6.8k cap resistor blowing

for Dynaco Mark II/III/IV and DIY PP monoblocks

Re: 6.8k cap resistor blowing

Postby 20to20 » Tue Jun 30, 2015 11:22 am

wungun wrote:The NTC CL80 thermistor is doing exactly what I want it to do, which is limit inrush current.


In order to protect what? You mentioned this in relation to the Q-cap. How much current is it limiting there? Do you want it to limit the current through the heater circuit? That would slow down the time to get the heaters up to tube conduction temp.
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Re: 6.8k cap resistor blowing

Postby wungun » Tue Jun 30, 2015 12:27 pm

Capacitors charge and discharge in 5 time constants with respect to the amount of resistance is present. Very quickly indeed with low resistance. And seeing how amps, volts and resistance are all mathematically tied together, low R and high I = high E. Or high E and low R = high I...neither good for the amps.
The heaters run right from the transformers....what 6VAC or so....? There is very little delay in their heating up. High resistance because they are purely resistive (heating coils) makes little difference when considering the low voltage
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Re: 6.8k cap resistor blowing

Postby wungun » Tue Jun 30, 2015 12:35 pm

A thermistor is an inrush limiter, and they are widely used as such. Key word here is inrush...
They quickly warm up and start conducting an n current is applied. It may be reducing my wall E down a volt or 2, which is fine, as the Dynaco were design in a world of 110 or 112V.
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Re: 6.8k cap resistor blowing

Postby 20to20 » Tue Jun 30, 2015 3:47 pm

wungun wrote:A thermistor is an inrush limiter, and they are widely used as such. Key word here is inrush...
They quickly warm up and start conducting an n current is applied. It may be reducing my wall E down a volt or 2, which is fine, as the Dynaco were design in a world of 110 or 112V.


Your rectifier is a better inrush limiter for the B+. It just drips out the voltage to the filter caps very slowly compared to the rate they are expecting (built to take) to be charged.

Your inrush limiter allows 3A @ 125V from the gito. The inrush limiter is up to operating temperature and its lowest resistance long before the GZ34 has slowly come up to full conduction allowing it to charge the first filter cap, and everything else downstream. By then, the initial rush through the heater circuit is over and the CL-80 is hot and is not limiting anything because the load should be lower than 3A by then.
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