10M ohm resistors in this circuit. Why so high? Can I use 4.7M ohm instead?

[Philtre]

I'm building the Giant Hogweed fuzz circuit. R19 and R20 are both 10M ohm. I'm wondering why the value is so high and could I use 4.7M ohm instead?

[Scruffie]

Should be fine I'd think, sets the input impedance of T3 and forms a filter with C9.

[EBRAddict]

It's fine. It's just setting a bias point or reference voltage. The extra 450nA isn't going to hurt anything or cause your power bill to skyrocket.

[Philtre]

OK, thanks. But why does it use such a high value?

[EBRAddict]

Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

[Philtre]

Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Interesting, thanks for that! Wonder what "large as possible" means? 10, 20, 30 M ohm?

[Adam_DIY]

Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Interesting, thanks for that! Wonder what "large as possible" means? 10, 20, 30 M ohm?

10 Billion Ohms

[Philtre]

Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Interesting, thanks for that! Wonder what "large as possible" means? 10, 20, 30 M ohm?

10 Billion Ohms

LOL! 

[reddesert]

10 Mohms is about the largest value you are likely to find in a resistor assortment bag or commonly stocked in small web stores, although large distributors may have larger values.

If you want to build the circuit as intended, just teepee two 4.7M resistors together in series for each 10M resistor. I do this frequently to approximate oddball values.

[somnif]

Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Interesting, thanks for that! Wonder what "large as possible" means? 10, 20, 30 M ohm?

10 Billion Ohms

https://www.mouser.com/ProductDetail/Ohmite/MOX1125231008F?qs=sGAEpiMZZMtlubZbdhIBIJUr%2fIAV1Mb2feZPNMSd1M0%3d

Easy enough, bit pricey, but readily available. 

[Philtre]

Thanks, all.

Tayda have 10M ohm 1% in stock. I'll get a couple on my next order.

[Adam_DIY]

https://www.mouser.com/ProductDetail/Ohmite/MOX1125231008F?qs=sGAEpiMZZMtlubZbdhIBIJUr%2fIAV1Mb2feZPNMSd1M0%3d

Easy enough, bit pricey, but readily available. 

[somnif]

We could even go for 10 TRILLION ohms!

https://www.mouser.com/ProductDetail/Ohmite/UM050E100BKE?qs=16s%2f1Hm5%252bncGdj%2fHS8ziZA%3d%3d

Pretty sure a straight up air gap would have lower resistance than that.....

[Adam_DIY]

Lol that's it I'm done now. 

[somnif]

I have absolutely no idea what a teraohm resistor would be used for.

BUT, I can guess it would probably be way overkill for setting the input impedance on a MOSFET.