Lessons learned: pilot lamp assemblies, amp techs, and voltage charts

[sds1]

Here is the wrench I got if it helps anyone:

FENDER STYLE JEWEL NUT WRENCH — TONECRAFT AUDIO

www.tonecraftaudio.com

 

[dan40]

60w, I think. I believe that, the higher the wattage rating, the less resistance there is from the filament. But I just had a 60w handy, so I used that. Would you have expected the limiter to glow, or is that really mostly going to reveal a dead short?

A 60 watt bulb is perfect for this circuit and it should have lit up and prevented the damage to the transformer. I'm surprised that the light bulb limiter didn't work in this instance but maybe one of the fellas has an explanation as to how this fault occurred while on the limiter.

 

[theprofessor]

Two outer tabs are soldered together as a filament buss. Center tab for 100 ohm resistors is grounded to the chassis. Transformer wires come in from right. Filament wires to tubes and wires to lamp holder go out left. Works great. Lamp holder wiring was as simple as it gets.

Excellent! Thank you for this!

 

[Burning Fingers]

Hmmm dodgy pilot light holders shorting and taking out power transformers.
I have never experienced that luckily but have been dismayed at the poor quality of lamp holders I have purchased recently.
Might be cheap insurance to put a suitable fuse in the 6.3 volt line in future builds/repairs.

 

[theprofessor]

Well, after all that, I ended up disliking the way I had wired the heaters to the terminals of that old fuse holder. So I decided to re-do them. In pulling everything apart, the fuse holder itself came apart. The terminals just pulled out of the cylindrical insulation. That may have been a mercy, if it was a weak point. So that left me in need of a new lamp assembly, and I only have more of the Aeronautical Supply types in my box. After doing a continuity check between the tabs and the frame multiple times, I decided to use it. Since I was re-doing the connections anyway, I decided to splice the power transformer heater wires to their respective power tube heater wires, and then do a lineman's splice at that junction with a single lead going to the lamp from each of the original splices. That means only one wire to each tab, and it allows you to keep the heat farther away from the insulators. Glad to say that the amp sounds great and is holding up just fine so far.

Here's a pic (and yes, I really do enjoy colored heat shrink).

 

[JPKmusicman]

After reading all these pilot light issues I decided to route my filament circuit through a terminal strip. Now all the current runs through that and not the pilot light connection tabs.

 

[JuneauMike]

A moment of silence for all the iron that died so that this thread may live.

 

[theprofessor]

A moment of silence for all the iron that died so that this thread may live.

Hahahaha! No doubt! It was THE WORST!

R.I.P.

 

[Peegoo]

@theprofessor

I think the source of the short is not your soldering work. I think the lamp assembly is defective, and the clue is in the pic you provided. There needs to be an air gap all the way around the rear rivet to the base of the lamp when it's popped into the socket.

 

[theprofessor]

@theprofessor

I think the source of the short is not your soldering work. I think the lamp assembly is defective, and the clue is in the pic you provided. There needs to be an air gap all the way around the rear rivet to the base of the lamp when it's popped into the socket.

Thank you, Peego! Some helpful folks here pointed out that there was a batch of these lamps that were known to be bad around the time i bought this one. Whatever the case, it makes me feel better to think it wasn’t my fault!

 

[King Fan]

@theprofessor

I think the source of the short is not your soldering work. I think the lamp assembly is defective, and the clue is in the pic you provided. There needs to be an air gap all the way around the rear rivet to the base of the lamp when it's popped into the socket.

Excellent, thanks, @Peegoo . I knew there was insulation, I knew these little cuties could short around it, but how? and how easily? those pics are worth 1,000 words.

 

[chas.wahl]

If that's the case, then it's a pretty poor design, that relies on an air gap that can be compromised, instead of having an insulative shoulder washer or sleeve that maintains the separation. Of course, it's "tried and true" as a traditional (nay, revered) item, right? But somehow, the (also ubiquitous) Littelfuse fuse holder from the same era seems to manage combining electrical separation and solderability without creating potentially dangerous electrical failures.

 

[jeff_e]

Good info here, thanks everyone. I think I'll just use the jewel for my little build.. with an led hidden inside. The lamp holder part seems like junk.

 

[King Fan]

If that's the case, then it's a pretty poor design,

The lamp holder part seems like junk.

Poorly designed? Yeah, probably in the 1920s or something. Junk? Yeah, junk that's still probably in service in hundreds of thousands of (admittedly old) cars, motorcycles, amps, and, yeah, planes. You guys should *really* stay away from tubes. Outdated, dangerous, totally unreliable, and prone to nasty distortion.

 

[jeff_e]

Poorly designed? Yeah, probably in the 1920s or something. Junk? Yeah, junk that's still probably in service in hundreds of thousands of (admittedly old) cars, motorcycles, amps, and, yeah, planes. You guys should *really* stay away from tubes. Outdated, dangerous, totally unreliable, and prone to nasty distortion.

I know , and I keep burning my fingers on them..

I imagine the pilot assemblies were better back in the day, but I wasn't impressed with mine. Can't remember where I got it. I had a closer look at it this evening, the crimp holding everything together was loose enough that I could slide the outer bulb holder enough to short it out.

 

[tubeswell]

bus wire leading off the terminals, around which I could wrap the 18AWG heater wires.

Or run one wire from each lamp terminal to a separate tag terminal strip (and mount your other heater/PT wires/balancing resistors etc to the tag terminal).

 

[Peegoo]

Poorly designed? Yeah, probably in the 1920s or something. Junk? Yeah, junk that's still probably in service in hundreds of thousands of

I'm pretty sure he was referring to that specific run of parts--not every one ever made since The Crusades

If that's the case, then it's a pretty poor design, that relies on an air gap that can be compromised, instead of having an insulative shoulder washer or sleeve that maintains the separation.

The fiber insulator washers are a compliant material, meaning they compress a little (you can see that in the pic above). When the unit is being riveted together, the washers are intended to bulge a little into the air gap and create their own shoulders, like this:

The run of bad assemblies is due to the stack of components not being properly lined up when they were riveted together. They were bad before they left the factory.

 

[JPKmusicman]

Or run one wire from each lamp terminal to a separate tag terminal strip (and mount your other heater/PT wires/balancing resistors etc to the tag terminal).

This is what I did. Worked great.