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In Reply to: RE: "Burning In" and Pre-Heating OTL Output Tubes posted by Cory M. on September 15, 2019 at 11:48:01
Up to doubling it.
If B+ is applied, the preconditioning process ends.
It is a good idea to warm up the filaments of any power tube prior to applying B+. In the case of the 6AS7G/6H13C/6N13 30 seconds is sufficient.
Follow Ups:
I would like to know more about "preconditioning" of power tubes. Could you point me to a place to find more information, please? I searched these forums, as well as the Atma-Sphere website but still have several questions about it and hope to find the answers:
1. Do NOS and tube of recent manufacture benefit?
2. How is the "preconditioning" performed.
3. How much is enough/optimal?
Ralph wrote in a 2008 post that most power tubes need preconditioning and most need 24 hours of heaters on without any plate/screen voltage. Is that the essence of it?
Edits: 01/04/21 01/04/21
I am not aware of a significant change in power tubes during the first few hours of operation-
There is an aspect of burn-in or Break-in with regard to operating the tube for a few hours - to get over that "New Tube" smell - making sure that the unit will not have an early failure.
There is also a bit of 'settling in' of the filament/heater
Beyond that it is a downhill journey-
one other note:
1. Do NOS and tube of recent manufacture benefit?
NOS = New Old Stock - by definition they can not be 'recent manufacture'... - to the extent that infant mortality and settling in of the heater/filament are affected by operation this will apply to new (never powered up) tubes and NOS tubes.
2. How is the "preconditioning" performed.-
Beyond the settling in of the filament / heater - I am not sure what the 'preconditioning' does or is
3. How much is enough/optimal?
I can't speak to #2, so #3 is also un-answerable by me-
Reach out to Ralph -
I have less experience w/ OTL amps -
Happy Listening
@6bq5, thanks for your reply.
In my question #1 I meant to inquire if only NOS tubes need this preconditioning, or, do tubes of recent manufacture also benefit from preconditioning.
Thank you, Ralph. Apologies for the uber-late reply ;-)...
I assume that this would be attributed to contaminants "burning" off of the cathodes and being absorbed by the getter? When B+ is applied, does it just "bake" them on instead, or is the reasoning so far unknown?
I'm just trying to understand the process behind preconditioning better.
Cory
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About to re-tube my M60 amplifiers.
I built them from kits long ago and they have the output B+ transformer fuse in line with the fuse of the heater transformer. The stated rationale was that a heater malfunction will shut down the output supply.
The latest schematics show the driver stage transformer, not the output transformer, fused after the heater transformer. Does that protect things in a different way?
The schematics, and Ralph's recommendations, no longer have the heaters for the driver stage powered from the transformer for the driver stage. The heaters for the driver stage are now connected to the heater transformer for the output stage.
If I did this, then can I wire the driver stage transformer to turn on later with the output B+ using the standby switch, and have only the heater transformer turn on at start-up? That way all new tubes can be conditioned easily, and all tubes can warm up before operating. As it is now, the driver tubes see operating voltages immediately the amp is turned on.
Advice appreciated before I do something stupid.
- Terry C
Hello, I have a vague recollection that several years ago someone posted a circuit diagram for a DIY tube burn-in device specifically for the 6AS7G (it may have been on the old ASOG site). Is this feasible? It this a recommended or foolhardy approach? Assuming sufficient skills to properly interpret the diagram and correctly construct the circuit, does anyone have any information they can share on how to construct such a device? Also, is there a way to match and/or sanity check 6AS7Gs prior to actually using them?
-john
Anyone can see the future, ..if they're willing to wait.
Hi John-
Sorry, just saw your post. For a preconditioning burn-in device, really all you'd need is a power supply for the filaments. Pins 7&8. 6.3 V. @ 2.5 A. each, use Ohm's Law for what's available for your PSU/ connection method and the number of tubes you're conditioning at once.
Or, just cut B+ from your amp by pulling the rectifier (tube rectified only).
You'd probably want to do quite a few at a time- the easiest dedicated apparatus that I can think of would be mounting 20 sockets to a chassis/ board, then wiring all of the heaters in series and plugging them in to the wall.
As for "sanity checking 6AS7G's prior to actually using them," Atma-Sphere has a great article on their website (you may be aware already) for visually checking tubes for having been arced over. Short of that (no pun intended), besides the basic filament continuity check with a meter or just observing the glow, you'd need a tube tester to do current matching and transconductance testing.
Cory
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I apply filament voltage to my 7241s about 3-5 minutes before applying plate voltage. Probably this is overkill, but it makes me feel good, and I like to feel that the tubes are truly warm to the touch before powering up.
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