Bulb across OT primary?

[hylaphone]

Anyone encountered this? What is the purpose?
I'm tinkering with an old Stromberg Carlson PA with a pair of 6550's. Though not indicated in the schematic, it has a small lightbulb soldered from plate to plate.
I can post pictures over the weekend, here is the schematic:

[John_H]

False center tap maybe?

[hylaphone]

Nope, the "real" center tap terminates at C22 just like the schematic.

This amp has a dead 5U4, and I am suspicious of this bulb lurking in there.

[g1]

Is it a neon?

[John_H]

The only lamp I see on the schematic is across the filament taps. The green wires. It looks like a false center tap to me.

[nickb]

Anyone encountered this? What is the purpose?
I'm tinkering with an old Stromberg Carlson PA with a pair of 6550's. Though not indicated in the schematic, it has a small lightbulb soldered from plate to plate.
I can post pictures over the weekend, here is the schematic:
[ATTACH=CONFIG]39090[/ATTACH]

I'd guess it's spark gap /gas discharge tube to protect the output stage from voltage spikes. Sometimes people use MOVs in that location

[hylaphone]

I believe so.

[Enzo]

I too would ask if it is a neon, and would agree that it is there as a spike limiter.

[hylaphone]

You guys are right, it's a neon:


Any advantage over MOV or flyback diodes?
Obviously, that few inches of bare lead needs some insulation..

[Enzo]

Why is that inch of bare wire any more in need of protection than the bare wire leads of the caps and resistors around it?

Advantage? Yeah, they are cheap and on hand locally.