If I have GZ34 rectifier and 450V filter capacitors is it allowable to have AC voltage at GZ34 anodes with peak value above 450V?
I already have transformer with 2x 310V rms winding for GZ34, so peak AC is less than 450V. However, GZ34 is indirectly heated so when it start to conduct current other tubes will also start to conduct which means GZ34 is never operating without load so voltage on capacitors cannot reach peak AC (unless all other tubes are plugged out).
What do you think, is it possible to slightly raise AC voltage at GZ34 anodes to say 350V, would it be safe to still have 450V filter capacitors?
I already have transformer with 2x 310V rms winding for GZ34, so peak AC is less than 450V. However, GZ34 is indirectly heated so when it start to conduct current other tubes will also start to conduct which means GZ34 is never operating without load so voltage on capacitors cannot reach peak AC (unless all other tubes are plugged out).
What do you think, is it possible to slightly raise AC voltage at GZ34 anodes to say 350V, would it be safe to still have 450V filter capacitors?
It depends on the capacitor. Most quality caps are designed with a "surge" voltage rating. In my experience, most 450V caps can handle 500V for a few seconds without problem, but it WILL shorten the lifespan. Consider replacing the cap with something with a higher voltage rating like this instead?
B43541B8476M000 EPCOS datasheet and CAD model download | Octopart
B43541B8476M000 EPCOS datasheet and CAD model download | Octopart
From circa 1977, the standard IEC 60384-4 that covers testing of fixed capacitors has included a surge voltage test (clause 4.14) that applies a surge voltage of 110% of rated voltage for cap rating > 315 V, through a source resistance where RC = 0.1 sec for 1000 cycles of 30 secs on and 330 secs off, and the outcome has to be the same leakage current spec and no more than 15% capacitance change.
That generic test requirement indicates no reduction in service life from short duration turn-on events such as for valve rectifier amps, assuming the manufacturer has designed for compliance. But it is not always the case that mains voltage is constant, or that a design's working voltage is exactly 450VDC, so judicious allowance for tolerances is appropriate, as internal arcing can start to occur above the 110% mark and it may not take too much additional over-voltage to cause degradation.
That generic test requirement indicates no reduction in service life from short duration turn-on events such as for valve rectifier amps, assuming the manufacturer has designed for compliance. But it is not always the case that mains voltage is constant, or that a design's working voltage is exactly 450VDC, so judicious allowance for tolerances is appropriate, as internal arcing can start to occur above the 110% mark and it may not take too much additional over-voltage to cause degradation.
Thanks kodabmx & trobbins, thats what I had in mind.
Done some simulations with PSUD2 and with 350V rms at GZ34 anodes at normal 180mA load I have 415V peak (DC+ripple) at C1 of the filter. Without any load, except for 100k bleeder resistor I have 485V at C1.
Caps are highest quality 450V KEMET(Rifa) PEG124 so they should whitstand this short overload during startup especially since GZ34 is indirectly heated so other tubes have enough time to heat up and put some load on it when it starts to rectify.
If I decide to go with this I will make an update!
Done some simulations with PSUD2 and with 350V rms at GZ34 anodes at normal 180mA load I have 415V peak (DC+ripple) at C1 of the filter. Without any load, except for 100k bleeder resistor I have 485V at C1.
Caps are highest quality 450V KEMET(Rifa) PEG124 so they should whitstand this short overload during startup especially since GZ34 is indirectly heated so other tubes have enough time to heat up and put some load on it when it starts to rectify.
If I decide to go with this I will make an update!
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As a note, you just need to be confident about how much your mains voltage changes - nowadays there can be a noticeable diurnal variation depending on your street distribution and solar generators.
How long is your warranty?
On 30-day warranty it was not unusual to find a 450V cap rising well over 500V in the few seconds until the finals warm-up and suck-down the rectifier. Some of this stuff does survive decades.
Better stuff, or self-warranted stuff (AT&T, IBM), had ample over-rating.
Electrolytics? They are cheap. Don't under-rate.
On 30-day warranty it was not unusual to find a 450V cap rising well over 500V in the few seconds until the finals warm-up and suck-down the rectifier. Some of this stuff does survive decades.
Better stuff, or self-warranted stuff (AT&T, IBM), had ample over-rating.
Electrolytics? They are cheap. Don't under-rate.