Modified Tubescreamer Popping

[sta63bmx]

Here is a picture of part of my modified Tubescreamer schematic.  This is R.G.'s drawing from the website, and the mod is drawn on there just to show what I did.



The second channel in this "two channel tubescreamer" does three things (four, if you count the popping).

1. Switches between the 51k drive resistor and the 1M "more drive" pot for an increase in drive

2. Switches between a jumper or a 1M pot going from ac ground to the first lug of the volume pot (effectively adding more resistance between AC gnd and the beginning of the "regular" volume pot) for an increase in volume.

3. Lights an LED, w00t.

4. Pops. lol

I built this same circuit (using fixed resistors for the "more drive" and "more volume")  in another box, no popping.  But this one pops.  The biggest differences are using pots instead of fixed resistors and here there is about 2-3" of wire between the second footswitch doing the switching and the "more" pots.  Otherwise they are the same.  It seems like...

1. I'm doing some switching in a high-gain part of the circuit, which could be bad.

2. Instead of doing some kind of parallel resistor arrangement in either case to increase or decrease resistance, I'm doing a "this or that" switching arrangement

3. There are no pulldowns to AC ground or real ground or whatever level these circuit locations are supposed to be at.

I only posted up this picture of the drive switching, since I think he is probably most likely the guilty party.  The pop is loud, much louder than the normal "pedal on/off" pops I have heard in other boxes.  Here is a list of when the thing pops/doesn't pop.

1. Turning the whole pedal ON does NOT make a pop, either in "regular" or "more drive".  In other words, no matter which option was selected, turning the pedal on or off is silent.  So that switch and switching arrangement seems to be working out ok.

2. Changing from regular to more drive or from more drive to regular while the pedal is on DOES make it pop.  Changhing this while the pedal is bypassed does not make it pop.

Should I be doing some kind of parallel resistor arrangement rather than this or that?  Should I be using an extra pulldown to something?  Any suggestions are welcome.  I am going to look at this tonight and try switching to a parallel resistor arrangement.  For instance, when I'm in "regular" drive, I'll have the 51k in parallel with the 1M pot, so the resistance will be ~ 51k.  If the more drive pot was ALL the way down, that parallel resistance would be ~ 0 ohms, but the regular drive pot is ALWAYS engaged.  I am gonna try that first, and then try the same deal on the volume, maybe putting a 100 ohm resistor in parallel with the 1M "more volume" pot to effectively short it when the "normal" channel is engaged.

This is why building pedals is fun.  I wanted a lead boost in the pedal, I built a lead boost in the pedal.  Bam, instant doubled usefulness on my original.  The gain is a little high when it's maxed, but it's really handy for kicking on a lead sound with more gain or volume or both.  This one looks nice, but pops.  It has five knobs across the long top edge of a white Taiwanese BB (the toadworks boxes), two footswitches (with blue and red LEDs) and two toggle switches (for more/less treble and different clipping diodes).  I jokingly called it The Walshinator since the "more treble" sound is real crass like the beginning of "Funk #48" or "Funk #49".  It's a nice box, really cuts through.  I am building it for a friend and trying to get rid of that stupid pop.

Thanks in advance.  I have enjoyed this pedal a lot so far, and the ribbon cable has made it a lot more manageable.

[sta63bmx]

Please allow myself to reply to...myself.    Ok.  I fixed the popping.  Here's what I did.  In that schematic, I modified it so the 1M pot is ALWAYS engaged.  So there should be a wire from the "bottom" of that switch to the pot.  I realized that on the 3DPT by wiring the pot right to the center lug for that pole.  When you turn the boost on, it switches from the 51k resistor over to an empty lug.  That takes the 51K out and the 1M is all that the signal can go through.  but when I switch back to normal mode (instead of boost mode), it switches the 51k resistor in parallel with the 1M pot, so the resistance is effectively 51k unless the boost drive is set REAL low, in which case the value would be a little less than 51k, but that's no big deal.  That takes care of the boost drive pot.

I did the same thing to the boost volume/level pot.  The 4.5V Vb is wired to one side of the normal 100k level pot.  Increasing the resistance between AC ground (Vb) and the wiper of that pot boosts the volume.  So I wired the boost level pot like the boost drive pot, in series there between Vb and the lug of the level pot (the CCW lug?) and then there is a jumper that can be switched on of off in PARALLEL with the boost level pot.  So when boost is on, the switch goes to an open lug and that boost level pot is all that's in the path between Vb and the normal level pot.  But when the boost is switched off, that jumper is put in parallel with the pot, reducing the effective resistance to zero.

This killed the pop completely.  Maybe it's "safer" to always have something connected.  This way no potential can build up, I guess since those pots are always connected?  I really don't know.  But it worked.  I don't completely understand, it just seemed like it would be less likely to pop if something was ALWAYS connected and it wasn't this-or-that switching deal.  Fixing that problem made me feel good.  Like a winner!

Unfortunately, I could not enjoy the pedal afterwards, because I don't have a decent working amp in my garage.  However, this clears the deck for the next couple projects.

1. Building another BSIABII for my cousin (hopefully with full gain and level this time)

2. Recapping and retubing both Ampeg Geminis that are currently dead on my workbench.

I can't hang onto the pedal, either, since it's going to someone else and it was kind of behind schedule.  Here's a quick ghetto-rific picture of it.  I know, the dymo is cheesy.  He's going to relabel it and do his own graphics but I got confused enough by the knobs that I had to label it.



EVERY pedal should have a Joe Walsh switch.

[Gilles C]

You made me curious... what is a Joe Walsh switch?

I couldn't find that detail in your explanations.

Gilles

[sta63bmx]

Not only does it make life good, it also enables you to walk the Rocky Mountain Way without falling into any type of funk, be it 48 or 49.  And it enables you to perform midnight man functions, ya dig?

Joking aside, all it does is switch between a 4.7k and 1k resistor on the input to the inverting (? minus...) terminal on the opamp.  This mod is an oldie mentioned here, I'm pretty sure.  Way more high end with the 1k.  It's not always super useful, but helps nail that nasty, biting, treble sound that Joe Walsh often had.  Intro to "Funk #48" and intro to "Funk #49" both come to mind.