JCM900 2500 SL-X mods anyone?

[autumnleaves]

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.

I believe modification to the negative feedback loop has been tried and found to be effective in boosting and tightening the bass. Any other mods found to actually work?

 

[Blacque Jacque]

I have a 5751 in V2 of mine, drops the gain nicely & reduces the fizz quite a bit.

 

[jensbrix]

I have a bunch of mods that worked out well for that amp, hear the difference:
pre: SL-X stock by jensbrix on SoundCloud - Create, record and share your sounds for free
post: SL-X final by jensbrix on SoundCloud - Create, record and share your sounds for free

 

[autumnleaves]

The modded version definitely has a sweeter ring to it. I wonder if this is something that can be done on a budget.
I plan to experiment with tubes too. Has anyone tried 12AY7/12AT7 for the gain, reverb or phase inverter position?

Thanks,

 

[RickyLee]

The modded version definitely has a sweeter ring to it. I wonder if this is something that can be done on a budget.
I plan to experiment with tubes too. Has anyone tried 12AY7/12AT7 for the gain, reverb or phase inverter position?

Thanks,

I am a bit lost with the "reverb" reference.

My amp is still apart as I am doing some heavy experimenting with it right now. But I found something that might help you out in search of a more Plexi type tone.

Run some good quality valves in the preamp for a start. I had NOS Sylvania 12AX7 in there for awhile but then pulled them out and was actually having good results with new production stuff. I had the New Sensor Mullard 12AX7 in V1 - V3 and a Sovtek in 12AX7WB in V4. Then the other night I put in some Chinese 12AX7's in V1 - V3 and it gave my amp some much needed midrange.

Last night I had the amp sounding real close to Angus Young's "Walk All Over You" intro tone. My settings were Master Volume on 8 and both Gains were at aprox. 9 to 10 O'Clock. This is amazing for the SL-X. LOL It even had that slight "just starting to feedback" tone at the end of my last chord on each cycle. And this is with Hot Humbuckers as well.

What my amp needed was more mids in the voicing. My amp already has plenty of bottom end. And most people complain that their SL-X has not enough bottom end.

To get yours to open up a bit with more clarity, change out those funky lime green capacitors that look like 1/4 watt resistors to Silver Mica or Ceramic Disk caps - I went with Silver Mica. And I also changed all the signal coupling caps to Sozo standard. This alone will give the amp more clarity and definition with a bit less fizz.

 

[RickyLee]

I am a bit lost with the "reverb" reference.

My amp is still apart as I am doing some heavy experimenting with it right now. But I found something that might help you out in search of a more Plexi type tone.

Run some good quality valves in the preamp for a start. I had NOS Sylvania 12AX7 in there for awhile but then pulled them out and was actually having good results with new production stuff. I had the New Sensor Mullard 12AX7 in V1 - V3 and a Sovtek in 12AX7WB in V4. Then the other night I put in some Chinese 12AX7's in V1 - V3 and it gave my amp some much needed midrange.

Last night I had the amp sounding real close to Angus Young's "Walk All Over You" intro tone. My settings were Master Volume on 8 and both Gains were at aprox. 9 to 10 O'Clock. This is amazing for the SL-X. LOL It even had that slight "just starting to feedback" tone at the end of my last chord on each cycle. And this is with Hot Humbuckers as well.

What my amp needed was more mids in the voicing. My amp already has plenty of bottom end. And most people complain that their SL-X has not enough bottom end.

To get yours to open up a bit with more clarity, change out those funky lime green capacitors that look like 1/4 watt resistors to Silver Mica or Ceramic Disk caps - I went with Silver Mica. And I also changed all the signal coupling caps to Sozo standard. This alone will give the amp more clarity and definition with a bit less fizz.

Also I need to add in here that I lowered the value of the first bright cap across VR1. That is something to experiment to taste. My value there is now a bit less than half of the original value. And I also lowered the value of the first gain stage cathode bypass cap to .47uF. And the other important one for my amp was I lowered the vlaue of the very first interestage coupling cap from .022uF to .0022uF Sozo standard. But these mods work for me because my SL-X is bottom end heavy already.

 

[jensbrix]

The modded version definitely has a sweeter ring to it. I wonder if this is something that can be done on a budget.
I plan to experiment with tubes too. Has anyone tried 12AY7/12AT7 for the gain, reverb or phase inverter position?

Thanks,

It's definetely done on a budget, parts for these mods are only like a few $$$

 

[autumnleaves]

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,

 

[mickeydg5]

The modded version definitely has a sweeter ring to it. I wonder if this is something that can be done on a budget.
I plan to experiment with tubes too. Has anyone tried 12AY7/12AT7 for the gain, reverb or phase inverter position?

