The following is a select gathering of articles, and writings, from various vintage electronics books and magazines that could help explain a few capacitor ‘mysteries’ There are a few things to keep in mind as we read these articles.
- We have to ask ourselves if these writings pertain to the Hi-Fi world, or to UHF and VHF engineers.
- We have to ask ourselves if these ideas are relevant today. Some of us like distortion, so trying to avoid it is a waste of time.
- We have to ask ourselves if the parts mentioned are available today. Trying to find paper capacitors manufactured today is an impossible task. And even the NOS paper capacitor may not be usable fifty years after the fact.
- Most importantly, keep in mind the pages presented were often unsolicited articles submitted by people just like you or me. The articles can generally be anecdotal, not scientific, and the opinions given are by definition subjective, not objective. The more I squint and move my lips as I read, the more technical it seems.
With that out of the way, let’s try to discover a revelation or two on the journey to tone truth!
There have been many ‘established’ authors of books and articles over the long course of history relative to vacuum tube theory. Names such as G.A. Briggs, Norman Crowhurst, and Howard Tremaine are held in high esteem with many modern ‘practitioners’, myself included. Another respected author, Frederick Terman, has penned more than a handful of tomes aimed at providing the Freshman a solid foundation in vacuum tube (and related topics) theory. From his excellent book, Radio Engineer’s Handbook (McGraw-Hill Publishing, 1943), we get the following excerpts.
- “A perfect condenser when discharged gives up all the electrical energy that was supplied to it in charging. Actual condensers never realize this ideal perfectly but, rather, dissipate some of the energy delivered to them. Most of the loss in ordinary condensers occurs in the dielectric. Other ways by which energy can be lost in a condenser are from the resistance of the leads and metal plates, from leakage resistance between the plates, and as a result of corona.” page 109
- “The voltage rating of a condenser at low frequencies is the voltage at which the dielectric will spark through. As the frequency is increased the voltage rating becomes less as a result of energy dissipated in the condenser.” page 125
What does any of this mean? Along with the article written by Jack Greenfield, there is much to consider. However, from the chart provided by Mr. Terman, the frequency at which this all comes into play is listed as 100kHz. However, other maladies can come into play with ‘older’ capacitors. But, rather than repeat myself, I’ll suggest you read up on capacitors as much as possible, along with my own ‘lesson’ on capacitors. It can be found HERE. Let’s utilize this space ‘wisely’ and move along, shall we?
In the May, 1960 issue of Electronics World, writer Jack Greenfield submits an article entitled ‘Low-Noise Construction Techniques For Audio’. Now, this article appeals to me on many levels. As outlined above, it is aimed at the audio technician, and not the radar engineer. The article mentions considerations for tubes, tube sockets, resistors, and a host of other ‘oversights’ that lead the DIY’er to construct an audio amplifier that is unstable and noisy. This leads to frustration, and a lack of enjoyment of the amplifier. However, we will deal with the section on capacitors for now. What does Mr. Greenfield advise? It has been copied verbatim below for your consideration. As you read the following passage, keep asking yourself if these are subjective opinions, or objective technical considerations.
- Replace paper dielectric coupling capacitors with plastic film dielectric units. Pick capacitors rated at a higher voltage than the paper dielectrics in the original circuit.
- Plastic film dielectric capacitors have a very high “figure of merit” compared to their paper dielectric equivalents. This means that plastic film dielectric capacitors are less “lossy” and tend to couple more of the applied signal energy rather than to dissipate it in the dielectric. The use of higher voltage ratings decreases the possibility of signal energy losses and noise generation due to corona discharges (ionization of the dielectric). In addition, these high-voltage plastic film units are often physically smaller than their paper counterparts, contributing to a more orderly parts layout and simpler wiring.
These simple sentences could open Pandora’s box, if we choose to let it do so.
While no mention is made that paper dielectric capacitors will make Count Basie 78’s sound ‘warmer’ on my Williamson (and after all, this is an article aimed at audio amplifier builders), the thought of paper actually being less ‘efficient’ at coupling the signal to that next tube grid, and the corona discharge bit, has me thinking. Maybe there is something to this capacitor jazz, although I haven’t figured out how to apply this revelation just yet. Is the corona discharge and the coupling efficiency frequency dependant? Is the corona discharge and the coupling efficiency voltage dependant? I may never know for 100% certainty, but I am not going to lose sleep over it. As I’ve mentioned; paper capacitors are not available today, except for NOS units. Secondly, paper capacitors always get leaky over time. Regardless of the implications from the above article, I’ll avoid paper capacitors for my own reasons. Most film or plastic capacitors available today are of similar quality, so you’d imagine that they should perform, or ‘sound’ identical, as well.
Way back in the April, 1955 issue of Popular Electronics, Sprague decided to tell us about ‘dielectric soak’. A sample of that advertisement can be seen below.
Here, Sprague tells us all about ‘dielectric soak’, and compares it to a spring. This can contribute to the ‘energy losses’ mentioned by Mr. Greenfield earlier. What does all this mean? Just as transformers can be made more efficiently, I do believe capacitors can also suffer from poor construction techniques. However, the capacitors we can buy today will all be of similar quality and construction. Any discrepancies would most likely show up beyond audio frequencies (other than ‘corona discharge’). Is this an objective look at capacitors? Or is the advertisement above designed simply to sell Sprague capacitors?
One last capacitor article, and the relevant information, includes the following. From ‘The Designer’s Notebook’ series found in Electronics Today (February, 1987) we have writer Jon Fairall telling us;
- “In theory a capacitor is simply a capacitor. In practice, a capacitor, like every other device on a circuit board, has capacitance, inductance, and resistance. The only difference is that the capacitance is controlled, and hopefully, the inductance and resistance are minimized.”
- “Series resistance is a measure of the loss in the capacitor. It is measured as the loss angle or dissipation factor.”
- “….. most dielectrics are unstable. They tend to be sensitive to temperature, frequency and voltage, and any or all of them can cause the capacitance to change quite dramatically.”
From the above ‘revelations’, you have enough ‘food for thought’, and you can go out and investigate whether this has any pertinent revelation to today’s tube guitar amplifier manufacturer. The losses attributed to series resistance (the ‘ESR’) may be very relevant with large value electrolytic capacitors (i.e. the LCR filter capacitors some would suggest have a ‘British’ sound), but I highly suspect that in small value coupling capacitors this resistance is negligible. As a final thought; these ‘problems’ are associated with paper dielectric capacitors, and not ceramic or plastic. Since most esteemed boutique amplifiers utilize the available Sprague ‘Orange DropTM’ capacitors, we’ve just argued about capacitors purely for a moot point. Yet, hopefully you begin to understand the amount of information that is ‘out there’ for the technician to absorb. I cannot see Leo and the boys arguing the ESR of Mallory filter capacitors, or asking Bill Carson if he can ‘hear’ the corona discharge in an amplifier, but I can see Leo on the telephone asking around how cheaply he could get 10,000 capacitors from his local ‘jobber’.