Variable capacitors

[RoadrunnerOZ]

I’m surprised no-one ever made a liquid electrolyte variable capacitor - bit of engineering as far as a liquid sealed tank for a 3 gang but in those days there was a lot of fine engineering just to make an air gap tuning cap and things were big. They could have made them smaller using liquid maybe larger values suited to audio……..

 

[Somkereki]

Well, my opinion is primarily based on an A-B test, as with all audio equipment. I do have quite a few good instruments, for example Neutrik A-2 D Audio Test & Service system, THD, IMD, phase etc. for measurement, as well as several generators, oscilloscopes, 6.1/2 digit HP multimeter. But the most important is the comparison test with switching. Fortunately, I have a friend with a very serious recording studio and several trained musicians. (I was a sound engineer in many rock and jazz bands when I was young.) I am 70 years old now. I always listen to their opinions. My "obsession" is that noisy things always sound bad. Maybe I have a prejudice about LF... and TL071 etc. from times. I remember building a 24-channel mixing desk for my band in 1980. Then I selected all the parts according to their sound. For example, there was the LM301 in both a plastic case and a metal case (military) version. Both were made by NS. The metal casing (M) had an incredibly nicer sound. Although according to the catalog data sheet, only the operating temperature range is different. My test material was a recording of jazz music recorded with a professional microphone from a Uher Report tape recorder, the power amplifier Crown DC 300 A The monitor speaker is a custom-built Isophone KK-10 dome tweeter. Also, the square signal (and sawtooth signal) transmission from the function generator. Checked with an oscilloscope. At my previous job, I learned what the various transmission distortions of a square signal mean. Transients constantly appear in the music, and very rarely a clear tone. That's why I wrote that I already bought several types of OP series FET input operational amplifiers, but I haven't had time to test them yet. (Unfortunately, my Chronic Lyme disease takes a lot of time, but I'm not young either.) For example, I recently had to repair the ATC monitor (active) speakers of my studio friend. I'm already retired, but my hobby is the same as my job, I can't stop. This makes it easier to bear the severe pain caused by my illness. I wish everyone good health!

 

[poppaflavor]

..it's really hard to predict from specs the ability to drive the G21 (output impedance does not specify output drive capabilities) - I'd recommend just trying it and see what it behaves like..

Thanks very much, I'll do some exploring and report somewhere here how it goes.

 

[vintageampfixer]

If you really want to use a variable capacitance, then a Varactor (varicap diode) might be an easier route to take because they are at least solid state, somewhat more reliable, significantly smaller and indeed readily available. That said, their capacitances still tend to be in the range starting with a few 10s of pf ... and to quote JR:

Most equalizers involve RCs... Its far easier to vary the R

 

[Somkereki]

Csodálom, hogy soha senki nem készített folyékony elektrolitos változó kondenzátort – egy 3 fős csoport folyadékzárású tartályának technikájával, de akkoriban nagyon sok finom mérnöki munka volt, csak hogy légrés hangolósapkát és hasonlókat készítsenek. nagyok voltak. Kisebbre tudták volna tenni őket folyadékkal, esetleg nagyobb értékekkel, amelyek megfelelnek a hangnak ……..

Elvileg tényleg lehetne, de több okból sem készült el. Rádiófrekvenciára teljesen alkalmatlan, mivel az ionvezetés rendkívül lassú. Már a hangtartományban is problémákat okoz. A nagyon erős hőmérséklet-függés miatt teljesen alkalmatlan ilyen célra. Csak az egyenirányítók utáni pufferkondenzátorra használható (néhány évig, utána cserélni kell)

If you really want to use a variable capacitance, then a Varactor (varicap diode) might be an easier route to take because they are at least solid state, somewhat more reliable, significantly smaller and indeed readily available. That said, their capacitances still tend to be in the range starting with a few 10s of pf ... and to quote JR:

If you really want to use a variable capacitance, then a Varactor (varicap diode) might be an easier route to take because they are at least solid state, somewhat more reliable, significantly smaller and indeed readily available. That said, their capacitances still tend to be in the range starting with a few 10s of pf ... and to quote JR:

Indeed, for many years at my workplace, we used Varicap diodes with a capacity of up to 500 pF. All of this is available in approximately 2mm diameter and 5-6mm length. (I don't know if this is still manufactured, because it was 30 years ago.) Even 10 pieces connected in parallel, that's already 5 nF capacity. Of course, this is also temperature dependent, like all semiconductors. This must be taken into account, but in the case of an RC filter, it can be compensated with a suitable NTK or PTK resistor. The bigger problem is that the audio frequency signal itself continuously modulates the capacitance value, so it can only be used at a low level, where we encounter the problem of noise. There used to be an elegant solution for tuning inductances. A variable inductance is obtained by placing a spherical coil part inside a solenoid coil and rotating it. Even as a child, I saw such a radio receiver.

