Re: dumping a lot of heat

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 said the same thing in my next interview. Melvin laughed and hired
me and I designed about $200 million worth of stuff for him.

This was back when I got tubes for free or nearly free, and a PNP
germanium transistor cost two weeks allowance.