JCM900 2500 SL-X mods anyone?

[RickyLee]

It's over in the JTM-45 Section, however in my personal experience I rarely go over 5hy, 3hy being my go-to. Large inductance chokes first of all I feel unnecessary for a guitar amp, and second super smoothed stiff screen nodes do something really icky IMHO.

Listen to this man.

I bought two Mercury chokes way back when I started working on this SL-X. I got the 10H MC10H and I got the 3H MAR100-C. I very briefly tried both chokes in my SL-X as I rigged up some special connectors to plug each one into my amp - one at a time of course. But to really compare them I would need to be able to do this on a switch. As just the short time it took me to pull one out and put the other in and turn the amp back on, it is hard to remember every little tone nuance and detail - because truthfully, there was no big difference that stood out to me between the two. Maybe I was not pushing the amp hard enough? But for a reason that I can't even explain, I picked the 10H over the 3H for that particular amp (SL-X). It did seem that the 10H helped the bottom end to be slightly tighter, and in my particular case my amp has a good abundance of it. Plus I have my negative feedback loop modified with the fixed resonance control as well.

The ClassicTone 3H 40-18058 would be a good all around choice to cover all your bases.

 

[mickeydg5]

That one got by me. Here's the fallback position then <http://heyboertransformers.com/tubeamps.shtml>

So the voltages are of no consequence in a choke?

Most dealers and manufacturers don't bother stating all the specs.
Note that that big 40H choke is only 50mA
The 20H is 70mA
Lots of others come in at 90mA
And you have the 200mA and 250mA too

But you will be replacing R35, 100 ohm 5 watt resistor, only capable of 22.4mA.

 

[autumnleaves]

Right then, 20 henrys it is, or nothing at all. I'll save this one as a last resort. I remain indebted to Mick, Rick, Joey and Jacques for their sage advice and helpful suggestions. I'll order some tubes and the basic components and see what we can do when I finally get my hands on the amp.
Best regards to all.

 

[mickeydg5]

Listen to this man.
I very briefly tried both chokes in my SL-X as I rigged up some special connectors to plug each one into my amp - one at a time of course. But to really compare them I would need to be able to do this on a switch. As just the short time it took me to pull one out and put the other in and turn the amp back on, it is hard to remember every little tone nuance and detail - because truthfully.

I have a little MP3 player that can actually record, mono and not great quality. But it works. I A/B recordings on the computer to listen for differences when comparing things.

 

[Joey Voltage]

Listen to this man.

I bought two Mercury chokes way back when I started working on this SL-X. I got the 10H MC10H and I got the 3H MAR100-C. I very briefly tried both chokes in my SL-X as I rigged up some special connectors to plug each one into my amp - one at a time of course. But to really compare them I would need to be able to do this on a switch. As just the short time it took me to pull one out and put the other in and turn the amp back on, it is hard to remember every little tone nuance and detail - because truthfully, there was no big difference that stood out to me between the two. Maybe I was not pushing the amp hard enough? But for a reason that I can't even explain, I picked the 10H over the 3H for that particular amp (SL-X). It did seem that the 10H helped the bottom end to be slightly tighter, and in my particular case my amp has a good abundance of it. Plus I have my negative feedback loop modified with the fixed resonance control as well.

The ClassicTone 3H 40-18058 would be a good all around choice to cover all your bases.

Only did it twice, once to a DSL I modded for a client, and 5 years ago to a 5150. In both cases the higher inductance choke seemed to have a weird effect on the treble response of the whole system, overly smoothed and linear, the stiffer feel of course I was expecting. Now the mod I did to the dsl was my three decades of decadence one that I have done many times prior, same test cabs everything. I found myself having to increase apparent treble response (kill it less rather), to compensate.

I was talking to my friend Dave who makes the Baron amps around the same time and told him about it, and he said he also experienced similar results, which lead me to conclude it was the theoretical "better" screen regulation, (with a super huge inductance you are basically passively regulating the screen supply, which in turn carries over into the preamp), which in turn, better filtered an already massively filtered preamp, that was behind the ickyness, and Since the principle of amplification in its most basic form is really just the modulation of a power supply, that this should matter in some way. Anybody who knows me knows I'm not one to make wild unfounded claims, although the YMMV adage certainly applies

 

[mickeydg5]

Joey, I have been thinking about that.

