Project Big Marf

[Ben Lyman]

Hey everyone, I have been making more music than pedals lately but I have a couple questions about this project. Probably been done already but this is what I have on the bb right now. It's the "47 Ram's Head" BMP with what I am told (by the intrawebs) is a Marshall 3-band tone stack, except with a 50k mid pot for boosting the mids.
I call it the "Big Marf Pi" and it's a heavy metal machine

D4 seems to reduce noise a lot, but does that mean it renders D3 inoperative and unneeded?

Have I correctly inserted the tone stack? I don't hear any crackle when I turn the pots but it seems almost like I missed something... maybe not, it sounds good.

Any thoughts?

[robthequiet]

Looks good to me. 2 questions, which trannys did you use and how did you arrive at the 500k value for R23, just out of curiosity?

[Ben Lyman]

Seems to work fine with almost any trannies, 2n2222, 2n3904, 2n5088, etc. I even tried some old npn Ge things I have and they worked too.

That 500k is in the "Marshal Tone Stack" schematic I found online, it is labeled "Load resistor" and the so called "Fender" schematic has a 1M "Load resistor"
I guess I actually used either a 470k or a 510k though.
Is it bogus?

[ElectricDruid]

Yes, I'd expect that D4 makes D3 fairly pointless. The Ge diode will have a lower forward voltage and therefore conduct first. Unless the signal still increases enough after it starts clipping to hit the threshold of D3 (and you do get some increase, especially with Ge diodes - it's not hard clipping, after all) then D3 will never do anything.

Dunno. If it's on the breadboard, does it sound the same without D3? That's the obvious way to find an definitive answer!
It will sound the same at low gain, most likely. If it were to make a difference it'd be at higher gains, where the other diode might kick in a bit.

Tom

[robthequiet]

Seems to work fine with almost any trannies, 2n2222, 2n3904, 2n5088, etc. I even tried some old npn Ge things I have and they worked too.

That 500k is in the "Marshal Tone Stack" schematic I found online, it is labeled "Load resistor" and the so called "Fender" schematic has a 1M "Load resistor"
I guess I actually used either a 470k or a 510k though.
Is it bogus?

I wouldn't think anything's bogus if it works. Just hadn't seen it before. Looks like the '67(?) Marshall stack, I think, where they went from the 250p to the 500p treble cap. I can see how it would keep your signal from floundering going into the final stage so it makes sense. I probably would have gone to ground on the treble pot and crashed the signal going into Q4, myself.

Rock on!

[Quackzed]

don't people usually 'scale' down the 'amp' tonestacks when used with trannys? i seem to remember that the impedance is very different when your feeding a tonestack from a transistor than from a tube...  i wanna say like 10x scaling type thing is the normal approach( resistors 10x smaller and caps 10x bigger)
if the tone stack 'as is' doesn't have as much of an effect as you expected,  it might need to be scaled down...

[Ben Lyman]

Tom- thanks, very good to know. I have not actually been hitting the strings while testing with and without  I will do that next.
Thanks!

Rob- I'm not quite sure what you mean about hooking the treble pot up like that but if you think it would be better, I could try it out. I think I would need a schematic of it so I could visualize it.
Thanks!

Gil- awesome info, thanks, I was not aware of that. The tone stack thing is still kinda new to me. I'd like to eventually put this tone stack in my Plexi-Drive and my ROG Thunderchief.
Thanks!

[robthequiet]

I think I mis-explained -- actually, looking at both the Marshall and BMP circuits, you would basically take the signal right off the tonestack output and put it into the Driver/Inverter or gain recovery stage Q4, so no 500K resistor, although better brains than I adapted the tonestack to work in a pedal. I also learned that the 50k mid pot is a popular mod for some Marshall amps, so you're onto something there.

[Ben Lyman]

Oh, okay thanks. So, I can probably pull that 500K without any ill effects. That's good, I always like eliminating parts whenever I can.
And yes, the mid pot mod is something a friend told me about for his JCM800 mod, it works great, I guess some might even go with 100k but 50k seems fine for me.
btw, I noticed a typo in my schematic in the first post, corrected now. C11 was supposed to be .047uF.

