Author Topic: A few Valve Facts  (Read 3069 times)

Offline WD

Re: A few Valve Facts
« Reply #15 on: May 29, 2011, 09:18:33 PM »
Is there a rule of thumb for how long it takes approximately for new valves to burn in? How big a change can I expect eventually in a KT88 based Primaluna? It's gained a lot of body in the three days I've had it. It's my first proper valve amp and I'm very excited! ;D

Offline Ampdog

Re: A few Valve Facts
« Reply #16 on: May 30, 2011, 03:37:11 AM »
.... but my reply will be very unexciting, if controversial in certain quarters. I do not know with present-day valves, but when we were working with valves in the hey-day (and lots of them), there were no 'burn-in' changes of practical consequence. For power valves - mostly EL34 and KT88 - some 8% settling in transconductance were observed over a few days; mostly <4%. In those days certain manufacturers had a pre-condition time (I hate the term run-in) of a few hours; I am not sure about present times. (And I am talking of instrumentation amplifiers in some cases; rather more demanding specs than for audio.)

The controversial implication of this statement: Any half-decent amplifier should have no audible consequences resulting from small valve parameter changes. The Quad II noted less than 0,18% increase in distortion with output valves mismatched by up to 25% - I would imagine a rather similar situation (meaning inaudible) with other valve amplifiers. Even with the few "zero feedback' models around there might be a barely noticable change in gain (volume), none audible in sound quality. (Don't underrate the conditioning ability of hearing!)
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline WD

Re: A few Valve Facts
« Reply #17 on: May 30, 2011, 12:51:31 PM »
I suspected pretty much that I was the one that was burning in! ;D
Find the dynamics and tone of the Primaluna to be very different from what I'm used to - the music lives and breathes!

Offline adolph

Re: A few Valve Facts
« Reply #18 on: June 07, 2011, 06:13:24 PM »
I have read with interest all of this thread and would like to offer a few answers.
The Getter has one powder charge it fires once and is then disconnected.
The correct way of limiting current in a valve heater to use a Varistor or VDR.
Cathode Ray Tubes do suffer from cathode poisoning, I designed a device which used unidirectional DC pulses to blow rubbish off the cathode for a company in Durban, they have sold several thousand.
Regards Bob

Offline handsome

Re: A few Valve Facts
« Reply #19 on: June 07, 2011, 06:37:18 PM »
i really am finding this topic fascinating glad to hear someone else has some input! the getter is disconnected after it's single powder charge but surely the getter material sprayed on the envelope continues to absorb gas throughout the life of the tube? i have read that valves stored in cold conditions can suffer from gas because the getter material cannot function under such temperatures. the cure was to pop the valves in the oven. also it was recommended to operate new valves for 30 minutes or so with just the heaters and no HT in order to fully reactivate getter material allowing it absorb any gas. how are the cathode of crts poisoned? is it from HT starting before LT? i have heard of rejuvenating valves is this what your device did for crts?

Offline adolph

Re: A few Valve Facts
« Reply #20 on: June 08, 2011, 01:33:16 PM »
During the manufacture process the barium coated cathode has to be activated by heating to convert the coating to oxide form.
This gives off a lot of gas, the getter is a small pan made of barium or sometimes magnesium which is fired by induction heating and then remains highly reactive and protects the cathode from gas or other molecules. So I suppose if the whole thing got really cold it would affect that process, I have not come across this but it makes sense.
The barium coating of the cathode is bombarded by positive ions, debris from the phosphor face and debris from untied grids. (Grids in any vacuum tube should not be left floating). Poisoning can also be caused by low heater voltage and long term storage.These all cause the cathode to become coated or poisoned preventing the release of free barium and thus low emission.
To get rid of this coating you raise the heater temp from 1100degC to 1300DegC and apply unidirectional DC to Anode1 in a short burst.
This boils the contaminant (carbon dioxide) off and sucks it to the anode and hopefully all is well.
Unidirectional DC is just a posh name for mains rectified but not well smoothed, lovely big fat 600V pulses.
Sorry this is such a lecture but it is really only part of the story.

Regards Bob

Offline handsome

Re: A few Valve Facts
« Reply #21 on: June 08, 2011, 09:25:27 PM »
lecture away mate, i love to learn!

Offline handsome

Re: A few Valve Facts
« Reply #22 on: June 13, 2011, 11:16:21 PM »
more:

-------------------------------------------------------------------------
A DEEPER LOOK AT THE PHENOMENON OF CATHODE STRIPPING IN THERMIONIC VALVES
-------------------------------------------------------------------------

                                                From a rec.audio.tubes
                                                article posted Dec.'96
                                                By: J.H. van de Weijer


One may assume that a thermionic valve's susceptibility to the stripping
phenomenon primarily depends on its cathode's design, and indeed that the
side effects caused by this are most prone to be seen in high grid
impedance low level signal tube circuits and their aging behavior:

Indirectly heated (generally Nickel) cathodes coated with a rare earth
metal oxide electron emission "cement" compound are prone to mechanical
stress caused by thermal cycling; i.e.: the heating and cooling of the
cathode. (Manufacture dependent).

The ceramic nature of the indirectly heated cathode's emissive coating
with its (from the metal cathode carrier) differing thermal expansion
coefficient may cause surface material to crack and become "loose". The
thus gradually "powdered" ceramic cathode emission surface may keep
minute amounts of electrical charge stored after cooling down; the
surface in cold state remains nonconductive. Minute amounts of these
cathode borne particles, either with remaining charge or electrically
polarized upon sudden apply of anode voltage, may "dust off" and clog
onto the most nearby "sieve" i.e.: the control grid; cathode stripping
has happened, and here it is that this less heard of tube
degradation/aging mechanism (not discussing others) occurs!

Consider this:

The grid clogged particles due to radiant heating from the nearby cathode
will start to behave as pointwise cathodes themselves, causing beyond
normal grid current, this has the effects of:

-Drift in those high impedance biased control grid circuits: And this
 just in the unwanted direction: Take a tube endstage which is
 capacitive coupled  from the phase  inverter and DC biased through (say)
 50 kOhms: Current runs from the anode into the control grid and
 therefore shifts the grid bias voltage to a less negative value...
 There you go...
 (Ever wondered why some tube manufacturers specify a maximum grid bias
  resistance?)
-Causing excess noise.
-Etcetera, you don't want to know.

Now, how to be most gentle to your indirectly heated tubes and give them
a long life: (and this also applies for all fellow guitar players having
a "stand by" switch at hand):

SWITCH ON:
 Switch on from standby mode: i.e. only fire the filaments, wait somewhat
 longer  than fully "glown" up, then switch from standby to power (B+). 
 (i.e.: B+ may be suddenly switched on, but only after full filament
 warm-up)  (with regards to "cathode stripping": The cathode is now
 conductive: All localized cathode charge will have drained).

SWITCH OFF:
 The same sequence reversed: I.e.: Turn off B+, wait, and only then turn
 off the filament supply; This will gradually and properly discharge all
 charges.
 With regards to cathode Stripping; this will assure no charge will
 remain stored locally on the susceptible cathode surface and so forth...


                                     Copyright 1996 J.H. van de Weijer

                                     This article may be freely distributed,
                                     stored, copied and printed for non-
                                     commercial use, provided the integral
                                     text including this notice is kept
                                     unmodified.

Offline adolph

Re: A few Valve Facts
« Reply #23 on: June 17, 2011, 12:53:28 PM »
Very interesting, but I don't think I have seen a commercial device that uses this as a safeguard and one thing for sure you can't "rejuvenate" a striped cathode. Maybe they want you to wear them out?