rectifier filament technical questions - resistance, inrush, etc

[Opcom]

Anyone know what material the filaments of rectifiers like the 866, 872, 575, etc. are made of and whether there is any reason at all to consider inrush current on them?

Since they run only at red heat, it seems less likely that there could be an issue of any kind, even over the long term.

[W7TFO]

In all my years of babysitting MV tubes, never have I seen a design using inrush protection. 

What is gravely important relative to filament life is the maintenance of proper voltage.

High or low, the tube suffers.  5% is the defacto voltage tolerance.

Set it right via a RMS Voltmeter, reading at the socket contacts.

Ambient temperature is another thing to watch; too cold or hot and they will fail. 

Treat them properly and they will last a long time.

73DG

[Opcom]

Sort of what I was thinking but glad to have good advice from experience on it.
They can go on the output of the Sola transformer to keep the voltage regulated.

The PA filament will get a soft start on the secondary side of the Sola transformer. The diagram shown what I believe should work OK.

It will need soft start no matter what there b/c the ferroresonant unit won't supply a big inrush, instead when the load goes to it's output will drop to almost nothing according to literature and I also tested this on the bench - it's very true. The output 'collapsed' as the datasheet said it would when the unit was overloaded.

The soft start resistor of 80 Ohms should limit the cold (300 deg K) 3CX3000 filament inrush current to 80A which should be 620VA surge on the Sola.
assuming 95% efficiency in the filament units:
Normal load should be 410VA plus about 30 Watts for the adjusting resistor to get right at 7.5V.
So, 440VA for the 3CX3000 filament
The Rectifiers add 210VA.

So on soft start, the Sola may see 830-1000VA for a moment, if the rectifiers are considered. It should be good for a surge of 130% over the 1KVA rating so that looks good.

In operation, about 650VA will be used. Kind of light but the next size down was 750VA and it might not deal with enough of an inrush to get the 3CX3000 hot enough to switch to full power without taking a prohibitively big surge.

It needs to be mounted to a heavy steel panel at 60 lbs. and the panel mounted to the rails with 1/4" bolts instead of the 10-32 stuff. It should be OK to do that. In some racks shown in old handbooks, the transformers were mounted to the backsides of steel panels.

Things starting to look good. This filament business has to be done before the next stuff is done. Can't do nothin with cold tubes.