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27 Special Offer – Fetid Dingo’s Kidneys

Here is a “special Offer that I received from audio equipment retailer “Audio Trends” this afternoon, and the email I have just sent them:

Special Offer

Here is the text enlarged so you can read it:

Special Offer text

Hello Audiotrends people,
I have been a recipient of your emailouts since purchasing a Tivoli Radio from you some time ago.
This afternoon, I will be clicking on the “Unsubscribe Instantly” link, but I wanted to write to inform you of why I am doing this.
I am an electronics engineer, and am qualified to make judgements about the credibility of claims for many of your products.
The claims made for the Isotek EV03 Premier Power Lead are so far into the fetid dingo’s kidneys domain, that it would amount to theft if you were to take a gullible customer’s money for this product. I do not wish to be associated with any company who would peddle such stuff.

Richard Schurmann

26 Colour

As you all know, our colour vision works by exploiting three different spectral responses of the cone cells in the retina.

Spectral response of human cones

I found this picture somewhere, and although it gives the general idea, I am not sure how accurate it is. In particular I am not sure that the “colouring in” is right. We generally consider these spectral responses to peak at Red, Green and Blue. Note that the optics guys like to characterize light by its wavelength: not frequency so the hoizontal axis is the other way around from what we would normally expect in electronic engineering spectral work. It looks here (on my screen, anyway) that the S curve peaks in the black, and the L in the yellow. Anyway, we assign Red, Green and Blue to them and this is why we regard these as the “Primary Colours” (Not to be confused with the Cyan, Magenta and Yellow used by printers, or the approximations to them: Red Blue and Yellow used by artists.)

Those of us with a little schooling in how television works sometimes have trouble convincing our fellow TV viewers that there is no yellow light emanating from the television screen: particularly when a vivid yellow image is portrayed. Reference to the above graph might help, with the explanation that what we perceive as yellow is the condition in which the M and L cones are (roughly) equally stimulated. This can happen with narrow band spectral light (sodium vapour), or with a bimodal spectrum with a peak in the green and a peak in the red. I find it nicely intriguing that we cannot tell these apart.

If either of our M cones or our L cones are missing, or perhaps they are both there, but for some reason have identical receive spectra, then we have red/green colour blindness.

There is another phenomenon that arises from the way we generate our perception of yellow by comparing the signal strength of our Green (M) and Red (L) sensing cones.

If we are presented with a spectrally pure red light, we see it as red (or, whatever we see it as, we have come to know that the name for it is red): similarly for spectrally pure green light. The intensity of the light has no bearing on the perceived colour. On the other hand, when we are subjected to light containing both red and green spectral lines, our perceived colour depends on the relative intensity of the two spectral lines.

If the relative sensitivity of the M and L cones varies from person to person, or perhaps the location of the spectral peaks of their sensitivity varies, then those effects will have little or no effect on the colour perception of red or green light, but will directly effect the colour perception of light containing both red and green lines.

My first awareness of this arose when reading the biography of Lord Rayleigh. When I write “Lord Rayleigh”, I mean the 3rd baron Rayleigh. His surname was Strutt. My interest in him was sparked in the days when my primary work interest was acoustics. Rayleigh made many advances in that field. Rayleigh’s son wrote a biography of his father, which I have and which I read every few years. In his time, the son became Lord Rayleigh himself, but to avoid confusion, I will restrict the name to the 3rd baron.

Rayleigh investigated red/green colour blindness (important because of the choice of red and green for railway signals) He made a significant observation (Page 46)

 

Lord Rayleigh P46

That this could be a “new” discovery may seem strange to those of us who are familiar with how colour television (or colour computer monitors) work, but here it is. No date given in the biography, but probably about 200 years after Newton’s work with a prism.

Lord Rayleigh P174
Lord Rayleigh P175
The fourth Baron was writing here as if the population (of people) could be divided into two groups, one of which had Lord Rayleigh as a member, and the other had the Balfours.

There is a delightful similarity here with Lord Rayleigh’s discovery of Argon. In an era when air was believed to consist of water vapour, carbon dioxide, oxygen and nitrogen, it followed naturally that nitrogen could be obtained by removing the other constituents from air. Rayleigh had set himself the task of weighing gasses, and he discovered that chemically produced nitrogen was lighter then nitrogen thus derived from the atmosphere. This led to the discovery that there was another gas present. This was produced by removing all the known constituents from air. What remained was named “Argon”. In the account of this discovery by his son, there is no mention of the possibility that this “Argon” might have more than one constituent.

