Prototype set is here

[cbenham]

Originally Posted by earlyfilm

<---> On a twin-tube "B" supply, if one tube opens a filament, the other tube promptly melts before the user notices and can shut the system off. <--->

Someone in this forum pointed out several years ago that wiring a 5U4's plates in parallel and connecting to *just* one side of the B+ winding, then doing the same thing with a second 5U4, but with both tube's filaments connected in parallel to the 5 volt PTX winding, will possibly save the other tube and the PTX in the event of a failure.

The idea is that if one tube shorts and blows the filament open, the remaining tube will be the only load on the transformer. The B+ will drop, but the transformer will not be overloaded.

A 6.6 volt filament winding is sometimes used if the power supply is on one chassis and the load circuitry is on another separated by 2 to 4 feet of heavy cable. The voltage drop through that cable can be in the range of ~0.3 volt if the filament draw is substantial.

Cliff