Blog posts of '2015' 'March'


Well, today, I had an e-mail from an 'expert' on valve vibration.  He attempted to 'correct' my last blog entry  but rather than telling him to buzz off, I thought I would clarify matters a little further in my own 'non expert' way

My last blog entry dealt with an example of swept frequency sinusoidal microphonic testing where the gross effects of vibration on valve performance was being assessed at typical conditions potentially found in an accelerating aerial shell.

There existed a number of direct excitiation methods of measuring the affect of vibration on valves and for completeness, I will describe just one which is a typical system for examining specific effects on electrode structure - and hence electronic performance - this one was used by Mullard Research Laboratories for both initial design, competitor analysis and complaints analysis.  Here below, we see a wonderful photo of this paragon of instrumentation:-

Perhaps a block diagram used in conjunction with the following text will help to describe how this contraption worked:-

As the frequency of the vibrator is varied, the microphonic output voltage of the valve will vary.  If the vibrator excitation frequency is lower than the resonant frequency of the system, the deflection will be in phase, conversely, if the vibrator excitation frequency  is higher than the resonant frequency, the deflection and force are 180° out of phase. The transition between these two states takes place in a narrow frequency band near the resonant frequency, at which a 90° phase shift between the deflection and the vibrator excitation force will exist. These changes of phase are visible on the oscilloscope as an oscillogram displaying Lissajous figures.

At the resonant frequency of a component, both the amplitude of the vibration of that component and the microphonic output voltage will be a maximum, accordingly, by varying the frequency of the vibrator until the microphonic voltage shows a maximum, and by observing the oscillogram in conjunction with viewing the valve through the microscope, it is possible to determine which component(s) within the valve envelope  are responsible for any microphonic effect.   As the vibrator and stroboscopic signal generators ar set to operate with a frequency differential of 10 Hz, any vibration of a component can be visualised as a slowly 'moving and blurring' artefact.



Oooh, what is this nice man doing........ well, you might think he's making some sort of mine though you would be wrong.  In actuality, here is a Philips technician, vibration testing a batch of experimental thyratrons and triodes for use in a European variant of the Mk 53 aerial mine's proximity fuse.

The working of these devices was rather elegant.  The shell contained a miniaturised transmitter using the shell body as an antenna to emit a continuous wave of approximately 200 MHz. As the shell approached a reflective target, an interference pattern was created.  As the distance to target decreased, a radiated power and corresponding oscillator supply current change would occur.    The Doppler frequency shift could be tuned between the ranges of 200 - 750 Hz to trigger a detonation once a chosen amplitude was reached. 



This is a very nice photo of the Philips Herleen B9A base making machine circa 1955 - see earlier blog entries about the three part base wires and glass eutectics.





Well, look at this,  I have known quite a few dummies, some of them even like valves..... and radios!!!!!!!   But the only Mullard dummy I had ever seen before was my reflection looking back at me from the mirror - haha - that was a Mr. Mullard Magic joke.

I wonder, was this just an empty space box filler?   Maybe  a delve through the Mullard archive I have here may tell me or even one of you customer type people may know the secret.