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MPF102 vs BF245
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author MPF102 vs BF245
fonik
any reason why a BF245 could not replace a MPF102?
i mean other than pinout.

i find the MPF102 a hard to get part here, the BF245 would be much cheaper and is widely available... however, some circuits still use the MPF102 as switch (i.e. in a VCO core).

i roughly compared the datasheets and i don't know if the differences are that significant. any insight? oppinions? experiences?
J3RK
Unfortunately I've never used a BF245 before, so I don't have much input to give on that. I use the MPF102 for quite a few projects, but didn't realize it was getting harder to find. I used to just order them from Mouser. (I haven't looked lately though.) I've seen the 2n3819 used in its place, but as an example, I think the MPF102 works a little better than the 2n3819 in the fader of the 258 clone.

Edit: just looked on Mouser, and they actually don't sell the MPF102 anymore. woah
fonik
i have a single let over 2n3819 here as well, alas, i need a bunch of MPF102 for a certain project NOW. i think i will use sockes and try the BF245.
nevertheless, i am still interested what the critical specifications are actually.
vozs
BF245 is made in A,B,C types therefore the devil sits in the details!
Soooooo, how much did you pay for the knobs? 8_)
Sebo
For VCO cores the On Resistance is the critical parameter, it have to be low (50 ohms or less if I recall right).
ElectroSteam
The BF245C seems to be a close equivalent for the MPF102, save for the pinout as you mention.

For JFETs used for audio, the critical parameters are usually the pinch-off voltage, drain/source resistance and the drain current at zero gate voltage. Given the usual very wide spread on these parameters on 'old' JFET types, then the two types you are interested in are pretty much the same thing. 8_)

Farnell still carries the BF245C (positively needs to be the 'C' type) in quantity.
fonik
yes, the BF245C seems close (and i believe that's what i have in my drawer).
i will use them as simple switches in the mutant vactrol filter. does not seem to be a critical application, they are used to switch a signal to GND.

thank you all for your comments so far.
Blinkar blÄ
maybe

http://www.musikding.de/Active-parts/Transistors/FET/MPF102::363.html? XTCsid=3a80bbfcae03dbfb320e9708650e4bc9

http://uk-electronic.de/onlineshop/product_info.php?products_id=996&os Csid=fd4d4c4fa5d40ac7370cff8b192b4d13

http://www.taydaelectronics.com/mpf102-jfet-n-channel-transistor.html

I think musikding is quite fast to deliver?
fonik
thank you. banzai (berlin) has them as well, alas, i have the BF245 in stock and will just try it. i mean, scott used 2n3904 on the breadboard first, and it worked as well in this application 8_)
ultrashock
you can also try 4393 and J310 widespread jfets which are available. work well in s&h circuits and seems will suit for simple switching operations as well. there are shitload of smt and still a few th versions of both.
J3RK
ultrashock wrote:
you can also try 4393 and J310 widespread jfets which are available. work well in s&h circuits and seems will suit for simple switching operations as well. there are shitload of smt and still a few th versions of both.


I saw a few recommendations to use the J310 as well in place of the MPF102. Let us know how the BF245 works out! I just ordered a bunch of MPF102s so hopefully I won't run out anytime soon. screaming goo yo
fonik
Sooo, how i expected: in this simple non-critical application of tmr scott stites mutant vactrol filter the BF245C works quite fine. w00t
TOPLEL
The sync FET (that FET across the timing cap) in J3RK's PicoVCO is a MPF102. I guess i need a FET there with high off resistance and low on resistance. Can i drop in any n-channel JFET in there from the following: J201, 2N5457, BF245C? (i have these laying around from my diy guitar effect days, by the way these also disappeared from the shops i used to buy parts from)
J3RK
TOPLEL wrote:
The sync FET (that FET across the timing cap) in J3RK's PicoVCO is a MPF102. I guess i need a FET there with high off resistance and low on resistance. Can i drop in any n-channel JFET in there from the following: J201, 2N5457, BF245C? (i have these laying around from my diy guitar effect days, by the way these also disappeared from the shops i used to buy parts from)


Most JFETs will work, but the ideal part will be a "switching JFET". Even the MPF-102 isn't the best part for sync or integrator resetting. It just used to be readily available and worked ok. The 4391 is the best that I've come across for this. I actually use the SMT MMBF4391 now. It will even solder easily enough to through-hole transistor pads (just make sure the pins are oriented correctly).

It won't hurt anything to try other N-Channel JFETs. Some may work better than others.
TOPLEL
J3RK wrote:
TOPLEL wrote:
The sync FET (that FET across the timing cap) in J3RK's PicoVCO is a MPF102. I guess i need a FET there with high off resistance and low on resistance. Can i drop in any n-channel JFET in there from the following: J201, 2N5457, BF245C? (i have these laying around from my diy guitar effect days, by the way these also disappeared from the shops i used to buy parts from)


Most JFETs will work, but the ideal part will be a "switching JFET". Even the MPF-102 isn't the best part for sync or integrator resetting. It just used to be readily available and worked ok. The 4391 is the best that I've come across for this. I actually use the SMT MMBF4391 now. It will even solder easily enough to through-hole transistor pads (just make sure the pins are oriented correctly).

It won't hurt anything to try other N-Channel JFETs. Some may work better than others.


Thanks for the answer! There are no 4391s here, but one of the remaining 4-5 types of JFETs here is a switching JFET. (J107) I'll try some of these. (i actually want to add a reset option to the CGS LFO)
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