I'm looking to build a drop-in reproduction of a Fender Elite preamp, which is nearly identical to the original Eric Clapton preamp from back when the EC strats came with Lace Sensors. I have a couple of questions - mostly looking for confirmation on a few things.
Schematics for reference:
Elite: http://www.eledar.net/Music/walnelite/photos/elite%20pre%20schem2.jpg
Clapton: http://i197.photobucket.com/albums/aa211/bajaman002/FenderEricClaptonActiveElectronics.jpg
- 2N6429 is a long-obsolete BJT that's used in both schematics. Is there anything special about this in the circuit, or would a 2N5088 work just as well? Only reason I ask is that there are a lot of wacky resistor values (46.4k, 1.28k, etc.) and they may be working together with the transistors in some regard. But the fact that they used a 2N5087 for the PNP means that they would have had access to 2N5088s just as easily, so it gives me a bit of hesitation about using them straight up without asking. From looking at the datasheets, 2N5088 does have a bit lower Hfe than 2N6429. Maybe BC549/550 would be a better choice?
I did find one source for the 2N6429s, so it's not out of the question, but they'd be $2.25 each after shipping and I have dozens of 2N5088s already.
- Speaking of the wacky resistor values... do these do anything special for the circuit? Again, since there are so many different ones used and since they're 1% metal film in the original, it seems like it was a conscious decision - they would have had to go out of their way to choose these values as they are pretty inconvenient. (Then again, Fender didn't really have much of a history with 9v circuits in 1980, so this may have been originally designed by an amp engineer or even outsourced.)
It's a mid boost and as the guitar's freq. range is rather limited, small variations in resistor values, even as simple as using 5% resistors, can throw the frequency of filters way out of whack from that intended.
As for the transistor - sub in any high gain, modern transistor or even use an opamp instead of the crude discrete opamp in the circuit.
Regards,
Jay Doyle
Confirmed my suspicions on both counts. Thanks!