Mindbender (NPN Tonebender MKII) Build with low gain transistors

[Exactopposite]

I recently built a Tonebender MKII using the Mindbender schematic from Fuzzcentral. The only exception being that I mounted the bias pot on the case instead of using a trimmer. I biased it much higher than the suggested 4.5 volts on the collector of Q3. I think Mine is set at about 7.5 or so. it sounds too thin to me at 4.5V.

I used some low gain GE transistors and it sounds phenomenal. I played my first gig with it on Sat night and I was really impressed by it.  I know people tend to build them with higher gain transistors, but it's got A LOT of gain. I keep the gain knob at about half way and the sustain is crazy as it is.

Here are the specs of the transistors I used:
Q1 HFE - 55 Leakake 150 uA
Q2 HFE - 60 Leakage 275 uA
Q3 HFE - 70 Leakage 300 UA

Anybody else here have experience with low gain transistors in a MKII?

[LucifersTrip]

I recently built a Tonebender MKII using the Mindbender schematic from Fuzzcentral. The only exception being that I mounted the bias pot on the case instead of using a trimmer. I biased it much higher than the suggested 4.5 volts on the collector of Q3. I think Mine is set at about 7.5 or so. it sounds too thin to me at 4.5V.

I used some low gain GE transistors and it sounds phenomenal. I played my first gig with it on Sat night and I was really impressed by it.  I know people tend to build them with higher gain transistors, but it's got A LOT of gain. I keep the gain knob at about half way and the sustain is crazy as it is.

Here are the specs of the transistors I used:
Q1 HFE - 55 Leakake 150 uA
Q2 HFE - 60 Leakage 275 uA
Q3 HFE - 70 Leakage 300 UA

Anybody else here have experience with low gain transistors in a MKII?

It's really only Q3 that's a little lower than the "norm". For instance, Fuzz Central says ~ 70, 70, 100

Also, Q2, Q3 are basically a Fuzz Face and I've made a bunch with 50/70, 60/60, 70/70, etc, that sound cool.

I think the biggest change you made is the 7.5v. Though, I believe there was a discussion here regarding that...that there were many variations of the TB and some do not have Q3C at 4.5v

According to FC, it is 4.5v for the Mindbender:
MindBender NPN Test Voltages
Vin
   9.65V
Q1 Vc (hfe=70)
   8.11V
Q2 Vc (hfe=70)
   1.10V
Q3 Vc (hfe=100)
   4.5V

[Exactopposite]

I think the biggest change you made is the 7.5v. Though, I believe there was a discussion here regarding that...that there were many variations of the TB and some do not have Q3C at 4.5v

According to the info on the DAM forum 4.5 on Q3C is way low and my ears agree. Stuart Castledine posted voltages from 3 vintage MKII's there which were as follows:
OC75 MkII (10K on Q1 base, 47K on Q2 collector)
Battery -9.01
Q1 c -8.50 b -0.038 e 0
Q2 c -0.13 b -0.076 e 0
Q3 c -8.29 b -0.13 e -0.08

OC81D MkII #1 (100K on Q1 base, 100K on Q2 collector)
Battery -9.55
Q1 c -8.51 b -0.06 e 0
Q2 c -0.13 b -0.068 e 0
Q3 c -8.72 b -0.13 e -0.07

OC81D MkII #2 (100K on Q1 base, 100K on Q2 collector)
Battery -9.50
Q1 c -8.76 b -0.062 e 0
Q2 c -0.22 b -0.075 e 0
Q3 c -8.13 b -0.22 e -0.14

[LucifersTrip]

I think the biggest change you made is the 7.5v. Though, I believe there was a discussion here regarding that...that there were many variations of the TB and some do not have Q3C at 4.5v

