Pete Cornish - G2 Guts, Schematic, Layout (from Tracer's Fund)  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
User avatar
Greg
Old Solderhand
Information
Posts: 3047
Joined: 03 Nov 2007, 09:35
my favorite amplifier: Tophat Emplexador & Supreme 16.
Completed builds: LOTS..
Location: Australia
Has thanked: 65 times
Been thanked: 164 times

Post by Greg »

A theoretical gain of 3 dB, in a heavily clipped and distorted circuit, is by no means the same as an actual 3db of gain.
Remember also that PSpice is a simulation.. very good, but it has certain limitations.

For me the "meme" in this is the commonly held belief that transistor type and gain are critical to the Big Muff sound.
culturejam wrote: We are equal opportunity exposure artists.

User avatar
culturejam
Old Solderhand
Information
Posts: 4062
Joined: 24 Feb 2008, 05:59
Has thanked: 433 times
Been thanked: 563 times
Contact:

Post by culturejam »

lolbou wrote:Which gives, no matter what hfe matching the previous criteria:

Code: Select all

Gv ~-Rc/Re
Precisely my point in asking the question. You've spelled it out quite nicely, sir. Well done. :thumbsup

User avatar
tschrama
Breadboard Brother
Information
Posts: 118
Joined: 17 Aug 2008, 08:39
my favorite amplifier: SE El84 DIY .. :D
Completed builds: Distortion +
Tripple RAT
about 5 Big Muffs
Tubescreamer
JFET simulator
5 Watt SE El84 versions Marshall 2204
5 Watt SE El84 versions Mesa Recto
100W 4xEl34 Mesa Recto
Has thanked: 24 times
Been thanked: 4 times

Post by tschrama »

Hi all,

I think you are all right to some degree but the in the first three big muff gain stages has two feedback mechanisms
1] the emitor resistor, 2] the input series resistor and collector bias resistor.

Without Collector feedback, you are right that the Rc/Re would determine this stage gain in a c. This is only thrue as long as 1/Re is much smaller than the BJT transductance. In a typical BMP, Re = 100R and Rc is 10-to-22K. Gain would then be 100 to 220. The gain of a BMP is about 30-to-35, so obviously somethin else is going on:

The Collector feedback resistor is typically 470K, input series resistor is typically 8k2. If the Vac at the base is small, we can state that Vin/8K2 = -Vout/470K, That leads to Vin-/Vout is equal to 8K2/470K, which is about 57.

Yet the gain stage is neither 100x to 220x, nor is is 57x is it about 30x to 35x. So both assumption associated with both these simplified gain calculations do not apply and transductance and Hfe of the BJT does is significant for the real realized gain.


PS
don't mean to hijack this topic; Mods: maybe we can move this discussion to a seperate thread?

PPS
Pspice is dead on near perfect for these simple, low freq, AC and DC calculations.

PPS
3dB is significant, unless so think that turning gain pot back to to 70%to80% is not significant. It has nothing to do with the fact that the signal might clip.

PPS, Hfe of 120 would lead to 6dB gain reduction in a typical BMP. Thats halve of your gain.
Build your own 50W, three channel, amp in just two days at www.sgravenmade.com

Copyright does not protect facts, ideas, systems, or methods of operation, although it may protect the way these things are expressed. US Copyright Office

User avatar
tschrama
Breadboard Brother
Information
Posts: 118
Joined: 17 Aug 2008, 08:39
my favorite amplifier: SE El84 DIY .. :D
Completed builds: Distortion +
Tripple RAT
about 5 Big Muffs
Tubescreamer
JFET simulator
5 Watt SE El84 versions Marshall 2204
5 Watt SE El84 versions Mesa Recto
100W 4xEl34 Mesa Recto
Has thanked: 24 times
Been thanked: 4 times

Post by tschrama »

lolbou wrote:
culturejam wrote:So then how is gain determined in a common emitter amplifier?

Code: Select all

Gv = (-Rc x hfe) / (Re x (hfe + 1) + h11)
For the first stage of a big muff, with h11 around 1k (typical), hfe over 100 (as R.G. suggested) and Re = 100, you can consider h11 as irrelevant (like 5% or less):

Code: Select all

Gv ~ -(Rc x hfe) / (Re x hfe)
Which gives, no matter what hfe matching the previous criteria:

Code: Select all

Gv ~-Rc/Re
:mrgreen:
Edit, sorry
This is emittor feedback gain stage right? For the BMP there are two complicatng factors 1.You did not inlcude the colledtor feedback to Base. 2. You did not inlcude the loading of the stage by the next Rbaseseries, usualy 8K2.

