MK I gain pot problem

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BMS1971
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Post by BMS1971 »

Hey guys I need help for more advanced guys in electronic than me

I made several MK I and found a strange paradox with my gain pot, I use the DAM Sola Sounds schematic.

I tried 3 combination of transistors with different results

1 MP26B X3, grain works fine but gates way too much and realy not good with strats and teles (HFE 25 to 80 range leak about 100)

2 Closest from original transistors: MP41A, GT403 X2. amazing sound BUT here my gain pot is quite useless fuzz seams to be at max all the time (you really need to turn down the volume on guitar to 3 to hear the difference) the GT403's always have MASSIVE leak usually 600 to 1400!

3 My "compromise version" MP41A/MP26B/GT403B. the "best" result so far but I'm a little less exited with sound

I use a tripot on Q2 and Q3 of the collectors side to adjust bias on them (Usually best result with gain at 10 beet on the -6 to -6.5V range)

So my question is: Do you have any suggestions to deal with this problem? The strange thing that is my Maestro (almost virtually same PCB) is much easier to adjust
Here is a website for the datasheets of the transistors I used
MP26B: http://transistor-spravochnik.ru/descri ... p26b/46651
MP41A: http://transistor-spravochnik.ru/descri ... p41a/46748
GT403B: http://transistor-spravochnik.ru/descri ... 403b/38471

PS: I know the GT403V is closer from 2G381, but they are virtually impossible to find with more Than 10 of HFE and less than 1000 is leak!

Any advice welcome
Cheers
Benoit

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Manicstrat2112
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my favorite amplifier: Egnater Rebel 30 MK I
Completed builds: Partial list: Creepy Fingers EQ Exciter Fuzz, Tonebender MK II, Silicon Tonebender MK II, Fuzzrocious Ram the Manparts, Thunderdrive, Fuzz Face. Silicon Fuzz Face, EHX Muff Fuzz, Brian May Treble Booster, DAM Grease Box, DAM Meathead, Lovepedal Woodrow, Lovepedal Eternity, Silicon Fuzz Master, Fuzz Factory, Mastotron, Super Hard On, 73 Rams Head BM, Katzenkonig, Phase 45, Black Dust, RAH, Tentacle, MXR Distortion +, DOD 250 Preamp, Distortron, Naga Viper, JHS Andy Timmons, Pro co Rat, Celine Mark 4, EQD Dunes, EHX OD Glove,, Lemon Drop, 59 Sound, Formula No 5, Formula No. 55, Black Ash, Plexi Drive, 65 Deluxe, Hummingbird, Fuzzolo, Univibe, EQD The Depths, Guzzling, Maestro FZ-1 and FZ-1A, Tonebender MKI, Harmonic Percolator, Zonk Machine, Hybrid Si/Ge Zonk Machine, MBP Harbinger 1.5, MBP Pork Barrel (to original CE-2 specs) and it all started with the Bazz Fuss =)
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Post by Manicstrat2112 »

Get403v....if your interested
https://www.amazon.com/Transistors-Germ ... 515&sr=8-2

IV had nothing but good luck with this seller.
".....and when I'm sad......I slide!"

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Manicstrat2112
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Joined: 23 Nov 2019, 05:49
my favorite amplifier: Egnater Rebel 30 MK I
Completed builds: Partial list: Creepy Fingers EQ Exciter Fuzz, Tonebender MK II, Silicon Tonebender MK II, Fuzzrocious Ram the Manparts, Thunderdrive, Fuzz Face. Silicon Fuzz Face, EHX Muff Fuzz, Brian May Treble Booster, DAM Grease Box, DAM Meathead, Lovepedal Woodrow, Lovepedal Eternity, Silicon Fuzz Master, Fuzz Factory, Mastotron, Super Hard On, 73 Rams Head BM, Katzenkonig, Phase 45, Black Dust, RAH, Tentacle, MXR Distortion +, DOD 250 Preamp, Distortron, Naga Viper, JHS Andy Timmons, Pro co Rat, Celine Mark 4, EQD Dunes, EHX OD Glove,, Lemon Drop, 59 Sound, Formula No 5, Formula No. 55, Black Ash, Plexi Drive, 65 Deluxe, Hummingbird, Fuzzolo, Univibe, EQD The Depths, Guzzling, Maestro FZ-1 and FZ-1A, Tonebender MKI, Harmonic Percolator, Zonk Machine, Hybrid Si/Ge Zonk Machine, MBP Harbinger 1.5, MBP Pork Barrel (to original CE-2 specs) and it all started with the Bazz Fuss =)
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Post by Manicstrat2112 »

As far as the gain pot issue goes......when I had that problem it was always a transistor issue. Not having them biased right...usually because I was using transistors with the wrong leakage and or hfe. Leakage seems to be the most crucial part of this circuit.

If you can post your transistor voltages it will make it easier to see what's going on.

This is from a different thread from LucifersTrip (he really knows his mki's :
LucifersTrip wrote: It's funny. There are long threads on various forums with no conclusive ways to build this one well. You usually get the standard "try a bunch of combos until it sounds right" comments.

Though, after building a bunch that sounded excellent, I was going to post a "guide" with some explanations...but the response for vintage stuff here is so underwhelming, I didn't...

I can still remember much, so I'll try to help from memory.

I think the first misconception is that it should sound like an FZ-1 or 1A. It doesn't. Secondly, many shoot for certain gains in the different slots (a common set would be 60 / 100 / 60), but I have found that leakage is far more important as long as the gains are 50+...and as usual, the model # transistor means almost nothing. I almost always use transistors with leakage of at least 200uA for the 3 positions.

The first thing I do is simply breadboard stage one and play thru it. You should get a decent clean boost that doesn't sound misbiased. You want the notes to fade out nicely. I've seen a large range of voltages for Q1E. The last one I built I marked down as .6V.

Secondly, a load of discussion goes into biasing Q2 and whether to use a 180K or 470K on Q2 base. It's no big deal whichever you use. When you're at lowest fuzz setting, Q2 collector is close to 9V and when you increase that fuzz pot, the voltage drops. The max fuzz is usually in the 3-5V range. If you use 180K, you'll probably hit 3-5v at only 5 - 15K instead of at 50K, so all you have to do is replace the 50K pot with a 10K one...very simple. Look at the Zonk. They only have around 3K there!
http://fuzzcentral.ssguitar.com/schemat ... Ischem.gif

With a 470K, you will be able to use most or all of the 50K pot range to get to 3-5v. So, just measure the K where it's max fuzz (around 3-5v) and put the exact value pot there (use a resistor across the lugs to get the exact value you want...remember, it's only a variable resistor). If you drop the C voltage much lower (1-2V and lower), you'll get a thin, synthy, gated fuzz...and it will also get noisy sometimes <5V, so set the max fuzz where it's cool for you. If you pick a Q with the wrong leakage, C will be too high when fuzz is at minimum, so you must choose one that sounds good at min (C voltage not too high). With 180K, the last one I set max at 4V with a 7K pot. At min fuzz setting, it was in the high 8V range (I think ~ 8.75 - 8.9V).

Q3 works similarly to Q2, but the C voltage is much higher. I have found it's best in the 7.5 - 8V range. So, I just choose a transistor that gets me in that range using the stock 15K. The leakage is usually 300uA+. I usually use a trimmer instead of the 15K to fine tune it, but you want to be close to 15K. If the transistor doesn't have enough leakage C will be closer to 8.5 or 9V and just like Q2, won't fuzz well and you'll get more gating. The last one I set to 7.75V with the trimmer at 13K. Set the voltage too low and the volume will drop and the tone thinner.

After I've chosen Q2 and Q3, I always throw a few different Q1's in with similar leakages, just to see if it sounds any better that my initial choice.

I still have these handy:

2N1144 (72 / 210uA)
2SB376 (69 / 250uA)
2N1038 (54 / 380uA)
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Post by BMS1971 »

Manicstrat2112 wrote: 27 May 2021, 22:13 Get403v....if your interested
https://www.amazon.com/Transistors-Germ ... 515&sr=8-2

IV had nothing but good luck with this seller.
Thanks I already have 3000 ;-)
Ben

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Post by BMS1971 »

Manicstrat2112 wrote: 27 May 2021, 22:24 As far as the gain pot issue goes......when I had that problem it was always a transistor issue. Not having them biased right...usually because I was using transistors with the wrong leakage and or hfe. Leakage seems to be the most crucial part of this circuit.

If you can post your transistor voltages it will make it easier to see what's going on.
all of them are on the -6.5v range. I copied that from a MK1 clone I just happened to repair.
Ben

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