Octavia Voltages

[jrem]

Can anyone point me to the right reference voltages for Justin Philpott's 10-97 schematic negative ground Tychobrahe Octavia?  I have it running but it's saturated, seems like there's just too much gain coming through one of the stages.

Ideas?  Voltages?  Or should I post them first?

Thanks, John.

[jrem]

Okay, I want to bump this thread (or lack there-of) and add voltages and scope traces.  Can someone please tell me where the gain is going wrong?  Else I'm going to just start changing resistor values to try and get it in control. 

J Philpott 10/97 Octavia schematic, MPS-A18 (T1), (2) 2N3906's (T2 & T3)


Signal = 1vdc p-to-p, 0.282vac home-grown signal generator

          Signal   No Signal
T1   E     4.85   4.23
      B     4.56   3.61
     C   3.6   3.18

T2   E      2.96   2.34
     B   3.6   2.94
     C   9.19   9.2

T3   E     3.01   1.654
     B   2.97   2.34
     C   7.14   5.08

Scope traces (under 100kb each)(the bottom trace is the input signal @1volt/div, the top trace is on 2v/div):

Base of T2 http://jrem.webhop.org/5788a.jpg
Base of T3 http://jrem.webhop.org/5787a.jpg
Collector of T3 http://jrem.webhop.org/5786a.jpg

Thanks . . . John.

[jrem]

hmm, nobody?

[johngreene]

You are calling it "Justin Philpott's 10-97 schematic negative ground Tychobrahe Octavia" but the only schematic I found was positive ground.

Your voltages have the MPS-A18's base lower than the emitter, this is reversed. The MPS-A18 is a NPN transistor. The collector shouldn't be lower than the emitter either.
The 2N3906's have their collectors higher in voltage that the emitters, this too is backwards. These are PNP transistors so the collector should be lower than the emitter.

Now, if the voltages you measured are actually 'negative', then the voltages are looking more correct (since you are getting gain I'm assuming this is the case).
What happens when you adjust the 'Intensity' pot? This should increase the negative feedback to T1 and reduce the gain.

--john

[jrem]

You are calling it "Justin Philpott's 10-97 schematic negative ground Tychobrahe Octavia" but the only schematic I found was positive ground.

yes, you are right, my bad.  overlooked again.  it is a postive ground circuit.

Your voltages have the MPS-A18's base lower than the emitter, this is reversed. The MPS-A18 is a NPN transistor. The collector shouldn't be lower than the emitter either.
The 2N3906's have their collectors higher in voltage that the emitters, this too is backwards. These are PNP transistors so the collector should be lower than the emitter.

Now, if the voltages you measured are actually 'negative', then the voltages are looking more correct (since you are getting gain I'm assuming this is the case).

yes, you are probably right.  It took me thirty years to realize that the Lectron set by Raytheon was positive ground, I think that perminently messed me up.

What happens when you adjust the 'Intensity' pot? This should increase the negative feedback to T1 and reduce the gain.

--john

the intensity pot seems to work like it's supposed to, it is just that the circuit is saturated and too distorted. When I hit a chord soft it doesn't peak, but if I drive it (single coil strat) it goes into saturation big time.  It doesn't sound like any of the clips on line or the hendrix stuff (nevermind I can't play . . .)

thanks . . .