Talking about collector feedback
Gv is -Rfb/Rseries. Only thrue if you assume the Vac at the base is zero, which it is not. :mrgreen:
Last edited by tschrama on 27 Oct 2011, 09:39, edited 1 time in total.
Build your own 50W, three channel, amp in just two days at www.sgravenmade.com

Copyright does not protect facts, ideas, systems, or methods of operation, although it may protect the way these things are expressed. US Copyright Office

User avatar
tschrama
Breadboard Brother
Information
Posts: 118
Joined: 17 Aug 2008, 08:39
my favorite amplifier: SE El84 DIY .. :D
Completed builds: Distortion +
Tripple RAT
about 5 Big Muffs
Tubescreamer
JFET simulator
5 Watt SE El84 versions Marshall 2204
5 Watt SE El84 versions Mesa Recto
100W 4xEl34 Mesa Recto
Has thanked: 24 times
Been thanked: 4 times

Post by tschrama »

Greg wrote:A theoretical gain of 3 dB, in a heavily clipped and distorted circuit, is by no means the same as an actual 3db of gain.
Remember also that PSpice is a simulation.. very good, but it has certain limitations.

For me the "meme" in this is the commonly held belief that transistor type and gain are critical to the Big Muff sound.
:hmmm: .. didn´t think about that.
Build your own 50W, three channel, amp in just two days at www.sgravenmade.com

Copyright does not protect facts, ideas, systems, or methods of operation, although it may protect the way these things are expressed. US Copyright Office

User avatar
Greg
Old Solderhand
Information
Posts: 3047
Joined: 03 Nov 2007, 09:35
my favorite amplifier: Tophat Emplexador & Supreme 16.
Completed builds: LOTS..
Location: Australia
Has thanked: 65 times
Been thanked: 164 times

Post by Greg »

There's a good discussion here (which is where I borrowed RG's comment from):
https://www.diystompboxes.com/smfforum/ ... ic=60143.0
culturejam wrote: We are equal opportunity exposure artists.

User avatar
engineelite
Information
Posts: 37
Joined: 11 Jun 2010, 22:47
Has thanked: 4 times
Been thanked: 41 times

Post by engineelite »

So I built a G-2 clone and it sounds excellent but a tad dark in "bypass" and "active" modes. Does anyone have any recommended value changes in the buffer to make it a little brighter?

Thanks.

User avatar
kleuck
Cap Cooler
Information
Posts: 469
Joined: 15 Nov 2008, 23:24
Completed builds: I build and sell my own :)
Location: France
Has thanked: 42 times
Been thanked: 27 times

Post by kleuck »

tschrama wrote:Hi all,

I think you are all right to some degree but the in the first three big muff gain stages has two feedback mechanisms
1] the emitor resistor, 2] the input series resistor and collector bias resistor.

Without Collector feedback, you are right that the Rc/Re would determine this stage gain in a c. This is only thrue as long as 1/Re is much smaller than the BJT transductance. In a typical BMP, Re = 100R and Rc is 10-to-22K. Gain would then be 100 to 220. The gain of a BMP is about 30-to-35, so obviously somethin else is going on:

The Collector feedback resistor is typically 470K, input series resistor is typically 8k2. If the Vac at the base is small, we can state that Vin/8K2 = -Vout/470K, That leads to Vin-/Vout is equal to 8K2/470K, which is about 57.

Yet the gain stage is neither 100x to 220x, nor is is 57x is it about 30x to 35x. So both assumption associated with both these simplified gain calculations do not apply and transductance and Hfe of the BJT does is significant for the real realized gain.


PS
don't mean to hijack this topic; Mods: maybe we can move this discussion to a seperate thread?

PPS
Pspice is dead on near perfect for these simple, low freq, AC and DC calculations.

PPS
3dB is significant, unless so think that turning gain pot back to to 70%to80% is not significant. It has nothing to do with the fact that the signal might clip.

PPS, Hfe of 120 would lead to 6dB gain reduction in a typical BMP. Thats halve of your gain.
Plus, you have to remember that the max gain for a single transistor (and a resisitve load) is limited by the voltage supply and "transconductance".
Randall Aïken said :
Q: Is there any advantage to using solder with a 2% silver content?
A: Yes. Silver solder keeps werewolves away from your amp.

User avatar
zappato
Information
Posts: 2
Joined: 15 Nov 2007, 17:34

Post by zappato »

I just built this pedal and the sound is great.
thanks

User avatar
PokeyPete
Resistor Ronker
Information
Posts: 371
Joined: 31 Aug 2008, 01:45
Has thanked: 11 times
Been thanked: 43 times

Post by PokeyPete »

Sorry for digging up an old thread, but I'm curious about something.

