Yeh the W5M schematic shows a nominal 117V mains, and the spec shows a 105 to 125Vac range, so presumably the PT's were designed for operation up to 125V - but time showed the PT as a part that ended up damaged after years of operation.
Hindsight reports indicate a number of mechanisms that could or were stressing some PTs, but not enough hard data to say it was a particular PT manufacturer or a particular stress (although the Heathkit service report pointed the finger at the high voltage caps getting stressed and that may have cascaded to the valve rectifier and the PT). So like many vintage amps, if there are simple ways to add protection then they are worth considering. Although imho adding a thermistor may be simple to do in practise, but appears to be often done without a good awareness of how the NTC will operate or how the amps performance may change.
Hindsight reports indicate a number of mechanisms that could or were stressing some PTs, but not enough hard data to say it was a particular PT manufacturer or a particular stress (although the Heathkit service report pointed the finger at the high voltage caps getting stressed and that may have cascaded to the valve rectifier and the PT). So like many vintage amps, if there are simple ways to add protection then they are worth considering. Although imho adding a thermistor may be simple to do in practise, but appears to be often done without a good awareness of how the NTC will operate or how the amps performance may change.
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I am not being paranoid, nor am I scared. And I am not staying awake at night worrying.
The turn on surge to the main power supply cap is around 447VDC, and the cap is rated at 450VDC. After 15 or 20 seconds when the output tubes are conducting, the voltage is around 400VDC.
The turn on surge to the main power supply cap is around 447VDC, and the cap is rated at 450VDC. After 15 or 20 seconds when the output tubes are conducting, the voltage is around 400VDC.
In this case just a resistor and a timed relay should solve this as i assume you use bleeder resistors over the HV capacitors anyway.
The Ultimate Guide to Soft Start Design – Neurochrome
The Ultimate Guide to Soft Start Design – Neurochrome
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