Adding gain stage to Marshall 2203; handling the 10k cold clipped stage

[stevehollx]

I'm planning to build a plexi that will essentially be a 1959 plexi bright circuit, with a switch to cascade into a 2203. The circuit will be modded to 12xxx spec and Rockstah's Mod5 modifications, following SDMs 2-in-1 layout for the switching of the stages (not accounted for in this schem).




I want to add another switch to add an additional 2.7k/.68u stage on the front of the 2203 from above. My inspiration is the Friedman HBE sound or that John Sykes Bad Boy Live growl. Looking at schematics, the main difference I see is with the HBE there isn't a 10k stage, and it is another 2.7k/.68u in place of the classic 10k unbypassed cathode.



I'm aware of the SLO style of cold clipping this stage at 39k here, too. I've seen discussions of going colder than 10k with others in a 2203 spec, but haven't seen anyone actually validate results.

I see the AFD100 AKA Caswell #4/#39 mod uses TWO of the 10k stages:


So, what are thoughts on how to handle this stage if I throw another stage in front of a 2203 type spec? Should I go 2.7k (or 4.7k which seems to be a common mod) w/ .68u, or leave the unbypassed 10k? Or make it colder to 39k? Any advantage to putting all three options on a rotary switch? I feel I need to leave the unbypassed 10k at least as an option for the classic 2203 sound, but interested what to do here when there is that gain stage in front of the 2203 circuit.

Also, any concerns with the stock 470k(w/470pf)/470k voltage divider here in any of the above?

[The Dude]

Welcome to the place!

Not to start off on the wrong foot, but you'll get better/quicker results if you post schematics of (or links to) what you are doing and/or trying to achieve. I can't speak for everyone here, but I've no idea without looking what 'x' mod is and 'y' layout without seeing it.

[stevehollx]

Thanks for the welcome. I took a 8 year break from building amps, and the metroamp forum isn't what it once was. Wasn't sure the etiquette of posting the BE100 schematic up, though also not sure it's right. :-P

I'll add schematics to the OP.

[Steve A.]

Not that it is relevant here but many years ago I tried switching the cathode resistor of the second stage of a 2203-ish design from 1.8k or 2.2k to 10k and there was a definite "pop!" I tried wiring up a variable resistor instead and you'd hear a swooshing noise around 2.7k or 3.3k which I guess accounted for the pop. (Since then I've wired up a toggle switch going from 10k to 39k with no pop as I recall.)

Steve Ahola

[Steve A.]

Here are pix inserted in-line into text for easier viewing...






Steve Ahola

P.S. OP is welcome to copy text and paste it into initial post (click on Reply With Quote to view text with syntax.) So why don't we refer to initial post as IP to differentiate it from OP (Original Post-er)?

[J M Fahey]

Ok, now that schematics are shown, we can start talking

modded to 12xxx spec and Rockstah's Mod5 modifications, following SDMs 2-in-1 layout

makes absolutely no sense, or, to be more precise as if I told you: "he looks like the guy who used to sit behind Ronny on 3rd grade at School. Cīmon, donīt you remember him? the one who brought the peanut butter and fried onion sandwich at Mr Jonesīclass" which of course makes all the sense ...... for the 30 kids who went with me to 3rd grade in 1961 ... and to nobody else.

To go on, this is a Tech Forum where , say, going from Mod 4 to Mod 5 , changing a bright cap from 220pF to 270pF (absolutely made up example) will not trigger World War 3 type fights with 1000 angry post long threads arguing that, weīll simply say: "ok, you rised its value a bit, youīll have a little brighter sound, no differece when volume on 10, so ... anything else?"

Getting into your question, I guess the main point is that you already have an amp you like, and want to increase gain, by adding an extra stage.
Is that so?
IF so, and the second schematic is what you want to do, I see a switchable first stage V1a .
It has Marshalish cathode resistor and bypass , and a weird split load, partially bypassed at high frequencies, doubt it achieves something useful besides "doing it slightly different" and gain is attenuated, from real around 50X to very humble 3.4X .

Are you asking what will happen if you add such a stage to your amp?
Well, youīll have more gain/compression/distortion/sustain by a moderate amount, it might not cause you big trouble.

Are you asking what will happen if you replace it with a cold biased stage?
Well, gain will drop drastically and considering passive attenuation, you will *lose* signal.

To boot, cold biased stages are very important to get certain Marshall sounds but do not act by mere presence, by sheer "magic", they are not gain stages (although they have a little gain) but clippers ... of course they have to be driven *hard* too clip and that will NOT happen in an input stage.

Well,thatīs what I understand from your question; if I misunderstood Iīm sorry.

In general, not dissing anybody but thatīs what I see, thatīs why I donīt visit such Forums too much, nobody has a clue on how to design an amp, not even a gain stage or a tone control, so they just cut and paste bits and pieces from different designs, and tweak some value , just by ear and not knowing what it really does or why.

In a way, the Mc Donaldīs sandwich design technology: just how many *different* sandwichs can you design using just *8* elements: buns/patty/cheese/lettuce/tomato/bacon/egg/special sauce.
Well, you can claim a huge number if somebody makes bun/patty/lettuce/tomato/sauce/bun and another thinks his is different because his is bun/lettuce/patty/tomato/sauce/bun and another skips lettuce and tomato but uses 2 stacked patties and so on and on and on.

I see all these "new designs" use is some "Mc Donalds combination" of Fender/Marshall gain stage plus Fender/Marshall tone control plus basically same power stage, or some mix a Plexi or Blackface preamp with a VOX power stage ... conceptually the same thing, as revolutionary as making a patty encased bun in the middle sandwich: yes ... it IS different

Back to your question, although I have already answered it: no, you have nothing to gain by switching cathode resistors in the first stage from normal values to "cold biasing" ones, and probably a lot to lose.