2 ch. rack preamp

[Zozobra]

Main board populated. Just the tone stack and board wiring to do.

[ajaxlepinski]

Looking good! Can't wait to hear it!!!

[Devin]

Super quick and shitty clip of the first power up of my nitro pre into my speedtwin fx return. The mosfet output buffer isn't swinging enough signal as it and is cutting bass so the output is taken straight from the volume pot here, which isn't great tonally. I should get that ironed out over the weekend then I'll do some proper clips.

[video]https://youtu.be/ZILOf3_T_OU[/video]

How many preamp gain stages are working here? Sounds pretty rad to me

[Zozobra]

Should be working by the end of the weekend if I don't end up partying for all of it

Devin: That is all all the stages but it never quite sounded right compared to the clips of the real thing. Splawn's have loose power amps which is a big part of the tone. I thought I'd give it a shot though. If I built it as a full amp I'm sure it'd work right.

[Devin]

Is it 4 stages?

[Zozobra]

Yep, 4 stages with a plate driven tonestack. I used a post tonestack MV whereas the nitro is pre tonestack.

[Zozobra]

Today's progress.

[ajaxlepinski]

Man, that looks so neat and tidy!

[Zozobra]

Thanks. Tag board is quite neat to work on once you get your head around it. It might still be a self oscillating mess at power up though haha! Cornford stuff is built on tag board and some of those are pretty high gain so it can be done.

[Zozobra]

After a long period of work induced procrastination it's time to pick this back up. In order to get the motivation to properly wire up the 1st channel and call it done I need to get excited about the 2nd channel. I think for this I want to start with a modded SLO and work from there. Crankys Cobra based rack pre sounded killer so I'm starting with that and then adding some known great SLO tweaks as options. First is a tight/loose switch that cuts the bass early on and the other is a 3-way toggle to swap the infamous cold clipper stage between a bit cold, fairly cold and full-fat SLO cold, where the less cold options will sound a but fuzzier. I still have an extra hole for a pot that I need to think about how to fill. Maybe I'll leave it as an unwired pot and label it mojo

[ajaxlepinski]

Every amp needs a Mojo pot!

[Zozobra]

I stopped being lazy and spent an hour or 2 on this today.



[video]https://www.youtube.com/watch?v=6wBlO7iAjt4[/video]

It's super quiet, which is surprising for 4 stages and bits hooked up with crocodile clips
First impressions are I like it and I can work with this for sure. Needs a little more saturation and the small change in the voicing. My Speedtwin doesn't have a depth mod but if it it (I can't be dicked doing it) then this would thunder like a motherfucker. I just remembered that I do probably have the dampening all the way up which thins out the tone. I'll give this a solid blast at volume tomorrow

[Zozobra]

Mods I'm gonna try tonight are adding cathode bypass caps to the 2nd and 3rd stage. This will add an assload of gain/saturation. The mig100h isn't as gainy as you might think to say it has 4 stages and this is because only the 1st stage has a cathode bypass cap boosting the gain. Even just adding a bypass to the 2nd stage should give the 3rd stage the kick in the ass it needs to clip harder. The 3rd stage bypass cap is just to make sure. These can be added using an on-off-on dpdt switch to provide an option for no boost, high end boost (0.47u or 0.68u cap) and full range boost (1µ+ cap). This may mean I have to add a bit of high frequency shelving for stability and tonal reasons, especially if both boosts are used. A 100 to 220p cap from grid to cathode on the 3rd and/or 4th stage or a big grid stopper on the 4th will do here. Goodbye clean tone though

[Zozobra]

Updated schematic. I added switchable bypass caps to the 2nd and 3rd stage and a big old 10u cap on the 4th to boost the output a little as I had to run the masters high to get enough juice into the FX return of my speedtwin. I'm really digging the tone. Sounds killer with my 72 but I definitely need to shield that motherfucker as all my other guitars are quiet through this (even my p90 tele) and my 72 has a slight whistle to it.

Other than maybe replacing one of the dpdt switches that seems a little flaky I should call this channel done and start on the 2nd. I can then voice them to do what I want when I have em both on the table.

[Zozobra]

[video]https://www.youtube.com/watch?v=77Z7Bs-g2TI[/video]

Quick mid volume blast post shower before heading for work. Them bypass caps add the brootz.

[Zozobra]

I'm now considering another 4 stage circuit for the 2nd channel as my speedtwin is very SLO so do I really need another one? Currently leaning towards the Engl blackmore as that will be great for questionable black metal

[Zozobra]

Thought about and I'm sticking with the cobra pre

I also adjusted the voltage down a little on the 1st channel. Sounds a bit gainer/thicker and less modern.

[Zozobra]

Tonight's work on this. Cobra with a parallel input stage as I have an unused triode. It'll thicken the tone out a little.

There are a few minor changes from the stock SLO/cobra circuit, e.g., switchable cathode resistor on the cold clipper stage and switchable bypass cap on the last stage.
I've also dropped the last stage plate load but I may put that back to stock.

The tone stack is james/bax with a bridged-t filter to scoop the mids. All of this can be done on the back of the pots.

I may need to put a resistor across the gain pot. A lot of people seem to favour the crunch channel boosted to the lead channel and the difference is the gain pot is of lower value and has a resistor in parallel. This makes the pot taper a lot more usable.

[Zozobra]

So when I've got ch.2 hooked in I think I'm going to rebuild ch.1 as a more standard 3+CF topology, most likely based on the low input of the GH100TI/S as I've have great tones with that amp and my pedals as after playing around with this 4 stager circuit I can't quite get it to be where I want. For shits and kicks I'll probably add some Jose diode clipping options. This way I have a JMP superbass, an 800-esque 3+CF and a full tilt gain monster for recording with. Options!

[Zozobra]

less talk; more do

[Zozobra]

This said, I do quickly want to convert a busted old Bugera 1960 I was given with a dead OT into a 2xKT88 power amp with all the trimmings: