Hello everybody
I'm new to the forum so apologies if I haven't found the relevant discussion!
I want to build a tonebender Mk1 using Russian germanium and have breadboarded it, but get a decay fiss (high frequency fizz 🙄) at high attack settings. Otherwise it sounds ok with gating at lower input levels, and lots of sustain at higher levels. It sounds more like distortion though with the tone knob all the way up.
The transistors used are fairly low leakage and lower gain, and I've been swapping transistor positions.
Questions are:
Is it possible to get a decent TB1 sound if the circuit is properly tweaked for bias?
How important is leakage for sonic quality (I understand that it is relied on for some transistor bias)?
On the question of biasing, does it really matter that much with hFE if the circuit is biased correctly ( again I realise that impedance may be affected by resistance values)?
Transistors are MP42B (hFE 40-70), and MP21A (50-80]. Leakage is generally <100uA.
Many thanks 😊
Quote from: Smeddling on July 17, 2020, 11:55:07 AM
Leakage is generally <100uA.
Pure disgrace for any self-respected Ge BJT.. :icon_lol:
P.S.
Welcome... :icon_wink:
Pure disgrace for any self-respected Ge BJT.. :icon_lol:
P.S.
Welcome...
:icon_smile:
That's. 'progress' I suppose :icon_rolleyes:
So I've tested a build using lower hFE and leakage transistors and biased it to try and answer my unclear questions :-[
The build is based on the DAM solasound Mk1 schematic.
Q1 (47/54uA). I added 2M2 collector to base to imitate greater leakage and pull the emitter up:
E -1.24V
B -1.28V
C -9.41V
Q2 (51/66). Collector load is now 5k6.
E 0V
B -0.038V (-0.134V @ max attack)
C -9.25V (-4.63V @ max attack)
Q3 (61/98). Collector load is now 47k.
E 0V
B -0.022V (-0.014V @ max??)
C -8.47V (-7.3V @ max??)
I noted that battery voltage drops by 30mV or so between min and max attack (quiescent current is 0.22mA to 1.04mA @ max) so I am assuming this is causing the variations in Q3 voltages?
I also noted that Q2 collector varies very little with the the last 50% of travel on the pot. My assumption is this is due to the shunting of the lowering forward resistance of Q2 be junction?
The sound is actually ok but possibly not as aggressive as I'd like. It is very saturated and compressed but maybe this is due to the fluctuating -9V supply?
What would be the effect of using higher gain devices? Is it a case of having more current driving into each subsequent stage and therefore clipping harder?
As far as leakage goes, I can observe the effect in Q1 by removing the 2M2 C-B and the falling emitter voltage. Sound wise the 2M2 didn't bring much to the party...
Q3 is basically in cut off so I can't see what high leakage buys you with the emitter grounded...
I would really appreciate some help with this
Many thanks
MP42B works fine.
I assembled all or nearly all of the germanium circuits when I first started learning about effects pedals and these transistors performed well.
There are GT402s, which have a high gain and many large manufacturers (MXR, Ramble FX etc.) use them, I compared them and realized that the MP42B has less low frequencies, but everything is easily corrected by post filtering.
The gain is over 60, which is enough for most circuits if the offset is set correctly.
These transistors are very cheap, now they cost about $ 0.27 and can be bought in major radio parts stores LOL
Thanks POTL. :)
Maybe that's why the circuit doesn't sound that aggressive and more bright.....
I want to keep the circuit as close as possible but will look into altering the filtering...
Do you have any experience with MP21A's?
I also bought some P416A & Bs which should give me higher gain but they haven't arrived yet so I'll have to wait and see how they sound.
Many thanks
No, I haven't used them.
They are still sold in radio parts stores and cost a ridiculous $ 0.1 but I had no reason to buy them since the MP42B and GT402 worked great for me.
In fact, I see no reason to chase certain components if you can adapt the circuitry to what is on the market.
It may sound different, but this is a germanium fuzz, each one is unique and unstable in its own way =)
Cheers
I guess even the original Mk1 tone Benders all sounded different due to transistor variability (even using the holy grail OC75's and 2G381's) ;) :icon_wink:
I just got hold of some GT402B's and got a good range of gains (mostly 90-140), but some are measuring leakage 500uA @ 25C.
Are these still useable? I guess I could use the leakage to give me buckets of headroom in Q1, but I don't really need 90+hFE in that position. If I use it in Q3, this is going to give me a lot of artefacts &sizzle right?
Any other circuits that will take the gain and leakage?
Cheers