ValveWizard PCB / BJ Hybrid Project

[acheld]

Many of us will make New Year resolutions -- for some, it might be "lose weight," or "no more amps."  That's just such a non-starter, I said to myself, you can't have too many amps!  And at a certain stage of life where being buff is not so relevant.

I have lying around my bench:
1.  Blues Junior PT (Mojo780) and OT (Mojo781).
2.  Merlin Blencowe's PCBs -- 1 through 5.  See http://valvewizard.co.uk/universalpcbs.html

His PCBs have intrigued me as a way of prototyping projects one step at a time, though haven't had the time to work up a design until now.  In truth, using these small boards (which are well crafted little buggers) takes some adjustment, as the components are generally different than what some of us normally carry in stock (ie, all caps are radial rather than axial, and the package size and config does make a difference).

My goal for this project is to build an EL84 push-pull amp, with an input / EQ section very different from my stable of BJs.  I generally play clean; my favorite amp is a Hoffman Deluxe Reverb (6V6).  I do intend to modify this project over time.

I adapted Merlin's "VWAC15" schematic to the BJ PT.  The PI, output stage and power supply are close to Blues Junior stock.  The Input and EQ section were taken directly from the VWAC15 schematic -- and these sections are quite foreign to me (as a Fender amp guy).

Apologies in advance for the schematic.  I've been using Jschem on a Mac, and when I get above a certain size, the schematic crashes -- so as you will see each page represents a separate PCB.  And as far as I'm concerned, the printing is too damn small.  Part numbers on the page correspond to Merlin's boards.   

Would appreciate comments/suggestions. 

[vampwizzard]

It's an interesting project. Cant give you any advice on the schematic etc but Id be interested to know how you like Merlin's input scheme with the lower resistor and added cap.

[acheld]

Won't know until I try it . . .

Honestly, the input section design is unusual to me -- I'm sure there are plenty of folks in the forum who will understand it.  I've been a Fender guy forever, so it is not what I expected when I looked at the schematic.

Again, apologies for the "board by board" schematic -- for some reason Jschem is giving me problems now.   Maybe I should move on (but, to what?).

The design of the PCBs -- which are not expensive, by the way -- is that you can switch them around easily. 

So, I'm going to give it a shot and see what happens.  If it doesn't sound great, then I get to change it. 

[FranciscoPerez]

Really interesting project !
Reminded me that I bought some of Merlin's PCBs as well, but haven't had the time to use them either.
It would be great to see your progress and end result.


About the input circuit, I guess you wouldn't notice much of a change, apart from maybe reduced hiss, depending on the resistor material used.
Merlin lowers the input resistor to reduce resistor noise, and then adds a small cap to mantain the cutting frequency you would have with a larger grid stopper.

With a 10K grid stopper, considering 200pf 12ax7 input capacitance, you will get a cutting frequency of around 79kHz.
Adding a 1nF capacitor, you change that cutting frequency to 13kHz, and that will be the roughly the same as using a 60K grid stopper.

All using the RC low pass filter formula:
R = 1/(2*pi*f*C)

[vampwizzard]

Again, apologies for the "board by board" schematic -- for some reason Jschem is giving me problems now.   Maybe I should move on (but, to what?).

I use LTSpice and it works pretty well.

[DummyLoad]

Again, apologies for the "board by board" schematic -- for some reason Jschem is giving me problems now.   Maybe I should move on (but, to what?).

KiCAD EDA is an option for Mac OSx. Upverter is browser based, so that's another option. 

https://kicad.org/download/macos/

https://upverter.com/features/tools/

--pete

[brewdude]

I have been using a program called ElectricDesign that I got from Apple’s App Store—it wasn’t free, but not expensive (~$10, if I remember correctly).  I’ve been mostly satisfied.


I had been using Jschem, but it had stopped working with an OS upgrade.
I don’t think I ever loaded the corresponding Jschem update, I moved on. 

[Crestone]

Subscribed.  Looks to me like the first board began life as a 5E1.