Fuzz Face bias - 9v resistor question

[Snufkinoob]

Typically I use a 10k pot in place of the 8k2, but I've found that having a pot in place of the 33k is also useful for fine-tuning the bias. I've always had a fixed resistor in series with a pot (e.g. 5k and a 25k pot) on the assumption that the fixed value will offer protection from the 9v to Q1 just in case the pot is turned to 'zero'.

The reasoning for this is that I've always gone by the rule of thumb that the 470 ohm provides protection against the 9v overloading Q2 if the 10k pot is put at 'zero'. So I've just assumed I needed to do the same for Q1. Is my thinking right on this or do I not need the fixed value on Q1 at all?

I don't think I've read anything about this or seen it on any schematics.

[PRR]

The ultimate limit is the 100K base resistor. If the Q2 stage were totally screwed-up, so that 9V appeared on the emitter, you could have 8.4V/100K or 0.084mA into the base. With a high hFE part, that could be 8mA or more collector current. Assuming "33K" were short, you could have over 9V*8mA or 72 milliWatts of heat in Q1. This will not harm modern Silicon. And to have high voltage on Q2 emitter you would have to have "1K" effectively open (this does seem to be a popular build fault) or outrageously low values at "470" and "8.2K".

So I think you could build and twiddle these for decades and never quite blow a Q1.

However I very much agree that adding a stopper resistor is more-sure and less brain-pain than computing all the worst-worst-cases.