Hi guys:
I wanted to replace the ballast tube in this AirMaster radio. (balast tube (BK38B2)
tube line up is:
25z5, 6sa7, 6sk7, 6u5, 75, 43 & (dual 6v #44 parallel dial lamps)
This heater string draws 300ma.

I saw this article at:
http://antiqueradios.com/chrs/journal/ballast.html

It shows how to use a 1n4004 in series with the filament string and a 6v Zener across the lamp.

It's replacing the AC with 1/2 wave pulsating-DC dropping the (120-VAC) voltage about 1/2 or less... so I guess that if you need to drop around "half" this works...
( I only needed to drop about 35-40 since my string is (25+25= 50v) + (4x6.3=25v) = 75v plus 6v for the lamp = about 81v total out of around 120Vac

They show a6-volt SK6X0 RCA Zener diode in series with the filament string to ground and ACROSS the lamp!
I can't find the data on that Zener... so I don't know how much power it can carry.
But I question how/why the Zener can be used properly in this circuit. It says "to prevent the lamp from burning out".
As I see it, if the lamp was to 'open' that Zener would have to carry the entire 300ma filament current. Most Zeners are not designed to carry that much current are they?
A 1-watt Zener at 6v (1watt/6v =.160a) would only carry 160 ma... and that's maxed out... right?... and safety dictates only 70% of max... right?

I measured the current in the string and it is 300ma.

I also measured the total available voltage as only 52vdc.
Things seemed to run alright but on the low /dim side.

I found that I could adjust it up easily by adding the approriate filter cap.
I tried several but found that a 15uf 160v elytic does a nice job of kicking it up to just under 80 volts.

I think the Zener should be removed for safety though and if the lamps go out then replace them.
I would like a better way however.

Knowing, of course, that it would "smoke" it...I tested it out.

I had a 6-volt, 1n754 (500mw..I think) zener laying around here and stuck it in.
Sure enough... as I pulled out the lamps... the zener smoked a bit and became a permanent short.
The set certainly continued to work ... as it drew the 300ma right through that shorted-out Zener very nicely!... lol

Any better ideas?

You could use a 50 or lesser ohm resistor of the 5 or 10 watt variety. 50 ohms is fine with a .15 ampere string. With a .3 ampere string, you may want to go with a lower resistance.

Thomas

ZENER to shunt the Lamp:
As a follow up on the Zener protecting the dial lamp.
In my last post I questioned it because the filaments require 250-300ma and that may smoke the .5 watt Zener if the lamp is burned out.
Well that is true but it actually works fine like this.
Well it works to protect the lamp by providing a temporary shunt only while the filaments are cold.
I tested it all again today.
My AirMaster uses two #47 lamps in parallel (combined current 300ma) and those are in series with the filament string ...also at 300ma.
I NOW have a 1/2 wave rectifier replacing the ballast tube. I put the Zener across the two lamps as a shunt. When heaters are cold initially the zener conducts drawing a brief 250ma or so... and slowly tapers off down to about 20ma and the lamps glow dimmly at first and as the filaments get hot the lamps brighten up nicely.
It works Fine!
(without that Zener... the lamps flash brightly and burn out right away on a cold power-up)

The lamps probably will not be burning out from cold starts now this way and therefore I don't think I have to worry much about the Zener blowing if the lamp opens... but just for safety, I decided to double-up and put two 1/2-watt 1N754(6v)Zeners in parallel to give me 1 watt. This will carry the full filament current even if the lamps are open or missing.

Doubling up is a great idea. You can also use a resistor in place of the diode, but obviously the voltage regulating type diode affords more protection.

Thomas

:Hi guys:
:I wanted to replace the ballast tube in this AirMaster radio. (balast tube (BK38B2)
:tube line up is:
:25z5, 6sa7, 6sk7, 6u5, 75, 43 & (dual 6v #44 parallel dial lamps)
:This heater string draws 300ma.
:
:I saw this article at:
:http://antiqueradios.com/chrs/journal/ballast.html
:
:It shows how to use a 1n4004 in series with the filament string and a 6v Zener across the lamp.
:
:It's replacing the AC with 1/2 wave pulsating-DC dropping the (120-VAC) voltage about 1/2 or less... so I guess that if you need to drop around "half" this works...
: ( I only needed to drop about 35-40 since my string is (25+25= 50v) + (4x6.3=25v) = 75v plus 6v for the lamp = about 81v total out of around 120Vac
:
:They show a6-volt SK6X0 RCA Zener diode in series with the filament string to ground and ACROSS the lamp!
:I can't find the data on that Zener... so I don't know how much power it can carry.
:But I question how/why the Zener can be used properly in this circuit. It says "to prevent the lamp from burning out".
:As I see it, if the lamp was to 'open' that Zener would have to carry the entire 300ma filament current. Most Zeners are not designed to carry that much current are they?
:A 1-watt Zener at 6v (1watt/6v =.160a) would only carry 160 ma... and that's maxed out... right?... and safety dictates only 70% of max... right?
:
:I measured the current in the string and it is 300ma.
:
:I also measured the total available voltage as only 52vdc.
:Things seemed to run alright but on the low /dim side.
:
:I found that I could adjust it up easily by adding the approriate filter cap.
:I tried several but found that a 15uf 160v elytic does a nice job of kicking it up to just under 80 volts.
:
:I think the Zener should be removed for safety though and if the lamps go out then replace them.
:I would like a better way however.
:
:Knowing, of course, that it would "smoke" it...I tested it out.
:
:I had a 6-volt, 1n754 (500mw..I think) zener laying around here and stuck it in.
:Sure enough... as I pulled out the lamps... the zener smoked a bit and became a permanent short.
:The set certainly continued to work ... as it drew the 300ma right through that shorted-out Zener very nicely!... lol
:
:Any better ideas?
:
:
:
: