- 2024
- Aug
- 4
The dial lamp can be more important than you think.
You’ll notice there’s a #47 lamp (Part# LM1) in the circuit for a lot of transformerless sets like the S-38C. It’s fed by a special tap on the rectifier tube:
It’s just a light bulb, right?
Yes and no. If you look at where it’s at in the circuit, it’s actually helping feed input current to the plate of the rectifier. This has the effect of both providing said current for the tube, and limiting the current available to the tube - you can draw about 60mA in this configuration.
So, do you need this bulb?
Again, yes and no. There are configurations where the tube is fed directly from the AC line, and this is a perfectly acceptable method. However, in our case of having the bulb, you have a bit of voltage drop to the plate which results in a lower output voltage, so you’d need to take this into account if removing the bulb - i.e. you need a resistor for some voltage drop. There’s also the fact that it is providing the AC current input - if you remove it, all of your current is now being sourced by the connection from the heater, and you’re stressing the tube. The radio won’t quit - but it just told you “Hey, I need service.”
In this case, the bulb is a component of the circuit that does more than provides light - it’s ballast for the rectifier and is providing a path for input current. It’s as necessary as the rectifier itself. You can’t replace this with anything other than a incandescent bulb without modification, so don’t grab an LED replacement for this one.
Fortunately, places like Antique Electronic Supply sell packs of 10 for under a fiver, so you can pick up a bunch and drop them in your parts bin. You probably want to do that anyway to get one that isn’t more tungsten boil-off than light bulb.