Advice.

[Jhexp75]

Hey guys just wondering if you can tell me if this is stock or anything has been messed with? It has a half-power switch but nothing else done according to the owner. Can that half-power switch be removed with no issues?

Thanks.

 

[dawnofdreamx97]

Looks mostly stock , seems they added a cap on the presence pot and that half power mod , annoying they drilled a hole they could have used the speaker jack easily ....

 

[Jhexp75]

Looks mostly stock , seems they added a cap on the presence pot and that half power mod , annoying they drilled a hole they could have used the speaker jack easily ....

Yeah, silly that they drilled into it. What would the cap on the presence pot be for?

 

[glpg80]

Honestly with a chassis plug and the dodgy half power BS removed, it really hasn’t affected value at all. It looks really clean.

A smaller presence cap shifts the shunt RC cutoff frequency higher, meaning the highs it targets are much higher. It’s basically making a low pass filter more like a band pass filter. But since this is in the negative feedback path, the majority of frequencies are mellowed out at the phase inverter. At the cutoff frequency or higher, signals are shunted to ground, eliminating them from negative feedback, not cancelling them at the phase inverter. Meaning they’re more “present” at the output.

It’s basically a way of making a Marshall less harsh by neutering the presence pot. Realistically you could just increase the negative feedback to kill more highs with a stock capacitor, but you also kill gain in the process. It’s all a balance.

Presence pot not being stock is easy to chance back and not as severe hit on value as that half baked switch design.

 

[RedPlated]

What is going on with the phase inverter plate resistors? They’ve been replaced by a variable trimmer. That is the weirdest thing I’ve ever seen. Not sure what is going on there. It’s like someone is trying to balance both sides of the tube. Stock uses 82k on one side and 100k on the other for that purpose.

There’s a capacitor on the mid pot. Not typical.

There is some strange stuff going on in there.

 

[311boogieman]

Honestly with a chassis plug and the dodgy half power BS removed, it really hasn’t affected value at all. It looks really clean.

A smaller presence cap shifts the shunt RC cutoff frequency higher, meaning the highs it targets are much higher. It’s basically making a low pass filter more like a band pass filter. But since this is in the negative feedback path, the majority of frequencies are mellowed out at the phase inverter. At the cutoff frequency or higher, signals are shunted to ground, eliminating them from negative feedback, not cancelling them at the phase inverter. Meaning they’re more “present” at the output.

It’s basically a way of making a Marshall less harsh by neutering the presence pot. Realistically you could just increase the negative feedback to kill more highs with a stock capacitor, but you also kill gain in the process. It’s all a balance.

Presence pot not being stock is easy to chance back and not as severe hit on value as that half baked switch design.

^ what he said

But seriously, what does it matter? Are you strictly looking to flip it? Play the damn thing and enjoy it.

Actually, the way glpg80 explained it I think I would enjoy it as is more. I'm not a combo guy though. I'd do a sexy head conversion and then a 212 cabinet. Dumb question. How do you do half power with 2 - EL34s?

 

[glpg80]

What is going on with the phase inverter plate resistors? They’ve been replaced by a variable trimmer. That is the weirdest thing I’ve ever seen. Not sure what is going on there. It’s like someone is trying to balance both sides of the tube. Stock uses 82k on one side and 100k on the other for that purpose.

There’s a capacitor on the mid pot. Not typical.

There is some strange stuff going on in there.

I thought it was part of the bias circuit just quick glancing but you’re right. Maybe to cancel noise? Why would anyone play with the plate resistors on the PI, they are balanced as is and tuned to even account for gain differences between triodes in a single envelope. Nothing there to mess with.

 

[RedPlated]

I thought it was part of the bias circuit just quick glancing but you’re right. Maybe to cancel noise? Why would anyone play with the plate resistors on the PI, they are balanced as is and tuned to even account for gain differences between triodes in a single envelope. Nothing there to mess with.

Yeah I’d scrap that trimmer, put the 82k/ 100k back and add a 47-68pf fizz cap. Don’t see one in there.

And….I just noticed, .022uf PI output cap on one side, .1uf on the other side.

 

[glpg80]

I’m trying to think why the cap is on the mid pot like that and what it would do. Wouldn’t that act like a permanent mid boost turning the mid pot into a half baked volume knob?

