I agree with Tom on this. Bigger is not better in the case of filament
transformer ratings.
If the tube requires 10V at 10A then a transformer rated at 10V at 10A
CCS, properly cooled, is acceptable. At the same time one must consider
the IR loss between the transformer and the tube socket and any thermal
rise due to restricted heat dissipation. Start up current limiting, if
desired, should be installed on the primary side only.
In my view, a 10V @ 20A or 30A transformer etc. is typically asking for
a hosts of problems.
Tom Rauch wrote:
>
> > >By far the biggest stress in amateur service is the
> > > constant thermal cycling of the filament
> >
> > That's where the rheostat comes in handy - assuming proper
> > proportions, it limits surge current.
> > Of course, so does a suitably designed surge limiting filament
> > transformer, but that can be a bit more expensive.
>
> Not really, a suitable current limiting transformer costs LESS than
> a transformer that allows excessive starting current. You do have to
> allow for temperature rise, however, and not box it in.
>
> > >provisions would have to be made for monitoring voltage
> > >accurately
> >
> > I assumed external metering, but point taken, especially on cost.
>
> Oh sure! Let the guy use his handy $25 dollar VOM to set voltage!
>
> I've seen six cases of low cathode emission in the past 18 years in
> 8877's. In five of those cases, the filament systems were modified
> to reduce voltage and they missed by a few percent.
>
> 73, Tom W8JI
> w8ji@contesting.com
>
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