The 20H is going to tighten everything up a lot. But it may be too much for some peoples taste on the top end.

The why is because of large capacitance in the filtering and the use of diode rectifiers.
The older amps had smaller caps and a tube rectifier.
Like RickyLee mentioned, he noticed the bass difference at 10H. So with this amp a 10H, 7H or maybe even a 5H choke will get closer to what is wanted if your looking for more of the Bassman clean and big bottom. That is another preference call though.

And, who uses that big Hammond 40H in a JCM800? :eek2:

 

[Joey Voltage]

And, who uses that big Hammond 40H in a JCM800? :eek2:

I think thats a typo if it describing their marshall replacement. I think it is really supposed to be 4hy. something like 40hy would be for a choke input filter for class A, which is rare since it doesn't really offer a lot of bang for the buck.

The why is because of large capacitance in the filtering and the use of diode rectifiers.
The older amps had smaller caps and a tube rectifier.

That, and the intended use for these amps (Bassman) originally was for bass, in which the additional filtering would be useful, and on top it was a Hi-Fi design that was licensed from Western Electric/AT&T which I'm sure was part of it as well. If Leo realized at the time it would become a well used guitar amp, and could have used a cheaper choke, or none at all he would have I bet.

 

[mickeydg5]

The Hammond 194C is noted as 40H on the site and its diagram. I would be inclined to say it is probably accurate because it is a 50mA 420 ohm dcr choke.

I know Fender acknowledges patents on their old stuff, but believe that there were no direct amp copies, just some circuits. He developed some from his own ideas.

None of the first Fender amplifier designs had chokes. Some later versions incorpoarated them.

 

[Joey Voltage]

The Hammond 194C is noted as 40H on the site and its diagram. I would be inclined to say it is probably accurate because it is a 50mA 420 ohm dcr choke.

The 50ma/420r may indicate thinner wire and more turns, and one might think higher inductance, but that's not necessarily true in all cases, the physical size has a lot to do with that, although i dont have the data sheet on hand. plus what Marshall is this supposed to be a drop in replacement for?

know Fender acknowledges patents on their old stuff, but believe that there were no direct amp copies, just some circuits. He developed some from his own ideas.

None of the first Fender amplifier designs had chokes. Some later versions incorpoarated them.

I think he did initially license the bassman from AT&T, and you sometimes hear the term the "western" circuit being used in certain circles. it was a slightly different circuit that changed during production. The earlier smaller pentode/triode amps didn't use chokes no but those to the best of my knowledge were designed to be bonafide guitar amps and the Currents are much smaller in 18w shitboxes so filtering doesn't need to be rediculous. I don't think Leo fender was much of an engineer either

 

[mickeydg5]

Yes, I get what you are saying, but I have only noticed 400 to 700 dcr in larger chokes. Oh well.

The tags on old Fenders state:
Licensed under U.S. patents of AT&T Company and Western Electric Inc...
But not specific as to what patents.

Well, I hope Autumnleaves got it all worked out. Let us know what you picked and get us some before and after samples if possible.

 

[autumnleaves]

Once again thanks to Mick, Rick, Joe and Jacques for their suggestions. I think I'm on the right track now.

 

[Markdude]

Hi guys,

I don't have a ton of amp knowledge but I am interested in trying some different preamp tubes in my SL-X 2100. Ideally I would like it to have a bit fuller midrange and a more open, clear top end, and just a more classic Marshall sound overall.