 

[RedPlated]

I’m trying to think why the cap is on the mid pot like that and what it would do. Wouldn’t that act like a permanent mid boost turning the mid pot into a half baked volume knob?

It’s taking frequencies from the tone stack to ground, bypassing the resistance of the pot. Which frequencies I don’t know since we don’t know the value.

Actually an interesting concept. Maybe it’s a mid cut mod.

 

[Jhexp75]

^ what he said

But seriously, what does it matter? Are you strictly looking to flip it? Play the damn thing and enjoy it.

Actually, the way glpg80 explained it I think I would enjoy it as is more. I'm not a combo guy though. I'd do a sexy head conversion and then a 212 cabinet. Dumb question. How do you do half power with 2 - EL34s?

I'll be keeping it, I just have no idea when it comes to the electrical stuff amps lol. I dunno, all this stuff is starting to put me off.

 

[glpg80]

It’s taking frequencies from the tone stack to ground, bypassing the resistance of the pot. Which frequencies I don’t know since we don’t know the value.

Actually an interesting concept. Maybe it’s a mid cut mod.

Upon closer look it does look like a mid cut though which as I eluded to earlier, depending on the value, it could be making the mid pot act more like a shitty volume knob of the cap is of relatively large value. I see the piece of the cap going to ground but no ground solder. Odd that you’d want to scoop a Marshall further than stock, they are already scooped with a bandaxall tonestack as is

 

[cardinal]

I’m trying to think why the cap is on the mid pot like that and what it would do. Wouldn’t that act like a permanent mid boost turning the mid pot into a half baked volume knob?

Cap on the mid pot is stock on these JCM combos. Supposedly let's more low mids into the signal to fill out the sound. I've had one of these and just took it out and rewired the mid pot to be like the heads, but it sounded fine either way.

 

[dawnofdreamx97]

What is going on with the phase inverter plate resistors? They’ve been replaced by a variable trimmer. That is the weirdest thing I’ve ever seen. Not sure what is going on there. It’s like someone is trying to balance both sides of the tube. Stock uses 82k on one side and 100k on the other for that purpose.

There’s a capacitor on the mid pot. Not typical.

There is some strange stuff going on in there.

totally missed that . You are correct! the cap on the mid pot is common on the combos, i've had a few just like that

 

[RedPlated]

That’s a good bit of info on the mid cap. I can honestly say I’ve never worked on a combo version. Just never come across my path.

 

[Briman24]

I had the same amp! Just traded it 2 weeks ago. It had many mods. KT66 tubes, clean chanel, effects loop with tube added and loop volume. Here's pics of it and guts for anyone curious.

 

[RedPlated]

I love seeing all these photos. It's so interesting to me to see the different mods and way the mods are done on these old Marshalls.

Your "Damping" control is an adjustable negative feedback control. I had to look at the guts to figure out what damping was.

 

[Briman24]

I love seeing all these photos. It's so interesting to me to see the different mods and way the mods are done on these old Marshalls.

Your "Damping" control is an adjustable negative feedback control. I had to look at the guts to figure out what damping was.

I know very little to nothing about amp mods. I basically change my tubes and bias them with multimeter after finding plate voltage etc.
Rick Onslow modded my amp. The previous owner had it all done. When i got the amp, i went to put all brand new tung sol tubes in it but couldn't bring the bias down far enough so took it to my tech and he changed bias resistors. Don't remember if they or it was faulty or just wrong spec. Amp sounded great. I never had the amp with el34's in it so i can't compare it to the Kt66's. I imagine the Kt66's are similar in mids but not as aggressive.
The guy i traded it to is super happy with it and said he has been searching for that tone for ever....

 

[Briman24]

I love seeing all these photos. It's so interesting to me to see the different mods and way the mods are done on these old Marshalls.

Your "Damping" control is an adjustable negative feedback control. I had to look at the guts to figure out what damping was.

Here's some more pics i snapped one day when i changed the tubes.
The 4104 is a great amp but very heavy for a combo! The clean chanel that Rick added with his mod was amazing.

 

[Jhexp75]

Thanks for the info guys, sounds like it's pretty much a stock 800. I'm going to grab this amp.