Using 4th preamp tube hole for footswitchable clean channel?

[cagenut]

Would it be possible to do this...is there an appropriate layout diagram for doing so..finally, would it be a truly usable circuit...somewhat robust sounding...or kinda thin and otherwise "weak"?

Im not looking for a "gainy clean"...a bit Fendery. Am I dreaming????

[darkbluemurder]

That should be possible. One tube will give you 2 gain stages which is what a Fender topology uses. You will need 3 holes for the volume, bass and treble pots. No problem on a 4-input model. Just convert to one input and you have three pot places.

It will not be exactly Fender though for the following reasons:

1) the typical preamp voltages in a Fender are way over 200V. In a Marshall they are only 160V. You should therefore use a separate resistor and filter cap. Hook the resistor to the B+ for the phase inverter to get more B+ for the clean tone.

2) the power stage in a Marshall is different (EL34s, 12AX7 phase inverter tube and a way bigger input cap to the phase inverter). The OT is also slightly different (Fender primary impedance 4.2k (2.1k for 100W); Marshall primary impedance 3.4k (1.7k for 100W)).

Nevertheless you should get a more than acceptable clean tone when everything is done right. In a 100W chassis space is not an issue. In a 50W chassis things may get somewhat tight.

Cheers Stephan

[cagenut]

Thanks so much my friend!

Using a full size 100w chassis isnt a problem...I have not ordered the chassis yet.
Who can I pay..or beg...to come up with a pictorial chassis layout based upon a 50w JTM..one which would show me exactly what to do regarding the clean channel integration and channel select switch for that?

A newbie thinking ahead (too ambitiously perhaps!)

Many thanks in advance!
Cagenut

[Cabbagebreeze]

Channel switching via the foot will be a large task. If you had 2 inputs you could use an ABY, but switching the inputs can be noisy, and most often is.

You want to get into relays and all the associated crap? Likely not.

Another way: You could have 2 separate pre outs then use an ABY to get to the return. But you will be changing the function of your current amp a whole lot

You also just add a blackface pre section and toggle between the current M pre and the new F pre at the PI input, but it ain't gonna be footswitchable.

[cagenut]

Thank you Cabbagebreeze!

Actually, putting a relay in there would be something I'd consider. Doent Mesa, Soldano, etc use a LED-based relay of some sort for silent switching??

Pardon my true ignorance my friends....

So...it wouldnt be possible to "toggle via relay" between the JTM pre and the Fender pre prior to the PI?

Many thanks for all the help given already!!!!
Cagenut