In my King radio there is a 90 volt connection and a 6 volt connection. Is it safe to connect 6 volts to 5x 5volt filements ?

Not that I have had a lot of experience with 20's radios but typical 20's radios have a rheostat in the filament circuit primarily to cater for changing voltage from the battery as it discharges. The photos in the link you posted show a rheostat as the RH control on the front panel. I might guess that this is in the filament circuit, might be worthwhile checking. Standard operating practice AFAIK, is to return the rheostat to max resistance prior to switching on, then adjusting the filament voltage to the minimum required to establish a comfortable listening level.

So yes, IMHO it's okay to connect a 6V battery (or power supply) to a 201A filament provided you drop the voltage at least 1V.

Joe

All my 20's radios (not many) have multiple rheostats in the filament circuits, typically one for RF valves, one for the detector and one for the AF

UX-201-A has 5V filament. The rheostat is usually in the negative rail. The voltage across the filaments is part of its bias and the volume is controlled by the rheostat by reducing the emission of the filament.

Some rheostats had an over-run and that was the off switch.

Radiotron data is specific as to not running them on excessive volts. Gilfillan GN1 actually has a voltmeter.

Beware: If you use a regulated PSU (Battery eliminator) it will need loading if it supplies the grid (7905) as the grid for purpose here draws infinitely low current, the regulator will not work.

If its running on mains ground only via a 0.01 line cap.

Do not run the detector above 45V

Marc

This set actually has a volume control which is the one just under the middle tuning knob. I believe this is a rheostat.

Its physical position is not as important as its function, which should be examined for function & correct wiring. Some of them had a bit of a habit of going open, often through wear.

The wiring of those could be rather haphazard and I would suggest that where there are two volume controls there was a risk of regeneration if the RF one was pushed too far & it would break into oscillation. Some I have repaired are a bit like that.

Sorry guys I must have been having a Grey moment. I edited my first comment. There is only a 90 volt and 6 volt connection.

Wiring in those things was fairly primitive but the same cannot be said of its circuit design.

If that has a UX-201-A as a leaky grid detector the tube cannot be run above 45 V without a separate grid bias of -5V. More commonly 22.5 V if there was a hint of instability.

Perhaps some reverse engineering if there is no circuit. It would not be uncommon to have an exclusive control on that 2nd detector.