According to the info on the DAM forum 4.5 on Q3C is way low and my ears agree. Stuart Castledine posted voltages from 3 vintage MKII's there which were as follows:
OC75 MkII (10K on Q1 base, 47K on Q2 collector)
Battery -9.01
Q1 c -8.50 b -0.038 e 0
Q2 c -0.13 b -0.076 e 0
Q3 c -8.29 b -0.13 e -0.08

OC81D MkII #1 (100K on Q1 base, 100K on Q2 collector)
Battery -9.55
Q1 c -8.51 b -0.06 e 0
Q2 c -0.13 b -0.068 e 0
Q3 c -8.72 b -0.13 e -0.07

OC81D MkII #2 (100K on Q1 base, 100K on Q2 collector)
Battery -9.50
Q1 c -8.76 b -0.062 e 0
Q2 c -0.22 b -0.075 e 0
Q3 c -8.13 b -0.22 e -0.14

right...like I said, there was an old thread here regarding versions of the TB that were not 4.5, but you wrote that you built it from Fuzz Central, which was 4.5 (last schematic).
http://fuzzcentral.ssguitar.com/mkII.php

...there was much confusion in that old thread because there are various versions of the vintage TB

[Exactopposite]

Yeah i get what you mean. I just used the fuzzcentral Mindebender schematic and ignored the part about 4.5 volts

[deadastronaut]

out of curiosity, i just made a layout for this npn version with onboard pots, and a 3 trimmer version to adjust the bias on all 3 trannies......would it be ok with say 2n3904's...2n2222a's or any npn's ?...

haven't breadboarded it yet!...cheers rob.

[Exactopposite]

Breadboard it and give it a shot. Once you have it built you can easily swap out transistors and see what works for you. Silicon transistors are super cheap, so I see no reason not to.

[mnordbye]

Just wanted to add that I've also had luck with using low-gain germaniums. I used 3 that all had 67HFE, bought them from Smallbear I think. Sounds great!

Though I do have mine around 4,5 volts, instead I did some "bass" mods, removed one of the resistors before the tone control. Used some of the Tonebender mods here: http://home-wrecker.com/fuzz.html

Magnus

[LucifersTrip]

out of curiosity, i just made a layout for this npn version with onboard pots, and a 3 trimmer version to adjust the bias on all 3 trannies......would it be ok with say 2n3904's...2n2222a's or any npn's ?...

haven't breadboarded it yet!...cheers rob.

It would probably work since with many Fuzz Face variants you can sub silicon for ge...but it most likely won't sound as good since the specific values of the components were designed for ge.

like a si Fuzz Face, you just switch the 440 to 330, maybe you can do something just as simple here.

[AnalogPackrat]

I've currently got one breadboarded as well.  It's setup for Si with Q1 Re = 2.5k and 470k Rbc on Q1.  I've tried a bunch of transistors, but the best sounding set so far is:

Q1  2N2369 (hfe = 40)
Q2  2N5856 (hfe = 94) (old TI epoxy dome package--I have a few with a range of hfe)
Q3  2N4124 (hfe = 135)

I've got a pot and 2k series resistor for Rc on Q3 and 220R in series with a 1k pot for Q3 Re.

The tone is thick and growly, but my fuzz knob doesn't do much.  Until about 1/3 of the way up Q3 seems to be cutoff.  It's the same with all transistors, so maybe I have a wiring error around Q3.

I plan to try out some NPN Ge next and maybe try to add a 50k pot in series on the input or something between Q1 and Q2 to allow for some taming of this wild beast.  I'll post some voltages from my build tonight or tomorrow if anyone is interested.

AP

[jrod]

out of curiosity, i just made a layout for this npn version with onboard pots, and a 3 trimmer version to adjust the bias on all 3 trannies......would it be ok with say 2n3904's...2n2222a's or any npn's ?...

haven't breadboarded it yet!...cheers rob.

You may need to adjust the bias on Q1. Not 100% sure with the Mindbender, but the TB MKII Q1 needs some leakage to bias well. I have tried si transistors in a MKII but needed a ge with some leakage to get it bias.