I've seen a lot of people recommend a 5751 in V2 and/or V3, but I just wanted to get more clarification about whether or not it would be helpful to have one in V1 as well. Earlier in this thread I saw someone say that putting one in V1 would cut down on gain overall, but then I saw someone say that V1 is also the cathode follower too and that only a 12AX7 should go there...but isn't the 5751 interchangeable with the 12AX7? Anyway, suggestions would be greatly appreciated. I don't necessarily want to lose all the punch and attitude of the amp (I still want it it be able to get raunchy), I'd just like the voicing to sound a bit more old school. I should also add that I DON'T want smoothness necessarily -- I want it to still sound pretty bright and have some 'clank' and 'chime' on the attack of the notes, but I'd prefer it too sound less compressed and have a more vintage midrange flavor like an old Marshall.

HOWEVER, at the same time I'm also wondering if maybe there would be any benefit in actually going with higher gain preamp tubes since my main complaint comes from the Sensitivity knob when it is set above halfway or so. I find that running the main Gain knob around 1 o'clock to 2 o'clock gives me the right amount of tightness/thickness in the lows/low mids, but then increasing the Sensitivity to my desired gain level seems to give a weird, fizzy character to the tone that sounds like an abundance of intermodulation distortion. Is my thinking above actually backwards and maybe I need higher gain preamp tubes to that I can get away with running the Sensitivity knob lower? I don't like running the main Gain knob much higher than 2 o'clock because the EQ structure gets kinda muddy and too loose in the bottom end IMO.

I should also specify that I'm honestly not interested in doing any mods other than tube rolling, even though I realize this is a thread about mods. I'm just replying to this one because I've seen some good tube discussion here.

 

[JCMDOUG]

I put a high gain in my PI tung sol and a sovtek 12ax7LPS in V3. Also speakers play big part in sound vintage 30s work well for pusing the mids. Is your amp EL34 or 6L6 for output tubes.

 

[Blacque Jacque]

The "fizzy character" you refer to from the sensitivity control is exactly why I tried the 5751 in the first place. It still allows the compression in that stage, but tends to take out some of the harshness.

I tend to run mine with lower settings these days to suit my playing style, but it will still hold together at higher settings.

 

[Markdude]

Thanks for the help, guys! Mine is an EL34 model.

The "fizzy character" you refer to from the sensitivity control is exactly why I tried the 5751 in the first place. It still allows the compression in that stage, but tends to take out some of the harshness.

I tend to run mine with lower settings these days to suit my playing style, but it will still hold together at higher settings.

Awesome, that sounds like it should be exactly what I want. Do you have the 5751 in V3 as well or just V2? Any chance you could share your recommendations for all preamp spots?

 

[Blacque Jacque]

I have an old Mullard I61 in V1, a JAN GE 5751 in V2, and GT 12AX7Rs in V3 & 4.

I'd advise you not to use a 5751 in V3, that's the cathode follower stage to drive the tone stack. While it should work, you may end up losing too much gain there, you may also exceed the cathode / heater voltage rating of the 5751.

 

[Kelia]

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?

Hi there !
I’ve read all pages and it stopped dead quiet on jan 2016 !

Any updates on your tone quest ?

 

[RickyLee]

Here is my Update lol.

I had my SL-X buried in the back of the Ol amp pile lol. I do not think I played it ,much since the beginning of this thread lol.

Well, I pulled it out a few weeks ago as I am trying to find an amp that will get me closer to Gary Rossington tones for my current project.

Well, I was stoked on how good my SL-X sounds. I am at the point where I want to start selling off some gear. I have way too many amps that are just not getting used and they should go to a good home, a youngster in a band, someone that could use them really lol

I did sell my 2nd stock 1988 2205 to a young kid in a band a few years ago, but that is all I have sold at this point.

Anyways, back to the SL-X. I was pondering putting that on the Sell list but it is a unique amp. The SL-X has a preamp design unlike any other I have seen except the Marshall 6100 CH3 that uses a few more cathode follower circuits in series in its preamp. The SL-X has one. But I can say, I was stoked how good my SL-X was coming across and then the sweet chewiness in the crunch. Awesome. I have JJ 6CA7 in that one as they seemed to really shine in that amp for some reason.

