substitute?

[bigchasbroon]

i am building a rat and dont quite have all the parts. So I reckon i can use some alternatives and just want someone a bit more clever and experienced than me to confirm that I can swap
a jrc4558 instead of Lm308
and a 2n3904 instead of 2n5458
and finally a 1n914 instead of 1n4002.

Also if you feel like teaching me something how far can i go from the specified value of a cap if i dont have the value required.

thanks a lot guys

[R O Tiree]

LM308 is a single op-amp. 4558 is a dual. Not a problem if you re-jig the layout but it is not a simple pin-for-pin drop-in replacement.

2N3904 is an NPN transistor. 2N5458 is an N-channel JFET. This definitely will not work as advertised.

The 1N4002 is there to provide a (very) limited form of reverse-polarity protection (in conjunction with the 47 Ohm resistor). It is a high-current device, so it can cope when someone connects the battery up the wrong way for a few seconds. The 1N914 is a small-signal diode and it is rated at about 20mA max forward current. It will not last long when asked to pass a high current (about 190mA - almost 10 times what it's rated for). At that point, the diode blows and you get almost 9V the wrong way around, which fries the opamp you were trying to protect in the first place. False economy, then.

As to cap values, well if they're de-coupling caps, it can sometimes be OK to insert a larger value, but on occasion the value of the cap is deliberately set to pass higher frequencies and attenuate lower ones to a specific degree before passing the signal on to the next stage of the circuit. Most often, though, the caps are in conjunction with resistors in order to form filters, which will definitely alter the voicing of the pedal. If you have a 1µF cap on the schem in a low-pass filter, for example, and you replace it with a 2.2µF cap, then the roll-off frequency will be more than halved. Oops.

Bottom line - Get the right parts, for now, until you have built a few of these things and figured out a little bit more about how they work.

[frequencycentral]

I'm sur I've read somewhere that you can sub a CA3130 for the LM308.

[bigchasbroon]

thank you R O Tiree for saving me from myself. i realise that i actually know nothing. 

And FC iwill further research the ic situation. I had read a list somewhere of possible replacements for the lm308 and im sure the 4558 was there. shows you not to beleive everything on the net

[Scruffie]

I used the NE5534AP with pleasing results (and it is the chip that the vodka mods company mods there Rats with) LM741 and TL071 will work aswell though and

I'm sur I've read somewhere that you can sub a CA3130 for the LM308.

Pretty sure i've seen that around too.

[bigchasbroon]

thanks scruffie. its getting better. i really need to keep the cost down since i find that when im buying caps etc i can only get a minimum of say 25 and sometimes 200 for resistors so it is working out very expensive which can create conflict in the home anyway i have had a look around and based on price (both the lm308 and the ca3130 are around 10gbp with post thats 17us) the NE5534 seems the best option at 1.40 gbp/2.30usd its cheap and seems to be a better quality than the 741etc. if anyone is interested theres a little bit about it in this catalogue http://mercury.maplin.co.uk/maplin_2009/book.html?page=814

[Scruffie]

I dunno where your from, but if it would be cheaper I could buy parts and ship them to you, i'm more than happy too if it'll cut your costs down?
Otherwise no probs, glad you got it sorted 

[bigchasbroon]

thanks Scruffie that is very kind of you. Im in  the UK but have just been directed to a couple of sites that are pretty reasonable so I think I will be OK. If im stuck I will PM you.

Also it is my wedding aniversary in 2 weeks and my good wife have just given me some money to get parts. Im in love again

[WLS]

...  the caps are in conjunction with resistors in order to form filters, which will definitely alter the voicing of the pedal.
...

Googel - "RC Calculator" on the web. It will help you understand low pass and high pass filters.


Bill

[Scruffie]

thanks Scruffie that is very kind of you. Im in  the UK but have just been directed to a couple of sites that are pretty reasonable so I think I will be OK. If im stuck I will PM you.

Also it is my wedding aniversary in 2 weeks and my good wife have just given me some money to get parts. Im in love again

No worries, i'm from the U.K too, www.bitsbox.co.uk is the cheapest source I have found in the U.K with www.bowood-electronics.com being the second cheapest.
Congrats on the aniversary, although I think that's the first time i've heard of an pedal building addiction re kindling love lol.