Plexi Cap value suggestions

[jlatrace]

I have been tweaking a homebrew Plexi that I built and am looking for suggested replacement values for the .0005uf cap on the bright channel volumn.

[dai h.]

you can go from no cap up to whatever value that doesn't cause any problems

[jlatrace]

Thanks. Help me understand what happens as the cap valuse are lowered or raised (e. g., what increases treble, bass, or mids)? I have tried it without the cap and I do not like it (unless the amp is wide open on 10, which makes sense to me).

I have been tweaking the V1 cathode cap and resistor and found (like many others) that the 4.7 uf cap and a 1610 resistor (happen to have one in my stash) sounds the best to me in the Treble channel. I'm not sure I'm unhappy with the C5 cap, but have seen posts by others suggesting changing the values and I want to experiment.

Thanks again!

[Enzo]

Nothing will tell you how it sounds better than your ears.

0.0005uf is the same thing as 500pf.. That sounds like something on an older schematic, from before the use of the term picofarad became common.

If you are talking about the little cap between two legs of the volume control, we tend to refer to that as a "brightness" cap. Get a selection of small caps - if you already have the 500pf, then get a 100pf, a 220pf, and maybe even a larger one like 750 or 820. Then try each one. You can just tack solder it in place, don't bother even trimming the wires down. Or use a couple clip wires. In either case, connect the various values in place and see which you prefer. Then take the one you like, and mount it properly.

All this part does is bypass higher frequencies around the volume control. SO when the volume control is only half way up, the real high frequencies are still going stong. This adds emphasis to the treble, making the sound brighter.

The larger the cap value, the farther down into the midrange this effect goes. SO a 100pf cap wil just add some real high end edge or shimmer to it, while a large cap will also add more upper mids.

Look at the circuit. If the cap is wired from the volume control wiper to the top end terminal, when the control is all the way up, then teh cap is essentially shorted across. SO the brightness effect is more pronounced at lower volume settings.

[dai h.]

fwiw I've got a bunch of caps with alligator clips on the ends to facilitate experimentation with the value so I can just clip them right on the pot lugs.

[Enzo]

Good idea.

SOmething we used to build from kits and you could also buy - probably still can - was a "substitution box." It was basically a box with a bunch of different value parts in it and a selector switch. A couple clip wires trailed out of it. Set the switch to the desired value and clip it in the circuit. then yuo could flip value to value to see what difference it makes.

Would be easy enough to build one on your own.

Kit makers like Heathkit and Eico made them. Typically there would be one for caps and one for resistors. Usually we'd make the cap one, and for all the world, I'd just use a pot and some wires for resistance.

[dai h.]

weber vst has some (cap and resistance). I bought some apparently really old (late 60s?) "Calrad"sub box. Have to recap it since it's got the japanese oil caps that seem to be guaranteed to be leaky or go leaky with use.

[jlatrace]

Thanks for the replys. This is the information I needed. You are correct that the cap I am talking about is across the wipers of the bright channel volume pot known as the bright cap.

However, I mis-typed the value, It's a 5n or .005uf cap, so from your posts I now understand that it is boosting the middle as well as the highs.

I have 500pf and 100pf, I'll check them out this weekend using the alligator clip suggestion.