trying to understand/plan mesa G-EQ (Mark V)

[jplebre]

Hey guys

So, got to play around a lot with a Mark V on a couple of sessions I did.
that G-EQ kinda got me...

Got the schems on the internet. And of course they have a lot of stuff in there (relays for different channels, the "preset" mode, etc.)
Below is the watered down version of it. Couple of questions come to mind:

1 - Most switching is done by J-Fets (J175 to be exact). At the bottom of the schematic there was a series of switching (EQ from Sliders, From "Preset", From whitch channel they'd come from) and an extra J-Fet labeled EQ.
I'm assuming that this is the "bypass" for the EQ. Meaning the signal is buffered constantly, regardless the EQ is on.
Am I getting this straight? Replacing the bypass with a switch would cause the EQ to come in/out, because the signal would have to go through the 1M resistor instead? (R15 and SW1, bottom right of schematic).

2 - The schematic as is needs a 24v rail. That is feeding Q1, Q3 and Q4 directly (MPSA20 and ECG159). Any way we could power this with 18v? (more reasonable for a pedal?)
Would this affect only the headroom?

3 - This schematic comes after 6 valves stages (for Ch3 MarkV mode, which I can also post soon). Right before this EQ, there's the master level pot, 100K - A. (after a coupling cap, of course) (btw, did I got this right? caps between stages are coupling, not de-coupling?) My worry is that the signal that would come from another pedal won't be enough to drive the MPSA20. Would it be wise to change the transistors for something else?

4 - Finally (I think, at least these are the most immediate things bugging my mind) if you look at the RC bit C2 and R16 -> GND. Between C2 and R16, this was the alternative "Output" point in case we were using channel 2. I am not certain wether to take it out as I'm not sure if that RC counts as parallel to C9 and R9. As in, effectively counting as a parallel set of resistors and capacitors.
If that's the case, I'm sure I can calculate it down to 1 set of RC?

I looked online there are some people that sell the inductors (£11 each). Mesa branded. Other than that is a bit hard to find those values. Would it be cheaper or different sounding to go for a gyrator design?
I spoke with a guy that had a long recording career in the states (let me just say he was hanging out with some of the coolest "cats" in the west coast) and he swears by the inductor based gear over the "simulated inductor" option. (Can't dispute that some of the best of equipment he has in HIS shed of dreams (the real thing, not the SSL shed of dreams) is some of my favourite gear).

Ideas, opinions?
Your help would be greatly appreciated. I'll then revise the schematic and play around with implementing a PCB layout for it (might wait for the inductors to arrive - if the discussion goes towards the "real thing" - to see if they are the same size and implement them on the board as well and all).

Thanks in advance

[CynicalMan]

Read this: http://www.geofex.com/Article_Folders/EQs/paramet.htm


1) The 1M resistor turns off the EQ when it's not shorted. The signal is still buffered.

2) The power voltage changes the headroom. 18V should be fine, you could even try out 9V.

3) The first stage is a unity-gain buffer. The input level shouldn't matter.

4) You can just remove C2 and R16 if you're not using that output.


Use gyrators instead of inductors. Much cheaper. You won't notice the difference. Tell you friend that they're real inductors and he won't either.

[defaced]

Try getting samples from Wilco Inductors.  This has been done by some other people building Mark series amps.  I've seen gyrators discussed many times, but not seen them implemented.  I have no opinion one way or the other, just this is what I've seen. 

[jplebre]

Hey guys!

@CynicalMan cheers for your replies! So I wouldn't need to change the transistor models at all!
@Defaced hmmm how do I figure which type of inductor I need?

Any good articles on inductor types?
not wikipedia like please :S

[CynicalMan]

The transistors are just generic AF models. You can use the original transitors, but you should be able to use 2N3904 and 2N3906.