That's why the old incandescent? 'fairy
lights' were so unreliable. From the point of view of any one
lamp, the much higher resistance of all the others in series
means that it is current-driven, so the lamp with the highest
resistance is in danger of thermal runaway.
On 2025-02-18 19:09, Jerry Lee Marcel
via groups.io wrote:
In real life, the problem with current drive is that it may
result in thermal runoff. As the temperature of the filament
increase, the resistance increases, resulting in voltage (and
power) increase, which in turn increases temperature, and so
on.Heating time is excessive, because at start-up, the
resistance (and power) is low.
Not a good idea.
Le 18/02/2025 ¨¤ 19:40, Carlo a
¨¦crit?:
Yes, exactly. The 12AU7 model includes only one-half of
the heater (pin H1 & H2 in subckt model). Therefore the
current through it should be designed to be about 150mA.
Indeed the ratio 1.25/7.5 for LM317 in "current source"
configuration is 166 mA.
?
On Tue, Feb 18, 2025 at 08:53 AM, Jerry Lee Marcel wrote:
Actually, it is not "driven" by a current source.
It is rather "biased" with a current source, which is a
perfectly legit way of operating a 12AU7.
The actual signal source is a voltage source.
Since only half of the dual triode is used, it's rather
normal to use only one-half of the heater, hence a 6V
connection. Again a quite common arrangement.
UIC flag for .TRAN analysis skips the initial DC operating
point solution step (aka ITS). All energy storage components
(i.e. capacitors & inductors) by default start transient
with 0 stored energy.
?
Therefore, do you think that the difference w.r.t. .TRAN
without UIC comes down to the fact that in the latter the
initial ITS step picks as stable operating point a nonsensical
one for the 12AU7's heater (as you said where the 12AU7's
heater generates energy from (very) thin air) ?
?
On Tue, Feb 18, 2025 at 07:16 AM, Andy I wrote:
With UIC, and no other changes:
Normal solver -> 1.165 V @ 186 mA into the
heater.? That's somewhat better.? But still not right.
Alternate solver -> 1.165 V @ 186 mA into the
heater.
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If something is true:
* as far as we know - it's science
*for certain - it's mathematics
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