JCM900 2500 SL-X mods anyone?

[RickyLee]

Good stuff Mickey.

And for autumnleaves:

Do you want your SL-X to be permanently cleaner as in you have no need whatsoever for the high gain capabilities?

If so, besides the obvious "get another amp that does that" advice, you could always bypass V1. I am experimenting with this right now as in possibly making my SL-X a channel switcher.

Have you tried turning the Master Volume up to aprox. 7 and keeping both Gain controls at 9 O'Clock? It works really well like that on my SL-X - but I have modified mine quite a bit to accomplish that as well. Mine is real close to that Plexi tone set this way. The first Gain adds more bottom end while the second Gain adds more Dirt and crunch. That is one of the cool features of this amp in being able to get a wide range of tones. If you were to run a 5751 in V1 or both V1 & V2 and set it like I posted above, it might just work out for what you want without doing any modifications.

 

[mickeydg5]

Crap, V1 is a cathode follower too in that amp, only 12ax7 there.

I hate that schematic. I never payed attention to the wacky connection tab callouts at V1 before.

looking at it.

 

[RickyLee]

Thanks Jens and Rick,
Sorry about the mind melt. I somehow got confused with the JCM900 dual reverb for a moment. The options I am looking at range from JCM's Mod G to tube swaps to fiddling with cap values or upgrading caps. And there was also a suggestion about tweaking the negative feedback channel. Jens, I am definitely going to get some 5751s and perhaps a 12ay7 for experimentation. Meanwhile Rick, would you be so kind as to list by name a few of the caps that could be switched to Silver Micas or Sozos? The sound I would like to achieve is more bluesy, not more ACDC, so something like the boosted clean on the Rivera Venus here <http://www.youtube.com/watch?v=CJ_ebGB7DGU>.

I also found the attached set of mods which someone proposed. Would you care to comment on it?
Thanks,

OK. Here is the list of caps I swapped out:

All these coupling caps to Sozo Standards,

C3 (I lowered from .022uF to .0022uF)

C14 .022uF

C5 .010uF

C7 .022uF

C9, C10 I raised to .047uF for just a touch more low end

Tonestack caps C19, C20 .022uF


All these to Silver Mica 470pF - C13, C15, C18

I think I covered them all.

Crap, V1 is a cathode follower too in that amp, only 12ax7 there.

I hate that schematic.

looking at it.

Yeah, I ran a 5751 in V1 with no issues. I also changed R5 & R7 both to 100K for a nice change for the better.

 

[mickeydg5]

Ricky
I don not have a SL-X, so a question.
What is your voltage at V1 cathode out to preamp volume?
A 5751 is rated at 100V h-c. You can push the envelope, but how long will it last?

Also, I am just reading the schematic. The SL-X is basically a high gain distortion amp with switchable master volume channels, correct???

 

[autumnleaves]

Mick,
The Marshall moniker for it was "gain saturated" or something like that. I think your description would fit.
Actually I'm trouble shooting an amp I picked up cheap for my nephew. Probably should have known better, but it was not until later that I learned of its enthusiastic following among the metallurgical guitar guild. My nephew is more inclined toward classic rock/blues. But then I discovered the world of tweaks and mods, and being ignorant of electronics, I find this stuff more entertaining than TV. Since this one is signed by Jim himself, it's probably worth putting some effort into tapping into its warmer side.

Rick, much obliged to you for the proposed cap changes. I'll look into acquiring the appropriate materials if you feel that the amp's personality can be altered in the warm/blues/sweet direction. BTW, do you happen to know which model of Orange Drop (715p, 716p, 418) is the equivalent of the Sozos?

 

[autumnleaves]

Of course half of the problem may reside in the 2x12 G12T-75 cab connected to the SL-X. Probably only amplifies the harshness.

 

[mickeydg5]

Mick, thanks for the detailed explanations. Forgive my ignorance, but which valve is the PI in this amp? If it is V3, what is the function of V4? If it is V3, what does V4 do? I assume that V1 and V2 are preamp gain and tone. Is a U really feasible here, with such a low gain factor?
Have you combined any mods with tube alternation to get results along the lines of what I sketched above?
In a similar vein, what tube complement would you suggest for a JCM900 dual reverb (50W)? (V1=preamp, V2=reverb, V3=PI??)

V1 is primary gain for preamp (clean)
v2 is secondary gain for preamp
***these are in series (cascaded gain stages) for that added distortion
V3 feeds the tone stack
V4 is the PI

A 12AU7 is required for some amps, not mine and do not use them.

I can not read your sketch. Its too small.

Generally, I stick to the intended designed tube. I have on occassion played around with trying different u-factor tubes in different slots, but always end up with the original. But I like the high gain on tap.

 

[mickeydg5]

Mick,
"gain saturated"
some effort into tapping into its warmer side.

Yes, some tweaks are nice to change to a tone you desire.
Tell your nephew to just back off the second gain knob (sensitivty).
The G12T-75 speakers are good all around speakers. But there are mellower ones out there.

 

[autumnleaves]

Thanks Mick. I'll try uploading the schematic, zipped this time. What you see appears to be a thumbnail only.
A tech did try giving the SL-X a boost by maxxing the bias, but that only got us a blown fuse.

 

[RickyLee]

Ricky
I don not have a SL-X, so a question.
What is your voltage at V1 cathode out to preamp volume?
A 5751 is rated at 100V h-c. You can push the envelope, but how long will it last?

Also, I am just reading the schematic. The SL-X is basically a high gain distortion amp with switchable master volume channels, correct???

My pin 8 voltage is 188VDC. But I have changed both plate resistors on V1 as well. And have not ran a 5751 there since all these changes I have made.

And when you say heater to cathode voltage, do you subtract the heater voltage to Ground from the cathode voltage to Ground?

Yes, the SL-X is one channel with switching between two master volumes with the IC circuit.

Mick,
The Marshall moniker for it was "gain saturated" or something like that. I think your description would fit.
Actually I'm trouble shooting an amp I picked up cheap for my nephew. Probably should have known better, but it was not until later that I learned of its enthusiastic following among the metallurgical guitar guild. My nephew is more inclined toward classic rock/blues. But then I discovered the world of tweaks and mods, and being ignorant of electronics, I find this stuff more entertaining than TV. Since this one is signed by Jim himself, it's probably worth putting some effort into tapping into its warmer side.

Rick, much obliged to you for the proposed cap changes. I'll look into acquiring the appropriate materials if you feel that the amp's personality can be altered in the warm/blues/sweet direction. BTW, do you happen to know which model of Orange Drop (715p, 716p, 418) is the equivalent of the Sozos?

Signed by Mr. Marshall! That's cool for sure.

As for using Orange Drop caps? Tone is subjective for sure, but I would think the Orange Drops might be quite high end spikey in this amp?? I know that the Sozo Standard caps actually darkened my amp a bit which gave it a warmer feel. But the main capacitors that you should concentrate on replacing at first, would be the ones i mentioned that I swapped to Silver Mica. You can also go with Ceramic Disk which some people actually prefer over Silver Mica and is what is in the older Marshall and Fender amps in these circuit locations to begin with. That change alone might be good enough to knock a bit of the fizz down.

Of course half of the problem may reside in the 2x12 G12T-75 cab connected to the SL-X. Probably only amplifies the harshness.

I can say that the T-75's were not a good speaker for my SL-X when I first got the amp and it was all stock. My SL-X voicing was very mid scooped to begin with and those speakers are a bit mid scooped as well. But my SL-X would sound good with the T-75's now after all the work I have done to it.

 

[mickeydg5]

My pin 8 voltage is 188VDC. But I have changed both plate resistors on V1 as well. And have not ran a 5751 there since all these changes I have made.

And when you say heater to cathode voltage, do you subtract the heater voltage to Ground from the cathode voltage to Ground?

Yes, the SL-X is one channel with switching between two master volumes with the IC circuit.

Yes, that is 188 - 6.3 = 181.7. Very high for a 5751.

 

[mickeydg5]

Thanks Mick. I'll try uploading the schematic, zipped this time. What you see appears to be a thumbnail only.
A tech did try giving the SL-X a boost by maxxing the bias, but that only got us a blown fuse.

Maxing the bias, all these tweaks. Tell me exactly what are you trying to accomplish. Half of this stuff negates half of what the other stuff is doing, sort of speak.

I looked at the diagram and changes. To be honest, I personally would not fool with most of them. I may understand some of the stuff being changed if it were just an attempt to put better quality components in there.

Add: I guess that may come across bitchy, but I am just trying to get a handle on what you guys are aiming towards, the final product.

