previous: R.G. Just so we recap this properly for ... -- 2/2/2000 5:03 PM

Re: the tubes, that is, if you overheat them - MOSFETs or not

2/2/2000 7:25 PM

lion

Re: the tubes, that is, if you overheat them - MOSFETs or not RG – please set me straight (once again)!     >The way you protect the MOSFETs from gate punchthrough is a gate-source 12V zener. The zener prevents any positive transients over 12V forward and clamps any reverse transients at a diode drop. That in combination with series impedance in the gate leads pretty much stops transients from exceeding the gate voltage.<     The zener goes direct between the gate and source legs on the IFR820’s – right?   Will any type of 12V zener do?   Maybe the orientation of the zener should be obvious – but I’m not sure?     >The series impedance can be the gate-stopper resistor in most cases.<     Most cases????     >Long term over voltages can exceed the power rating of the zener and/or resistor and lead to their destruction, but zeners are pretty good with transients.<     Meaning not to worry – or that we need to take other safety precautions?     Last thing – re Steve C’s comments about the implications for tube life, I’m with you that this is a quest for tone – and I’m willing to trade the ultimate tone for say a shorter tube life – but on the other hand I’d like to know what I’m up against.     >Using a follower, whether tube or MOSFET, can drive a power tube beyond its normal dissipation in a circuit and cause heat death by increasing its power dissipation.<     ???     Running in class AB2 with a follower doesn’t chance the rules:   EL34: (0.7*25/plate voltage)=bias current – right,   but it’s the added heat we’ll have to deal with?     Better get that fan working.     Regards   Lion

 

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R.G. q{The zener goes direct between the... -- 2/2/2000 7:58 PM