Hey /diy/, /g/ here.
I've always been a digital logic fag, so my analog skills are subpar. I could use some help with what some components of this circuit do. Here's what know:
C1 prevents reverse flow out the input
R1 biases the input
The gain potentiometer controls gain by controlling the amplification factor, preventing electrons from flowing across the cathode.
C2 and R4 do the same as C1/R1 for stage 2
4,5 heater
C3, prevents reverse flow through the circuit
C4, VR2 adjusts tone by controlling how much low-band FQ can drain to ground
VR3, self explanatory
What I dont understand, is where R2/R3 come from, how those values were determined and what they really do.
>>916571
C1 and R1 are an input signal filter, I think high-pass to get 60Hz and DC out of there.
C2 and R4 do the same
I think C3 is a coupling between the amp circuit and the tonestack/volume control to keep the two from interfering with each other (hard to explain I'm drunk)
C4 you understand. Look at C1 and 2 the same way, just with fixed resistors.
R2 and R3 affect what voltage actually goes on the tubes. Look up a voltage divider and it'll give you a rough idea, not exactly the same, but the ballpark concept.
>>916601
Oh, C3 also keeps DC bias out of the output signal.
>>916571
> What I dont understand, is where R2/R3 come from, how those values were determined and what they really do
Look up "common emitter voltage amplifier".
>>916601
>I think high-pass to get 60Hz and DC out of there.
It has no effect at 60Hz. It's just a simple DC block, like OP suggested. Same with C2 and C3.
>>916571
R2/R3 are part of the reason the circuit amplifies. The grid voltage changes the current through the tube and this changing current causes a changing voltage drop across the resistor. This same current to voltage drop effect is the reason why VR1 changes gain as well: the voltage across that resistor is in the same phase as the input voltage, thus weakening its effect. Well, there's also that reason you mentioned.
Considering your amp runs the tubes at a tiny fraction of their normal plate voltage, I suspect the principal design method has been trial and error. Other than that, the tube datasheets have graphs for Ia/Vg and also list gain, transconductance and internal resistance. Graphical methods were popular when people actually used tubes, but the published curves aren't really useful for misuse like in your pic.
Why the variable resistors that connect to themselves?
>>916693
Ordinarily, it makes no difference. But having the wiper connected to one end of the track means that if the wiper breaks contact with the the track, the resistance will be the total resistance of the track, rather than the near-infinite resistance of the air gap.
>>916571
Tubes are voltage controlled current devices. When you place a small voltage at the grid of a tube from input to pins 2 and 7, that voltage causes a large change in the current from cathode to anode. R2,R3 convert this large current swing into a large voltage swing at the anode. and that's amplification.
>>916656 is right on the money. These little 9 volt tube crash and burn fuzz boxes look pretty ill conceived. At least incorporate feedback like you'd see with any standard fuzz face. Maybe add some voltage doublers or more supply voltage to get closer to a more workable plate potential.
