Jose Plexi mods

[Matador]

Referencing the secondary of the output transformer (regardless of NFB) also simplifies debugging, in that you can always just attach a scope probe to the 'non-grounded' side of the output transformer and see what is happening referenced to the common ground potential: in other words, you can see something in the preamp and the output signal on a common scope shot (provided the vertical scales are appropriate, etc).

Utilizing chassis points as circuit return paths is a poor solution, as one loses the ability to predict and control the return currents in the circuit(s).

It does then bring me to think where is the best noise option for gnd for the speaker jack?

There is no (appreciable) current flowing in the ground reference for the speaker jack: the current circulates through the secondary and the speaker only. Since most of my designs use NFB, I always return the feedback signal as a twisted pair from the speaker jack back to the point where it's injected in the phase inverter, so that's where the speaker's ground reference is attached.

 

[Spencerleehorton]

Ah ok so do not put the speaker gnd to the filter caps gnd.

 

[Matador]

Ah ok so do not put the speaker gnd to the filter caps gnd.

In a typical Marshall LTP with a presence control, there is a 5K pot to ground right at the tail of the LTP. There is also the 47K resistor that is fed from the secondary 8 ohm tap. I typically feed the presence ground and 47K attachment point as a twisted pair out to the speaker jack, and attach the two sides to the two sides of the secondary (keeping in mind the phasing is important). So the 'ground' of the speaker jack is actually the same ground as the bottom of the presence pot. This Almost Always (TM) works fine.

https://groupdiy.com/threads/pcb-ground-planes.83954/post-1089789

 

[john12ax7]

It might be helpful to consider what is actually going on. For a NFB amp you essentially have a transformer balanced differential signal that gets fed back and inserted single ended. How would you handle a transformer coupled mic pre feeding a single end unbalanced eq? It's the same thing. Looking at it that way logically gets you to what Matador is describing.

 

[Spencerleehorton]

Just wanted to check I’ve got this right before I move forward

 

[Spencerleehorton]

On the OD channel I want to adjust the bottom end a little and have gone with 1.5k cathode and 0.68uf bypass caps but there it’s still a little muddy if I change the plate 22nf to 2.2nf will that cut a bit of bottom end as well?

The cathode cap calculator on amp books seems to look like it should sort it!!

 

[Alva Goldbook]

On the OD channel I want to adjust the bottom end a little and have gone with 1.5k cathode and 0.68uf bypass caps but there it’s still a little muddy if I change the plate 22nf to 2.2nf will that cut a bit of bottom end as well?

The cathode cap calculator on amp books seems to look like it should sort it!!

Take note of which frequency is each guitar string.

https://homes.luddy.indiana.edu/donbyrd/Teach/MusicalPitchesTable.htm

Amp books has a good coupling cap calculator. I think you gotta figure out your output impedance I can’t remember. For a 12ax7 with 100k plate it’s 38k. But there’s an amp book calculator for that too in case you need it.

At 22nF you basically pass the full frequency range of the guitar. So run a simulation of 10nF or 4n7 before you use crazy low values of 2n2, 1nF or 500pF like Vox did.

I don’t recall what Soldano did off the top of my head, but Marshall also boosted treble using a RC filter of 470k/470pF.

Marshall traditionally used cathode bypass caps and RC filters to boost treble, Vox boosted treble using coupling caps and Orange did both. I think Marshall’s approach gives a more aggressive sound.

 

[Spencerleehorton]

yep thats done the trick, 22nf to 2.2nf on the last OD ch preamp, sounds great now.
still a few bits to tidy up but looking and sounding so much better now.