Question for all: what is the point of C11 in the schematic JMF just posted?
C12, C13 and C14 keep any DC out of the tone pots and IC1B is referenced to ground at pin 5
(why do I ask? C11 stops one replacing the two LEDs with an assymetrical clipper)
C12, C13 and C14 keep any DC out of the tone pots and IC1B is referenced to ground at pin 5
(why do I ask? C11 stops one replacing the two LEDs with an assymetrical clipper)
Thanks - I was just wondering if I was missing something obvious - in a budget amp I don't expect to see redundant bits 😕
That's the plan!😀
Well, finally got to tear it down again today (busy life and the amp's been busy too). Bypassing C11 has resulted in the desired asymetric behaviour.
Now I just need to work out what;s gone wrong in the reverb recovery circuit - if I unplug the tank I can see the signal come back out, plug it into the circuit and there's just a bit of noise. :-(
First thing is to see if pin 5 of IC5B is shorted to ground. R57, C41, C42 will need checking. Other half of the op amp is working fine. Pot is fine (first thing I checked). Not sure what else could cause this. 🤔
Some gut shots before I put it all back together. Left to right, veroboard with diode/resistor array hot glued onto edge (dodgy as, bro). DPDT/centre off switch which gives the choice of smooth clipping, no LED or factory default (LEDs are mounted on the switch under tape). C11 bypass is circled. (Behind the LED is the boost gain control pot wiring).
Screenshot of smooth clipper doing it's thing to a 600ish Hz signal. Not a whole pile of headroom above this (about 3db) before you get limiting. Very different to both default and LED-removed sound
Screenshot of smooth clipper doing it's thing to a 600ish Hz signal. Not a whole pile of headroom above this (about 3db) before you get limiting. Very different to both default and LED-removed sound
If signal is too asymmetrical sound like farts or broken.
But can be desirable for Fuzz type distortion.
Basically the trick to get the real buzzy splatter type fuzz
change transistor offset with bias.
can also be done with opamp with single supply.
easier with inverting stage. virtual ground is not set to mid supply
on non inverting pin.
so causes dc offset. when opamp driven to distortion
clipping asymmetrical.
C11 DC blocking cap.
typical fet opamp like TL072 etc etc
3 to 5mv DC offset on output. block DC
from changing clipping diode.
with single supply more important
since half supply on output. clipping diodes
dont work right.
But can be desirable for Fuzz type distortion.
Basically the trick to get the real buzzy splatter type fuzz
change transistor offset with bias.
can also be done with opamp with single supply.
easier with inverting stage. virtual ground is not set to mid supply
on non inverting pin.
so causes dc offset. when opamp driven to distortion
clipping asymmetrical.
C11 DC blocking cap.
typical fet opamp like TL072 etc etc
3 to 5mv DC offset on output. block DC
from changing clipping diode.
with single supply more important
since half supply on output. clipping diodes
dont work right.
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That's about as close as I could get the switch without hitting the PCB. You might be clever than me
Funny you should ask: I've been looking for the sketch 🙂
Fundamentally it is two sets of diodes + resistors, four in each set IIRC. Each diode is in series, but the resistors are in parallel.
The fun bit is selecting the diode voltage and resistor values to play off the 1K drive resistor (R15) and the c. 80K tone stack impedance..
My mistake so far was to not use all of the available headroom, using only plain diodes through to green LEDs.
Next time it'll be schotkys and zeners. to use up the whole 15V on each rail and get more db between the first and last.
Excellent. I have a Vox Cambridge 15 - like yours but no reverb but a 12ax7 in the preamp. I have to keep going back to my notes to remember all the mods I have made. Biggest mod was to hardwire a preset delay pedal after the preamp to make up for the lack of reverb.
To join two threads together (see here), someone suggested another option
)
The schematic and mod is below, the pot in this schematic is not there in the stock amp.