Hi Tassie,
          All good, like you I've not ever been inside a JC amp and just looking at Schematics tiss easy to miss things. I had to scratch my head for a bit to work it out.

Thanks for the info on Enzo, yeah a great mind who was always helping us lesser minds learn from his wealth of knowledge. He will be missed.
Phil.

Go here for real in depth insight into guitar amp tones.
The end will likely make you wonder why no one ever researched this before.
I've been quietly babbling on about this for years but this chap pretty much puts it all together well.

I'm not a marshall fan mainly because they only have one trick and the tone controls are pathetic. (though I'm sure many will argue) lol

https://www.youtube.com/watch?v=wcBEOcPtlYk


I use an old Laney Keyboard amp which is super clean.
Everything else is done with my pedal circuits, half of those are my own designs

I can produce Hank clean all the way to ZZ top at the press of a button or 2.
If you have built amp/pedal circuits then it will not be hard to work out how it's done when you see the end of the video 

The game of guitar tone is in understanding the frequency range the guitar needs. most gear today has insanely wide bandwidth and it totally destroys your tone (Especially for hard driven OD tones).
You have to dramatically limit the bandwidth for rock guitar sounds otherwise it's just one clangorous mush of eFd up noise.
which is why I steer clear of all in one pedal like Boss GT's as they have way too much hi freq crud and tonal adjustment is limited.
Go build some pedals and save a fortune and your back ,,,winky Phil

Hey Guzz,
I assume when you say *Terrible Freq response and then Boss GT8 in the next sentence* then you are maybe chasing the old sweet magic of limited bandwidth valve gear.
This can be done with SS gear but you have to in a different manner than you do with Valves.
Phil.

Also the Fet pair phase splitter Q1 and Q20 form a limiter circuit so inserting after that may (or may not) cause strange things to happen.

Teemu mentions this limiter but I'm still not sure how it works.
A note,
Sadly Teemu along with Enzo and J M Fhaey don't post much these days but they have a wealth of knowledge if you go searching for older posts. plus a few others, whose names I can't recall right now.
These wonderful members have helped me many times when I've been stuck.
Phil.

Hi Tassie,
Note;
Vr4 And VR10 is the mix point for the 2 power stages, trying to insert a loop at C26/C126 means you will only be able to use one power section. (I'm assuming a mono FX loop is being asked for?)
I think you will find the 84 schematic is a single channel circuit which makes your idea possible but this one is a different setup.
 I assume by the schematic the in/out wires are easy to lift on the unit posted so less chance of stuff up.

For a full run down on all the JC models try this page;
https://www.ssguitar.com/index.php?topic=2243.0
Thanks to Teemu
Phil.

Yes output of VR10, cut before R19 and R121.
Should work and easy to reverse if it does not work.
Phil.

Quote from: guzzis3 on January 18, 2023, 09:05:19 PMHi,

If you had to order extra PCBs that you want to sell I'd be very happy to buy them from you. I am in Brisbane,

This is my first post to this forum. I was pointed to this thread by a person on the marshall forum. I've been building electronic stuff since I was a kid but while I have built quite a few valve guitar amps I've not built a SS one.

 Have you determined if the 5005 etc develop their distortion in the preamp, power section or both ?

I realise you haven't finished your design, and there may be problems, but starting even with a flawed PCB is probably easier than laying it out on tag strips or breadboard.

Thank you for reading.

Hello Guzz, just note on SS rigs,
Most do not respond to power stage distortion, some rare ones try to do that but likely not as convincing as a full Valve powerstage.

With SS Amps all the dirt, grit, compression and like attributes of Valve amps needs to be done in the preamp stages.

My main rig is just a basic SS amp and all the magic is done with my pedal board. Most of the pedals are my own design, some of my schematics are posted on this site.

40 plus years back I wanted to build an amp that did it all in one rig.
Took me many years to realize that I was just building land fill.
I now  have draws full of failed ideas (other wise called spare parts )
I have built both Glass and Sand designs and although I like Valves there many benefits to SS designs.

If you are lucky enough to find an amplifier (Glass or Sand) that works for you then lucky you, sadly more often than not you'll spend a fortune trying to find it.

BTW I'm up the road in Morayfield if you want to make contact I'm happy to help with my years of researching. Just pm me for my phone number.
Phil.

Looks like Ukraine shot it down
Phil.

Hi TassieV,
           I guess it depends on what the High/Low is referring to?
The size of the source signal or the sensitivity of the input, take your pick

In the case of the Lab circuit
The LOW input would be much louder than the HIGH input for a given voltage.

But as the Lab uses *Inverting* inputs then yes the Hi input is HiZ  but with much less gain.

BTW The Lab input design is not an ideal setup, there are better ways.
Phil.

Hi Joe, Yep can be confusing.
My understanding, it's about the Z (impedance) of the input.
A low Z input on guitar amps is for big signals or even line input.
While a Hi-Z input is very sensitive so as to pickup very small signals.
In the early days guitar PU's were low output and needed a very sensitive input to amplify well. (Hence the 1Meg grid resistor on the old Valve gear)
These days even the cheapest guitars have much bigger outputs than the 40's 50's era so the preamps will redline much earlier on the dial using the pu's around nowadays.

The old Fender circuits used 68k resistors on the Grids of V1 for LOW input. while the HIGH input switched the input Z to 1Meg. far more sensitive.

With super hot pickups you are likely better served by limiting the input Z to about 220k.
I've noticed this with some of the hotrod boutique Amps.
A Carvin Legacy is an Example of this where in put grid R is 220k.
With multi stage preamps this keeps thing from going stupid and also helps keep circuit noise lower.
Phil.

Agree, Mr Thud
If it has master volume then likely it has a gain pot as well
Try turning the master to full then turn down the gain pot.
Phil.

And after that I'd be looking into the limiter wrapped around the Power amp section, as it maybe faulty.
The master volume switch might be connected to that as well ,, hard to tell.
Also there is a feedback setup in the power amp but not connected to the output?? maybe a drawing omission.

Limiter is Q1&2, &Q11
Feedback is at R15

Phil

Thanks G1,
          Yep your eyes are better than mine.
Hopefully *Victor* might like to take some close up Clear Pictures of the Inside of the Chassis might help us to confirm just how to help.
Phil.

Victor,Regards the Loop;
Signal splits at pin 7 of IC4B. (see TP7)

Dry goes through R73 & R77 across to input of IC4A.
Meantime signal also goes through R84 to input of DSP (the Send). The DSP FX is then sent back via Return line through R101 where it is mixed with the dry signal.

The CD or aux input is also mixed at this point.

Note Q24 is the mute circuit,, you will find label D is linked right at the input socket.
So when you remove the guitar lead it kills all signal.

As you seem to imply that you don't really need the DSP then it's a fair bet some of the channel selections may work without the DSP.
Which is why I mentioned joining R to T.
It may at least bypass some of the DSP control over the analog parts of the circuit.
Of course I'm just guessing as there is not enough info on the Schematics to work out how the DSP interconnects.
Be aware that a lot of those pin header connectors are very cheap and often don't like being plugged in and out many times before they no longer connect. A gentle push around the wires and plugs can often find a faulty connector and amp comes to life again.  they are not really suited for guitar amps which tend to get a lot of abuse due to speaker vibration and road handling.
Phil.

Hum,,My thought, What about Bridging R and T?
Find them at Test point 8 after IC4A also at J21

The DSP seems somehow interconnected with the channel switching. Hard to tell?
Though it seems to me that Modes A,B & C are direct to circuit, while Solo OD, B2 & A-B modes are activated via the DSP chip.
Maybe Post some pictures might give us some better plans.
Phil