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Caps in Preamp Designs

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9/28/2000 2:55 AM

Caps in Preamp Designs

I have 2 Questions related to preamp design:

1) In some designs, I have seen a .001uf cap (Sometimes other values), in parallel with the 100k plate load resistors in preamp circuits. Mesa Boogie does this some in their lead channel. What function does this .001 serve? Reduce high frequency?

2) What function does a 120pf cap have when it's connected between the input grid and the cathode in preamp circuits. I have seen this used in Boogies. Is this used for killing some of the highs in the high gain circuits?

I've built a couple of tube preamps and the highs have too much of a razor thin edge. I'm looking for some way to smooth out this a little.

Any suggestions?

9/28/2000 6:38 AM

Gary:

Any suggestions?

All of the above! In example #1 the .001uF cap across the plate resistor is actually going to AC ground as far as the signal is concerned. In example #2 the cathode is fairly close to ground. You can also put a 150pF to 350pF cap between the plate and the cathode to cut highs. Or run a cap to ground from the grid or after the coupling cap. Each one of these high-cut caps seems to work a little bit different...

Are you using a tone stack after a cathode follower like a 5F6A or early Marshall? If so you might want to see how caps can be added to the tone stack to cut the highs in the Pignose G40V, Soldano Atomic 16 or Electroplex Rocket 50 drawings on my site:

http://www.blueguitar.org/

--Good luck!

Steve Ahola

9/28/2000 4:10 PM

quote:
"You can also put a 150pF to 350pF cap between the plate and the cathode to cut highs."

Steve,

I'm curious. I've never heard of a 350p cap. That particular value is neither a standard 10% decade value (the closest being 330p) nor an "old fashioned" value (what I mean by this are the non-decade values you see in vintage guitar amps, such as 250p and 500p).

Do 350p ceramic caps really exist?

9/28/2000 10:28 PM

Steve, Thanks for replying... I'm using a standard Fender tone stack. .1uf bass, .047 Mid, & 250pf caps with a 100K slope resistor. The preamps I've built so far are too sharp on the high end. Like a razor... I was looking at some of the mesa lead channel schematics and noticed these 2 circuit designs and thought that they should cut the highs a bit, but I didn't know if there was other functionality on not.

I think I'll play with these and maybe adjust the slope resistor. It's almost like the highs (Sounds like the 5-10khz region) gets amplified too much. Really thin sounding on the G, B, & little E strings.

On one of these discussions, I found an excellent article ("Picking Capacitors" by Walter G. Jung and Richard Marsh) on the differences in capacitors. It showed how the impedance varied with frequency and a lot of other moving factors due to temperature, frequency, etc. I used polyester metalized film caps in my projects, & probably should have used polypropylene instead. I probably should have read this before I built my projects, but learning something new each day gives me a challenge. :-)

:-)

Thanks, Gary J.

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