I have also tried low gain ge's in an MKII with great results. I have low gain OC71's I got from SmallBear and they work well in the Q1 spot.

[jrod]

out of curiosity, i just made a layout for this npn version with onboard pots, and a 3 trimmer version to adjust the bias on all 3 trannies......would it be ok with say 2n3904's...2n2222a's or any npn's ?...

haven't breadboarded it yet!...cheers rob.

You may need to adjust the bias on Q1. Not 100% sure with the Mindbender, but the TB MKII Q1 needs some leakage to bias well. I have tried si transistors in a MKII but needed a ge with some leakage to get it bias.

I have also tried low gain ge's in an MKII with great results. I have low gain OC71's I got from SmallBear and they work well in the Q1 spot. They are all 30ish hfe.

[deadastronaut]

cheers guys, i'll give it a go with the 3 trimmers to tweak/bias too....

[AnalogPackrat]

Here are my voltages:

Q1 c: 7.3 b: 1.0 e: 0.36
Q2 c: 1.6 b: 0.55 e: 0.01
Q3 c: 4.5* b: 1.6 e: 0.9

* With my bias control I found usable sounds with Q3 C between ~3V and 6.5V.  Below 3V it gets spitty/gated which some people might find useful.

I also found my problem with the fuzz pot--bad 'lytic on the bypass.  Too tired to mess with it tonight--will fiddle tomorrow.  This thing sounds really good when followed by my DLS clone.  Anyone else try stacking?

A P

[Exactopposite]

Here are my voltages:

Q1 c: 7.3 b: 1.0 e: 0.36
Q2 c: 1.6 b: 0.55 e: 0.01
Q3 c: 4.5* b: 1.6 e: 0.9

* With my bias control I found usable sounds with Q3 C between ~3V and 6.5V.  Below 3V it gets spitty/gated which some people might find useful.

I also found my problem with the fuzz pot--bad 'lytic on the bypass.  Too tired to mess with it tonight--will fiddle tomorrow.  This thing sounds really good when followed by my DLS clone.  Anyone else try stacking?

A P

What was the source voltage?

[AnalogPackrat]

I forgot to measure it--I'm using an MXR DC brick for power, so I imagine it's within a couple of tenths of 9V.  I'll measure tonight when I mess with it again.

It is interesting to monitor Vc on Q3 while playing.  It really varies a lot which probably accounts for the dynamic feel of the thing.  I'll be interested to see if this behavior is different with Ge transistors.  I've got some 2N1306, 2N1308, a 2N1310, and a couple of others to try.

[AnalogPackrat]

My MXR brick output is dead on 9.0V.

Spent a little while on the thing tonight.  Swapped in a 2k fuzz pot and replaced the problem 'lytic--now I get a lot of adjustability.  The total range of the bias pot covers Q3 Vc from 3.6 to 7.6V.  At 3.6V the tone is kind of smooth and the dynamics compressed and squishy.  The voltage doesn't vary much between silence and hard strumming--about 0.3V.  At the higher end of Vc the volume is higher, the tone is more aggressive, and the dynamics less compressed.  At the highest fuzz settings I get some oscillation with the high bias voltage.  The bias voltage drops over 2V when I really frang the strings.

Summary: really cool circuit and great sound.  Wish I had built one sooner.  I have to work a lot this weekend so the Ge experiments will have to wait.  Hope you guys have a good DIY weekend.

A P

[Exactopposite]

Well, with the storm approaching I don't think I'll be doing any building this weekend.

The way I determined the bias point was simply that the pedal had the most sustain set that way. If it's set much higher or much lower there is decent sustain, but at the current setting it's  more sustain. It's like focusing the lens of a camera (I also dabble in photography) and it seems like that setting is where the circuit came into focus.

When I was using it live I could get sustain and controlled feedback on demand. It was bananas. I was using a semi-hollow (ibanez as73) and a Jet City 20 watt combo with the gain set low and the volume at about 7.