Dirk, when you were degooping and tracing this pedal, did you happen to note the brand/type of coaxial cable used?
“No man is so foolish but he may sometimes give another
good counsel, and no man so wise that he may not easily err
if he takes no other counsel than his own. He that is taught
only by himself has a fool for a master.”
–Hunter S. Thompson

User avatar
Dirk_Hendrik
Old Solderhand
Information
Posts: 4193
Joined: 03 Jul 2007, 08:44
Location: Old Amsterdam
Has thanked: 232 times
Been thanked: 887 times
Contact:

Post by Dirk_Hendrik »

Nope. Sorry.
Other than that, while re-reading the last pages of this thread, I have never had the feeling that at any point real effort was put in component selection. All seemed "oft-the-shelf" stuff form the first available supplier.
Sorry. Plain out of planes.

http://www.dirk-hendrik.com

User avatar
PokeyPete
Resistor Ronker
Information
Posts: 371
Joined: 31 Aug 2008, 01:45
Has thanked: 11 times
Been thanked: 43 times

Post by PokeyPete »

Dirk, thanks for that super fast reply! I remember reading that Pete C. had said that he tested numerous
cables before he found the perfect one (my phrasing). I was looking at cables today for a project and this
came to mind. I'm not mojo minded, but I was curious to know what cable that was, so that was the
reason for my asking. Thanks again.

PP
“No man is so foolish but he may sometimes give another
good counsel, and no man so wise that he may not easily err
if he takes no other counsel than his own. He that is taught
only by himself has a fool for a master.”
–Hunter S. Thompson

User avatar
mike76700
Information
Posts: 1
Joined: 29 Jun 2012, 12:15
Has thanked: 1 time

Post by mike76700 »

thanks it's great job!!!!! :D :D

User avatar
andregarcia57
Cap Cooler
Information
Posts: 572
Joined: 16 Nov 2008, 15:42
Has thanked: 73 times
Been thanked: 109 times

Post by andregarcia57 »

thanks!

User avatar
fakcior
Breadboard Brother
Information
Posts: 68
Joined: 31 Oct 2010, 12:22
Location: Poland
Has thanked: 4 times
Been thanked: 20 times

Post by fakcior »

I bet Kitrae could solve diodes mystery by degooping Toptone DG-2 or at least just little part with diodes and post some high-res pictures. What you say, Kitrae?
Cheers :)

User avatar
sinner
Old Solderhand
Information
Posts: 4709
Joined: 06 Nov 2008, 17:16
Location: ...no more
Has thanked: 1031 times
Been thanked: 909 times

Post by sinner »

How about that - sell two more Meathead copy's, buy one yourself and make a contribution?

For fuck sake, anyone with a pair of ears and half of brain could do better than Mr.Cornish as we already learned - nothing groundbraking, just well know circuits with double input buffers

I already started a tread with some suspicions based on Cornish description, scraps of info and what Gilmour was using, with hope we can figured things together, but you bitches are sadly to lazy

User avatar
fakcior
Breadboard Brother
Information
Posts: 68
Joined: 31 Oct 2010, 12:22
Location: Poland
Has thanked: 4 times
Been thanked: 20 times

Post by fakcior »

Unfortunately in Poland nobodys interested in Meatheads ;|
I would contribute, but at this moment I'm busy tracing and looking for right components of headphone amp.
..And can you post link to your topic, Sinner?
edit: okay, got it, but nothing revolutionary there.:(

User avatar
sinner
Old Solderhand
Information
Posts: 4709
Joined: 06 Nov 2008, 17:16
Location: ...no more
Has thanked: 1031 times
Been thanked: 909 times

Post by sinner »

fakcior wrote: ..And can you post link to your topic, Sinner?
edit: okay, got it, but nothing revolutionary there.:(
Like I said - Pair of ears and half of brain is minimum to figure it out

User avatar
fakcior
Breadboard Brother
Information
Posts: 68
Joined: 31 Oct 2010, 12:22
Location: Poland
Has thanked: 4 times
Been thanked: 20 times

Post by fakcior »

Like I said - Pair of ears and half of brain is minimum to figure it out
Maybe that's why nobody hasn't found a solution yet

User avatar
sinner
Old Solderhand
Information
Posts: 4709
Joined: 06 Nov 2008, 17:16
Location: ...no more
Has thanked: 1031 times
Been thanked: 909 times

Post by sinner »

Again, because you guys are lazy bastards.

I'm no brainiac in "EE" world, but Im not as blind to still believe in "magical diodes". There is at least three other spots in BMP schematic you can bright up the circuit, and having Kit's website you mentioned, with semi complex BMP technology page I can't believe my eyes you still seeking for solution around the diodes.

Dude, you're AGH student, you can do better than this

Post Reply