Roger Mayer Octavia - on buffers and more

[Renegadrian]

Folks, a guy asked for a mxr octave JH in exchange to one of his pedals.
I found the mxr schem at the other forum, basically it's justa a RM octavia with an IC buffer (in and out) in place of the transistor based out buffer of the RM. I proposed him the RM, as I have a compact layout of it, and it seems super easy, while there is no vero of the mxr and I want to waste zero time on it. Now, problem is that I only have 2 MPSA13, so the 3rd one has to be changed for another tranny. I *think* that there should be zero problem, as after all it's just the buffer part, so putting a different tranny would sound the same. Is some BC5** a good choice?! or even a common 5088 3904 and so on?! I got several of the cited trannies, so no problem. Guess the BC5** are closer to the 13, thinking about their gain.
Shall I have to change something in the buffer circuit part?! or another tranny would fit right away?!
also, thinking about that mxr, what's the point of having a dual IC buffer?! do you have that pedal?! do you think it makes a notable difference?! thx for the copious replies!!! :-)

[Kipper4]

Are the MPSA13 not Darlingtons?

[rutabaga bob]

Yes, they are...I guess you could do a 'roll-your-own' without any problem to substitute.

[Renegadrian]

would it matter so much which transistor is used for output buffer?!

[antonis]

would it matter so much which transistor is used for output buffer?!

In theory, No..!!

In practice, the transistor hfe sets the values of voltage divider base bias resistors in the way that 10 x (R1//R2) < hfe x Re
(for a "stiff" voltage divider..)

So, if we use a high hfe tranny (like a Darlington pair) we are able to also use high value resistors so the preceding stage "sees" a high total impedance..
(for a little signal loss..)

[Gus]

Q3 and Q4 as Darlingtons does not make sense to me because of the two Vbe drops and this is working at 9VDC

I would use a high beta NPN for Q3 however you will need to adjust the 470k 330k voltage divider for the best octave

Q4 at 9VDC with two Vbe drops lower than 1/2 the supply voltage  at the emitter is odd.
I would use a high beta/hfe transistor with a 1.8meg or lower at the "top" with a 2.2meg at the bottom

[Renegadrian]

ok thx for replies - q2 and q3 I am gonna go safe as I have two mpsa13 for these slots, while I have to fill the buffer slot with some other tranny - think I will go BC547 or 548 and leave the other components the same, changing them just in need.

[Gus]

I think Q3 as a Darlington is a design mistake at 9VDC.  Q3 is where you get the octave from, two VBE drops at 9VDC limits the phase splitter outputs.

[Gus]

this thread was the inspiration for this http://www.diystompboxes.com/smfforum/index.php?topic=114636.0