First of all the voltage at the output of the filter network is a little over 500V. The maximum current that can be drawn from a pair of 81 tubes is about 180mA. So the with a little fudge factor added use a 250mA fuse or give it a little more range use a 500mA fuse rated for 600V. This size fuse is about ½"Dia x 1 ½"Long and cost is between $20 & $30 each. Another $10 to $20 for a fuse block or fuse holderstartgroove wrote:Wayne, If you go the F1/F3 route, it would be better to put F3 in line with the plate circuit, between the output transformer and the power transformer. That way, if there is an overload, all DC power will be disconnected from the amplifier.
Keep in mind that the voltage along that circuit will be about 400 VDC above chassis ground, so isolate the fuse holder well and try to find a fuse that is rated above 400 V.
Cheers, Russie
Bussman #FNQ-R-½ SB
Bussman #KTK-R-½ FB
Bussman #FNQ-R-¼ FB
Put it in the return line and you only need a 150V rated fuse at the same current. $2 for a regular 3AG fuse at list prices and a $3 fuse holder.
At the rate this discussion is going you might as well put a fuse in every node that has voltage on it to protect every component, I count 10 needed to protect everything.
Chuck