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Silver Mica Caps... revisited!

9/29/1999 7:22 AM
Steve A.
Silver Mica Caps... revisited!
    I'd been having a lot of trouble getting decent sounds from the hotrodded "Abnormal" channel of my 65 Pro project amp (aka Son of Pro Maniac) and while Gil's tips about using ceramic caps instead of silver mica seemed to help a bit, I never did progress any further to the great blues sounds I'd been getting out of my other amps using a similar preamp.  
    Well, the basic problem finally occurred to me last night: I've been hot rodding the wrong channel on a Fender XXX Reverb amp! The 3M3/10pF reverb splitter does some serious filtering on an overdriven signal, and although Mesa Boogie has managed to get some good sounds with that design, I was more interested in a cranked and raunchy Tweed Bassman sound and was only getting buzzy, fizzy distortion. So by rerouting a few wires I was able to run the Abnormal channel after the reverb circuit and the Dumble-ized Normal channel ahead of the reverb. Well, the OD channel sounded much better, but it still wasn't quite there. I put the mica caps back in and the amp has gotten the great clean and overdriven blues sounds I was never able to get from it when it was trying to squeeze all of that signal through the 3M3/10pF RC network.  
    I've had a DPDT toggle switch wired up so that I can select between different valued caps for the typical 470k/470pF RC network that Marshall uses between stages. So I wired it up with a ceramic cap on one side and a mica cap on the other, for a direct comparison of the two caps. With the ceramic cap the sound is muddied up; with the mica cap it is clear as a bell. (With the OD channel running through the reverb circuitry a week or two ago, when I did the exact same test the sound was very harsh with the mica cap, and it was mellowed out quite a bit with the ceramic cap.)  
    Anybody wanna buy some ceramic caps... cheap? Just kidding! I'm still using them on the tube pins, etc., and will undoubtably need them when I build my full Dumble ODS clone since Gil mentioned the problems with mica caps in the high-gain preamp of the Dumble ODS.  
Steve Ahola  
P.S. I tried switching the Dumble-ized Normal channel in and out of the reverb circuitry, and I'd guess that the clean signal is degraded maybe 10% when it goes through the 3M3/10pF network (and pre-driver stage), but it sounds so great with the reverb engaged that I don't really mind that (I may wire in a back panel switch to optionally bypass the reverb circuitry on the Normal channel, too.) While the Jazz position of the Jazz/Rock switch usually sounds a bit thin (it uses a major cut in bass and lower mid frequencies), it works great with reverb (you don't have to worry about those thick bass notes slugging the hell out of the reverb springs).  
P.S.S. While it could be said that an amp never left the Fender factory with a silver mica cap, I bet that some of the Fender amps used by the great blues guitarists had at least some of the ceramic caps replaced with mica caps (which have been used in radio transmitters for years).

Bruce Thank you Steve!I've been b... -- 9/29/1999 2:04 PM