"Patrick Turner" wrote in message
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:
:
: Ian Iveson wrote:
:
: "Patrick Turner" wrote
:
: RDH4 says +/- 10%. ...
:
: Brimar says mains-derived heater voltage should be within 5% at
: nominal mains voltage, as long as mains itself remains within 10%
:
: Does that mean 15% is the limit?
:
: I can't quote the exact reference from RDH4, but its in there alright.
:
: And there is no mention of +/-15% afaik.
:
: So whatever that book says is about right.
:
: I try to make the heater voltage for all gear stay within +/- 2% of the
: official proper voltage.
:
: Mains might vary +/- 5%, but I have never seen more than
: between 235v and 253v for 240v nominal, so I guess heater voltage would
: vary say +/- 7%, but over time its not enough to significantly shorten
: expected tube life.
:
: One can place 12.6 volts onto a 12AX7, and gee it looks bright, but not
: for very long,
: and you might stuff the emission of the cathode....
:
: The tube will also work with 8 volts on the heater, but it'll be noisy,
: and Gm will be low...
:
: Patrick Turner.
:
Constant power is a good thing to strive for - the cathode coating material
doesn't 'like' thermal fluctuations. A 12AX7, run below 11 V will have
altered characteristics: the electron emission is lowered, therefore the
'virtuel cathode' due to the space charge will be physically closer to the
cathode: Gm will get lower (curves will tilt to the right), depending on the
bias this may actually increase linearity. Even lower, say below 10 V or
so, the cathode will no longer be uniformly heated, resulting in increased
noise and the risk of cathode sputtering - this *does* decrease lifespan!
Rudy