Wikipedia says:
Constituents of Air
Lord Rayleigh’s Argon was mostly argon (99.7%), but it was 0.19% neon, 0.056% helium and 0.019% methane. You can’t really blame Lord Rayleigh for not noticing the others.

Similarly, Lord Rayleigh assumed that there were people with normal eyesight, and then one group (“this peculiarity”) (which included those Balfour brothers) which was different.

As with the argon, he was right to identify an anomaly, but wrong to assume that there was only one. The feature that Lord Rayleigh noticed in the Balfours is these days called Anomalous trichromacy. Wikipedia says:

Anomalous trichromacy is a common type of inherited color vision deficiency, occurring when one of the three cone pigments is altered in its spectral sensitivity. This results in an impairment, rather than loss, of trichromacy (normal three-dimensional color vision).
Protanomaly is a mild color vision defect in which an altered spectral sensitivity of red retinal receptors (closer to green receptor response) results in poor red–green hue discrimination. It is hereditary, sex-linked, and present in 1% of males.
Deuteranomaly, caused by a similar shift in the green retinal receptors, is by far the most common type of color vision deficiency, mildly affecting red–green hue discrimination in 5% of European males. It is hereditary and sex-linked.
Tritanomaly is a rare, hereditary color vision deficiency affecting blue–green and yellow–red/pink hue discrimination. Unlike most other forms, it is not sex-linked, it is related to Chromosome “7”.
Here is a summary of the frequencies of occurrence (not agreeing exactly with the above text.)
Anomalous Trichromacy Table
Two Consequences for us

Consequence 1.
Years ago, I was putting the finishing touches to a design that has three LEDs on the front panel. These were red, yellow and green. As the LEDs of different colours have different forward volt drops, one would never imagine that they would have the same value of series resistor. Factors of variation in LED brightness, and variations in the sensitivity of the human eye were to be considered as well. I had read Lord Rayleigh’s biography many years before, but did not think of it then. I selected the resistors to give much the same brightness to the three LEDs (to my eyes). I proudly submitted my work to my mate Tino Vescovi, who was my boss at that time. He asked why I had set the brightness of the three LEDs the way I had. I told him that I had set them for equal brightness. “No”, he exclaimed, “The brightnesses are so very different.” Tino changed the resistor values so that the brightnesses looked the same to him. They now looked very different to me. Unfortunately, I do not recall whether Tino’s changes made the red look brighter than the green or the other way around. I conclude that either Tino or I (or both) have anomalous trichromacy.

The lesson is, when you set the brightness of LED lamps like this, get a consensus of your work mates. Don’t rely on one person’s view. It is better to optimize your product for 96.665% of the population than the other 3.335%. You cannot get it right for everybody.

Consequence 2.
These days most people in the photography game are using digital equipment. They view their work on a computer monitor (often a very expensive “calibrated” one) which uses red, green blue primary colours. Then they print it out with a colour printer that uses cyan, magenta yellow and black inks.

No matter how well calibrated the monitors are, they cannot (even in principle) provide an image that will look the same as the printed one for 100% of the population. I suppose pleasing 96.665% is good enough. However it would be good if people whose profession is the presentation of colour understood this.

CIR Chromaticity Diagram

CIR Chromaticity

Somehow I love this. The diagram is presented as occupying x – y axes. Of course there is a z axis representing brightness. I have drawn in a triangle. I understand that if the screen has three phosphors (red, green and blue) then the colour on the screen can be any colour within the triangle.

Somehow the shape of the curve must follow from the spectral response curves of the S, M and L cones (first picture in this post). See http://en.wikipedia.org/wiki/CIE_1931_color_space

I wonder if it would be practical to use a similar “colour space” in other communication systems which used a three channel receiver with partly overlapping passbands. Hmmmmm!?

 

Flame Loudspeaker

Recently, I was sorting out some old material and I came across a few documents from what now seems to be the distant past. It is hard to believe this these days, but before the days of the personal computer, blokes generally did not type for themselves, and if a document was required to be well presented, one had to find a friendly typist to type it out. In 1975, which just before the “Personal Computer revolution”, it was my job to write programs for a Wang personal computer. The term “Personal Computer” wasn’t even coined then, and they called it a “Programmable Calculator”. It was programmed in BASIC. So great was perception of the barrier between a male “user” of this machine, and the qwerty keyboard, that Wang sold this machine with two keyboards: one of conventional “qwerty” style, and one in which the keys were in alphabetical order. Each key doubled as a BASIC command name when used with the <Ctrl> key.

So in 1973, if one wasn’t a typist (and the vast majority of us were not) then one wrote out a document in long hand first.

My interest had been aroused by an item in a column called “Real and Imaginary” by “Vector”, in the August 1973 issue of Wireless World. In those days, it was very common in Britain for regular contributors to magazines to use a nom-de-plume. I don’t know why this was, but “Cathode Ray”, “Vector”, and in another magazine, “Tubal Caine”, “Duplex” (This was actually two people who shared the name), “LBSC” (and others) became well known. I wonder where this idea came from.

Here is the item:

flame0002

I determined to try it. I had an oxy torch, and it was in at work for some reason at the time. I do not have a copy of the typewritten letter: indeed it might not have been copied. Here is my handwritten report.

flame spkr letter

For you young blokes who are not familiar with the common arrangements of valve a.m. radio circuit, I include one here. It was universal to have a radio that provided for a.m. only, as there were no FM broadcast transmissions in those days!

Schematic

This circuit is much older than the one I used, but all the essential features are there. It will be seen that if one of the wires to the speaker is cut, then the last valve has just an inductor (the speaker transformer primary) for a load. A connection to that anode provides a high voltage audio signal superimposed on a d.c. bias (in this case 250 volts). This is a good first guess for the signal requirements for the flame speaker.

Here was a speaker with no moving mechanical parts, and thus no resonances of moving mass and mass suspension stiffness. Yet…. completely impractical!

flame0003 (800 x 1037)

Flux Strap / Flux Band

You know the thing I am referring to. A strip of metal (usually copper) is wrapped around a transformer to provide a shorted turn to stray field. A Flux Strap or Flux Band?

 Transformer 2
Transformer with Flux Strap or is it a Transformer with a Flux Band?

What do you call it?

I have searched in vain for a Standard way of depicting this thing (whatever you call it) on the schematic symbol. In the face of this vacuum, I have adopted my own which seems to be clear to people who have never seen the symbol before. This is an advantage over some other attempts to depict it, which have been misinterpreted as a metal shield between primary and secondary.

Flux band symbol (500 x 435)My way of depicting a Flux Strap on the schematic symbol

Are any readers aware of a Standard way of showing this, or even one that is commonly accepted in some circles?

20-07-2014 I return to report that nobody did come back to me with the details of a Standard way of depicting a flux strap, so I used the style of depiction that I had sketched out above. Here it is as used in my work.

Circuit Diagram

April Fool

In the last post, I wrote that a friend had sent me a job ad “not because it was an interesting job, but because it was amusing.” This friend of mine is a “grass is brown on both sides of the fence” sort of guy. He characterized the ad like this:

“Greedy entrepreneur with no management skills embarking on a start-up venture, in a field he has little knowledge of, seeks a high-calibre Professional Engineer with experience in all facets of business management, project management, systems engineering, electronics design, CAD design, embedded firmware engineering, computer high-level application software development, electronics prototype testing and debugging, components procurement, verification testing, etc, willing to work long hours under pressure of unrealistic short-term delivery schedule, for a salary commensurate with a recent graduate or technician.”

Maybe he was right, but it is not necessarily mischief (implied by his words “Greedy entrepreneur”) that can lead an advertiser to grief. Maybe sometimes, he is genuinely out of his depth.

Yesterday another job ad came to my attention. I couldn’t help reading between the lines. Here is how the ad started out:

 

I have removed the identity of the advertiser. There are some interesting points to note here. The first is the use of the word “Graduate”. This is a perfectly fine word with a well understood meaning. It means that someone is required who has little experience, and little pretensions for being high up in the peck order. It also means, of course, that this job will be near the bottom of the pay range.

The ad goes on:

We see that this person (who is a junior (“graduate”)) is to work in the “Engineering Division”, and yet reports to the State Manager. Where are all the other people in the “Engineering division” in the hierarchy? It looks to me as if this “Engineering Division” is to be a one man show. Where is the graduate to turn to for engineering mentorship? To the State Manager, we have to suppose. How qualified he might be for this is something we will have to judge as we take in more of the ad.

The ad has a block of “motherhood statements”. I suppose that they have to be put in, but they don’t help much.

Is this person to be a completely raw graduate? Well, no. The ad does state that they want a person with one to two years experience. How should the person have filled in the last one to two years? The ad gives us a clue.

What is an applicant for this job who claims to have the required (or even most of) the required experience in the last one to two years? Superman? No. A bullshitter.

