With regard to a.c.- only powered communications receivers, especially those from Eddystone, if necessary, I usually modify them so that there is one mains input fuse in the incoming "L" line and another fuse between the centre-tap of the HT transformer and chassis. If the set has a removable mains lead, I always re-work the mains input arrangements so as to make this lead non-removable and ensure that the incoming earth wire is securely bonded to chassis. Similarly, I also replace the front panel mains on / off switch - if it is a single-pole - with a double-pole switch, thus switching "L" and "N". All exposed joints that carry mains voltages are always well insulated with heat-shrink or something similar. (This is very relevant in the case of many Eddystones, since the connections to the mains on/off switch is dangerously close to your fingers during maintenance! Those who have been there will know what I mean! 
)
As for fusing and the mains switch, there is a good degree of disagreement here. My personal choice is to wire the mains on / off switch before the mains fuse, simply because if the fuse needs to be tested or replaced and I have switched the set off at that switch, (but have forgotten to unplug it from the mains source
) then the fuse can be safely removed. However, if the fuse and switch are transposed, there is a risk of electric shock when the fuse (even if O/C) is removed. The counter arguement runs that that method fails to protect the switch, since the fuse comes after it. My preferred method seems to be favoured by U.K. designers; the latter by those in the U.S.A. I have never had a mains switch explode under a fault condition (this is the usual claim against my method) with my preferred method; obviously, the switch must be adequately rated for the set in question.
All of those comments apply when a valve rectifier is in use and are equally applicable to those sets where silicon diodes are used for HV rectification. However, in this latter case, the need for surge limiters in series with the diodes is required to eliminate switch-on surges blowing the fuse, or worse still, damaging the rectifier diodes. There is also an arguement that can be made that by only fusing the centre-tap of the mains transformer, a failed bridge rectifier (or bi-phase rectifier) can put a direct S/C across the whole of the HT winding of the transformer. :@ For myself, I have never experienced this mode of failure, but then I always incorporate surge-limiting resistors and a snubber network (R-C series combination) across the whole of the transformer secondary. The obvious method of preventing such a disaster as a burnt-out mains transformer in that scenario is to fit fuses in each 'leg' of the HV windings to the rectifiers.
As for fuse ratings and types, I use an 'ordinary' 1.25" fuse for the incoming mains supply and an anti-surge 1.25" fuse for the HT connection to chassis; it is frequently necessary to incorporate electrical and mechanical changes to achieve this. The ratings of these fuses will simply depend on the steady-state current that flows when the set has stabilised, voltage and temperature-wise.
I have never found fuses in the 6.3v a.c. lines in such sets and have never found such a fitment to be necessary.
As regards a.c. / d.c. sets, I have little experience, so I can contribute little of value here, except to say that I always run such sets from a suitable rated & fused isolation transformer.
Al.
)As for fusing and the mains switch, there is a good degree of disagreement here. My personal choice is to wire the mains on / off switch before the mains fuse, simply because if the fuse needs to be tested or replaced and I have switched the set off at that switch, (but have forgotten to unplug it from the mains source
) then the fuse can be safely removed. However, if the fuse and switch are transposed, there is a risk of electric shock when the fuse (even if O/C) is removed. The counter arguement runs that that method fails to protect the switch, since the fuse comes after it. My preferred method seems to be favoured by U.K. designers; the latter by those in the U.S.A. I have never had a mains switch explode under a fault condition (this is the usual claim against my method) with my preferred method; obviously, the switch must be adequately rated for the set in question.All of those comments apply when a valve rectifier is in use and are equally applicable to those sets where silicon diodes are used for HV rectification. However, in this latter case, the need for surge limiters in series with the diodes is required to eliminate switch-on surges blowing the fuse, or worse still, damaging the rectifier diodes. There is also an arguement that can be made that by only fusing the centre-tap of the mains transformer, a failed bridge rectifier (or bi-phase rectifier) can put a direct S/C across the whole of the HT winding of the transformer. :@ For myself, I have never experienced this mode of failure, but then I always incorporate surge-limiting resistors and a snubber network (R-C series combination) across the whole of the transformer secondary. The obvious method of preventing such a disaster as a burnt-out mains transformer in that scenario is to fit fuses in each 'leg' of the HV windings to the rectifiers.
As for fuse ratings and types, I use an 'ordinary' 1.25" fuse for the incoming mains supply and an anti-surge 1.25" fuse for the HT connection to chassis; it is frequently necessary to incorporate electrical and mechanical changes to achieve this. The ratings of these fuses will simply depend on the steady-state current that flows when the set has stabilised, voltage and temperature-wise.
I have never found fuses in the 6.3v a.c. lines in such sets and have never found such a fitment to be necessary.
As regards a.c. / d.c. sets, I have little experience, so I can contribute little of value here, except to say that I always run such sets from a suitable rated & fused isolation transformer.
Al.
