Filament wiring to jewel lamp that is entirely outside the chassis.

[Jatchley]

Hello so I’m building a project amp and I’m curious what you genius people would do in this situation. Will the twisted filament wires be extra susceptible to EMI if they exit the chassis? Or is the twist good enough? The PT has a filament CT so I don’t have to worry about the filament resistors. The only part that is entirely outside the chassis in reference to the filament wiring is the lamp.

[sluckey]

Still got 70VAC?

[Jatchley]

Still got 70VAC?

LOL oh! i forgot about that post. NO. i was referencing to ground not neutral.

[sluckey]

Still got 70VAC?

LOL oh! i forgot about that post. NO. i was referencing to ground not neutral.

The variac power switch is probably on the neutral rather than the line. If it were mine, I'd fix that.

[Jatchley]

Still got 70VAC?

LOL oh! i forgot about that post. NO. i was referencing to ground not neutral.

The variac power switch is probably on the neutral rather than the line. If it were mine, I'd fix that.

I’ll open it up and see what’s up. So any advice on this lamp?

[sluckey]

I was waiting for the geniuses to chime in. Meanwhile...

[Williamblake]

I guess this means old radios got away with it but nobody knows where your out-of-chassis-filament-winding is going exactly.

[Jatchley]

I was waiting for the geniuses to chime in. Meanwhile...

Haha wow! So I guess it’s totally fine?

[Jatchley]

I guess this means old radios got away with it but nobody knows where your out-of-chassis-filament-winding is going exactly.

to a jewel lamp That doesn’t have the lugs inside The chassis. Every amp I’ve seen has the entire length of heater wires inside the chassis... that’s why I asked. This project has the lamp and it’s lugs outside the chassis. I wasn’t sure if the full shield of the chassis around them was needed to keep noise down. The design seems to me like it might have some noise issues to be addressed but I thought about it as a possible issue but wanted to confirm if it was frowned upon or not. Sluckeys example makes it seem like it’s no big deal...

[acheld]

Those wires seem awfully exposed. 

I guess if this was a project for my basement, and only my basement, then ok.   

But I would not let a project like this out into the wild . . .   Yes, I get it that these are heater wires, low voltage, but I have seen crazy stuff happen, and would not take the risk. 

As to noise, I doubt that the relatively low current AC outside the chassis would inject noise.   On the other hand, those wires could set up a great antenna.    You might just hear the Stones pulsing your heaters!

[Willabe]

Haha wow! So I guess it’s totally fine?

It's a risk.

I wouldn't wire it outside the chassis.

[sluckey]

Those wires seem awfully exposed. 

Those wires are more than adequately protected once the chassis is mounted in the cab. It's a table radio! Was never meant to be in the wild. Millions of radios and other electronics from that era ran filament wires outside the main chassis for indicator lights, dial lights, tv tuner lights, etc. Lot's of other wires too. Heck, they even ran B+ wires to the speaker! An antenna coupled to the filaments, coupled into the signal path? Not likely. You ain't gonna ever hear Stones pulsing your heaters! However, I have heard music being reproduced in long blades of grass.   

This was a restoration turned conversion project that started in the '60s, was forgotten, and finally completed in 2012. I posted that pic just to show Jatchley that it can safely be done, especially in his case where it's a short run to his faceplate.

Here's the rest of the story....  http://sluckeyamps.com/RCA/RCA.htm

[Jatchley]

Haha wow! So I guess it’s totally fine?

It's a risk.

I wouldn't wire it outside the chassis.

The lamp? I know the current is high. This is why I was asking. That contraption will be inside a wooden head cab rack case. I was going to heatshrink the pilot lugs after soldering over to prevent shock risk. Input signal and reverb wires will be shielded cable. All those leads sticking out are just trafo/choke wires I didnt run inside yet or hook up yet. But it is a noise risk?

[sluckey]

There is no risk of noise. And if done properly, there is no safety issue.

EDIT... And a #47 bulb only draws 0.15A

[AmberB]

All of the Bogan CHB-35 PA amps, and the Lafayatte PA amp that I've worked on all have their power indicator lamp mounted outside the chassis, with the twisted wire pair running to it.  I would say that it was pretty common back when tube powered products were being made.

[j_bruce]

I recently rehabbed a Gretsch Dual Bass that had the pilot light outside the main chassis mounted on a face plate. It was a similar arrangement to the one Sluckey showed for a radio except that the pilot light and PT were on the same end of the situation so the wiring was only 4 or 5 inches. It still had the original lamp cord and is as quiet as can be and has a great tone.

[tubeswell]

Morgan (amongst others) do this in their amp cabs without a problem.


Makes for convenience if you have a banana-type 2-prong male/female plug to easily disconnect the lamp from the chassis 6.3VAC supply wire when hauling the chassis in and out of the cab.