This is absolutely impossible. The ad writer betrays a complete lack of understanding of what an engineer does, and how long it would take to gain that experience. I do hope that the ad is not written by or approved by the State Manager, for, if so, he will be completely incapable of providing this graduate with the necessary engineering mentorship.

Yet, I do not see evidence of a “Greedy entrepreneur” here. I fear that the advertiser is just out of his depth if he has aspirations of having an “Engineering Division” under his control.

I am glad I am not a candidate, and are consequently not in danger of being the successful candidate. I am glad I am not that State Manager as well. He will be saddened by the experience he has laid out for himself.

This ad is not amusing, but it certainly leaves one thinking.

I fear it will all end in tears.

Post Scriptum.
The name of this post? My friend (the one who chose the term “Greedy entrepreneur”) also took a different slant on this ad.  He wrote “Richard, Have a look at the date on the ad. I think you might have your answer.”

 

Deliver us from Evil

I am interested in words, but usually this is not the forum for exercising that interest. However, one example of the use of words has cropped up a few times that might push its way into this place.

This is the use of the words in the expression “deliver the project”.

“Project” is an abstract noun, so one thing you can’t do with a project is deliver it. You can certainly deliver the fruits of your labour if you have been working on a project. When I was at a very early stage in my project oriented career, a wise colleague taught me that right at the start of a project, one had to define and agree on the “deliverables”. This is really good advice, as this is a strong defensive move against those who seek to move the goal posts after you kick the ball. My colleague wasn’t as wise as he looked, however, as he drove the lesson home by showing me what happens when you don’t “define the deliverables”.

It became clear that those who say and write “deliver the project” do not mean “deliver the deliverables”. What they are imagining is much more akin to “perform the tasks that make up the project”. At first, I thought that this must have been a sort of insider talk that belonged to one particular group, but since then I have encountered it in other places. This usage seems to crop up amongst those who have little focus on delivering the deliverables. It certainly is not an established usage. It is not supported by the Oxford English Dictionary, or even by those American on-line dictionaries that define “leverage” as a verb.

Just today I was sent a job ad by a friend; not because it was an interesting job, but because it was amusing. Here is part of it (not the amusing part):

At any time in the past when I read “from concept to delivery”, I would have been sure that I knew what was meant. I would have said that the writer obviously means “from beginning to end”. Do we need to ask if, these days, it might mean only “from first thought to execution”, a span of focus that leaves off the important step of actually handing over something at the end. The delivery of the outcome of the work is the end of the work. This idea of what delivery is is too important to be diluted by variants.

Didactic Diversion

I am not sure whether I have mentioned in this forum that I write a column for a hobby magazine. The magazine is Australian Model Engineering. My column is called “Sparks ‘n’ Arcs”, a rather uncomplimentary way of suggesting that the column focuses on electrical matters. The readers are mostly skilled artisans, but not people well versed in electronic matters. I started writing the column some years ago when I felt compelled to rescue the model engineering fraternity from some very strange ideas about how to provide variable speed drive for a traction motor.

I have just signed off the galley sheets for the third of a series of articles in which I attempt to explain why some control loops are stable and some are not – all for a readership with very little mathematics.

After considerable thought about how to tackle the problem, I came up with a sequence of concepts that ran like this:
1. Integer exponents. You know the sort of thing: x times x times x times x is equal to x to the power 4 because we multiplied together four instances of x.
2. Non-integer exponents. For this, I chose adiabatic expansion of a gas. You remember:
Pressure times Volume to the power Gamma is equal to a constant.
adiabaticThe value of gamma depends on the number of degrees of freedom of the gas molecules. It has the value of about 1.67 for a monotomic gas. I defined a very simple model in which a piston moved along in a cylinder to allow the gas inside to expand. the gas in the cylinder consisted of only one atom that bounced back and forth between the cylinder cover and the piston. Very simple equations of motion describe what is going on. If the atom velocity is such that it will bounce back and forth about a thousand times during the piston stroke, then a thousand points of data will show how the pressure varies with volume. Snap! It obeys the simple gas law! I was quite pleased with that. I have never seen anything like that before. Anyway, this showed the mechanically minded viewer how very simple equations could give rise to non-integer exponents, even though the notion of exponent was originally defined (in my work) by how many instances of x one multiplied together (a definition that seems to be restricted to integer exponents).
3. On to logarithms. Not the treatment that my readers would have got at school.
4. Very quickly moved on to graphs with logarithmic scales. Showed how a power law comes out to be a straight line on such axes. There is an example of the use of this in steam engines so that was not too far off the familiar ground for the reader.
5. From a power law is a straight line, I moved on to the way some equations come out to have two straight lines with a “bend where there is a transition between them.
6. Straight from there, we had a Bode plot representing a single pole.
7. It wasn’t hard to devise some Heath Robinsonesque mechanisms that would exhibit a single pole response.
8. Then multiple poles and their effect on amplitude and phase. It is pretty easy to explain how 180 degrees phase shift turns negative feedback into positive feedback, and the notion that if this occurs at a frequency at which the loop gain is at or more than unity, we have instability, does not stretch the imagination too far.

