Marshall Split channel again (some interesting information)

[Pedro Vecino]

Some important things I've discovered in the Split Channel over time:
* There are two basic versions of the amplifier. The first (was manufactured for a year and a half aprox. and have seen very few units) differs from his previous equalization. Impossible to get decent lead sounds.
* Second version is the most popular and has some variations between units:
- Switching system through transistors or an CA3046 integrated circuit
- Different distribution functions in the tubes 3 and 4. Some assign the last two steps of the lead channel in V3 (V4 is only for the reverb) and others distribute among them: penultimate lead step on the first triode of V3 and final lead step on the first triode of V4 (driver and recovery are allocated in the second triode V3 and V4 respectively). First version is desirable (with lead steps located in V3) to assign in more positive way the tubes, but you can rewire them without problem.
- Different driver circuits in 50 watts version. Some traditional, some asymmetric. 100 wats are always not traditional (assymetric)
- Different location of the three 0.22uF capacitors to mute channels. Some use little multilayer ceramic in the bottom of the PCB and others use polyester capacitors located on the top of PCB.
- Existence in some units of a 47K resistor in series with one side of the lead potentiometer gain.
...

*With the original value of these capacitors (0.22uF) is not possible to filter to ground (mute) completely all frequencies. Being active the lead channel, if the volume of the clean channel is raised, keep sounding low frequencies of that channel added to the lead (!)
The same applies in the opposite direction (low frequencies of lead added to clean sound with lead master raised)

There is another more serious variant (I've seen in one unit): In the footswitch jack 100uF electrolytic (channel selection) and 22uF (reverb on/off) are missing. During the lead / clean change, sounds a distortion in clean channel for the first four or five seconds that gradually fades. It can be a huge headache if goes unnoticed.

...

The best way to have a real and definitive clean channel on this amp is completely removing the first and second steps of the original channel and freeing V2. Inject the signal from the input jack directly to it and after the first triode install a Fender/Marshall tonestack type but adapted to the contours of the other channel to find a musical balance between them.
The one I've used is this. It has an internal adjustable resistor for the mids and a slightly cropped profile in the lows.


It´s better not to use the PCB to install the circuit. Simply remove a few components to disconnect the original circuit, remove the potentiometers and to do an external wiring. 100n capacitor is added to avoid DC voltage in the new circuit.



Accompanying this circuit, driver (when is asimmetric) is transformed as follows:



And a small modification in the loop circuit.
Capacitor C24 produces unnecessary emphasis for the lead in the highest treble/harmonics. With the new tonestack is also not necessary for clean channel.
Raising the value of resistor R29 the whole amp acquires higher output without compromising anything. It is good fot the clean channel to develop more volume.



I hope this can be useful to someone.
Best regards.

[Dre Oliveros]

Hello I'm very interested in this, but i do have trouble to identify some of the components in the pic vs the scheme, would someone be so kind to label them in the pic? thanks a lot.

[Pedro Vecino]

I'm sorry but if with the schematic and the amp in front you have doubts, it´s better not to try it. It´s necessary to know interpret exactly what you are doing at each moment.

[Dre Oliveros]

I'm sorry but if with the schematic and the amp in front you have doubts, it´s better not to try it. It´s necessary to know interpret exactly what you are doing at each moment.

Pedro, gracias por contestar, quisiera ponerte en contexto de mi situación he reparado o modificado antes algunos modelos de amplificadores vale decir que con layout, fotos y apoyando poco en esquemas ya que es cierto no he estudiado esto mas bien lo he hecho por necesidad a falta de soluciones, veras en mi país aunque parezca dificil creerlo no hay nadie especializado en electrónicos para instrumentos, los que leen diagramas o son "técnicos" (de electrodomésticos, vehículos, etc) siempre me han dado muchísima desconfianza ya que en el pasado por consultarles he perdido plata y tiempo incluso en una ocasión dañaron un ampli, es mas en este diagrama especifico un "técnico" me brindo un layout dibujado de este post, que a mi parecer no estaba correcto y mejor decidí preguntarte, obvio y me sirve mucho mas un layout o preguntar por que quisiera estar al 100% pues seguramente esta mod la haría yo, y debo pedir los componentes a USA, he visto reviews de tu trabajo y me parece muy pro, y por eso solo quería corroborar componentes, insisto gracias por haberte tomado el tiempo de contestarme, cualquier ayuda me sirve, de todos los amplis que he visto este siempre me ha resultado el mas complejo pues he notado tiene distintas revisiones el circuito, pero en serio agradezco tu respuesta ya que los amplis no son juego por sus voltajes,

saludos.

[Dre Oliveros]

I'm sorry but if with the schematic and the amp in front you have doubts, it´s better not to try it. It´s necessary to know interpret exactly what you are doing at each moment.

Pedro thanks to reply, i would like to put you in context of my actual situation, i been done some mods, and repair in tube amps myself in the past, but i do have to admit that i find more useful layouts and photos since i have not study electronics and i use the scheme merely for backup to check the info said that has been for pure necessity or hobby and i have succeed in all my past amps repairs, you see i know its difficult to understand but in my country we do not have any techs specialiced for amps, the ones that i have disgracefully meet are very sketchy, lost of time and money since they repair other stuff (home electronics, cars, etc) and barely repair em, in fact one "tech" draw me the layout of this post, but i checked out and i didn't was quite sure that he is correct so thats why i asked because I'm afraid that i would have to do it by myself (again) and i wanted to be sure before ordering the components since i have to order to US.

i have seen a lot of reviews of your remarkably work and you're a pro, so again thanks for taking the time to reply any help sure its well received, this particular amp model has always give me some difficulty since its different from scheme to scheme, but thank you and yes you're right amps are no game because of their high voltage.

thanks.

[Pedro Vecino]

That is what I meant. Layouts facilitate things at a certain level but it also complicates understanding what is being done and is not a method to get a global idea of the design. In this case it´s necessary read the schematic and follow it on the circuit itself. There are several components that can have different appearance, physical location, etc. and there can be several problems with it if they are not perfectly identified and associated with the real circuit (the one in the schematic).