[pinkjimiphoton]

Yes, I'd expect that D4 makes D3 fairly pointless. The Ge diode will have a lower forward voltage and therefore conduct first. Unless the signal still increases enough after it starts clipping to hit the threshold of D3 (and you do get some increase, especially with Ge diodes - it's not hard clipping, after all) then D3 will never do anything.

Dunno. If it's on the breadboard, does it sound the same without D3? That's the obvious way to find an definitive answer!
It will sound the same at low gain, most likely. If it were to make a difference it'd be at higher gains, where the other diode might kick in a bit.

Tom

hi tom,
that's my noise reduction mod. it really works, at the expense of a little bit of sustain. takes most of the "ocean" out of the sound.
no idea why it works, and it only works in that position. but it mos def makes a difference in the amount of noise generated.

it makes a diff regardless of the gain. my theory is that it helps gate the signal, but no idea if thats accurate.

been thinking maybe a falstaff sim would be worthy, but WAYYYY above my paygrade!!

ben,, excellent work as always bro 

[Ben Lyman]

Thanks Jimi! And btw, thanks for the tips on this project. The mid boost is awesome and the noise reduction Ge diode works great. The only question is whether or not I still need the 914 laying parallel with it. Not for the sake of functioning as a noise reduction, that works no matter what, but for the tone of the guitar, I foolishly had not even tried playing while removing the Si out and back in again.
I'm gonna go test it again right now.

I also looked around on the compu-webisphere and can't find any info on changing amp tone stack values for adapting to pedals. I did, however, notice that the ROG Thor uses these exact values and they also omitted that 500K "load resistor" just as Rob said. I'm gonna go do that now. Stand by

[thermionix]

Actually, disregard.

[pinkjimiphoton]

makin' popcorn

i've found also that a clipper made of led's in the feedback loop of a germanium boost or fuzz has a similar noise gating effect!

no idea why. just something i found messing with my breadboard.

i try adding shit to everything

i don't think you need to worry about changing the values of the tone stack if it's working.
rock on bro!

[Ben Lyman]

Actually, disregard.

Shit! I missed it... I wanna know what you said 

Late night demo, can't crank it up but halfway through I show with and without the Ge diode noise reducer, actually 2 different kinds.
Tone stack is still functioning fine without the 500k "load resistor"

A couple more days of testing and playing loud just to be sure I like it, then I think it will be ready to commit to solder. I have a pretty good feeling about it after watching Andy do a Pro Guitar Shop demo of the BMP w/Tone Wicker... I think mine is better

[anotherjim]

Maybe the reverse leakage of the Ge diode is adding some negative feedback. We know that more NFB=quieter? That's my guess anyway.

Re-scaling tone stacks has, I think, 3 purposes.

1: A low voltage solid state stage before the TS  often has about 1/10th collector or drain resistor than a tube anode resistor. So the source impedance is lower & that changes the frequencies of the tone controls. Play with the Duncan tonestack calculator to see the effect and know why it lets you enter the source resistor value. So if the tube original has 100k anode resistor and your SS has 12k, well that's near enough 10k to stick with the x10 rescale of the tonestack.

2: Resistor noise & cap quality. We know smaller resistors make less noise. Larger caps mean we won't have to use ceramics in pF values, but nicer film caps will be easier to find for the x10 new value.

3: Load resistor on the tone stack is the grid bias into the next tube stage. Often 1M (usually that is the volume control too). Simple BJT stages are hard to make with 1M input, often 100k is more likely. The Tone stack performance is altered again and fixed by making it's resistance 1/10th to make the 100k load change the tone less. Again, that tone stack calculator lets you enter the load value and see what happens. We know these tone stacks have a high insertion loss, too much load will make that worse.

All that said, the tone stack was designed for an amplifier. You already have an amplifier, this is a distortion box. So if what you have suits the distortion very well, why change it?

[ElectricDruid]

Maybe the reverse leakage of the Ge diode is adding some negative feedback. We know that more NFB=quieter? That's my guess anyway.

Interesting thought. You might be right. I wonder how we'd prove it?