 

[sahib]

Elvileg tényleg lehetne, de több okból sem készült el. Rádiófrekvenciára teljesen alkalmatlan, mivel az ionvezetés rendkívül lassú. Már a hangtartományban is problémákat okoz. A nagyon erős hőmérséklet-függés miatt teljesen alkalmatlan ilyen célra. Csak az egyenirányítók utáni pufferkondenzátorra használható (néhány évig, utána cserélni kell)

 

[CJ]

Any stereo from the mid 70's and back will have variable capacitors in the tuner section. I still have to deal with them when restoring Marantz receivers.

These ganged caps can get noisy and have dead zones in the tuning band, the solution is to spray the brass axle with some type of cleaner that will not stick to the gangs and alter the frequency range.

It is sometimes hard to hear the variance in response even while turning the freq band knobs on a Pultec at the high end, and it is only a 7 or 9 position switch, I forget,

So if you were to load up a 24 position Goldpoint or Lorin with caps, you could have an equalizer with very fine tuning, something that George Massenburg might like, who I saw the other night in a Lowell George documentary of all places.

 

[RoadrunnerOZ]

My original post in English:

I’m surprised no-one ever made a liquid electrolyte variable capacitor - bit of engineering as far as a liquid sealed tank for a 3 gang but in those days there was a lot of fine engineering just to make an air gap tuning cap and things were big. They could have made them smaller using liquid maybe larger values suited to audio……..

Your reply:

Elvileg tényleg lehetne, de több okból sem készült el. Rádiófrekvenciára teljesen alkalmatlan, mivel az ionvezetés rendkívül lassú. Már a hangtartományban is problémákat okoz. A nagyon erős hőmérséklet-függés miatt teljesen alkalmatlan ilyen célra. Csak az egyenirányítók utáni pufferkondenzátorra használható (néhány évig, utána cserélni kell)

(Could you please check your posts - they are not coming through in English) Google translation of your reply:
In principle, it really could, but it was never completed for several reasons. It is completely unsuitable for radio frequency, as ion conduction is extremely slow. It already causes problems in the audio range. Due to the very strong temperature dependence, it is completely unsuitable for this purpose. It can only be used for the buffer capacitor after the rectifiers (for a few years, after which it must be replaced)

Not referring to RF here
I was more referring to the possibility of using liquid electrolyte variable capacitors for audio as this type of electrolyte was already being used for wrapped fixed value electrolytics.
Varying the R in an RC filter is obviously a far simpler solution, but it would be fun to take an old tuning capacitor, get the plates coated with the appropriate oxides and polymer and use a liquid bath of a water based electrolyte (like used in the old water based electrolytics with a paper spacer soaked in electrolyte).
The use of carefully profiled metal plates with close spacing was convenient for RF tuning and using the right gap for high voltage applications, the ones used for domestic receivers had closer spacing and could have 3 gangs of up to 1nF each, in parallel would equate to 3nF - if this could be multiplied by 10, 20, 100 times by using liquid you’d maybe end up with useable values for audio purposes. There were also differential variable capacitors that as one stage increased in capacitance with rotation, the other would decrease. You can buy kits to build your own variable capacitor as well and with a bit of modding they could have very close spacing but there are plenty of old as well as new ones for sale.
The plate spacing air gap for a 1.25Kv tuning cap is around 0.03”/0.76mm - with closer spacing a much higher capacitance could be achieved (as there is no worry about high voltage here), C being proportional to the gap.

 

[Alva Goldbook]

I've often thought about this, but given most EQ circuits are some form of RC filters, it's just far easier to vary the resistance than the capacitance.

 

[RoadrunnerOZ]

Given that there are trimmer capacitors that have a very thin film of plastic between the plates it could be possible to make a variable plate capacitor with extremely fine plate clearance using Teflon like bearing material as a slip coating thus upping the capacitance considerably, possibly even a helical type arrangement for a multi turn device with a geared down dial to display 270° of rotation - like the multi turn coarse/fine pots with the coarse knob geared on some synthesizers. These incorporated with variable inductance threaded slug tuned chokes could make for an interesting EQ.