I gave them an example of a Watt governor on a steam engine.

It will be interesting to see what sort of reception these articles get. It has been a very interesting challenge.

Didactic Diversion Closer to Home – The role of this Blog.
I do get some interesting comments to this Blog. Of course there are all those who think that I will be wanting to have my wedding tackle modified. WordPress provides means to save me from bothering with those. Some comments are from those who understand what I have written but feel the need to straighten me out on some detail. Many of these people are my friends and acquaintances who tend to send me a personal email rather then using the Blog comment facility. Often I bring them out into the open by responding to their remarks in a later post. I do get a few, however that do not fall into either of the above category. Is the writer seeking a tutorial response? Sometimes this is not clear. If the real drift of a comment is not clear, but I suspect that there is a genuine query in there somewhere, then I respond directly to the comment author. Only if some point of more general interest emerges, would I then bring the matter up for us all to see.

 

Transformer B-H curve

In the last post, I quoted Brian Magee. Part of what he said was this:
“It goes without saying that a fuse should not be placed on the secondary of a transformer driving a full wave rectifier if its failure produces a DC component in the secondary.”

He was, of course, considering the once universal arrangement of full wave rectifier that used a centre tapped secondary and a dual thermionic diode.


This circuit diagram is taken from a book of published prac. notes that I used when I was an undergraduate student at Swinburne. It shows its age with the selection of “8uF” as a value for the electrolytic caps. In the earliest days of valve equipment, high voltage electrolytic capacitor values went up in octaves from 1 uF. The location that Brian warns us about is the location with the “link” symbol designated “A” in the above figure. Note that this circuit uses an indirectly heated rectifier as recommended by Brian. When I was a youngster and used to play with “wireless sets” for a hobby, the rectifier was nearly always a directly heated type, 5Y3 or 80. It must have been that in the olden days it was cheaper to put an extra secondary on the power transformer than to provide a heater to cathode insulation with a rating of 450 volts or so.

I notice that, although it is not quite the same, I came perilously close to doing the wrong thing by Brian’s missive in the circuit that I showed in my post: “Fuses 2”. That circuit would have imposed DC on the transformer if one fuse on the bottom winding was open circuit and the 7812 and the 7912 drew different currents. I was only young at the time.

The idea that we avoid drawing DC on a transformer winding (unless we really have to) is not new to any of us. I have to admit that I have never looked into this in any detail. In the same published prac. notes that I used to extract the circuit above, I find an experiment (it was to do with playing with Lissajous figures) for displaying the B-H curve of a transformer iron circuit.

It would be fun to conduct this experiment with an without a half wave rectifier and load applied to the secondary. Any one out there actually done this?

In the days when domestic radio sets were vacuum tube designs, it was almost universal to have a single ended output with a speaker transformer that (obviously) carried the class A output tube anode current. We coped. Right near the very end of the valve era, I owned a Philips brand valve mantle radio that derived the HT (B+) supply from a half wave rectifier. (Hey you young blokes! “HT” and “B+” are both designations for the anode supply in valve equipment.) The reason for this, it was said, was that it was easy to design the speaker baffle for a null response at 50Hz, whereas eliminating hum at 100 Hz would be more of a problem. Although the cost of a centre tapped secondary was saved, surely the power transformer would have had to have more iron to cope with the DC bias. This would be fun to investigate more fully some time.

Fuses 3

A correspondent of mine, Brian Magee, has been drawn in on the discussion about fuses, with particular application to valve equipment that had been so ably prodded along by Nigel Machin.

