[quote author="Tim Campbell"]To quote Bernhard Vollmer from a thread at Klaus Heyne's forum:
Underheating of tubes has one main reason. These tubes do not have a sufficiently high impedanced input and normally would not be suited for a microphone design. But if you underheat them (and accept the disadvantages that comes along with that procedure) their input impedance will raise and may reach acceptable values for a condenser microphone capsule.
[/quote]
Yes, it is "impedance" caused by grid current.
If we underheat tube, we have lower grid current and
then greater input (current noise) resistance.
and then smaller input current noise.
But parameters of tube as (voltage) noise resistance are worse(r).
But we have differend (and worse) tube (electrical) characteristics.
And tube with different characteristics must be differently typed.
for EF86 at 4,5 V heating
type BV1 (Bernard Volmer tube No 1)
typing EF86 in schematics complicated reading.
Follow mr. Hammond.
And what is it?
[/quote]
Note: Not all tubes accept underheating and those who accept it will show different behavior. Some will simply fail as their cathode changes chemically with a too low temperature at its surface. This may last some hundred hours but it will occur (for example with an AC701 with 3V heating instead of 4V) [/quote]
Is this "fail" equivallent to rise of 1/f noise corner?
xvlk