Saved by the Dim Bulb Tester

I was looking forward to some sweet tube sounds today from my Magnavox 9300 amp but it was not to be.

I did, however, learn to make and use the Dim Bulb Tester that was fittingly attached to a piece of wood from the old console.
I found that my transformer is OK but discovered a short circuit in the Filter Capacitor.

At least this will give me the chance to tidy it up cause it was rather sloppy looking to begin with.

Handy lil devils, aren't they?



Being a dim bulb myself, I even labeled mine so I could remember what it was. <G>

Talk about dim, I was gonna test it and see if the light came on when I plugged it in until I realized this was not how it functioned. Doh!

Mine is 3 separate pieces unlike your all in one box. Regretfully, I should have done the same but you live & learn.

loopstick said:

Question for you hollow-state guys. Could the DBT have killed that capacitor? Could judicious use of a variac have saved it?

I'm assuming the OP didn't first fire up the Magnavox when it was plugged directly into the wall outlet, then noticed a problem (fuse blew maybe), then used the DBP afterwards for troubleshooting.

IOW could the cold filament in the DBT have passed on to an old fully discharged electrolytic a destructive level of voltage? If so then the DBT both caused the fault condition and "saved" the gear from further damage. A mixed blessing perhaps.

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Wouldn't the same thing happen to old capacitors in solid state gear? I suppose this COULD be possible, but I suspect not very likely. I'm not an expert, though.

loopstick said:

Question for you hollow-state guys. Could the DBT have killed that capacitor? Could judicious use of a variac have saved it?

I'm assuming the OP didn't first fire up the Magnavox when it was plugged directly into the wall outlet, then noticed a problem (fuse blew maybe), then used the DBP afterwards for troubleshooting.

IOW could the cold filament in the DBT have passed on to an old fully discharged electrolytic a destructive level of voltage? If so then the DBT both caused the fault condition and "saved" the gear from further damage. A mixed blessing perhaps.

Click to expand...

no, and unlikely. The rectifier tubes will not switch on instantly like solid state diodes do. There is a ramp-up of voltage, though it is a bit rapid with a directly heated rectifier. You're not going to get a tube rectifier to do very much under about 75-80 volts AC line anyway. They just don't conduct enough to reform anything effectively. Truth be told, re-forming caps works better with SS diodes in place of the tube rectifier, and then you can bring it up real slow on a variac. Honestly though, 50 year old caps are usually just done for and well overdue for replacement. Even if they worked today for testing, I'd go with new ones if I wanted to make any use of the unit on a regular basis.

Power up can take out electrolytic caps if they've been sitting long enough. Helps to ramp up slowly and allow them to "reform". Different schools of though as to the best way to do that, but typically, you fire the equipment up on a variac at around 80 and work your way up to full vac in a few steps, a few minutes each.

Then again, was your filter electrolytic or film?

Either way, spilt milk and all that ...

The filter is electrolytic. I don't own a variac but would like to get one in.

sKiZo said:

Handy lil devils, aren't they?

Being a dim bulb myself, I even labeled mine so I could remember what it was. <G>

Click to expand...

Yep - best piece of equipment I own. Do need to get a variac though.

hogdaddy said:

The filter is electrolytic. I don't own a variac but would like to get one in.

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When you did your test did the bulb light up immediately or did you have to wait about a half minute before the bulb began to glow? The reason I ask is that it's probably not a good idea to slam full voltage on old unused electrolytics. In your case the capacitor may have been "ramped up" by rectifier warm-up but I don't know if we can assume all tube audio gear has tube rectifiers.

IIRC when I was a kid there was a lot of top hat style solid state bridge rectifiers in old TV's people used to toss into this ravine. That's how we did e-waste in those care-free years. I asked you this before but I thought you had abandoned the thread.

Hi,
I have trying to read up on these. Could you point to a good post that explains the use and building of a unit? Seems like I might be overlooking a sticky since so many hits come up within the search tool. I like that your unit is nice a tidy.
Thanks,

EDIT:
Seems some off site searching helps,
http://www.antiqueradio.org/dimbulb.htm

sKiZo said:

Handy lil devils, aren't they?

Being a dim bulb myself, I even labeled mine so I could remember what it was. <G>

Click to expand...