Marshall DSL401 Mod Idea - Advice Required!!

[diddymix]

Hi folks,

I was hoping people with technical knowledge of valve circuits could chime in for me and offer me some guidance or tips on something I want to try with my Marshall DSL401 combo amp. I've attached the schematic, if someone could have a look and see what they think I would really appreciate it!! Basically I can replace parts etc but I do not have a good knowledge of circuits and how the impedances work between stages etc, and how that translates to performance. I want to try running a wire from just after the V1 plate driven tonestack... straight into the V4 LTP phase inverter... thus the whole of V3 stage being removed from the signal path. I know this is a cathode follower and part of the marshall sound but I want to try removing it completely for a purer more raw clean tone, without the cathode follower signature clipping and character... which frankly Im not a big fan of..
I have found the points on schematic where I'm thinking of taking the signal... it would be from after just downstream of VR1 (Gain Knob)... the signal then hits a couple of transistors (T12 - T13)... this is the point I would like to re route the signal just before the 22K resistor. From here I would like to connect it straight to the input capacitor C93 (22nf) feeding the phase inverter input.
My question is mainly this... After the plate driven tone stack and VR1 gain control..., would I need to add a resistor in order for the signal impedance to properly interface with the phase inverter input?? I think there is some loading down of the V1 signal from the plate driven passive tonestack... is this potentially a problem with going directly into the phase inverter in terms of gain/level etc??? Does the output impedance depend on the V1 plate resistor and then slope resistor R118??

Thanks for reading and please any advice most welcome! I'm going to try this any way with no resistor as Im sure it cant hurt anything being just signal path but I guess if it doesnt work Ill be a bit lost as to why... Hopefully its doable could be interesting and if I need to add a resisor of any kind would love to find out the optimum value.

Cheers

[Phil_S]

Take the signal from the wiper of VR1 and input to the top side of C93. This is a one wire mod if you can find the take and insert points. Just tack in a wire and see what you think. If you like it, you can install a switch to break or make that mod whenever you want. if you don't like it, you can clip it, and forget you ever did it.

[Jered]

Whats going on with the dual pot VR12a and VR12b, is that 4 lugs on each pot?

[diddymix]

Thanks Phil appreciate it!, could I ask what you mean by the wiper.. is that the main signal path pin on the pot?.. what do you mean by the input as well?? Dont I just connect the switch to one point?
Also, might this be a problem in that the tonestack just before has robbed alot of gain from the V1a stage..?
Cheers!

[Phil_S]

Thanks Phil appreciate it!, could I ask what you mean by the wiper.. is that the main signal path pin on the pot?.. what do you mean by the input as well?? Dont I just connect the switch to one point? Also, might this be a problem in that the tonestack just before has robbed alot of gain from the V1a stage..?

Oh gosh. I was like you at one time. I remember well. People need to be patient with beginners.

Wiper = the middle lug on the pot. Inside the pot, there is a "wiper" that moves back and forth as you turn the knob and it connects to the middle lug. Find the wire that connects to the middle lug and find a safe place to connect the jumper. That is the "take off point." It might be where the wire that comes from the wiper of VR1 attaches to the next component. One end or the other, it is the same, but closer to the pot is preferable.

The input is the top of C93. By that, I mean, when you look at the schematic, C93 is vertical, and there is a top and bottom. It won't be that way wherever it is located. C93 needs to be between the wiper and the grid of the next tube.

I'm saying "wire." If this is a PCB amp, there are not many wires. The traces on the board are the wires. In such a case, see if you can slip the end of the jumper wire under/around the component lead and tack it with a bit of solder being careful not to overheat the component (pot, resistor, cap.)

If you like what it does, you can figure out how to mount a switch if you like, and then you can turn this feature on or off. When you do that, you should do better than simply making a temporary solder tack to see what it does. You'll need to figure out how to properly attach and solder the wire at both ends. Good practice says there should be a solid mechanical connection. I can't say what it should look like. I have no idea what the inside of this amp looks like. I am an amateur builder (maybe a dozen amps) and I don't do repairs for money.

I can't say about this particular tone stack bleeding off gain, but many amps will need a post tone stack recovery stage to boost the signal. It is reasonable to think this is applicable to your amp. What I am suggesting keeps the tone stack in the circuit, so it may not be what you want. If you really want to bypass the tone stack, see if taking the signal from the wiper of VR2 will accomplish that. I am not quite sure about how much of the tone stack that eliminates, but it is a better choice than the wiper of VR1.

Just be careful to work in a way that allows you to reverse what you've done. Only change one thing at a time, otherwise, you have no idea of cause and effect.

[Lynxtrap]

If you don't mind me asking, why don't you just use the clean channel instead?

[diddymix]

Thanks Phil!!

That's what I thought ok.. so the wiper is the signal path pin. Well I think I've got it now.. I just have to try it. I think that yes the tone stack does have insertion loss, if you follow on afterward it would normally hit V3a which I think is a driver stage before the cathode follower. What I may try is a DPDT switch that can do two moves in one... so what I think is on one pole.. re route the signal from before the tonestack (perhaps 0.1uf coupling cap) to the VR1 gain pot position.. Then on the other pole of the switch re route from VR1 wiper to the PI input cap as you say. Apperently one can also effectively remove the tonestack by eliminating the ground connection on VR3.
Theres only one thing at the moment I'm not sure about and would like to know... that is if I have to a add a resistor between VR1 and the PI input cap. Would there be a need for this?? I'm thinking perhaps there might be considering theres usually one before a preamp tube (grid stopper)?
Thanks again.
To the chap below This is the clean channel I will be using, only I am rerouting the signal as to remove V3 cathode follower... which I believe is a big part of the signature marshall sound. Im looking to try this as an experiment for a more pure and raw tone for clean channel only.

[roberto]

Have you tried it? What are your impressions?