Battery eliminator for simple one-valvers

[bileaflet_valve]

I am considering building the power supply for battery valve radios as described by Paul at
http://www.vintage-radio.com/project...y-set-psu.html

I understand the reason for separate HT and LT sections to cater for radios that use biasing resistors (typically in output stages). But I want to use it to power simple (leaky-grid with reaction) one-valve receivers where HT- and LT- are connected within the set to its ground line, which is of course earthed.

My one-valvers each incorporate an on-off switch in the LT+ lead. In normal use with batteries, HT is cut off when LT is disconnected.

I would built the unit exactly as described (in case I ever want to power a radio that does use bias), but the negative sides of the two outputs would be connected by the connection within my simple receivers.

Am I right in thinking that no harm will come to power supply or receiver in this application, or are there any precautions I should take?

Thanks,
Les.

Indeed no harm will result, using the LT switch will be ok too but leave the PSU running consuming a negligible amount of power. One valvers are fun and deliver amazing performance considering their simplicity, I must knock another one up soon just for fun.

[bileaflet_valve]

Thank you, MM. I was fairly sure it would be OK, but your re-assurance is very welcome!
Les.

[bileaflet_valve]

Following on from the above query (thanks again MM):-

As something of a novice with theory, the discussion in this thread:
https://www.vintage-radio.net/forum/...d.php?t=174818
regarding possible instability of the LM317 regulator, caught my eye.

It prompts me to ask whether I need to take any particular precautions with the LT section of the circuit as Paul describes it (link: see Post 1 above), either in terms of modifying the circuit or in choice of components.

As mentioned above in this thread, I intend to use the eliminator to power simple one-valvers, in which HT- and LT- are both earthed.

Also in the above thread, "crowbar" circuits are mentioned as a way of guarding against overvoltage to valve filaments. Is a crowbar circuit advisable in this instance and, if so, what form might the crowbar circuit take?

Any advice much appreciated.

Les.

[Herald1360]

Re the crowbar circuit, that's also discussed in the thread linked. Three reasonably hefty silicon diodes in series forward biassed across the LT output is suggested. You could fine tune the clamp voltage a bit by experimenting with substituting one or more Schottky diodes for standard silicon ones, this will add forward voltage of around 0.4V as an alternative to the silicon's 0.6V or so.

Adding a suitably rated fuse upstream of the diode string take-off point would make it a bit more bomb proof, though quite possibly diodes might be cheaper than fuses!

[bileaflet_valve]

Thank you, Chris. I am swotting up on diode theory!

In this application, I assume that the diodes should be chosen so that when connected in series, the forward voltage drops add up to a total a little in excess of the maximum desired voltage across the load, leading to a short-circuit (crowbar) across the output terminals. Presumably one can expect some variation in forward voltage, even among components of the same type? However (I presume again) that individual components can be tested for forward voltage, and chosen to add up fairly accurately to the desired limit.

Any corrections to my assumptions, and further comments from members, will be much appreciated.

Les.

[Herald1360]

That sounds about right. The clamping action is a bit "soggy" but probably good enough for the application. Anything more precise gets complicated quite quickly.

The filament drive circuits in '50s mains/battery sets tended more towards being current sources with series filaments fed via a large(ish) resistor from the mains derived HT supply. 7.5V worth of filaments from a 90V rail. Cheap and effective but not particularly efficient!

Another way used by some continental makers was parallel filaments fed from a NiCd cell used as a shunt regulator- its "on-charge" float voltage of 1.3V or so being just right and its on discharge voltage around 1.2V being OK and within the usable range of filament voltage for valves designed for use with a single dry cell filament supply.