Marshall JCM diode clipping bypass mod

[bluesbreaker18]

Marshall JCM 602 combo.

I've heard some interesting things about bypassing the diode clipping circuit. Supposedly the OD channel becomes less harsh and exhibits better dynamic response, at the expense of some perceived gain. I really love the clean channel on this amp but the OD channel is difficult to use with pedals -- I suspect this mod would help.

Can someone please comment on the portion of the schematic I believe to be relevant for this mod? I think installing a simple switch at the annotated place would effectively bypass the diode clipping circuit. Is there anything else I need to consider?

[Enzo]

I would move your break point down a little so R104 stays in the circuit.

If you want to see how it sounds, just unsolder one end of C105 and lift it from the board. That will let you listem. If that doesn what you want, then wire it up, but if not, then just solder the cap back to the board.

I am not sure those diodes are what you think, but it is a simple experiment.

[jstbrowsin]

Just a little note here if there is any interest in this still.
But that schem is for a JTM not a JCM, forgive me if I'm barking up the wrong tree here but they are two really different amps right???

I have a JCM TSL602 and that part of the schem doesn't match up with anything on the schem. In-fact from what I can see C105 is for the switching network in a JCM TSL 602. But I'm no expert far from it really

Hmmm then again. . . maybe it was a typo when he wrote JCM . . .but I never heard of a JTM 602??????????????


Ahhh well just a heads up anyhow. . . have a nice day y'all

[Enzo]

Well, good point. One must make sure the schematic covers the amp in question.

The JCM602 uses the JTM600 schematics.

[jstbrowsin]

Ooop's I maybe should have googled JCM602 before I posted
I thought bluesbreaker meant the JCM 2000 TSL602 I had never heard of a JCM602 before now. . . damn I feel like a real noob now

Oh well sorry to hi-jack the thread but is this diode bypass a possiblity in a TSL602? If so will it give the OD a smoother tone?

I can supply schematic if necessary?

just curious really. . .

[Enzo]

Oh, this thread has been dead for 6 months so I don't think there is much to hijack. Welcome to the place.

Offhand I'd have to say you couldn;t do it to the TSL602. mainl;y because as far as I can tell the TSL602 doesn;t have any clipping diodes to remove.

You see any in there?

[jstbrowsin]

Thanks for the welcome Enzo.

Wow man I did it again doh!!!

You are correct there is no diode clipping in a TSL60 (I call mine a 60 now because I put it in a head shell) but there sure as hell is a huge amount of diodes in this amp. . . whoa

Maybe you can help me with this little tidbit if you don't mind Enzo err or anybody. I spotted this bit of advice re; reducing fizz in a TSL100 and was wondering how I would go about applying the same mod to my amp.

quote: Putting a 390pF cap across R1 on V3 on the 'Circuit Diagram' TSL100 schematic page, and a 47pF cap across VR2 on the "Overdrive Channel/Front Panel PCB' page should work well. Both these caps work as a team, greatly reducing fizz without killing the 'good' treble you want to keep, especially for live work.

This is a 12dB/octave low-pass filter starting at roughly 7.5kHz; using different cap values can slide this frequency up or down if needed, but the 7:1 cap proportions should remain constant for 12dB/octave filter performance.

I believe this bit of advice was given by Ray Ivers

I was wondering if you could tell me how to apply the same mods to my TSL60 please. And again I can send you the schematics if you like.

TIA for any help. . .

and whoa I just hijacked a thread twice hehe maybe I should flag myself

[Enzo]

Hold the schematics side by side and note the similarities. R1 on the TSL100 looks to become R74 on the 602. And just looking, to me it appears VR2 on the TSL100 becomes VR10 on the 602.

Tacking a small cap across any of these parts is simple to try, and easily removed if not delighted.

As to the control, if tacking a 47pf cap across it will do something you like, then just do it to any volume control involved.

[jstbrowsin]

Cheers Enzo,

I'll give it a go and post back my results.