Tom

[digi2t]

Sorry for the highjack, but just out of curiosity, has anyone tried FET/MOSFET's in place of the usual suspects of clipping diodes? I haven't tried it myself, but I've read that they give a bit smoother feel. If anyone's tried it, what were your impressions.

[anotherjim]

Maybe the reverse leakage of the Ge diode is adding some negative feedback. We know that more NFB=quieter? That's my guess anyway.

Interesting thought. You might be right. I wonder how we'd prove it?

Tom

Maybe find 2 Ge diodes, one very leaky and one not so much, and see which is quieter? Something tells me that high leakage ought to be a noisy device in itself, so maybe not. One thing about diode reverse leakage current though, is that it can be higher than forward leakage is below Vf and happens with reverse in mV. Also a glass diode getting light is acting funny anyway and things could change in the dark.

[pinkjimiphoton]

Maybe the reverse leakage of the Ge diode is adding some negative feedback. We know that more NFB=quieter? That's my guess anyway.

Interesting thought. You might be right. I wonder how we'd prove it?

Tom

only way i can prove it is to tell ya to try it. it mos def works.

i think cuz ge's clip faster, it's adding a bit more compression to the signal and that alone is doing it. but i have many theories, but no way to scientifically prove it. way above my paygrade. all i can tell ya is it works.

i've reccomended it to peeps now for a couple years. most of the EE types look at me funny. i just say try it.

it works on every big muff i've tried it on. its on my hot p*ssy 47 rams head clone on my board, and has been for years now.
i did it to my NYC bmp, too.

i just did a guitar show on sunday. as soon as people heard that thing, even tho i was asking too much for it (didn't really wanna sell it but cash talks) and there were a tonne of muffs there.. old ones, new one, cheap ones.. and mine sold over all the others that didn't move. its a substantial noise improvement that really doesn't @#$% with the tone too much. it makes it sound different by taking the noise out.

curious thought, but if ge's conduct at about 1/2 the voltage of silicon, perhaps what it's doing is actually phasing it... if the signal arrives in that particular clipping circuit even one cycle out of phase on one side that may be enough to cancel the noise...

i'm really reaching here, guys... lol... all i can do is say try it!! it works!!
.
dino, i did try mosfets in the hot p*ssy pedal originally but i didn't like them. kinda changed the character of the fuzz too much for my liking. you can run ge q's too like that and they sound good too but i prefer the silicon with the added ge.. that's just me

i found i prefer low gain q's too. sounds more like the originals and less like a buzz saw.

most folks say use higher gain, use higher gain... i say... try some around the 200-300 range or LOWER.
suddenly the liquid sustain sound of the original comes back.

the distortion will go down substantially, and ya end up with a very clear thick compressed distortion  that cleans up well and sustains like you wouldn't believe! not right for everyone... sounds like my original one i got in 1970 something. i love it.

but anyways, the ge diode trick does work. but peeps gotta try it.

i posted it here as "stupid big muff noise reduction trick" or something like that about 4-5 years ago now.

nice clip ben. i can mos def see the diff with and without.

jimmy, i tried it with several diodes. it only works in that one position, and only with ge diodes, i don't think light enters the equation once the box is sealed up. i have used 1n60 and 1n34a for this, as it's all i had.

but i do it on all the muffs i build. i suspect it will work on other fuzzes in a similar way. lately i've been using an led clipper in the feedback loop of some transistor circuits to nuke the noise... it works there, too. i found the leds there don't change the tone much but do make a significant difference in the noise level.

leaky germanium adds hiss and frying bacon noises. this mod kinda takes that away at least in THIS circuit

but... why does it work? i asked that years ago, all i got was crickets

[Ben Lyman]

Jim- a while back I remember I couldn't do the Duncan TS calculator because I have a MAC 

Anyway, I could just as easily try a bread board test, so should I change all the pot values too?
That would mean a 5k mid pot for my hot rod mod, even smaller for the "regular" re-scaling value.

Also, should I use a 50k in place of that 500k to ground? I'm not sure if you are suggesting it may be important or not.
I took it out completely and everything seems the same to me without it.

I'd like to mention again that the ROG Thor does not rescale anything