Surge current protection

[catalin gramada]

Hello. I installed a voltage doubler bias PS in my Sound City 100 to be more usable with kt88. As normal the capacitors value was doubled to keep AC ripple as same amount. But now a question coming cause I have no ideea what current rating this secondary have (maybe between 50-100 mA) and I cannot estimate if it is en danger by startup current. Any series resistor will cause slow charging which is not what I want in this position. I instaled series 100ohm to limit the startup current a bit for a resonable time charging. Suppose those winding have 50mA rating (worst scenario) how much it support for instant peak charging currents in this old trannies please ? Thanks

[nickb]

To damage the transformer you would need to have the current high long enough to heat it enough for the wire or insulation to fail. A few high current pulses at 50Hz is not going to be a problem.

[J M Fahey]

Agree and add: that does not happen only with the tiny Bias supply: main caps themselves are *shorts* across the supply until charged, they would take Amperes if allowed to, only limited by winding resistance (meaning transformer wire dissipates the *full* initial charge peak until main cap voltage rises and lowers or stops it) ... yet power transformers do not explode at turn-on .

In part their huge thermal mass (they weigh many pounds, don´t they?) allows them to stand scary current/power peaks, as long as the integration of such power along the charging time which is very short does not really rise temperature to dangerous levels.