One final question if I may. A while ago I saw on the SLO forum (I think thats right as it was some time ago)
that someone advised changing the bridging diodes D9,D10,D12,D13 all 1N4007 to faster diodes HER 108's for smoother highs. Does this have any validity? I have some UF4007's here which might do the trick am I on the right track here?

Err ok now that's really 3 questions now isn't it

yeah, nah oh well. . .

Ok thanks again Enzo, back into the pit for me

[Enzo]

Oh I don;t know, I don;t get into all the little tweaky things.

[Steve Conner]

The answer is a resounding "Maybe"

From my own experiments (I've used diode clippers in other non-audio circuits) high-current, high-voltage diodes clip quite softly. So I'd expect a 1N4007 to sound less harsh than a 1N4148, which is a tiny high-speed signal diode.

The softest clipping action I ever saw was with a high voltage diode out of an old microwave oven, which I believe was rated at 12,000 volts.

I've never investigated the difference between ultrafast and regular diodes in audio clippers.

If the diodes aren't in an audio clipper circuit, but in the power supply, you're not going to hear a difference when you change them.

[jstbrowsin]

Hi guys,

Well I decided to give it a go so I swapped the standard 1N4007's in my TSL60 for UF4007's (UF means ultra fast) and funnily enough I can hear the difference it's like night and day. . . and before you ask sorry I don't have the facility yet to record clips for you believe me though, I to didn't think I would notice as greater deal of difference but it has smoothed the highs really nicely and made the tone controls more usable. : thumbsup:

The other added bonus is previously I could only get reasonably fat tones by cranking my amp but now I can get that tone at both loud and soft volumes.
its makes me wonder though how much more of a difference the "microwave diodes" or "F.R.E.D's" will make now?????

My next thing I would like to do is to change out the different cap types to Orange Drops, Mallory 150 and silver micas.
But its gonna cost me a arm and a leg to get them to me here in NZ because I would have to get them from possibly 3 different oversea's sources. So I trolled the sites and I stumbled across Weber capacitors here: https://taweber.powweb.com/store/capord.htm
It looks like they make their own caps based on the OD's, Mallorys and the silver micas, but has anybody here used these caps or can anyone vouch for their validity please?

Ok ciao for now

[Steve Conner]

FRED is a manufacturer's trade name for a fast recovery diode. (It stands for Fast Recovery Epitaxial Diode.) So the difference between a FRED and a UF4007 is roughly the difference between "Rice Krispies" and "Puffed rice breakfast cereal".

My personal opinion is that there are no audible differences between capacitor brands. I would never change capacitors unless they were actually faulty, or old enough that I thought they might be unreliable. It's very easy to convince yourself that changing out parts makes a difference, especially if you just spent a lot of time and money sourcing the parts.

[jstbrowsin]

Thanks for clearing that up for me Steve as I had completely forgotten what Fred's meant again. . .

AFA cap changes are concerned I won't be paying an arm and a leg anymore so I think I soldier on ahead with my original plan.

I have seen both sides of the fence about cap types and there differences and there may be some truth to both sides of the argument, however the one common factor that everyone agrees with is the difference in tonality when using tighter tolerances 5% - 10% as apposed to the more common 20%+. So on that basis (and the fact that the same guy who recommended the diode changes also told me about the cap type changes) I feel the result should be a positive one. Again I will post back the results.

[jstbrowsin]

I forgot to ask Steven, your analogy
"the difference between a FRED and a UF4007 is roughly the difference between "Rice Krispies" and "Puffed rice breakfast cereal"."
Are you saying here that there is a slight, huge or no difference at all?

Why I ask is because the "Rice Bubbles" we have here are same as your "Rice Krispies" small grain puffed rice but the "Puffed rice breakfast cereal" here in NZ I believe has a sugar coating on it. So by that analogy I take it that the UF4007's are nice like the Rice Krispies but, the FRED's are in essence the same but with a bit more detail (sweetness) added to them?????

Just curious really