120 volt battery

[ssaunders]

All,

I've just got a GEC 4046 on the workbench. Problem 1: what to do about batteries. I've seen a number of designs for 90volt eliminators but does anyone know the whereabouts of a 120 volt eliminator (perhaps with 2 volts for the LT)? I'd prefer not to take the multiple PP3 route if at all possible.

Thanks

Simon

[Ed_Dinning]

Hi Simon, cheap PP3's are a good start for testing, but for real use you need an eliminator.
A lot of the designs use voltage multipliers from a lowish voltage to derive the 90 volts; simply add an additional stage of diode and resistor. The theory is normally found if you Google "cockroft/ walton voltage multiplier".

The LT supply is usually from an LM317 regulator chip with a bit of crafty circuitry to get it to work down to 2 volts, with possibly a crowbar circuit to protect valuable filaments.

You may also need a GB supply, which can be virtually anything with a chain of resistive dividers for the voltages required.

Ed

[Dave Moll]

Originally Posted by ssaunders

I'd prefer not to take the multiple PP3 route if at all possible.

Why not?

[vinrads]

You could use an LT mains transformer and back feed the secondary with the appropriate voltage to arrive at 100/110V secondary. It works for me.

[Nickthedentist]

Good quality PP3s need not be expensive, e.g. http://www.ebay.co.uk/itm/10-DURACEL...item230b9cf162

A 20V RMS transformer with two voltage doublers one positive and one negative would give about 120V. With modern cheap large capacitors there will be no hum. Or this https://www.vintage-radio.net/forum/...ad.php?t=66030 would last for ages.

[robjkmannering]

I tend to put two transformers back to back, i.e. the 2nd in reverse. The low AC voltage can be rectified and regulated for the heaters (and any dial lamps) and the higher AC voltage can be half wave rectified and a single smoothing cap used for the 120 volts. A suitable resistor will drop any excess voltage as the current is low.
I buy cheap wallwart adaptors from car boots for this purpose.

Rob

[audion_1908]

I have tried cheap wallwart type adapters but most have terrible regulation, but I have used two wallwart type transformers back to back and neon voltage regulation tube controlled transistor to make a 230v in 90v isolated and regulated power supply.

The older heavier wallwart type adapters have better regulation.

[robjkmannering]

Regulation of the ht supply does not seem to much of an issue as long as it does not rise too high. I did try using an LM317 but it didn't offer any improvement in use.
The current drawn by these battery sets is only a few milliamps and easily supplied by a small transformer.
Regulation of the LT side is of course far more important as the battery valve filaments are fragile but a LM317 copes well as it only has to drop a couple of volts at low current.

Rob

[Herald1360]

Originally Posted by Ed_Dinning

The LT supply is usually from an LM317 regulator chip with a bit of crafty circuitry to get it to work down to 2 volts

Not particularly crafty. A LM317 works down to 1.25V (tail direct to 0V a la 7805 etc) and any higher voltage within its limits just needs two resistors to program. Add a couple of capacitors for stability and a heatsink if it's dissipating more than about a watt and that's it- apart from extras like crowbars or why.

[ssaunders]

Originally Posted by Dave Moll

Why not?

I guess I'd like to get a more permanent solution for use with other battery sets - which kinda gives me an idea for a design project where those interested on the forum perhaps could collaborate - a universal battery eliminator. I class myself as on the less technically qualified side but happy draw and write for those with less ability in that area. I'd be happy to build the prototype to prove it suitable for er... the less able?

Any volunteers or thoughts on specification beyond covering HT 90 and 120V and LT 2V?

I made this https://www.vintage-radio.net/forum/...2&d=1227122816 it has 60, 90 & 120V HT, 2V LT and variable negative grid bias, one of the advantages of using the voltage multiplier approach is that you can easily tap down for various HTs.

[TrevorG3VLF]

And here is another. There are many ways of doing the job, just use what components you have.

[kalee20]

Originally Posted by robjkmannering

Regulation of the LT side is of course far more important as the battery valve filaments are fragile but a LM317 copes well as it only has to drop a couple of volts at low current.

One caveat - the LM317 is a bit noisy. I used one once to regulate LT on a battery radio and was very disappointed with the radio - lots of mush but only a few stations - until I stuck a big capacitor across the LM317 output.

You do need to do this as a few microvolts of broadband noise on the LT line is effectively in series with the input signal!

[Ed_Dinning]

As Kalee20 has said, LM 317's need a big cap on the output. Best combination is an e-cap and a 0.1 plastic type. You should then also add a diode in inverse parallel back across the chip. This is to discharge the output cap at the same rate as the input.
If the input goes down first the chip is quite likely to die.

Ed

[julie_m]

Not a plastic 100nF, ideally, but a ceramic one; which has a lower self-inductance.

[kalee20]

That may be an old wives' tale. Most small plastic capacitors are metallised film with sprayed-on end electrode connections, thus effectively forming the same geometry as multilayer ceramics.

[Variometer]

Small PCB transformers are quite cheap. I just take three of them with 250V primaries and 15-0-15 secondrys. Connect the primaries in parallel across the mains. Connect the three ends of the 15-0-15 (30V) in series and you get 90V AC. Feed this into a normal bridge rectifier and smoothing circuit and it comes out at about 120 Volts. Rather simple, but it works OK for me.
Bob