Thanks,

Hi guys
Definitley do not put a 12ay7 or a 12at7 in the V3 slot. Those tubes cannot handle the heater-cathode voltages.

Add: the V3 slot is a cathode follower with higher voltage on the cathode.

 

[autumnleaves]

Mickey,
Thanks for the alert. Can I assume that either of these tubes would be suitable for v1 and v2? What other options apart from 5751 exist for v1? Any suggestions would be much appreciated.

 

[mickeydg5]

You can use those in V1 but will only be cutting your gain down. It is always best to put a good higher output low noise 12ax7 type that is tailored to your desired tone in V1. The 5751 will have less output (gain) if you are looking for cleaner tones. But honestly, in a way that is what the gain knob does. Check out the stuff on the preamp tube thread.

 

[RickyLee]

As Mickey states, I would avoid the 12AT7. The 12AY7 is slightly closer to a 12AX7 in it's capacitive characteristics as well as resistance, current ect. - but not by much.

You end up not just lowering the gain, but creating different EQ reactions between the levels of audio signal you put through the valve in relation to the operating voltages as well.





Huh?


Well, something like that.

I have tried all the various odd valves -- trust me.

 

[mickeydg5]

As Mickey states, I would avoid the 12AT7. The 12AY7 is slightly closer to a 12AX7 in it's capacitive characteristics as well as resistance, current ect. - but not by much.

You end up not just lowering the gain, but creating different EQ reactions between the levels of audio signal you put through the valve in relation to the operating voltages as well.





Huh?



Well, something like that.

I have tried all the various odd valves -- trust me.

Ok, I will bite. How do you figure?

 

[RickyLee]

Ok, I will bite. How do you figure?

Data sheets and my Ears.

LOL

 

[mickeydg5]

Data sheets and my Ears.

LOL

Ricky, your into learning everything. I like that.

What your ears hear is a difference in both gain and tone do to manufacture and amplification factor.

Talk to me more about what you think is going on with the capacitance between the different tubes according to the data sheets.

 

[RickyLee]

Ricky, your into learning everything. I like that.

What your ears hear is a difference in both gain and tone do to manufacture and amplification factor.

Talk to me more about what you think is going on with the capacitance between the different tubes according to the data sheets.

I would have to go through a few of the data sheets to compare again, but it had to do with things I was hearing when experimenting with some of the various odd valves: 12AV7, 12BZ7, 12AD7 ect.

I was not just hearing the obvious gain differences, but very strange EQ responses with some of these valves. A friend of mine some time ago explained a few things on band pass filters and capacitance that went straight over my head, but basically told me that I need to also look at the capacitance specs on these valves as well and compare them to the "circuit designed for" 12AX7. Things I would hear for example would be harsh high frequencies or extended broader frequency range and such.

School me here, as I am all Ears.

I will be back later as everyone just left my house and I have to crank up a few amp heads for some experimenting . . .

 

[autumnleaves]

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. That said, I have read elsewhere about using a Y in V1 for radical gain reduction. Is this dangerous in all X tubed amps? Maybe a full trio of 5751s would be the perfect solution. However, can a Y be used in V2, for example? (Didn't the old Fender Bassman have a Y in V1.)
Now what about the PI. If we want to hear power tube distortion, would a T work in that position (10 times the current going through to the power tubes as an X, and considerably higher transconductance for a bigger push on the power tubes). Or are there peculiarities of the SL-X that prohibit this? On the other hand, would a T work to tame the preamp distortion on a JCM900 Dual Reverb?

 

[mickeydg5]

autumnleaves
If your aim is to lower gain and distortion across the board, then try a 5751, 12at7 or 12ay7 in V1.
12au7 = 17u amplification factor
12ay7 = 44
12at7 = 60
5751 = 70
12ax7 = 100

You can try that in V2 as well.
For V3 only use 12ax7 or 12au7 type
For V4 the 12at7 can handle more current, it does not give you more current or a bigger push. The push comes from before V4.

As you drop the amplification factors down, your output signal will also drop, therefore ultimately dropping your total volume.

 

[mickeydg5]

RickyLee
What your friend told you is true, but only for radio frequency or above amplifiers. Your guitar and Marshall will never, never, never, never, never touch those frequencies. Did I say never?

The differences in eq or tone you hear are from manufacture only.

3 tubes, all 12AX7/ECC83 type
mullard
sylvania
telefunken
all have the same specifcations, yet all three sound different

you know what I am talking about, it is all over the preamp tube thread

 

[autumnleaves]

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??)