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Silver Mica Caps... revisited!


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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).
 
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9/29/1999 2:04 PM
Bruce

Thank you Steve!  
I've been biting my tongue and trying to stay out of this one 'cause I know what sounds good in my stuff.  
Hey, you aren't still using a ceramic cap across the 3m3 dry/wet mixer resistor are you? Yuk.  
I use two 27pF Sm caps in series across a 2m7 carbon comp resistor for, what I think is, a pretty nice sound.  
 
Fender used plenty of sliver mica caps BTW!  
Some newer tech guys just don't recognize the package yet.  
 
Bruce
 
9/29/1999 4:11 PM
Gil Ayan

quote:
"Thank you Steve!  
I've been biting my tongue and trying to stay out of this one 'cause I know what sounds good in my stuff.  
"
 
 
Bruce, have you wired up the Dumble amp stuff? If not, maybe go back to biting your tongue mode. :) Really, I had replaced my caps with SM in a Deluxe Reverb and in a Mesa Boogie Mark I which I tweaked to death (and as far as SM in that amp, I replaced the 250pF treble cap AND the 20pF dry/reverb mixer in the Boog). I liked the sound in both.  
 
However, on the Dumble suff, the SM just doesn't work. And it IS noticeable, not prychoacoustics... The Dumble has an unfettered top end in lead mode that the Fender or Mark I do not have. So I don't believe the offensive frequencies are there to begin with in those amps, so comparing the application of SMs in these different projects is pointless.  
 
It's like Steve said. If you add the 3.3Meg//10pF network feeding yet a new preamp stage in a Fender, you get a nicer sound. More tubey, warmer, crispier, whatever. Now, add the SAME thign to ANY distortion amp and, IF IT WAS sounding good before, it won't now -- geardless of the cap you use. You WILL have to attenuate the high end fizz.  
 
quote:
"Fender used plenty of sliver mica caps BTW!  
Some newer tech guys just don't recognize the package yet.  
"
 
 
I suppose it could be. Sadder still is that the majority of the techs are tone deaf. :)  
 
Gil  
 
9/30/1999 2:51 AM
Bruce

Well, sometimes I feel my tongue has been biten off a few times too many as it is and I still get just as tone deaf as the rest you guys playing too long with the same amp and circuitry over and over.  
If I spend too much time on any tone circuit, my ears get fatiqued and I end up making them too bright.  
 
Yes, I have my share of parts drawers full of 27pF to 680pF 500V/1Kv ceramic caps and I do use them too.  
But, I find that more often then not, a SM cap is either transparent or adds shimmer in a nice way to those amps.  
But remember what I said about fat Willie here and the harp players, they hated the SM caps in their Fender amps.  
Now I mentioned to Steve a year or two ago that I also have found that I can use less top end or reduce the effect of a presence control with the amps that have SM caps in the tone circuit.  
Bob Swanson, DynaQ Amps, has a theory about spreading high and low freqs that is very interesting and has proven to me a few times that some amps sound great when tweaked to freqs I know I can't even hear!!  
BTW, I also frequently use a SM 47pF to 110pF cap (68pF being the cap used more often) in series with a 68K resistor shunted to ground, right after the plate load resistor and before the coupling cap from the tone recovery to the next stage, as a way to cut some of the same shimmer I guess I just like it. Sorta like Dolby?  
 
Not that I think I am too good to try any.... :>) No, I have not tried to mock up any of the new Dumble tone tweaks yet, except a loose adapatation of a modified treble pot to bass pot deal that worked very nicely and I really liked the added flex it gave my amp.  
 
I'm afraid the thread original Dumble Amps ran so long, I became Numble. pun intended  
 
Bruce  
 
 
9/30/1999 2:59 AM
Gil Ayan

quote:
"Well, sometimes I feel my tongue has been biten off a few times too many as it is and I still get just as tone deaf as the rest you guys playing too long with the same amp and circuitry over and over.  
If I spend too much time on any tone circuit, my ears get fatiqued and I end up making them too bright.  
"
 
 
Isn't that the truth! Yes indeed, breaks a re mandatory and putting projects down for a while are a must IMHO. And in the end, there is no point in tweaking an amp to death at home, only to have it sound bad at the gig.  
 
quote:
"BTW, I also frequently use a SM 47pF to 110pF cap (68pF being the cap used more often) in series with a 68K resistor shunted to ground, right after the plate load resistor and before the coupling cap from the tone recovery to the next stage, as a way to cut some of the same shimmer I guess I just like it. Sorta like Dolby?  
"
 
 
Yes, I understand that too. The problem with gain amps, however, is that they are extremely sensitive to what goes on beyong the guitar's fundamental frequencies. I have heard it said many times, and concur, that to make a great clean sounding amp is a lot easier than to make a great sounding overdirve amp.  
 
quote:
"No, I have not tried to mock up any of the new Dumble tone tweaks yet, except a loose adapatation of a modified treble pot to bass pot deal that worked very nicely and I really liked the added flex it gave my amp.  
"
 
 
Hopefully, you will, and then came back and share your experiences with the rest of us here. :)  
 
Gil  
 
9/30/1999 1:40 PM
Trace

quote:
"Well, sometimes I feel my tongue has been biten off a few times too many as it is"
 
 
Don't like that Bruce. You are one of greatest cats there is!!! If I can learn 1/10 of what you forgetten then I'll feel pretty damn lucky!  
 
quote:
"and I still get just as tone deaf as the rest you guys  
playing too long with the same amp and circuitry over and over. If I spend too much time on any tone circuit, my ears get fatiqued and I end up making them too bright. "
 
 
Bruce this is pretty common! In the recording studio we call this "ear-burn" because after so many hours your ears just get burnt and the first thing to go is the high end and then just as you have found, you start boosting the high end. That's when it's time to walk away and catch a movie! (ha, ha) You need to know when you've been at it too long and simply come back later and listen with fresh ears. This is a VERY important point that oddly enough I do not recall being brought up before (?)  
 
quote:
"Now I mentioned to Steve a year or two ago that I also have found that I can use less top end or reduce the effect of a presence control with the amps that have SM caps in the tone circuit."
 
 
I can totally agree with this!  
 
quote:
"Bob Swanson, DynaQ Amps, has a theory about spreading high and low freqs that is very interesting and has proven to me a few times that some amps sound great when tweaked to freqs I know I can't even hear!!"
 
 
Bob and I briefly touched on this topic on the phone and I agree with him on this. Bob also engineers at a studio and efter doing that for a while you being to realize that certain frequencies work very well for guitar and not only sound pleasing to the ear but also sound "musical" over all. I've found myself voicing amps around those frequencies. Bob has a pretty good ear and therefore I'm VERY curious about how his amps sound! :-)  
 
quote:
"BTW, I also frequently use a SM 47pF to 110pF cap (68pF being the cap used more often) in series with a 68K resistor shunted to ground, right after the plate load resistor and before the coupling cap from the tone recovery to the next stage, as a way to cut some of the same shimmer I guess I just like it. Sorta like Dolby?"
 
 
Bruce that's pretty interesting...I'll have to give this a whirl. What amps do you end up doing this in most of the time...Fenders?  
 
Trace
 
9/30/1999 1:42 PM
Trace

Opss....I meant to say "Don't think like that Bruce"
 

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