The one thing I need to verify on this SL-X is: Was some of the MaGiK I was hearing a few weeks ago due to the first 500pF treble peak cap being out of circuit? What happened was, while testing it a few weeks ago, thought I heard something odd. I could not remember all the work I had dome to it over 10 years ago. So opened it up to see I had went berserk in there lol. Sozo coupling caps, swapped out those little lime green resistor looking caps, fixed resonance in the NFB loop. And I think I changed the bright cap on one of the gain pots too. Was checking my work and noticed the treble peak cap was a bit loose. Flipped the PCB to discover a possible lost solder pad. So I repaired that, made sure treble peak cap was in circuit.

So, then when playing it again I thought I lost a bit of the great tone I had before. It was a bit brighter. So with all the changes I had made, it could be that it does not need that first treble peaker cap. It is something I need to get back on testing later. Having it on a switch would be sweet but could cause lead dress issues.

I then drug out the 900 MKIII and was comparing it. I did put a 3H choke in the MKIII around the same time I put the 10H choke in the SL-X. I can say that I was liking the MKIII as well. It is a totally different sound from the SL-X due to having the same diode clipping circuit as the 2205/2210 amps. It is has a brutal low end emphasis and hitting it with a Tube Screamer type OD pedal gives it an added tonal option for sure. But I am really thinking I need to let that one go, put it up for sale. We will see what these next few weeks bring . . . .

 

[jagrenet]

Here is my Update lol.

I had my SL-X buried in the back of the Ol amp pile lol. I do not think I played it ,much since the beginning of this thread lol.

Well, I pulled it out a few weeks ago as I am trying to find an amp that will get me closer to Gary Rossington tones for my current project.

Well, I was stoked on how good my SL-X sounds. I am at the point where I want to start selling off some gear. I have way too many amps that are just not getting used and they should go to a good home, a youngster in a band, someone that could use them really lol

I did sell my 2nd stock 1988 2205 to a young kid in a band a few years ago, but that is all I have sold at this point.

Anyways, back to the SL-X. I was pondering putting that on the Sell list but it is a unique amp. The SL-X has a preamp design unlike any other I have seen except the Marshall 6100 CH3 that uses a few more cathode follower circuits in series in its preamp. The SL-X has one. But I can say, I was stoked how good my SL-X was coming across and then the sweet chewiness in the crunch. Awesome. I have JJ 6CA7 in that one as they seemed to really shine in that amp for some reason.

The one thing I need to verify on this SL-X is: Was some of the MaGiK I was hearing a few weeks ago due to the first 500pF treble peak cap being out of circuit? What happened was, while testing it a few weeks ago, thought I heard something odd. I could not remember all the work I had dome to it over 10 years ago. So opened it up to see I had went berserk in there lol. Sozo coupling caps, swapped out those little lime green resistor looking caps, fixed resonance in the NFB loop. And I think I changed the bright cap on one of the gain pots too. Was checking my work and noticed the treble peak cap was a bit loose. Flipped the PCB to discover a possible lost solder pad. So I repaired that, made sure treble peak cap was in circuit.

So, then when playing it again I thought I lost a bit of the great tone I had before. It was a bit brighter. So with all the changes I had made, it could be that it does not need that first treble peaker cap. It is something I need to get back on testing later. Having it on a switch would be sweet but could cause lead dress issues.

I then drug out the 900 MKIII and was comparing it. I did put a 3H choke in the MKIII around the same time I put the 10H choke in the SL-X. I can say that I was liking the MKIII as well. It is a totally different sound from the SL-X due to having the same diode clipping circuit as the 2205/2210 amps. It is has a brutal low end emphasis and hitting it with a Tube Screamer type OD pedal gives it an added tonal option for sure. But I am really thinking I need to let that one go, put it up for sale. We will see what these next few weeks bring . . . .

I have both of those amps (MKIII and SL-X). I have modded both of them as well. Gotta say .... they are definitely tone beasts for sure. I love them both equally, but for different reasons.

 

[Kelia]

I was stoked how good my SL-X was coming across and then the sweet chewiness in the crunch. Awesome.

Curious on what you did that made it sound sweet and chewy ! I really know what you mean about this but the both SL-X I had
were not chewy , more on the shrill side , you did something good for sure !