From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: rec.audio.high-end
Subject: Re: Re:Re: twin triode crosstalk question
Date: Tue, 15 Dec 1992 14:23:05 EST
Organization: The Software Engineering Institute
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu


Excuse me, folks, but you need to get your hands on a tube manual and stop
speculating!


In article <1g7j13INN1fp@uwm.edu>, peterca@ento.csiro.au writes:
|> >In article <1fvlg0INNkdi@uwm.edu> peterca@ento.csiro.au writes:
|> >>When the valves are getting old swap left for right
|> >>channel to give each channel a new, unused triode.
|> I'll revise the suggestion to swap the valves occasionally to keep both
|> halves similar in wear.
|> >
|> >I'm sure someone will correct me if I'm wrong, but my understanding is
|> >that most tube wear is caused by start up (i.e. the tempature change),
|> >not by actual "use" (i.e. signal being amplified).
|> I assume it is both.  The first can be reduced by various means of slow
|> turn ons but I gather the cathode "wears" down with use also.  I also saw a
|> suggestion in one of the early issues of this digest that the regulated DC
|> volts should be reversed in polarity occasionally for long life.  I power
|> amps I gather that AC is used to avoid having more current flow
|> consistently from one end of the filament/cathode? due to being closer to
|> the anode voltage.  I'm not sure if that is correct but it sounded
|> plausible.

Not correct, and not plausable because of the design.  There are no filaments
in the tubes under discussion -- directly heated coated emitters are used only
for low-power battery applications.  Most of these tubes start with a "1", and
there the examples of the 2A3 and 5Y3GT.  The manual will show those pins as
"F", and not "H".

Indirectly heated cathodes are used for several reasons: electrical isolation
of the emitter from the heater circuits provides circuit design flexibility,
the cathode is shielded from the hum induced by AC heater power, lower
temperature emitters can be used to allow the use of "dark" heaters of
stronger wire, the tubes are mechanically sturdier and less sensitive to shock
and microphonics due to vibrating filaments; and in rectifier tubes, closer
spacing between the plate and cathode is possible providing lower vboltage
drop; and in amplifier tubes, closer spacing between the grid and cathode is
possible increasing the gain.

DC is used to further reduce hum over a balanced AC supply, and not to extend
life.  The heater is center-tapped to allow flexibility in heater voltage
choice and the parallel connection usually provides lower hum. Running one
half of the heater will not heat the cathode to its proper emitting
temperature!


|> >In my own experience the most common complete failure mde of  preamp
|> >tubes is a burned out filament, which would affect both halves of a
|> >12a?7.
|> The filament is center tapped so you can run each half from 6V in parallel
|> or 12V in series, or only one half.  

Wrong -- see above!

|> However if one half went it might
|> effect the other half so I have revised my suggestion to occasional swaps
|> between channels.  I would expect that with only one half producing heat at
|> any particular time the pair of triodes would have increased life.  

It would last forever, but there wouldn't be any free electrons to put to
work!


Rich