Hi,
Would it be a reasonable thing to do? I'm under the impression the reservoir cap charges slowly as the rectifier heats up. So, I don't see anything weird that could happen. Am I missing anything?
Does the answer change if solid state rectification is used? Can there be a resonance between secondary leakage inductance and the reservoir cap, causing it to overcharge? I would imagine this would also be a problem even with load, so the transformer is designed against it anyway?
Are there any other failure modes of the power supply I should be thinking about?
Would it be a reasonable thing to do? I'm under the impression the reservoir cap charges slowly as the rectifier heats up. So, I don't see anything weird that could happen. Am I missing anything?
Does the answer change if solid state rectification is used? Can there be a resonance between secondary leakage inductance and the reservoir cap, causing it to overcharge? I would imagine this would also be a problem even with load, so the transformer is designed against it anyway?
Are there any other failure modes of the power supply I should be thinking about?
The output voltage will be higher than when under load, so make sure the input capacitor
is adequately rated in voltage for the rise. Some of the rise is due to the power transformer.
A 350V AC winding will make 495V DC UN-loaded, but under normal load and vacuum rectification will sag well below 440V DC. We often find 450V caps here. The few seconds they rise over 450V (when rectifier is nearly hot but power tubes are not yet sucking normal current) does hardly-any harm because it is short. Leaving it this way no-load at 495V sure will be bad for a 450V cap.
Do it, but watch the voltage and don't leave it over-voltage more than maybe a minute.
If long-term no-load operation is needed, consider smaller VAC or a 500V cap. (Note that the 2nd 3rd etc filter caps no-load will also rise to full 495V.)
For testing, it is very useful to have a many-Watt 10K resistor. At 450V this is 45mA which is around half-power for many amp plans, and tells if the supply basically works under some load. 450V*0.045A is 20.25 Watts. A 20W part will stand this for many minutes, but don't set it on paper. A 50W unit will safely test all week long. (If it is honest 50W. There are "50W" resistors good for 20W until they are mounted on a large slab of metal.)
Do it, but watch the voltage and don't leave it over-voltage more than maybe a minute.
If long-term no-load operation is needed, consider smaller VAC or a 500V cap. (Note that the 2nd 3rd etc filter caps no-load will also rise to full 495V.)
For testing, it is very useful to have a many-Watt 10K resistor. At 450V this is 45mA which is around half-power for many amp plans, and tells if the supply basically works under some load. 450V*0.045A is 20.25 Watts. A 20W part will stand this for many minutes, but don't set it on paper. A 50W unit will safely test all week long. (If it is honest 50W. There are "50W" resistors good for 20W until they are mounted on a large slab of metal.)
Hmm, interesting... wouldn't this shorten the life of the filter caps after many on/off cycles? My first thought, especially if not being a large amplifier manufacturer, would be why not just use the conservative rating all the time to increase their lifetime, decrease the chance of collateral damage to the capacitors if the amplifier fails elsewhere and looks like an open circuit, etc. Did the manufacturers cut corners, or is it a waste of money for negligible benefit in practice?
> Did the manufacturers cut corners
Do bears poop in the woods? {actually, no: the local bear poops behind our chicken shed, next to the woods.}
More so than in any other branch of engineering, electronic engineers customarily work right TO the limits of damage. When you design a bridge you take a good safety factor off your steel to get the safe load. Houses have a larg safety factor built into lumber grading. EE margins are far slimmer. An old EE (my dad) excused it saying EEs know their stresses better than most other fields. True. But the era of $9.98 AC/DC AM table radios (and later TVs) really pushed zero-margin penny-pinching to extremes.
> wouldn't this shorten the life
30-day, 90-day warranties.
Also when hollow rectifiers were common, caps had Working voltage and Surge voltage ratings. A 450V Working rating might be a 525V Surge rating.
The cap makers' motives were mixed. Yes they wanted the amp maker to have little trouble inside warranty. OTOH, electrolytic caps were always considered "wear parts" to be replaced every few years. So the cap makers would rate them to be good but not "too" good, so there would be repair replacement business.
> say 10ma
Little point. Plot the chart. Say the working condition were 390V 90mA. 5Y3 will cover that, using 350VAC. 350VAC peaks at 494V DC. A steady 10mA load "loads" that down to 480V. Not much drop at all. Still over the long-term rating of a 450V cap.
Do bears poop in the woods? {actually, no: the local bear poops behind our chicken shed, next to the woods.}
More so than in any other branch of engineering, electronic engineers customarily work right TO the limits of damage. When you design a bridge you take a good safety factor off your steel to get the safe load. Houses have a larg safety factor built into lumber grading. EE margins are far slimmer. An old EE (my dad) excused it saying EEs know their stresses better than most other fields. True. But the era of $9.98 AC/DC AM table radios (and later TVs) really pushed zero-margin penny-pinching to extremes.
> wouldn't this shorten the life
30-day, 90-day warranties.
Also when hollow rectifiers were common, caps had Working voltage and Surge voltage ratings. A 450V Working rating might be a 525V Surge rating.
The cap makers' motives were mixed. Yes they wanted the amp maker to have little trouble inside warranty. OTOH, electrolytic caps were always considered "wear parts" to be replaced every few years. So the cap makers would rate them to be good but not "too" good, so there would be repair replacement business.
> say 10ma
Little point. Plot the chart. Say the working condition were 390V 90mA. 5Y3 will cover that, using 350VAC. 350VAC peaks at 494V DC. A steady 10mA load "loads" that down to 480V. Not much drop at all. Still over the long-term rating of a 450V cap.
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As someone who used to repair LOT'S of valve items in the past, many used to use too low a voltage reservoir electrolytic's, relying on the current drawn, and the losses in the rectifiers, to drop the HT to a safe level.
If this is the case, then running with zero load will destroy the electrolytic's.
'Back in the day', a VERY common repair was replacing metal rectifiers with a silicon rectifier, during which process it was essential to add a suitable value wirewound resistor to simulate the large voltage drop of the original metal rectifier. As the metal rectifier was a large component, this was easily down with a piece of tag strip, a BY127, and a 10W resistor - however, I can't remember what value resistor we used to use?.
If this is the case, then running with zero load will destroy the electrolytic's.
'Back in the day', a VERY common repair was replacing metal rectifiers with a silicon rectifier, during which process it was essential to add a suitable value wirewound resistor to simulate the large voltage drop of the original metal rectifier. As the metal rectifier was a large component, this was easily down with a piece of tag strip, a BY127, and a 10W resistor - however, I can't remember what value resistor we used to use?.
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