Simple PSU for a few valves

[Skywave]

Amongst items of test kit, the one thing that there never seems to be enough of are power supplies! This one was built many years ago as a simple unregulated supply for a few valves: it is of use for the odd experimental lash-up when a nominal HT of about 250 v.d.c. at a few mA is required and a stabilised, lower HT for things like oscillators. Plus, of course, provision for 6.3v heater supplies. Basically, it was not so much as a 'design to spec' job as "Here's an assortment of useful-looking bits; I need a PSU. What can I put together?"
The main HT is a double-pi capacitive input L-C filter. I suspect that the rect. valve is a GZ34; the markings have come off. But its shaped like a GZ34; pin-outs are as a GZ34 & it behaves like a GZ34; it's a GZ34!
The +100v O/P is derived from a stabilizer valve. Note the switching of the meter, enabling voltage or current to be monitored. It took me quite some time working that arrangement out, using an ordinary DPDT switch.

Al.

[OldTickTock]

Excellent Al, I like that very much. One day when I'm more confident about home-brew I would like to be able to build stuff like this. H.T. PSU would come in very handy.neat with the switchable meter too

Well done.

Chris

[Skywave]

Thanks, Chris.

Amongst the wide-range of test kit that one can make, linear PSUs must about the easiest to design and build. Layout is not particularly critical and choosing the necessary components is fairly straight-forward if the basic electrical parameters of the devices are known; most calculations are simply applications of Ohm's Law and a few power calculations. Adequate mechanical strength, insulation & safety, surge protection, adequate voltage, current & power ratings and the provision for the escape of excess heat seem to be the limiting factors. Being tight-fisted as I am, it's nearly always a question of "What have I got in the junk box?" when the need arises.
The only other point I will stress is the importance of thoroughly 'soak testing' the finished item on full load before declaring that the item is Fit For Purpose. This is particularly relevant when solid-state devices are employed, since Vbe junctions (and similar - like Zener diodes) can vary substantially at elevated temperatures compared to the 'nominal' 25°C.

Al.

[tiv48]

nice i need to build one so i can get round to checking some of the old valve tuners annd pre amps that i have lying around ie quad fm2 rodgers cadet tuner and preamp , ps does anyone know if the umbilical cords used a common wiring scheme they all seem to use a form octal or jones plugs
rob t

[glowinganode]

The only other point I will stress is the importance of thoroughly 'soak testing' the finished item on full load before declaring that the item is Fit For Purpose. This is particularly relevant when solid-state devices are employed, since Vbe junctions (and similar - like Zener diodes) can vary substantially at elevated temperatures compared to the 'nominal' 25°C.

Al.

Also a failure of the series regulator or control circuit of a stabilised power supply can lead to the full un-regulated voltage appearing at the output terminals.
Take it out on a lamp bulb for a few hours first.
Rob.

[Joe]

This is probably going to sound really stupid to those who know more about such things than I but I beg your indulgences.

HT power supplies seem to be something of a recurring theme and to me there is an answer so obvious that there must be a good reason for not doing it of which I'm unaware, namely using a thyristor.

I've never dealt with them in practise but from what I remember being taught about them they are a four-layer, three-legged semi-conductor and otherwise known as a Silicon Controlled Rectifier whose function is that of a diode but with the ability to switch off part-way through the positive half of the cycle meaning that the output potential can be controlled.

Is the DC thus obtained just too rough or is there some other problem of which I am unaware?

From a personal point this enquiry is purely academic as it's something of which I have no need but it is something I'd thought about if I came across a set which had originally had a line-cord dropper as it would seem more straight-forward than using a capacitive dropper in this situation.

Please enlighten me.

Cheers
- Joe

[tiv48]

hy joe durin the 70s a lot of solid state tvs used thyristor reguated power supplys eg phillips g8 decca 70 thorn 8000 and GEC Model C2115 very easy to repair then thorn came up with swiched mode power suppy and life was no longer simpel , thyristors are not verry good for the mains suppy
rob t

[Skywave]

Thyristor power supplies.
The only low-frequency version I have met was when I was a Field Service Engineer, computer maintenance. Amongst the products that I serviced was a huge line printer (intended for use with mainframes) manufactured by an American firm: Printronix. Apart from the novel printing mechanics, the PSU (which featured a huge mains-isolating step-down transformer) used a thyristor-stabilised arrangement to produce +30v at about 15 amps for a huge set of solenoids and their drivers. The basic idea was that of the more conventional arrangement that we find in high-frequency switch-mode PSUs - but in this affair, the thyristor was switched at 50 Hz. Consequently, the series smoothing choke was enormous - as was the main filter capacitor: 0.1 Farad at 50 v working; about 12 inches high and about 5 inches in diameter. :s The recovery diode was mounted on a huge heat-sink also.

Needless to say, extreme care was required when working / servicing that beast: 30v d.c. regulated @ 15A capacity can do a lot of damage! Moreover, I dread to think what the a.c. mains supply looked like, especially when about a dozen of these printers were going flat out! I do recall that the racket was absolutely deafening!

Al.

[Yorkie]

Very neat and compact layout Al - what's not to like about that?

(Are we allowed to look underneath!??)

David

[Skywave]

The underneath: well, I actually took a photo of the underneath - but it came out badly out of focus. Oh, what the hell? Here it is anyway - warts an' all!

The white numbers on the transformer is simply where I've been there with the 'Tippex' - identifying the tags. The vertical green bar in the centre is a Welwyn 12-watt wirewound resistor - the dropper to the OB2 stabiliser valve. The tubular green thing, centre left, is an 8uF TCC capacitor: the first C in the LC filter network. At bottom centre is the HT fuse - from transformer HV secondary centre tap to chassis / 0v. The item with the yellow label is one of the inductors in the LC filter.

As you can see (even with the appalling focus), I didn't spend too much time 'n' trouble with neatness of construction. ISTR that when I built this, many years ago, it was done in a Hell Of a Rush - and considering that the chassis originally housed a home-brew PSU for a National HRO receiver, there's a strange uncanny poetry there. (Enthusiasts will know what I'm barking on about )

Al.