Bassman/Marshall switch in a Fender

[chocopower]

After read something similar in another web, yesterday i did that mod in the normal channel of my Silverface Pro-Reverb.



After test the sound i think .022 in the bass cap is too much bass for my speakers. Tomorrow i´m gonna install a new pair of Weber 12F150.

I´m really interesting in any coment or correction!!!!!

[MWJB]

This doesn't actually look like any of the Fender to Marshall mods I've ever seen. Usually they include changes to cathode bypass resistors & caps, changes in plate resistor values, even an additional gain stage.

I would question the practicality of a cheap, simple mod like this, that then makes your expensive (in comparison) speakers obsolete (assuming that they sounded OK before the mod).

Both the 5F6A bassman and classic Marshall circuits use a cathode follower tone stack, after an additional stage with a volume control - which sounds different to the plate fed tonestack that you have. In short, I'm not sure that there's a simple DPDT switched mod that can get your Normal channel in that territory.

Being more realistic, I would be more inclined to mod the normal channel to a more Marshall inspired channel (as you have, but with fixed treble cap & slope resistor - values decided by listening tests) and maybe have the 2nd stage cathode bypass cap for that channel at .68uf and use your switch to switch between the .68uf & 25uf. For this, you will need to provide each cathode (pin 8) at both V1 & v2 with its own cathode resistor (1500ohms at V2, 8 with a 15uf/25v bypass cap and perhaps stick with 820ohms at V1,8?).

For the cathode bypass cap at V1,3 try 220uf, 100uf & 25uf and see what you prefer, A/B a 2.7K cathode resistor against the stock 1.5K also.

Depending on your preamp voltage (I'd expect to see around 220-235v on the preamp plate of a 5F6A with 12AX7 in V1), larger power supply dropping resistors (in the cap pan) may also be in order.

[chocopower]

Hi!!

Thanks for your corrections.

Is really hard for me understand it (you can see my bad inglish), but i´ll try it.


Thanks!!!

[HTH]

by the look of that diagram, all your doing is switching between the 470pF/33k Marshall tonestack and the 250pF/56k Fender tonestack. pretty usefull for getting different tones.

you could take it even further - try a rotary switch with various slope resistor values in there, works very well to change the amp's tone (a pot will work here too although it's gonna see pretty high voltages).

again, you could do the same thing with the 250pF treble cap in the tonestack - Lee Jackson has this feature on his amps and it really does give a wide range of tones. try 250pF, 470pF, 1000pF, 2200pF, 3300pF

[chocopower]

Hi!!

I test this mod last weekend and after restoring the bass cap to a 0.1 (the 0.022 is too much bass for my taste) it sound better...
Both sounds are really useables with my Guyatone TO-2 (valve overdrive) and Rat2.
I use a Morley A/B/Y box. Clean sounds in "Vibrato" channel and rock sounds in the new "Bassman/Marshall" channel.

mmm....Rotary swicht...... Its a god idea!! I´m using one like this for diferent cap values in my variable NFB loop (it work great cutting the highest frecuencies when NFB is off)....

Just one more question...... Wating for a new 25K midd pot...... if i put a 10k resistor betwen bass and middle pots.... do it up the mids? I use to dial mid pot to "10" and is no enought for me....

I´m not a pro, so is easyest for me, change just resistors and cap values. Maeby in 2-5 years i´ll be ready for try your corrections, MWJB.

best regards from Galicia!!! (spain)

[HTH]

to increase the mids you can lift the ground connection on the mid pot and place a resistor between the bottom of the pot and ground. 10k would work fine but I personally like 22k for some serious grunt

[chocopower]

Thanks!!! 22k will be PERFECT.....