What I was told is that the primary reason was the need for a flame-proof resistor at those locations in the circuit.
That is IMO the purpose in most cases.
Shortread: printed circuit boards combust pretty well, with a risk of fire propagation to the surrounding furniture, and then your residence
Longread.....
Manufacturers must by legislation and associated electrical code ensure that domestic electronic/electric equipment is safe in case of an internal failure.
Hence, an amplifier must be able to fail (a shortage within for whatever reason) without setting itself into fire.
A simple igniting resistor might (with some help) set a PCB into fire, then there is no stopping it anymore.
PCB's are pretty much what aluminium is for airplanes.... in fact the last material you should have in case of fire...
In case of a VFET failure, or any other transistor failure, that will be in 99% of the cases a plain short, creating an overcurrent in the transistor's associated circuit
When a B-2 VFET fails, it will pull a very high amount of current out of the associated driver stage, which would set regular emitter resistors into fire (at least the 70's regular resistors).
As such, all the driver's emitter resistors are "fusible". In this case the word fusible is ambiguous: the function should be non-flammable, but the white resistors that Yamaha used are indeed containing a sort of fuse in series with a resistor (note that Sony did not use those, they used truly non-flammable resistors in similar situations). And hence this is where "fusible" and "non-flammable" are swapped/misunderstood.
Proof of those fusible resistors blowing? Oh yes...... I have a friend's B-2 on my bench here, in which the 2SK and 2SJ VFETs where swapped in one channel during a simple revamp job (not by me).
When the unit was powered on, the VFETs in that channel fried in a millisecond and blew 3/4 of all fusible resistors in the driver stage.
Noting that.... fuses are by far the slowest failing element in a circuit, and the circuit short(s) caused a couple driver and VAS transistors to blow as well (prior to the fusible resistors....)
That's the reason behind the saying : "
components are there to protect the fuses", but we are drifting off here....
Anyway, the "exposed" resistors blew without setting off any fire; exactly what ought to happen.
In various other cases I have experienced fusible resistors to act as a true circuit protection, mostly in the Sony TA-N7B.
Example one:
One AKer used an OTL tube pre-amp to drive the N7B.... in direct coupled mode.
The pre-amp being a semi-professional built (or semi-amateur depending how you see it)
OTL with bad coupling caps and no start-up/shutdown muting circuit.... can cause nasty DC currents or spikes...
And one day.... poof it went....
As a consequence the TA-N7B's input stage saw some high DC voltage levels and went into overload, which resulted in high currents in the input and notably the VAS part.
Luckily Sony has put fusible resistors in the power rails to the input/VAS parts, which blew and paralysed the amp.
Simple replacement of fusible resistors was enough to get it working again.
Conclusion: be VERY careful what you connect on DC coupled input, and certainly never OTL tube pre-amps.
The B-2 has DC coupled input option as well, and when a DC is present (or nasty transients), some transistor may fail and consequently overload some resistors again. Those resistors are made 'fusible" again in the B-2.
Example two.
I was finishing a TA-N7B rebuilt and making final checks when a probe slipped in the driver stage area.
poof poof poof.... and a few cute little white smoke plumes.
Various fusible resistors from VAS to driver stage / inside the driver stage / and one VFET gate resistors blew....
So yeah, those (new, made in 2016) non-flammable resistors did make a very little bit of smoke when blowing, but that was it. No fire.
And no VFET dead...
So, it doesn't matter on which side of the PCB you put your fusible/non-flammable resistors: it's function remains the same, and the risk of fire using regular resistors as well.
As much as I like Takman resistors,
they are not fusible/non-flammable.
In such case, the excellent Vishay Dale CMF comes in (excellent audio resistors); they are "flame retardant", which means they won't keep burning.
Vishay Dale CPF is "Flameproof, high temperature coating" The advantage of the CPF series is compact size for the rated power: VERY useful in the very dense B-2 unit with small leg spacing. So, besides for "fusible" function, they are also usefull for the places where high wattage is required in the B-2, such as the original red-colored power resistors in the power supply section.
That's why I recommended the CPF series to Mr.Yamaha for the power supply, and a few more on the driver boards.
The large CPF in the PSU used are 3 watt !
More than enough to replace the original 1 watt; no need to go to exotic and larger body resistors.
Other options are (for "fusible" functions):
TE Neohm FRN series (Flame-proof fusible metal film resistors)
Vishay BC NFR series (Fusible Power Metal Film resistors)