kee--(at)--ustin.ibm.com (RG Keen)
>writes:
>>
>>Nathan mentions a good trick (Hi, Nathan. Oh, yeah, drop me a note on
>>transformers...). Use a tube rectifier to drop B+. You can keep your
>solid
>>state rectifiers there, just stick a tube in series. This will help
>>extend the life of the other tubes, BTW, by not allowing the B+ to be
>>present before the heaters are warm, inducing cathode stripping. You
>could
>>even stick in two in series :-) . You'll drop about 50 volts or so per
>>tube. You could also stick in a -*power*- resistor and drop some volts
>that
>>way, too.
>Does this mean amp is running hotter at idle due to rectifier tube
>current?
No. The heater circuit has little to do with the plate current, it relates
to 'thermionic' effects.
All metals when heated in a vacuum exhibit this effect over a small range of
temperature, some are better than others at it. The operating temperature is
around 1200degC although that is variable dependant on the metal, some are
down as low as 500degC, others are nearer 2000degC. What happens is this.
As the metal is heated in a vacuum it reaches a point at which electrons start
to migrate off the surface and collect in a cloud. The size of the cloud and
its distance from the surface is dependant on the metal and its temperature.
Eventually the cloud becomes so negatively charged with electrons, it repels
any further electrons trying to leave the surface and equilibrium is reached.
Now introduce a positively charged electrode and electrons will start to leave
the cloud and head for the electrode. For each electron that leaves the cloud,
this leaves space in the cloud for one from the heated surface to move up and
join. A third electrode is used to control the rate of flow of electrons from
the cloud to the positive electrode. The heated electrode is the cathode and
is usually made out of thoriated tungsten. The heater is just a filament like
a light bulb to warm up the cathode and the positive charged electrode is the
anode or plate.
The current through the plate is, to a certain extent independant of temperature
(it isn't really, at extremes it obviously matters) thus if you switch on
an amp with both heater and HT (B+) off, the HT (B+) comes on immediately
with a solid state rectifier but the cathode is not up to temperature yet.
The anode is trying to grab electrons that aren't there. As things hot up,
there is a rustling on the ol' cathode as the electrons slowly start to warm
up, the anode grabs them before they even get a chance to move to the cloud
ripping them out the cathode and literally taking lumps of the cathode with
it - cathode stripping. Once it's all up to temperature though, everything
is ok and settles down as this only happens during the 'warm-up' phase.
That's the reason tube amps are usually fitted with two power switches, the
main power starts up the heaters, while the 'stand-by' switches on the HT(B+)
By incorporating a tube rectifier into the main supply, it takes a while to
heat up before allowing HT to pass through, giving the main tubes in
the amp a chance to get up to working temperature before HT is applied thus,
less chance of cathode stripping, longer tube life. Dropping the HT by 50 volts
will cause a slight fall in the quiescent current through all the tubes.
Lowering the HT voltage will dramatically reduce your power output as the
relationship between power output and voltage is a square law.
--
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| Where are they now? | Dave Morning |
| No.52.....................Gonks | da--(at)--cs.gla.ac.uk |
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