Brian writes:
Fuses in Valve Amplifiers.
I have never had any problems, so have not been forced to think too deeply about the matter..
The following thoughts come to mind when designing power supplies for valve gear :-

(a) Any starting transients in the high tension supply before cathode emission starts may generate large grid – cathode fields in the tube.
This can reduce cathode life.
In frame grid tube such as the ECC88– EF184 and the E180F the grid wire diameter and grid spacing is so small,  field emission occurs and this  usually destroys the tube.
To prevent this I always put a small neon between the grid and cathode of frame grid tubes.
To delay the onset of high tension I use indirectly heated cathode rectifiers such as the 5AR4.

Under these conditions the greatest peak input current occurs when the switching occurs at the start of a cycle, driving the transformer core into saturation. Toroidal core transformers tend to have smaller primary resistance and so greater peak input
currents.

To set the fuse rating on any new design, I simply pull the rectifiers out and switch the thing off  and on about fifty times upping the fuse rating until it withstands the turn on transient. It goes without saying that a fuse should not be placed on the secondary of a transformer driving a full wave rectifier if its failure produces a DC component in the secondary.

<end quote>

I do remember learning, years ago, that the initial magnetizing current in a transformer primary will depend on the phase of the mains cycle when the switch on occurs. My memory had been that the magnetizing current transient peak could be up to about twice the continuous peak magnetizing current. One would not expect this to exceed the sum of the continuous magnetizing current and the load current.  I have to admit that I had never thought of saturation in this context. Something new (for me) to think about. I have never been aware of any evidence of excessive current on transformer switch-on. I have been caught out, however, with a potential problem when a transformer is switched off. If there is no load on a transformer, and it is switched off with a switch with low arcing losses, then there is nowhere for the magnetizing current to go, except into the charging of stray capacitances or the creation of a carbon track through the insulation. A bit of a lottery as to which will happen first. After having lost a high quality and expensive transformer by doing no more than swithing it on and then off again with no load, I now have a rule that I never switch a mains transformer off without a load on it (a MOV on the primary will do), or if just powering it up to check secondary voltages etc. then use the variac so that the magnetizing current can be reduced at a slow pace compared with a mains half cycle.

 

 

The signpost on the side track says “Nigel”

(Prerequisites for this post are the previous two posts: “Fuses” and “Fuses 2”)

The only picture I had of Nigel was not suitable, so I have used a picture that Nigel might like instead.

Nigel Machin seeks to sidetrack us with an interesting comment about my last blog posting. I reproduce part of it here:

“I think that for a multiple output transformer individual output fuses make sense to save the transformer when a regulator or diode bridge fails – this does (rarely) happen. A single fuse at the 230V input may not do this because the secondary attached to the fault may greatly overheat without tripping the input fuse which is sized for the whole transformer’s current consumption. However a thermal cutout inside the transformer could suffice I reckon. In the special case of the teensy weensy winding (a technical term) only a secondary fuse could help. I have had this experience with the transformer for a valve amplifier that has a TWW for -47V bias, its diode failed and it burned into a short and the whole transformer was lost. No primary fuse or thermal cutout would help here.”

As is often the case with Nigel, we might be rewarded with some interesting thoughts if we follow him up his side track.

The example takes us straight back to what I (rather inelegantly) called “fault scenario analysis”.

The whole idea of installing a fuse is to minimize the damage when there is some failure that causes an increase in current. This design task would normally arise in a context in which a kindred task is to try to minimize such failures in the first place. Perhaps when each failure mode is identified, and it is being evaluated to see if it is suitable for fuse protection, it could also be considered for elimination or reduction in the probability of a failure.

Take Nigel’s example of the valve audio amplifier with a separate bias supply. The following considerations immediately come to (my) mind.

1. A bias supply is usually a supply with a quite low current. Maybe it would be acceptable to place a resistor between the transformer secondary and the rectifier in such a supply, with a value high enough that the transformer could provide the limited current into a shorted rectifier indefinitely. Power rating of the resistor to take the short circuit current indefinitely. If a vitreous wire wound (brand with good reputation, or tested in real circuit) type or other with a good surge rating is chosen, the resistor should be reliable. The presence of the resistor would surely reduce the probability of rectifier failure.

2. What about the amplifier?

How will those output valves cope with the bias supply removed? Will the plate current increase? Is the output tranny primary rated for the plate current of a pair of output valves with no control grid bias? (I have looked carefully at valve push-pull output transformer data sheets. I do not recall a continuous primary fault current rating.)

Maybe the increase in plate current when bias is lost will be sufficient to promptly open a fuse in the B+ supply. Indeed, if carefully contrived, perhaps the output stage would be just the sort of fault current multiplier that we might like to ensure prompt fuse opening.

As with many Nigel Machin comments – leaves you thinking.