Hey Guys 🙂 I'm trying my hand for the first time at some circuit design. I'm nearly a complete noob on electronic theory. So I need help...
Looking at these two projects schematics from Oately's, I think I can use these to learn how to make a new design specifically as a replacement preamp for my guitar. I recently burnt out the jfet based one... erwps.
Here is a really complicated instrument preamp:
Here is a simpler headphone amp:
My goal is to produce something like the first circuit, without the complicated filter banks. Still would need the two stage amplification in the first circuit and using it's Solid State output transitor. So going off the first circuit, basically anything left of V1 seems like it could be eliminated, except R1. Easy enough.
Next, I have some questions about some things in the first circuit that aren't in the second.
The amplifier stages look very similar, except in the first circuit, pin2 of the tube is coupled to ground by a capacitor c9, c11 and connected to +9v by R4, R6. But on the second circuit, pin2 of the tubes are connected straight to the pin1. This essentially seems to connect it to the +9v through the 180k resistor. Probably not a huge difference.... There's also a difference in resistance between pin1 and pin2: the first circuit has 450k of resistance between the two, the second has zero. What is the purpose the caps c9,c11 in the first circuit and the resistors r4, r6?
What is the purpose of c8 in the first? Is that to block DC? Why does the first circuit use 10nf for c8 and the second use .1uf for c1, c2?
What is the purpose of c10 in both circuits?
If I can get these questions answered I'll post a prototype diagram and order some parts! Pretty excited. Thank you!
Looking at these two projects schematics from Oately's, I think I can use these to learn how to make a new design specifically as a replacement preamp for my guitar. I recently burnt out the jfet based one... erwps.
Here is a really complicated instrument preamp:
An externally hosted image should be here but it was not working when we last tested it.
Here is a simpler headphone amp:
An externally hosted image should be here but it was not working when we last tested it.
My goal is to produce something like the first circuit, without the complicated filter banks. Still would need the two stage amplification in the first circuit and using it's Solid State output transitor. So going off the first circuit, basically anything left of V1 seems like it could be eliminated, except R1. Easy enough.
Next, I have some questions about some things in the first circuit that aren't in the second.
The amplifier stages look very similar, except in the first circuit, pin2 of the tube is coupled to ground by a capacitor c9, c11 and connected to +9v by R4, R6. But on the second circuit, pin2 of the tubes are connected straight to the pin1. This essentially seems to connect it to the +9v through the 180k resistor. Probably not a huge difference.... There's also a difference in resistance between pin1 and pin2: the first circuit has 450k of resistance between the two, the second has zero. What is the purpose the caps c9,c11 in the first circuit and the resistors r4, r6?
What is the purpose of c8 in the first? Is that to block DC? Why does the first circuit use 10nf for c8 and the second use .1uf for c1, c2?
What is the purpose of c10 in both circuits?
If I can get these questions answered I'll post a prototype diagram and order some parts! Pretty excited. Thank you!
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Relocated to the more appropriate Instruments and Amps forum per forum policy.
Those tubes are pentodes, and pin 2 is the screen. The screens are biased by B+. the cap to ground is a bypass cap, think of it as a filter cap. It keeps the screen voltage steady, so it doesn't modulate the signal.
In the lower circuit, the tubes have their screens wired to its plate. That just turns the tube into a triode. If you want to know more about that, look up triode-connected pentodes. The difference is the difference between a triode and a pentode.
C8 passes the signal from the plate of V1 to the grid of V2, yes it blocks the DC from that plate while passing signal.
I don't see the purpose of comparing the input cap of one circuit with an interstage cap in the other.
C10 is just a filter cap across the power supply, I'd imagine mostly there to decouple.
In the lower circuit, the tubes have their screens wired to its plate. That just turns the tube into a triode. If you want to know more about that, look up triode-connected pentodes. The difference is the difference between a triode and a pentode.
C8 passes the signal from the plate of V1 to the grid of V2, yes it blocks the DC from that plate while passing signal.
I don't see the purpose of comparing the input cap of one circuit with an interstage cap in the other.
C10 is just a filter cap across the power supply, I'd imagine mostly there to decouple.
Ok, another series of dumb questions, why is c15 hooked to the filament? Is that because VR1 is hooked to the filament as well? Could that be replaced with a resistor instead of the same resistance as the filament? Or is that a glass of water in the ocean because of the 1mohm pot?
Thanks!
Thanks!
Alright, So I had to change quite a few things up because parts are not available.
See my latest diagram as I'm messing with it:
Edit/Fork/See parts list/PCB here:
https://123d.circuits.io/circuits/852190-jan-6418-vaccum-tube-valve-preamp
First, the power supply, I found a 1.25v regulator available from mouser. The tube heaters are now wired in parallel so they should also see the same current/voltage across them. I also just grounded the gain pot, I hope that doesn't have an ill effects.
The part I can't find now is the BS245a N channel follower mosfet. Right now I just have T1 sitting in the diagram, but the circuit will need to be replaced...
Any advice on going about designing a mosfet or jfet buffer? I like the current design because someone it gets away without have a coupling cap between the tube and the output...
See my latest diagram as I'm messing with it:
An externally hosted image should be here but it was not working when we last tested it.
Edit/Fork/See parts list/PCB here:
https://123d.circuits.io/circuits/852190-jan-6418-vaccum-tube-valve-preamp
First, the power supply, I found a 1.25v regulator available from mouser. The tube heaters are now wired in parallel so they should also see the same current/voltage across them. I also just grounded the gain pot, I hope that doesn't have an ill effects.
The part I can't find now is the BS245a N channel follower mosfet. Right now I just have T1 sitting in the diagram, but the circuit will need to be replaced...
Any advice on going about designing a mosfet or jfet buffer? I like the current design because someone it gets away without have a coupling cap between the tube and the output...
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