TB MK2: what R2 does

[Mugshot]

last night i desoldered all the components of my Tonebender MK2 PNP negative ground build after i drilled a modified PCB (using MS Paint) of the one from GGG; i made it smaller so it can fit on a regular 1290NS enclosure (horizontal placement) since the original was kinda longer.

i was transferring all the components one by one when i found out that the resistor value for R2 (10K) i installed was actually 100K! when i was building the TBender, i have no solid background knowledge on determining resistor values through color bands (currently, i have to consult an online site to get the value of each).

yeah as reported, my build works. i just want to know what R2 does and what will happen if this is increased 10-fold. thanks!

and oh, yes the schematic, http://www.generalguitargadgets.com/diagrams/tonebender_m2p_sc_pn.gif

[mac]

If Q1 is low leakage, a 100k decreases collector voltage Vc. The working voltage is around 7.5v to 8.7v depending on leakage. As you lower Vc there is more gain, but some tonal characteristics begin to change.
Check Vc using a 10k annd see if you are near the above values.

mac

[leeyoungun]

If Q1 is low leakage, a 100k decreases collector voltage Vc. The working voltage is around 7.5v to 8.7v depending on leakage. As you lower Vc there is more gain, but some tonal characteristics begin to change.
Check Vc using a 10k annd see if you are near the above values.

mac

So theoretically, if Q1 has high leakage, 10K would be a better choice? Would this also be dependent on gain/hfe? I have a 100K in my BYOC MKII and threw some OC76s with the lowest gains being around 130, but leakages around 400uA and the thing sounded nearly identical to my vintage Supa Fuzz. I originally had a 10K, but with low gain and low leakage OC75s, the fuzz was muted and sounded more like an anemic Fuzz Face. 

[petemoore]

  There were threads, even charts and an Hfe calculator Joe Davisson devised.
  Early experiments with FF and Ge transistors seemed to indicate that when I had to alter bias resistor values to a great amount in order to get 'x' sound or bias voltage, I was better off sorting through to find transistors that biased easily with values that were closer to those commonly seen in schematics.

[Mugshot]

wow so many replies! makes me hold back from messing with my TB MK2 and alter R2 since it is already working. the only problem (if that's what it is for other people) is that mine is very raw and i cannot be dialed in to get it into "clean" territory even if the 1K pot is full CCW.

im planning to build another TB MK2 using NPN silicon, and by that time i think i have learned more than what i have when i assembled my first TB MK2.

fun project FTW! 

[Joe Hart]

If you like it, I would leave it... then just build another one!!
-Joe Hart

[leeyoungun]

There isn't much clean up with a MKII. It's not nearly as dynamic as a Fuzz Face. Try replacing the 47K resistor(R3) off Q2 with a 100K. It should smooth things out a bit.

[Gus]

The output resistance of Q1 is ~=R2 the 10K.

  Q2's gain is partially set by R4/R2 (you also have the open loop gain of Q2) 100K/10K

So instead of the guitar source Z going to the input imperfect summing junction of a ff you have the output resistance of Q1 (about 10k) as the source resistance.

There are three gain stages In the Mk2, Q1 and the gain is affectted by the operation points(closer to sat higher, closer to cutoff lower), Q2 open loop and closed loop and Q3 and the gain pot setting.

[Mugshot]

i have decided not to mess with it  i'll be building a silicon version of the MK2 Pro and house both in one enclosure, and im gonna call the TWINBENDER!


as for the technical replies =  i do have to learn much  but im slowly learning (im less than 2 months into pedal building mind you) but somehow im starting to grasp some basic concepts. thank you guys, to the DIY community. this is gonna be fun! 

[Gus]

Check Doug H's Hot Si for a hint for the first gain stage and the two transistor Si distortion section.

Three transistor 1st stage controlled gain and input and output resistance, 2nd transistor controlled open loop gain and feedback resistor picked for closed loop gain, third transistor gain and bias control.