Circuit Analysis

[Nathan]

Hey all, I've been planning a new build borrowed heavily from the Mesa Blue Angel.


I wanted to ask about the 'master volume' found in the schematic (see pics).


First off, the little trim pot on the PCB has been called a Master Volume but I question the placement? I'm still a novice but have heard about the different kinds of MV and how some are better than others. What would you call this one? Would it be as simple as replacing the trim pot with a full size one or would I be better using a different style of MV? 


Next I also notice it feeds into the Mojo Module which I will not be including in my build (I bypassed it on both of my Blue Angel Combos). Do you see any potential pitfalls of feeding the Phase Inverter directly from the MV?


Really appreciate your input!

[pdf64]

I guess that the trimmer is intended to be set to avoid the module / fx loop being overdriven.
The best place for a master volume is probably between the preamp and phase splitter input, a type 4, to use the Trainwreck Pages nomenclature https://robrobinette.com/images/Guitar/Trainwreck/The_Trainwreck_Pages_searchable.pdf

If you're including an fx loop that'll work best.
The vast majority of commercial amps arrange things along those lines.

If there's no fx loop, and you want more preamp overdrive, then a type 2 or its LarMar variant would probably be better.

[PRR]

I'm wondering if the trimmer balances levels so V2b overloads just before/after/sameas the power amp. The 680K to ~~20K divider, 34:1, would bring 100V at V2b down to 3V at V5b, which is not unlikely overload level there.

Sure, use a knob-pot. Randall Smith may shudder but its your amp now.

[Nathan]

Man coming back tot this topic because I'm having one of those cold feet moments... Thanks for bearing with my newbie a$$!


I am leaning toward keeping the pot where it is but there are 2 resistors before the 20 kL trim pot adding up to 4k and a 330 ohm after it. Add these all up and you basically get a 25 k load. Would I not do well to simply use a 25 kL full size pot?


I dropped the fx loop and mojo module, so the output will go directly into phase inverter pin 2 as shown on p.2. I do realize that some of these impedances might not balance properly but my simple logic tells me it would be ok?


 


   

[sluckey]

Would I not do well to simply use a 25 kL full size pot?

That's fine.

...so the output will go directly into phase inverter pin 2 as shown on p.2. I do realize that some of these impedances might not balance properly but my simple logic tells me it would be ok?

Page 2 of what??? You must put a coupling cap between the wiper of the MV and pin 2 of the PI.


 


 

[Nathan]

Thanks Sluckey, should have read photo 2 and there is a coupling cap on the wiper! I'm going to try it like this.