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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Tip: Safe Power Up of old Tube Gear posted by Jimmy on January 12, 2004 at 12:18:06:
You can keep a very close eye on things by using both a variac and an ammeter in series, watching for spikes and excessive sustained charging current as the ramp up proceeds. Also, it is very good practice to greatly derate the chassis fuse as an added safety measure should a short quickly develop, if only to attempt to save the power transformer. Obviously, out of chassis reforming is best, but is a lot of work and the effort expended might as well be spent on installing new or NOS stock replacements.About the old industry reforming guidelines: they really don't apply to 40, 45 or 50 year old specimens as we now encounter them in the older tube instruments. The filters we deal with are now relatively fragile and well beyond their design lives. The manufacturer guidelines were intended for reforming "stale" new production and relatively newer NOS examples and the engineers who formulated them never imagined them to be applicable to anything this old in the field.
This does not mean that an old filter can won't reform up fine and be a sturdy performer thereafter, but only that one should coax them out of their sleep more gently, as the initial chemical and mechanical composition of the capacitor has declined from extended aging.
Go gentle and go very slowly, and most older filters should reform up well if not previously abused. Some of these old runners do take a very long time to really rebuild up the dielectric oxides to good levels. If one wants to go faster or follow the "guidelines", so be it. But the extra time (and added care implied) never hurts anything and assures that you won't have reason to second guess if one begins to short or develops higher than anticipated leakage.
Whether a reformed filter can put up with today's higher line and working voltages is another matter, however.
A variac also permits a progressive measuring of working voltages throughout the circuits, and can help detect other passive failures and defects before a critical voltage or current is reached, and real damage sustained.
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Follow Ups
- Use both a variac and an ammeter. - sgmlaw 14:56:08 01/13/04 (1)
- Well said!! - koo 18:35:02 01/13/04 (0)
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