 

[autumnleaves]

Thanks Rick, if it is possible to tame the tone without having to find another cab that would be an alternative worth trying. I imagine most of those components can be sourced at Weber or Mouser. I really do appreciate the suggestions.

 

[RickyLee]

Yeah, to be honest about it, I do not think you need to worry about those stock coupling caps just yet. Now if you decide you want different values of coupling caps in specific places, then go ahead and go with something you may prefer. On the subject of cap brands such as the Sozo's, the more experienced guys have covered this over and over again about the "cork sniffing placebo effect" of these designer caps. That is something you need to let your ears be the judge of. In my case, the swap to Sozo's did give me a noticeable audio change making the amp darker. And I did change some values in a few spots. And to add to the cork sniffing subject, my 2555 Clone has Mallory signal and tone caps and I am loving them big time. LOL

As for this tech you mentioned that maxxed out the bias to give the amp "A Boost"? That dude can't be a serious tech? That's freakin' Nuts!

 

[RickyLee]

Yeah, to be honest about it, I do not think you need to worry about those stock coupling caps just yet. Now if you decide you want different values of coupling caps in specific places, then go ahead and go with something you may prefer. On the subject of cap brands such as the Sozo's, the more experienced guys have covered this over and over again about the "cork sniffing placebo effect" of these designer caps. That is something you need to let your ears be the judge of. In my case, the swap to Sozo's did give me a noticeable audio change making the amp darker. And I did change some values in a few spots. And to add to the cork sniffing subject, my 2555 Clone has Mallory signal and tone caps and I am loving them big time. LOL

As for this tech you mentioned that maxxed out the bias to give the amp "A Boost"? That dude can't be a serious tech? That's freakin' Nuts!

I forgot to mention that for the Orange Drop caps, I have read that the 225P series Polyester is good for adding some midrange to the signal. Anything that will enhance the mid voicing in this amp is a plus. I even like the cheap Mylar Film caps as well.

 

[autumnleaves]

Mick, Sorry if we seem to be getting off track. Here is how I described the program in posts 1 and 17. Perhaps the reference to a Tweed Bassman on steroids is also germane here. My nephew usually dimes the bass just to make get something near acceptable.
"Does anyone have any PROVEN mods/alterations to the above mentioned amp that can eliminate some of the excessive gain and thinness to allow more clean headroom and warmer/bluesier overdrive. Or even something closer to Plexi tones."
"Lowering the gain is a big part of the aim here. The SL-X has a "sort of" clean sound with gain rolled way back, but push the gain even a little bit and you start moving into deep dirt in a hurry. What I want to achieve is a sweeter clean and a less steep slope up to the dirt pile, i.e., with many nuances of distortion and tone so that the amp's pallet of sounds is expanded. Of course I am prepared to sacrifice the extreme metallicity for all this."

 

[autumnleaves]

Rick, The bias trick does seem a bit oddball, but it did emit some interesting sounds before going quiet. Luckily no tubes were fried, though I'm not sure what kind of health they are in now. I've not heard of that series of Orange Drop. Will look around, but I think your Sozo suggestion will be easiest to implement.
Here is what the kid sounded like 3 years ago. I don't know what he used to record though. <http://www.youtube.com/watch?v=6MxLT7bdvVw>

 

[mickeydg5]

Man, you guys keep going back and forth between threads, it is making my head spin.

 

[RickyLee]

I know one thing for sure and this is something I have considered being I already have way too many amps capable of Brutal Filthy Dirt.

Being there's three 12AX7's (not counting the phase inverter) this amp can be altered in quite a few very cool ways. Being that you are looking for a more open clean and bluesy overdrive, you can reduce the amount of gain stages and put a few of the remaining ones in parallel - as in use both triodes of V1 in parallel for the first stage and same thing with V2. Then on to the '59 Bassman/Marshall V3 and V4 driver config. It might not be too tough with this PCB as it's quite spacious compared to some of the newer amps.

To go even further I have thought of this config, with V1 in parallel with V2 and each having a Gain control to blend. I guess that would be more towards a Plexi, but with a bit more Grunt.

Just an idea . . .

 

[mickeydg5]

Nice idea. Lots of work. It is not a SL-X any longer.

Besides, his nephew may change his likings as he gets older. Believe it or not, I did not like Metallica or Megadeth or Sevendust or BLS or Pantera or Chevelle or Disturbed or Godsmack up until about 10 years ago when I decided to try stuff like that. Now I can not do without stuff like that. See where I am going.