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Re: Silver Mica Caps... revisited!

10/1/1999 4:07 AM
Re: Silver Mica Caps... revisited!
This 68pF/68K combo is not some kind of genius secret tone thing, it just shunts extremely high freqs to ground and for the most part, in a non evasive way.  
But, I've found the 68pF/68K thing works really good in amps when hitting EL84s too hard and it seems to get rid of some of the harshness in hard driving Fender amps too.  
I started using it on BF Fender Deluxes when the cheap Chinese 6V6 tubes I was getting sounded harsh and fizzy on the extreme top end.  
I couldn't think of any other way to slope that off without killing the livelyness of the amp.  
It can go on any plate load stage I'm sure, but I like it a little better when it is right before the PI driver cap.  
In a Fender with AB673 circuit with reverb, it would be on the triode section right after the tone stack and before the 3m3/10pF combo.  
So yeah, on pin 6 right to the cathode or ground is OK.  
On my custom amps, I have an eyelet on the board to run them right after the coupling cap directly back to the ground of it's cathode resistor/cap...if I need to use it, I can.  
Boy, it sounds like you totaly get it when it comes to the small plate cap to ground, the cap across the plate load resistor or the small plate cap to the cathode.  
The main thing is to get the cap AC grounded on one end.  
Applying it to the cathode probably injects a little more negative feedback at that freq though.  
I bet the cause and effects are nearly the same.  
Another interesting thing to try is splitting the anode resistor into two different equal values and bypass one of them with a 500pF cap or split the anode load and use a small value resistor on the plate (20K)+ (82K) and connect the coupling cap between the two plate load resistors and then bypass the second one... that is feeding B+.  
I guess there would be a number of different things to try...if you even need to do it at all.  
"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"
Hmmm. I'm not sure. I 'll try it tommorrow and see.  
I think I prefer using the resistor and cap in series because I can build a thru RC circuit and limit how much of the unwanted audio is passed to ground.  
A few months ago when you asked me if I had seen the 22m/.05uF NFB thing used before, I told you about old ham radio tricks like the "gimmick" and plate neutralizing.  
It's all part of the same thing only at audio and sometimes one can extract great sounds from basic/simple ideas.  
10/1/1999 4:43 AM
Steve A.

    This has come up at least a dozen times before, but where can you get eyelets to add to a Fender-type fiberboard? A recent post mentioned using pop rivets and then punching out the "nail", but I was looking for a way to add a few eyelets to a stuffed board. (I also like the size that Fender uses because you can stuff a lot of leads into them.)  
    As for that 68pF/68k resistor trick, I suppose that you could also use a 100k trim pot, fine-tune it for the results you want, and then replace the pot (actually a variable resistor) with a fixed resistor (or network of resistors) that match the value you measured on the trimpot.  
    So how does the frequency equation for RC networks work when the resistor and cap are connected in series rather than in parallel? And that formula is for the frequency that the signal level drops off 3dB- right?  
Steve Ahola
10/1/1999 4:59 AM
Gil Ayan

This has come up at least a dozen times before, but where can you get eyelets to add to a Fender-type fiberboard? A recent post mentioned using pop rivets and then punching out the "nail", but I was looking for a way to add a few eyelets to a stuffed board. (I also like the size that Fender uses because you can stuff a lot of leads into them.)"
Not Bruce here, but I get my eyelets from Mouser. They are the same kind Fender used, 1/8" type (that is, the hole they go into), and the length will vary depending upon the bpard thickness. Now, while putting them in is a snap with a centre punch and a hammer, I don't know how you could do so "in place." You really need to get to the underside of the board to crimp them in place. I had to add some after my board was "finished," and even then, with the board outside the chassis, you need to be careful if you already have components mounted on the other eyelts.  
My only tip: whack the things gently, no need to strike hard. They do open up nicely and then another gentle tap with a hammer and they ain't going nowhere -- I know, I had to take some out too and it was not a walk in the park, nip, nip nip.  
10/1/1999 5:08 AM

I hate to admit I've done this, because this seems a bit hacked as I look back on it...but the only way to do it with the board in the amp and fullof parts is to slide the eyelet in the new hole upside down from the bottom (betwen the insulator board).  
The the head is under the board now, but the shank is exposed so you can whack it with Gil's punch to flare it out so it won't go anywhere.  
The eyelet hole is the same hole no matter what side you insert the wires!  
10/1/1999 5:02 AM

The nice thing about RC time constants is the R and C don't seem to know the difference if they are in series or parallel.  
The same formula I gave you works and I think of it in terms of 3db too.  
So I would look at the RC as a circuit not particulary in parallel with the plate||plate load... etc.  
68pF||68K = abt 34KHz to 35KHz.  
The 100K pot with max R would make the circuit act like the capacitor was bigger in value and lower the frequency.  
68pF||100K = abt 23KHz to 24Khz  
But the resistance to ground is now higher so it will have less shunting effect.  
So if you really wanted the 23KHz to 24Khz shunt with about the same attenuation efffect, I'd just use a bigger cap.  
100pF||68K = abt 23KHz to 24KHz.  
22m||.05uF = abt .1Hz!!  
So ALL the audio frequencies are shunted back to the grid.  
That's negative feedback from almost DC to all freqs!  
But then there's that darn 22M series resistor sitting on top of the grid load resistor... hmmm, a voltage divider.  
Get it?  
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