Tube Amps / Music Electronics
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|9/29/1999 5:02 PM|
||Re: Silver Mica Caps... revisited!|
.. I was going to mention that Fender, as far back as you wanna go used SM caps. My '52 uses one across the PI as a MOF. You can see on some of the factory darwings that mica caps are spec'd. Where did the erroneous notion that they *never* used them come from?
I don't think it's good to label them as good or bad vis other types. It's all about the amp, the player, the application.. may not work in Dumbles, but may work wonderfully in BF or tweeds, again depending on the rest of the circuit and what you're going for.. (which is pretty much what Steve is saying IMHO)
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|9/30/1999 4:35 AM|
I'd like to pose the question "why do SM caps sound different than ceramic?". Isn't the cap value based on frequency roll-off?
|9/30/1999 1:46 PM|
Excellent point Speed. I think the micas work well for certain amps and it really does come down to "where" they are placed in a circuit. The use of a mica in a gain-amp can be more obvious but it all depends on where it is in the circuit as far as if it sounds harsh or pleasing to the ear.
|9/30/1999 6:28 AM|
Hey, you aren't still using a ceramic cap across the 3m3 dry/wet mixer resistor are you? Yuk.
No, that had been a 10pF mica cap for the past two years (although I did briefly change it with a ceramic cap during my "mica cap burning" phase). I will try your suggestions.
So did Fender use those old mica caps that looked like dominos (with the 6 dots)?
In any case, I have no doubt that the ceramic caps are needed in Gil's Dumble clone, with the higher gain structures, etc. I was just posting the results of a specific preamp based on a hotrodded 5F6A circuit, which is quite unlike the Dumble ODS.
|9/30/1999 7:36 AM|
I didn't think you were still using the ceramics in the reverb.
Yes, the old time domino silver mica caps!
At one time I had a large cigar box filled with them from previous past lives!
I think almost every tweed Fender amp I've worked on had at least one in them.
I think it's pretty cool you could duplicate a bad sound!
Personally I had been thinking it is harder to make a great clean sounding amp then a "distortion" amp.
But , yes Gil, a great distortion amp is probably just as hard anyhow.
Heck they're all hard!
I'd like a little more analysis on the effect of the reverb mixing section in the Fender amp and the distortion problem.
I wonder if you are hearing the preamp's odd upper harmonic distortion products leaking right past the 10pF SM cap unattenutated.
The cap and resistor form an RC 5KHz.
My little 68pF/68K shunt is about 34KHz.
Those small SM caps with short leads can probably pass RF with relative low ESR and all that.
I wonder what would happen if you put a small inductance in series with one and possibly a few pF of a gimmick to ground before it.
You remember the gimmick conversation Steve?
|9/30/1999 5:37 PM|
Consider shifting frequencies as opposed to hiding them. I use silver micas alot, but if one of them doesn't sound good I usually shift the sweet spot instead of covering it. But ceramics will work well to mellow ulgy top end. Whether it's ceramics, cement, or siver mica caps, it's about proper frequency presentation.
|10/1/1999 3:18 AM|
I'll have to pass on any further experiments with running the OD channel through the reverb splitter right now since I really like it better going straight to the PI. While I love the effect of reverb on a nice clean sound, it seems to just muddy up the OD tones (IMHO). On some of my non-reverb amps at certain settings I seem to be getting a slapback echo if I hit a note sharply- you really only hear it if you mute the strings quickly right after the "zinger". Or am I just drinking too much Diet Coke?
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.
Can you spell out exactly what you mean on a SR AB763 schematic? By "tone recovery" do you mean the "V4B" (aka "1/2 7025") with the grid connected to the 3M3/10pF splitter? And the 68pf/68k combo connected in series to ground from the #6 tube pin? Or am I reading that all wrong? (Does that trick work on other stages as well- assuming of course that you don't overdo it?)
BTW I had asked about hooking up a 500pF cap from the plate to the cathode in the D-ODS schematics (instead of connecting the cap to ground). Gil mentioned that the cathode is at AC ground and is almost at DC ground, so there should not be much of a difference. My guess is that it is easier to hook up the cap to the cathode since you can do it right on the tube pins (if you were to follow a strict star ground scheme, you might have an 8" lead going from the cap (hanging in mid-air) to the star ground point. Or can you ground it right to the chassis without adversely affecting a star ground arrangement?)
The tone shaping cap that really had me buffaloed was the cap across the plate resistor... but I guess you just ignore the DC component and look at the power supply as being very close to AC ground- right? Once again it might be easier to locate the cap right across the plate resistor and not have to worry about finding a good ground point to connect to.
Question: is there any noticable audible difference between adding in a 100pF cap:
1) from the plate to ground
2) from the plate to the cathode
3) across the plate resistor
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