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culturejam wrote:That would certainly put an end to my blind, fumbling suppositions.

aww, I was having fun.

drawn. Some stuff to check though. At least 2 suspicions of design errors and one major piece of wondering (elctrolit with normal cap in series).

FET input buffer, Ge gain stage, asymm. clipping.

oh, thank you

I would have put the gain control after the buffer. Between the 1uf and 0.1uf.

Maybe they originally had the gain control between the 1uf and 0.1uf but decided they liked it better on the input and then never bother changing the 1uf/o1uf in series because they liked how it sounded.

whups!
Minor errors corrected:

Some comment on those big ass electrolits.

The Ge stage has gain. Shiteloads of gain. This means that any power supply, including my lab-supply introduces hum over the voltage supply lines which is automatically amplified in the Ge Stage. (It's fine with a battery). Considering the supply filtering I suspect that the builder of this thing ran into the same issue and added extra supply filtering.

I suspect that C4 and C5 in series could probabaly be left over from the original design this circuit was made from that didn't incorperate true bypass. Marshall use a similar config to power the led when using a dpdt instead of a 3pdt, usually there would be a 10k ish resistor between the two caps.

Just speculating a reason for these caps, I could be way off and he might of just stuck C5 in there to change the sound a bit.

Couple electronics-noobish questions...

I've never seen anyone use a power supply filter cap like how C7 is used. At least I assume that's the point of it (to reduce noise)?

Q1 is used as a buffer, right? So Q2 is doing all the "work" in terms of amplifying the signal?

R5 and R8 are there for stabilizing the bias of Q2, doing away with the need for a bias trimmer?

D1 and D4... Surely you would want those the other way round? Give it 9V the wrong way and the signal diode would short out before you could say "oh bollocks" wouldn't it?

Also does the current setup not lead to Z_in of only 100k?

Better learn to say bollocks fast then. What you ask is what i didn't expect either (and therefore doublechecked).

And... yes. So what? 100k is a perfectly round and easy to remember number. That's simple so it must be tone heaven.

culturejam wrote:Couple electronics-noobish questions...

I've never seen anyone use a power supply filter cap like how C7 is used. At least I assume that's the point of it (to reduce noise)?

The use of AC decoupling on emitters in amplifier stages is a fairly common thing . C7 does not function as a PS filter cap here.

culturejam wrote: Q1 is used as a buffer, right? So Q2 is doing all the "work" in terms of amplifying the signal?

R5 and R8 are there for stabilizing the bias of Q2, doing away with the need for a bias trimmer?

yes, Yes, No (setting the bias, not stabilizing) ,Yes but be careful. With the wide variation between Ge Trannies a fixed setup will not always give optimal performance.

Dirk_Hendrik wrote:drawn. Some stuff to check though. At least 2 suspicions of design errors and one major piece of wondering (elctrolit with normal cap in series).

FET input buffer, Ge gain stage, asymm. clipping.

The electrolytic was probably an output for buffered bypass, with the 100n as the input to the gain stage.

Dirk_Hendrik wrote:The use of AC decoupling on emitters in amplifier stages is a fairly common thing . C7 does not function as a PS filter cap here.

Ah yes, how silly of me.

Dirk_Hendrik wrote:With the wide variation between Ge Trannies a fixed setup will not always give optimal performance.

That's what I was thinking, which is why I questioned the lack of trimmer.

Thanks, Dirk!

mictester wrote:

Dirk_Hendrik wrote:drawn. Some stuff to check though. At least 2 suspicions of design errors and one major piece of wondering (elctrolit with normal cap in series).

FET input buffer, Ge gain stage, asymm. clipping.

The electrolytic was probably an output for buffered bypass, with the 100n as the input to the gain stage.

Yeps. With a possible bypass mute in between as suggested above. Would make sense with the electrolit orientation like this.

So what exactly does C7 do?

As Dirk said, it's an AC bypass capacitor. Remember, Q2 is PNP.

Oh crap, you're right. Totally overlooked that.

kind of like a buffered Ge electra. as simple as this circuit is, at least it's somewhat "original".
isn't that anti-pop 2M2 a bit redundant with the gain control on the input? or would the coupling cap negate that feature?
when using a PNP in an NPN circuit like this, it can help to separate the supplies for each with their own current limiting resistors and decoupling caps. this can help with the "motorboating" oscillation you might get.
my guess on those series caps is the gain control move- i seriously doubt a pedal co like this would have buffered bypass in any of their pedals. either that, or it's just there to fuck us up!
i'd like to see what's happening inside their Mantra box. i kind of like the dark snarly tone that thing has.

RnFR wrote:kind of like a buffered Ge electra. as simple as this circuit is, at least it's somewhat "original".

Yeah, my first thought was a buffered Beavis Trotsky (germanium Electra).