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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: dumping a lot of heat
Date: Mon, 9 Dec 2024 09:33:35 +0000
Organization: Poppy Records
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john larkin <JL@gct.com> wrote:

> On Sun, 8 Dec 2024 21:08:42 +0000, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
> 
> >john larkin <JL@gct.com> wrote:
> >
> >> On Sun, 8 Dec 2024 17:34:56 +0000, liz@poppyrecords.invalid.invalid
> >> (Liz Tuddenham) wrote:
> >> 
> >> >john larkin <JL@gct.com> wrote:
> >> >
> >> >> On Sun, 8 Dec 2024 16:36:25 +0000, liz@poppyrecords.invalid.invalid
> >> >> (Liz Tuddenham) wrote:
> >> >> 
> >> >> >Jan Panteltje <alien@comet.invalid> wrote:
> >> >> >
> >> >> >> On a sunny day (Sun, 8 Dec 2024 13:33:39 +0000) it happened
> >> >> >> liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
> >> >> >> <1r490yz.1xraied16vto76N%liz@poppyrecords.invalid.invalid>:
> >> >> >> 
> >> >> >> >john larkin <JL@gct.com> wrote:
> >> >> >
> >> >> >[...]>RL
> >> >> >> >> 
> >> >> >> >> Tubes were awful. Still are.
> >> >> >> >
> >> >> >> >The techniques for designing with them are quite different from
> >> >> >> >transistors and ICs, you have to think a different way; they aren't
> >> >> >> >just poor transistors, they have a different lifestyle altogether.
> >> >> >> >They also have some advantages over semiconductors:
> >> >> >> >
> >> >> >> >1)  Withstanding short term overloads without damage.
> >> >> >> 
> >> >> >> Thermal overloads depend on teh heatsink.
> >> >> >
> >> >> >Only slow overloads.  Fast ones depend on the thermal time constant of
> >> >> >the bit being heated by the overload.  Some time later the energy
> >> >> >reaches the heat sink but but then the damage is done.
> >> >> >
> >> >> >I've just accidentally mixed up the anode and grid pins of one of the
> >> >> >triodes in an ECC91.  It drew about 100 mA for a few seconds with no
> >> >> >damage.  That's equivalent to mixing up the Base and Collector
> >> >> >connections on a transistor and subjecting the Base-Emitter junction to
> >> >> >about 10 times the rated maximum Collector current.  How many
> >> >> >transistors would survive that, even with the biggest heatsink
> >> >> >available?
> >> >> 
> >> >> Mosfet data sheets usually have SOAR curves.
> >> >> 
> >> >> IXFH400N075T2 is rated for 1000 amps and 1000 watts (with astericks)
> >> >> and 30 kilowatts for 25 uSec. 
> >> >
> >> >Into the gate?
> >> 
> >> Don't do that.
> >
> >...but that is the comparison with what I did by accident.  I did it to
> >a device that was designed as an RF amplifier with a rated dissipation
> >of less than 3 watts and you were comparing it with a semiconductor that
> >was massively bigger - and now you say don't do that - and the device
> >needs extra protection components.
> >
> >That was the point I was making: you can get away with mishaps in a
> >valve circuit that you can't get away with in a comparable transistor
> >circuit.
> 
> My first job interview, I told the guy that I preferred tubes to
> transistors because it was harder to blow up tubes. He sniffed and
> said "That won't do" and dismissed me.

I was asked the same question at my first job interview.  I just said I
understood valves better, which was true at the time.  The Chief
Engineer, who was interviewing me, then produced the circuit diagram of
one of the firm's valve communications receivers and asked me to go
through it and tell him what each part did.

I took one look at it and said "Yuck!  It's chassis-live".  My
interviewer let me continue and then gave me the job.  It turned out
later that he hated having to make the set that way but for commercial
reasons it had to work on 110v D.C. and therefore had to have the
chassis connected to one pole of the mains.  (The chassis was well
insulated from the casing and all the outside fittings and connections.)

Throughout the entire job with that firm, I never had to design with
valves - but I did ten years later when I had to make some high voltage
research equipment.


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk