Hi there.

I have a couple of questions concerning the use of a Dim Bulb.

I have just applied power to my Calstan radio for the first time. So far no smoke or sparks. Not many lights either.

It would appear that the rectifier valve is not overly healthy.

I had a couple of 40W / 60W incandescent bulbs in series, a variac and an isolation transformer.

What size bulbs do folks usually use for these. What I found happened was that I was unable to apply sufficient voltage to the unit. I’m assuming that after the voltage drop across the bulbs there was little else left for the radio. The best I could get with the bulbs in circuit was about 120V AC.

Does it matter what order I have these devices. I’d assume that the isolation transformer would be last but should the variac be before or after the bulbs?

If anyone is slightly interested I recorded my activities as I first powered up the radio. The video files are quite large but you are welcome to have a look and provide any critique. They are still loading as I write this so they will not be there for a little while. Here is a drobox link to the videos. You will need a good internet link.

https://www.dropbox.com/sh/pc2pvs4hk77an1b/AABGb7SyUp7rx2PRvnlHJlvra?dl=0

I finally took the bulbs out of circuit once I had determined that there appeared to be no significant current being drawn.

Clearly this is not what folks normally do so I’m wondering what I have done wrong.

Peter

For general valve radio work a 75 watt globe works best for me.
Your setup in series is approx half this.
Put them in parallel.
Leave variac out altogether, for normal testing purposes.
JJ

I have never used a dim bulb in decades however, the general idea is to not exceed the current rating of the transformers. Clearly you have too much resistance with two globes.

Do be aware that on the output side of a Variac (aka slide transformer / Dimmerstat) which is an "auto transformer" below 130V an RCD will not trip. An earth leakage on the secondary of a transformer will not trip an RCD.

Running too low a voltage can poison some cathodes

Marc

A 100 watt bulb was always my size.
The smaller bulbs have much higher resistance filamants.

The last couple of years I just use a Variac and monitor the input wattage as I stage the voltage upwards.
I use a power board with an inbuilt LCD. Easiest to use and you get a figure to work from.

Fred,

As the whole place is either new wiring or updated every fuse box is riddled with RCD's, so nothing to do there. So the isolation transformer for radio work is a modified commercial lighting job. 3A now with CB or Fuse input & output sides. Plus MOV's & caps for the RF & Lightning.

There are Neon Bezels either side of the transformer to show if both sides are powered, or a fuse has blown. The main addition is a big ugly "Kill Switch" on the top of the box. That's the quickest way of killing the circuit if there is a problem. There is always a meter on the HT on start up of a repaired set or one considered safe to power.

Marc

The lamp itself has always been my kill switch. It will react much faster than you can, actually faster than a fuse.

Choose the lamp wattage rating to be about equal or double the normal running current of the device, depending on how confident you are on the first power up.

Equal watts will not normally supply enough current to operate the device but you can measure voltages knowing you shouldn't damage anything and know if it's safe to move to the next step.

Having not used it for 25 years I gave my variac away to someone!

But everyone has their own way of doing this.

I just use 3x 60 watt bulbs in mine. Never really had a issue. Thats after the variac.

I use a stop / start button contactor after my bulb set up. With the 240V coil dropping out the contactor at < 130V I found it quite useful. With the series bulb test, any shorts or loadings that causes the bulb to light will drop the contactor out. Also I find the button won't hold in for the first few moments unit the inrush current has settled. This is a nice feature. If I test anything with a fault, the contactor won't hold in at all, alarming me to a problem. I also don't use my variac too often. It is handy on transformerless portables when making adjustments for filament circuits etc. Living in rural Vic, supply voltages can get quite high during sunny days, 255 - 260 volts so it has it's use here also.

Tallar Carl: Those 3 x 60W bulbs are they all in series or parallel?

I can help there.

Parallel. The 3 lamp idea is useful as it allows you to increase the current limit point just by plugging in an extra lamp or two.

No so good putting them in series 'cos the filaments won't get hot enough to invoke the high resistance current limiting part of the lamp's VA curve. The lamp must of course be rated for the supply VOLTAGE you are using.

The only time one would consider series, is if you put a switch across one only, of two, to short one of them out.