Marshall TSL100

[JoeRocks]

Hello out there, I know there was a really old thread out there in regards to Marshall TSL100 mods. I thought I would take a chance to see if I can get someone to byte here. I Have a Marshall JCM@2000 TSL100 that since I had bought felt was fizzy sounding on the lead channel. I recently found this link: http://www.anchorstates.net/2009/01/...-tsl-mods.html and have actually made all the mods. step by step includding the Mercury Magnetics Tformers and Choke. 100% improvement. The question I have, there was an update 4/4/09 in regards to removing even more fizz which I have also done. I changed the vr2 cap from 47pf to 68pf on the lead channel board. I think the lead channel now sounds fine to my liking. What I don't like is the clean channel has now become almost lifeless. I will sometimes play over the clean channel with a distortion pedal. The notes tend to die very quickly, no sustain. This didn't start until the last mod. Could the mod I made to the lead channel affect the clean channel? Any info or suggestions would be greatly appreciated.

[Enzo]

Many parts of the amp are used by more than one channel. ANy change to those parts will then of course affect any channel.

[JoeRocks]

Marshall TSL100

I was under the impression by the threads I have atached in my original post that the changes I made were pretty much isolated to the lead channel. Is that not correct?

[Enzo]

I fixed your link.

I am all about helping, but you are asking me to go to another web site and read through all his changes textually, then look on schematics and make the changes so i can look at what they do, plus THAT web site has links in it for furhter explanations. I don't have the time to do all that. Not for free anyway.

Can you make up a schematic noting the changes so we all can look at it?

[JoeRocks]

Marshall JCM 2000 TSL

Enzo, sorry not to get back sooner, I was out of town on vacation with family. Sorry didn't realize would be so much trouble to look at the weblink provided to see the changes I made. Anyway here is a schematic link http://www.drtube.com/schematics/mar...tl10-61-02.pdfand a picture od the capacitor I changed. Here is a picture link http://www.anchorstates.net/images/tsl/vr2-47pf.jpg (The picture shows a 47pf. I used a 68pf cap. As stated from the Anchorstates webpage this is what I have done Put a 390pf cap across R1 on the main circuit board (the one the tube sockets are on), and a 47pf cap across the lead channel volume pot -- VR2 on the lead channel panel PCB. Note that the pots used have 4 terminals; one of these is part of the pot case, for grouding/shielding. Counting from the side toward the gain pot, use terminals 1 and 3. There should be only one pin (pin 2) between the two pins connected to the cap, and one pin on the side toward the treble pot. You will have to remove the chassis, tubes, aluminum top plate, and panel PCBs to do this mod --- that includes all the knobs and pot washers. You will want to put pieces of tape on each of the wires you disconnect, labeling what they connect to. There is also a later mod which calls for changing the 47pf cap to a 68pf which I have done. Thats when the clean channel went to hell.

[jamesmafyew]

Joe,

I wrote the post at the site you linked to. Based both on the schematic and my experience, changing the value at VR2 does NOT affect the clean channel. On the DSL it does, but NOT the TSL. That pot is isolated to the lead circuit.

The 390pf cap across R1 on V3 WILL affect the clean channel. Essentially, when you're in the lead mode, the VR2 cap and the R1 cap each create a 6db/octave lowpass filter, giving you a 12db/octave total response curve. When in the clean channel, you still have the 390pf cap in-circuit, giving you a 6db/octave lowpass.

I would guess that you may have accidentally done something else while making the change. Try switching the 68pf cap back to 47pf and see what happens. Record the amp before and after the change, on each channel, so that you can evaluate the change impartially. My guess is that it won't fix the clean sound. You can also try removing the 390pf cap from R1, which WILL change the clean channel sound, but will also mean less fizz-reduction on the lead channel.

[JoeRocks]

Hey JamesMafyew thanks for replying. I haven't been on too much lately or I would have replied back sooner. I did switch the caps back from 68 to the 47 pf and the clean channel came back to life. Don't know why it acted like thatr but now seems o be ok. I assume you read my initial post on what happened to the clean channel? Actually, the mods you posted have helped out the TSL100 a ton. Are there any other mods you have done to it lately? I am always willing to try and clean up more fizz but don't want to lose the clean channel because I like to use pedals over the clean channel or sometimes play some clean jazz guitar over it.

[jamesmafyew]

Joe-

That's really strange that the clean channel would act that way. I actually have a 100pf cap across VR2 on the lead channel now and haven't had any issues with the clean channel... which is what the schematic would suggest. One thing I've thought of is that there are several different revisions of this amp's boards and some of them are undocumented by Marshall. There was a lot of variability through the different manufacturing runs, as they addressed design flaws in a pretty ad-hoc manner. For example, the EL34 grid blocker resistors have always been spec'ed at 5.6k, but for the first few years they were something absurd like 100k. Mine are correct as I have a 2005 (last revision) model, but older ones can be improved by making that change. Some older mainboards also have runaway bias problems due to carbon film resistors that have negative temperature coefficients. The later revisions are all metal film resistors and have better board materials.

See here for more details:
The Marshall TSL122 JCM2000 Repair/Mods Page

In any case my clean channel is still very sparkly and not very Marshall at all -- as it was before the lowpass mods. I run it with the gain up at about 8 to get just a little bit of breakup when the amp is turned up loud.

I'm glad the mods have helped. I enjoy playing it now more than any of my other amps, and the reliability has been fantastic. Four years, two albums, two full US tours, and a European tour (voltage conversion and all!), and never a single issue, rock solid.

[Dave Curtis, dB AudioTech]

"5.6k, but for the first few years they were something absurd like 100k."

Even more absurd; they were 220K.

"Mine are correct as I have a 2005 (last revision) model, but older ones can be improved by making that change. Some older mainboards also have runaway bias problems due to carbon film resistors that have negative temperature coefficients. The later revisions are all metal film resistors and have better board materials."

The runaway bias was attributed to the board itself, not the resistors.

[JoeRocks]

a quick question which i'm confused about on the resistors, are they getting replaced with the new values or in pararrel or piggy backed? I would guess they should be removed and replaced with 5.6k also will amp need to be rebiased, If so, should I bias to 80 to 90 mv?

[bluesfreak]

I would replace the 220k on the board with a jumper and put the 5.6k directly on the socket of the tubes. Bias the amp to 80mV first and then test, you can still adjust a bit according to your taste and the sound you want to achieve.

Dave,
which component should also contribute to the bias runaway? I can't see any huge design flaw on the bias circuit layout and there is not much else within the bias circuit beside R67,68,69 and 77?

[Dave Curtis, dB AudioTech]

I would replace the 220k on the board with a jumper and put the 5.6k directly on the socket of the tubes. Bias the amp to 80mV first and then test, you can still adjust a bit according to your taste and the sound you want to achieve.

Dave,
which component should also contribute to the bias runaway? I can't see any huge design flaw on the bias circuit layout and there is not much else within the bias circuit beside R67,68,69 and 77?

I've ended up replacing all bias circuit components to no avail. It's the board itself. Marshall knows about it, and will sell a replacement for a fairly reasonable price.

[bluesfreak]

So this means the board itself gets conductive when heated up ?

[JoeRocks]

Do you know the wattage of the 5.6 ohm resistors. I am going to attempt to replace mine and am not sure wattage to get. Thnx

[jamesmafyew]

Joe,

They look like 1/2-watt to me. Wouldn't hurt to go with 1-watt metal film though.