[Amps] MTBF - was Ameritron Amps

[Bob Maser]

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" <g3rzp at g3rzp.wanadoo.co.uk>
To: "Tom W8JI" <w8ji at w8ji.com>; "mike kendall" <ke6cvh at yahoo.com>; "Doug 
Renwick" <ve5ra at sasktel.net>
Cc: "'Amps Amps'" <amps at contesting.com>
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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