Gareth Magennis wrote:



** If you look at the schem, the HT fuse is in the rectifier ground path
where the voltage and current levels drop to zero at twice the supply
frequency.

So for a couple of mS each 10mS, there is not enough of either to sustain
an arc.

Yes, I noticed that. It is much better than the original arrangement. If
C23 and C24 are in good condition and the B supply rail doesn't get
shorted, then the fuse (and new switch) should only be carrying current
for a quite small fraction of the time, so arcs should extinguish well,
perhaps even better than with a resistive load on AC.


Thanks Chris and Phil, that's very interesting.


** Nice example of how so often, the devil is in the detail .....


Why wasn't this practice adopted decades ago? Were Fender (Bassman) really
that clueless?


** Fender originally placed the Standby switch after the main filter electros in consideration of the valve rectifiers being used. Placing it earlier creates a large current surge every time the switch closes while the electros charge. Such surges well exceed the maximum ratings for 5U4s or GZ34s and will drastically shorten their life spans.

When Fender changed to using strings of silicon diodes, the problem vanished and they ought to have reconsidered the standby switch scheme as Marshall did.

Ferder's "The Twin" model ( 1993) finally has a double pole switch in the AC like Marshall.


..... Phil