Re: dumping a lot of heat

Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Sun, 8 Dec 2024 16:36:25 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1r499nc.1l79pftrqcriyN%liz@poppyrecords.invalid.invalid>:
 

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?

 
The 100 mA is a current limit due to the limited electron emission
possible from that cathode at those settings. You can get flash-over with
anode voltage at the grid of toobs.

There was a 1k current-limiting resistor in the HT supply (it changed
colour).  At least I had time to get to the 'off' switch before the
valve could be seriously damaged.  Luckily I was testing with reduced HT
(about 100v) so the current would have been limited to about 100mA by
the resistor, even if the maximum emission had been greater

I understand you love toobs, sure toobs were nice

I enjoy designing with them, rather than just copying well-worn
circuits.  There are still niches to be explored.

A few years ago I set myself the task of designing a low-noise audio
preamp using a grounded-grid first stage - the result was far better
than I had expected because it had repercussions on the second stage and
the mixing stage which greatly improved the overload performance and the
S/N ratio of the whole amplifier.  If I had invented that in the 1950s,
I would be a lot richer now.

I sent copies of the 'before' and 'after' circuits to a friend who is a
great believer in the mystical properties of valves, but is otherwise
quite sensible.  It took him two days to get his mind around how the
circuit actually worked.


[...]

9) touch screen????

I have never needed a touch screen.  The only time I have used them is
at supermarkets and that was for their convenience, not mine.

10) X-Ray emission from the monitor HV stabilizer tubes..

Grossly over-hyped.


[...]

I modified it to drive that real old  magnetic deflection TV set with
transistors HV tronsformer, the works. Was an interesting learning
curve... Never longed back to toobs after that.

My first experience with transistors was building a simple automatic
parking light for a car from a circuit in a magazine.  It used a
photoconductive cell to switch a chain of ever-larger transistors and
supply a 12v bulb.  The idiot who 'designed' it had added negative
feedback instead of positive feedback, so the bulb never went right out
and never came full on.  The heatsink got hot and it ran down the car
battery in daylight.

It took a long time after that before I felt happy with transistors.


--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk