GK rails in a pedal?

[johnk_10]

i'll take the voltages now but here's a quick clip of what it sounds like at this point.
first bypassed and then clicked in at the 7 second mark.
then I adjust the gain pot (you should be able to hear the static as I adjust it).
as the gain is increased you can hear the distortion.
I also switched the foldback in and out at various times but you can't tell when I did it since it has no effect.

[Invalid or Expired Link Removed]

 

[johnk_10]

with a 1spot power supply @ +9.42V DC in at the DC jack.

U1 Voltages:
Pin:
1: -2.786V
2: 0V
3: -.001V
4: -8.70V
5: -.088V
6: -.092V
7: -.092V
8: +9.43V

C11: +7.63V
C12: -6.89V

TP3: +.006V

 

[johnk_10]

when you get a chance Frank, any idea what value of resistors to try instead of the 10K/100K in the foldback circuit?

 

[tallboybass]

Sounds really good, you're definitely on the right track!

 

[boomertech]

okay. I changed C3 to a .1 but there's still static on the pot and it still surges & dips, just not as drastic.

i'll take some voltages next.

IMO, it's not just rolling off highs for the fizziness, since the distortion itself is kind of' sputtery' sounding.

I will draw up a simple modification for the input level pot so that static will be gone.

i'll take the voltages now but here's a quick clip of what it sounds like at this point.
first bypassed and then clicked in at the 7 second mark.
then I adjust the gain pot (you should be able to hear the static as I adjust it).
as the gain is increased you can hear the distortion.
I also switched the foldback in and out at various times but you can't tell when I did it since it has no effect.

[Invalid or Expired Link Removed]

Sounds like your getting close. You are going to have to look at the waveform at TP3 to see what the clipping levels are.

When we were going from V1 to V2, I wasn't sure that the foldback would make much of a difference sound wise... Since the foldback should only be clipping the signal at a slightly lower voltage than the Q1/Q2 drivers when foldback is off. The cool thing is, if the foldback doesn't make much of a difference now, we can tweak the R19/R20 values to lower the clipping levels and also make them asymmetrical clippers... But you need to see the clipping.

with a 1spot power supply @ +9.42V DC in at the DC jack.

U1 Voltages:
Pin:
1: -2.786V
2: 0V
3: -.001V
4: -8.70V
5: -.088V
6: -.092V
7: -.092V
8: +9.43V

C11: +7.63V
C12: -6.89V

TP3: +.006V

DC Voltages look beautiful!!!!

when you get a chance Frank, any idea what value of resistors to try instead of the 10K/100K in the foldback circuit?

Are you talking about R23 and R24?

-Frank

 

[johnk_10]

Are you talking about R23 and R24?

-Frank

yup.

 

[boomertech]

I have used the original circuit for controlling levels before, but there wasn't a huge gain circuit hanging off the wiper. This modification should quiet the control down:

-Frank

 

[Jared Lash]

Just wanted to say that this thread has been a lot of fun to follow with a couple of my favorite TBers collaborating on something very cool.

Nice work John and Frank.

 

[boomertech]

yup.

In the original GK amp, the foldback connects directly to the collector of Q4 you can try making R24 zero ohms if you want. The foldback protection limits the signal to the output transistors to prevent speaker and transistor damage.

R23 is only there for D8 when the positive terminal on the opamp exceeds the negative supply and causes phase inversion. Otherwise the circuit never "sees" R23 and D8.

-Frank

 

[boomertech]

Just wanted to say that this thread has been a lot of fun to follow with a couple of my favorite TBers collaborating on something very cool.

Nice work John and Frank.

Thanx! It has been a lot of fun for me too! I just hope John won't need therapy after we get it finished.

-Frank

 

[johnk_10]

here's a few pics of it on the scope. sine wave at 400H,z measured at the output jack.

bypassed:

engaged clean (input level at about 50%):

just starting to clip:

input level at 100%:

foldback engaged with light clipping(???!):

full clipping with foldback engaged:

i'll have to take some measurements coming out of the input level gain stage to see at what voltage that the clipping starts (since these were all taken at the output jack).

 

[johnk_10]

Thanx! It has been a lot of fun for me too! I just hope John won't need therapy after we get it finished.

-Frank

no problem. I've been called a 'pit bull' by my friends when it comes to this stuff.

oh, and judging by the scope, there definitely is something going on with the foldback switch, but I can't hear much of a difference when it's in or out thru my amp/speaker cab.

 

[boomertech]

here's a few pics of it on the scope. sine wave at 400H,z measured at the output jack.

bypassed:

engaged clean (input level at about 50%):

just starting to clip:

input level at 100%:

foldback engaged with light clipping(???!):

full clipping with foldback engaged:

i'll have to take some measurements coming out of the input level gain stage to see at what voltage that the clipping starts (since these were all taken at the output jack).

We need to find out where that oscillation is coming from. I will draw up a test mod in a few mins.

We will also need to see the signal from the junction of R1/R2 (TP3) to determine the DC clipping levels and any DC shifts in the circuit. Wait until I have the testing mod done before taking any more measurements.

-Frank

 

[johnk_10]

okay. BTW, the anti static mod didn't work at all. with it, the input level doesn't work anymore and is a heavily gated mess.
I personally don't mind a little static (it's very quiet) but IMO, there must be another way to get rid of it.

 

[boomertech]

For testing and isolating that oscillation, remove R23 and R31. Make R24 zero ohm, then place a temp jumper from the collector of Q4 to the junction of D2/D3.

This will eliminate any affect that U1b has to the circuit.

***REMOVED IMAGE***

-Frank

 

[boomertech]

okay. BTW, the anti static mod didn't work at all. with it, the input level doesn't work anymore and is a heavily gated mess.
I personally don't mind a little static (it's very quiet) but IMO, there must be another way to get rid of it.

We will find a way to make that pot quiet.... But let's keep working on that oscillation first.

-Frank

 

[boomertech]

For testing and isolating that oscillation, remove R23 and R31. Make R24 zero ohm, then place a temp jumper from the collector of Q4 to the junction of D2/D3.

This will eliminate any affect that U1b has to the circuit.





-Frank

John. Hold off connecting the collector of Q4 to the diodes... there needs to be a buffer in there. I'll have another drawing in a few mins... I need to run simulations first.

-Frank

 

[johnk_10]

okie dokie.
IMO, we're really not that far away from this circuit sounding great.

 

[boomertech]

OK this works in spice....

Remove R23 and R31
Make R4 and R5 47K (the 10K's were too much of a load for Q4)
Connect the collector of Q4 to the base of Q2.

Measure the DC at pin 1 of U1 (it should still be only a few volts negative)
Check signals for oscillation again TP3.

-Frank

 

[DirtyDuke]

Mad geniuses I tell ya....