# What is the Mofeta Preamp?



## jessemhopkins (Apr 23, 2021)

What is it? Need deets.


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## Devoureddeth (May 3, 2021)

Model T preamp variant


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## Feral Feline (May 5, 2021)

Is it a brass-era that has to be hand cranked, or is it a '20s with a starter button?


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## Dali (May 5, 2021)

I just found this super interesting and in DEPTH article about another source for Model T preamp:









						Coda Effects: Black Hole : Sunn Model T preamp
					

Learn more about electric guitar related electronics: DIY guitar pedals, from fuzz faces to delays and reverb, cables and circuits theory




					www.coda-effects.com
				




I'm not good enough to compare PedalPCB/EAE and Coda's version tho.


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## Feral Feline (May 5, 2021)

Dali said:


> I just found this super interesting and in DEPTH article about another source for Model T preamp:
> 
> 
> 
> ...


Don't be so modest, sure you're good enough! You've been here longer than I have.  😸



For comparing circuits/schematics what I do is open up a table in a text document.
Choose one Schematic to be the master and make that the first Column and then however many circuits I'm comparing will be how many additional columns included. Very much like how different Muff circuits are often compared, but in this Sunny case...

*MODEL T*

*CODA*
has bright channel below normal*EFFECTS LAYOUTS*
has bright channel *above* normal*PEDALPCB (TBA)**MY BUILD PREFERENCES / NOTES*R1 68kR5 68kEL's signal path R4>R5R2 1MR4 1MCoda's signal path opposite of ELR3 68kR2 68kR4 1MR1 1MR5 680rR6 680r***EL has 1k5 for Laney SupergroupR6 470kR3 470kR7 470kR7 470kETC repeat rows for caps and ...all other components...
*** Already found a huge discrepancy between the two schematics: Coda's first JFET in its bright channel has a resistor and cap from Source to Ground, but the Normal Channel's first JFET's source goes straight to ground. According to the Effects Layout schematic, the channels' JFETs are opposite with the Bright channel's 1st JFET going straight to ground. Both schematics appear to be similar at first glance, with both showing the lower channel as having the resistor and cap, but the schematics upper and lower channels are labelled opposite of each other as to which is "Bright" and which is "Normal".

~~~~~~~~

Well, that's how I compare things, as per the purple-prose example above. I'll put the board I intend to use in the first column as my master, and that way the component numbering is more easily followed if I mod something to have another circuits' values. While I'm building I can refer back to my table & notes easily, helps cut down on mistakes creeping in while making mods.

For the table, inserting a line break every 5 or 10 rows on large tables makes it easier on the eyes and easier to find/keep-track of component values and reduces errors.
A single or double line break goes between each type of component: resistors and caps, pots, ICs.
Sometimes I'll even alternate the background colour if it's a big table, again helping keep things organised visually.

Caps is usually where I find more variance between schematics, resistors less so. Differences in pot tapers are common, and pot values too.



Anyway, I found it interesting that just comparing the first few components between two PCB suppliers' schematics has already turned up several anomalies. Which supplier is correct about which is the "Bright" and which is the "Normal" channel? I don't know, I haven't compared the above two schematics/circuits completely yet, nor compared them to the original "Model T" schematic. Does it even matter that they differ as to which JFET gets the resistor/cap while the other channel's JFET goes straight to ground if both circuits work and each Bright channel sounds more trebley than the normal channel?
Again, I don't know, I'm not a circuit designer.

How you get from A to B doesn't matter, unless you're low on GAS!


PedalPCB is accurate to the original circuits, from what I've seen, but some other PCB suppliers not so much. I've found reverse HPF and LPF a lot — putting a cap before the resistor or after the resistor can potentially make a huge difference in sound, no? In a simple DOD 250 circuit, reversing the order of two components like that would, IMO, potentially make a significant difference in the way the pedal sounds or the way a pot behaves etc. Could be an improvement over the original circuit, a mistake, or an intentional change so as not to be a "clone"... whatever the reason, I like to know if it's original and if not then why it's not. I'm always looking to learn something new.

Hope the above helps somebody.


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## Dali (May 5, 2021)

Feral Feline said:


> Don't be so modest, sure you're good enough! You've been here longer than I have.  😸


Look again at my signature.

I'm still not sure what a diode do... (please don't answer me).


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## fig (May 27, 2021)

Don't forget Wilkie1s . It specs J201s.


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## mdc (May 27, 2021)

I think the big difference between the approach of the ModelFET and other sunn-ish pedals is that the ModelFET attempts to model the power section as opposed to just the preamp.


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## vlad7329 (Aug 28, 2022)

Has anyone checked the schematic from the builddoc?

I want to make my own board, but would appreciate verification on whether it works as intended


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