BUSH DAC90A - dial lamp idea

[6SN7WGTB]

Lots of posts on this radio but I have an idea on dial lamps which I'd value some opinions on.

The original bulbs are 3.5V 150mA as we all know.

Hard to find, but easy to find 200mA versions.

My though was to put a 10R resistor in series with the bulbs.

Logic being that they're prone to blowing. When hot they're obviously ca. 24R each. But when cold probably I'm measuring 3R.

If I use 200mA then hot will be 18R each, and cold really low.

So, if I put 10R in series I will be approximately restoring the equiv hot resistance of the two 150mA bulbs, but when cold will be adding some surge protection.

Or am I barking up the wrong tree?

(BTW my dial shunt is the original 75R version).

[Ed_Dinning]

Hi, note that the lamp is in the heater series chain, which has a low cold resistance and adds to the problem with it's hig inrush

Ed

[6SN7WGTB]

Originally Posted by Ed_Dinning

Hi, note that the lamp is in the heater series chain, which has a low cold resistance and adds to the problem with it's hig inrush

Ed

Precisely why I'm suggesting using the 200MA bulbs and adding an inrush limiter resistor.

[Ed_Dinning]

Inrush limiter may do little except reduce the volts across the heater chain; a suitable NTC and adjustment to the heater dropper valve may work better, or simply fit LED "bulbs" and power them from the bypass resistor (LED of AC type and without an internal resistor)

This was a problem I addressed when I designed the autotransformer to reduce heating in these sets; a 6.3v supply tap is provided to give a "stiff" voltage for the lamps and no bypass resistor is needed.
It also keeps the innards a lot cooler

Ed

[6SN7WGTB]

Sorry, perhaps I wasn't clear.

My suggestion is to put the 10R between the two 200mA bulbs, thus giving an almost exactly hot equiv to two 150mA bulbs BUT providing an enhanced resistance when cold.

By 'AC type' LED I assume you mean an LED with 4007 in reverse parallel and a dropper cap in series? I did consider LEDs but without adding an array to get suitable illumination it seemed a bit of a faff. Plus getting a sensible retro colour etc.

So your modification is to effectively to run the HT off of 'US' voltage (in which of course the filaments add up to in a 'All American 5' config) plus have a separate filament tap for the bulbs - I can see how that works well. I suppose you could even have a LDO reg and run 6V bulbs...

It seems we're effectively 'burning' the 120V left over!

[kalee20]

10 ohms on a 240V string will add so little extra resistance that it's negligible! I can't see it making much difference to a surge.

[Ed_Dinning]

AC type LED's were available from RS at one time, probably with either an inverse parallel or series diode and did not have a series resistor (sort your own to adjust brightness)

My auto transformer fits in place of the dropper resistor; it takes in 240v and gives out a feed (200V) to the rect anode, so HT resistor not needed.
It also has tappings for 117V and 79 V (DAC90a & DAC90 heaters)
The 6.3v feed goes direct to the "hot end" of the dial lamps and the bypass/ shunt resistor is removed.
Greatly reduces the heating in the cabinet and prevents heat induced cracking in cream cabinets

Ed

[6SN7WGTB]

Originally Posted by kalee20

10 ohms on a 240V string will add so little extra resistance that it's negligible! I can't see it making much difference to a surge.

I was thinking about trying to up the cold resistance of the bulb chain only, in parallel to the scale lamp shunt.

Agree obv. with what you say on the whole chain, but I wasn't envisaging affecting that.