cutting capacitors?

I ´ve been reading some old neumann eq schematics and I found some weird values.  .078u, .117u  I suppose in big quantities custom values were available at the time.  I remember some teachers at my university telling histories of going to jewelers to cut cristals to achieve certain frequency.

I wonder if it is possible to cut caps to get more precision.  those old neumann used to use siemens mkt those look like a good candidate to try.

there are people out there building their own capacitors. I feel is a bit too much but also sound weird to put 2 or 3 caps to get the correct value.

have someone tried something like this?

I would not attempt physical modification but it is trivial to "build" odd capacitance values by paralleling smaller values together.

A 0.068uF + 0.01uF = 0.078uF easy peasy...

JR

back in the 80'es I was with a company doing mos-fet power amps. At that point in time it was not really a predictable and stable technology  : . So we trimmed individual filter poles by clipping off edges of ceramic caps, applying epoxy to them after the unit passed testing.

This does NOT work on any type of polyester or film capacitor that we found.

Jakob E.

And btw, for strange-values-in small-space in my commercial Gyraf range I simply buy a big bunch of 10 or 20% capacitors, then measure and bin then according to value. "Unused" values goes to non-critical applications.

Jakob E.

yes the construction method is important. In some of the mkt caps I have seen they even look like they were sanded in one of the sides.

I suppose on differential stages or mastering EQs precision is important.  The 10% or 20% idea is interesting too.

Due to space restrictions adding caps is something that I would like to avoid. But of course valid too.

12afael said:

In some of the mkt caps I have seen they even look like they were sanded in one of the sides.

Click to expand...

They're actually sawn; the electrode is deposited later. That's the part that's difficult to DIY.

They're actually sawn; the electrode is deposited later. That's the part that's difficult to DIY.

Click to expand...

that have sense.

I saw a similar thing with carbon comp resistors in older gear where they would trim the resistor to get the exact value they wanted.

I stand by my advice... Mechanically trimming capacitors sounds like an opportunity to cause problems.

Laser trimming resistors is already mature technology made less  valuable as processes get better controlled.
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I recall last century when pot manufacturers would cook potentiometers to pull the final "cured" nominal resistance into tolerance. We had a quality issue where one pot manufacturer's green resistance was so bad that they over-cooked their pots making them brittle and leading to too many factory assembly failures.  (That vendor dropped us [Peavey] because we were too picky.  )

JR

12afael said:

I wonder if it is possible to cut caps to get more precision.  those old neumann used to use siemens mkt those look like a good candidate to try. ..there are people out there building their own capacitors. ....

Click to expand...

If you have Russian PTFE caps you can "easily" unwind it a bit to get the value you need.  8)
http://moxtone.com/PTFE_COND.htm

I would give the same advice provably John.  I think is interesting to know how thing were made before. Make a resistor, a cap, a transistor, an op amp, sound crazy for modern days when you just can order them online. At some point it was not impossible, it was probably the only way. 

I had no idea that pots were cooked.

If you have Russian PTFE caps you can "easily" unwind it a bit to get the value you need.  8)
http://moxtone.com/PTFE_COND.htm

Click to expand...

wow I would not call that easily hehe but hey if works needs to be done...

I saw a video of an Argentinian guy who was using aluminium foil from an old capacitor to repair some old Coles ribbon mics.

12afael said:

I would give the same advice provably John.  I think is interesting to know how thing were made before. Make a resistor, a cap, a transistor, an op amp, sound crazy for modern days when you just can order them online. At some point it was not impossible, it was probably the only way. 

Click to expand...

One of the first radio boom I read, I had found in a cellar, dated from the early 1920's. They described how to make resistors with paper and a pencil and capacitors with paper and copper foil. Variable caps were deemed too complicated for DIY so all their designs were based on variable inductors, which used two coupled inductors, one rotating inside the other.
In another book, they explained how a variable cap could be made by recuperating the aluminium disks used in power meters.
I've never seen a book explaining how to DIY vacuum tubes though.

> I've never seen a book explaining how to DIY vacuum tubes though.

Book?? Isn't everything worth knowing on YouTube now?

There's a video of a guy making a working triode out of a fluorescent lamp-tube end, some bits of wire, and fire.

There is a video of a guy who makes Giant Nixie tubes a dozen at a time in a shed. Yes, they wind up with Neon in them, but he has to suck the air out to a pretty good vacuum first.

Artisanal Vacuum Tubes: Hackaday Shows You How

Ah! I have a book Experimental Electronics, Bachman, 1948, which is actually about vacuum devices. Somewhat past simple tubes, somewhat short of a Synchrotron. (catalog link) $20 on ABE.com

Not all paralleled caps are shown in all Neumann schematics.

And there was a short time they indeed trimmed MKTs.
I just filed down the 33nF from 33,4nF to 32,0nF:

I still do not like this, but do not leave capacitor plates exposed to the environment. Perhaps seal with nail polish or something similar.

JR

Oh, it was just a test, I need my nail polish for other things  ;D