[Amps] MTBF - was Ameritron Amps

[mike kendall]

Hello all,
    I could not help breaking out my "west coast handbook" Radio Handbook by Bill Orr 23rd edition.
    I looked at Chapter 19, power supplies and also Chapter 17 constructions projects.  They always use a little "safety margin" with working volts DC going from 20percent to even higher above the actual DC supplied (some were 5kv and 5.6kv worth of caps on the projects with 4.5 or 4kv supplies). In one project,  I especially liked the 53uF 5kv peter dahl capacitor that is still available on their web site last time I looked.
    I grabbed the first ARRL handbook I could get my hands on (got a good collection of older ones) and 1988, Chapter 6-10 on power supplies said "In a capacitor input filter, the capacitors should have a working voltage rating at least as high, and preferrably somewhat higher than the peak voltage from the transformer". I suspect that I would have a really hard time finding either an ARRL or a Bill Orr radio handbook project that did not have the power supply capacitance comfortably higher than the HV supply.  It makes me wonder on the other amp manufacturers like Alpha, Henry, QRO, etc.. what kind of margin they go above the power supply voltage with their capacitor bank.  Any comments from the reflector on other manufacturers wvdc of their capacitor bank compared to the HV supply voltage?
  73,
  Mike.
  
Bob Maser <bmaser at tampabay.rr.com> wrote:
  MTBF simply means "Mean Time Between Failure", Mean is the arithmetic 
average. On the average, the unit in question will fail at that point, some 
less, some more and the variation of when the item will fail is dependant 
upon the sigma of the distribution. Mean is only a good measuring tool if 
the failures are occurring in a symmetrical bell shaped distribution. In 
the case of electronic devices, they tend to have an infant mortality, then 
random failure and lastly wear out. This is called the "bathtub curve". 
Anyone with statistical knowledge would know all this.

Bob Maser W6TR
----- Original Message ----- 
From: "Peter Chadwick" 
To: "Tom W8JI" ; "mike kendall" ; "Doug 
Renwick" 
Cc: "'Amps Amps'" 
Sent: Monday, July 31, 2006 8:04 AM
Subject: Re: [Amps] MTBF - was Ameritron Amps


> Tom,
> You've just provided 'Reliability 101'.
> MTBF is a figure often bandied about, but without the correct assumptions, 
> numbers and inputs to the equations, can be very misleading. For example, 
> the MTBF of a tube amplifier with tubes changed on a time basis can be 
> very different to the MTBF if tubes are run until they fail - unless you 
> count tube change as a failure, in which case the MTBF is much lower. Back 
> in the days of aircraft radios using tubes, one company at Heathrow paid 
> the service techs on a piecework basis - they got so much per radio fixed. 
> So the first thing they did when a radio arrived on the bench was to 
> change every tube, whether it needed it or not. Thus those radios showed a 
> horrendous MTBF on tubes....
> Another complication with electrolytics is the 'memory' - use a 450 volt 
> part long enough at 250, and putting 450 on it gets an enormous leakage 
> until it's reformed at the higher voltage.
> 73
> Peter G3RZP
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