Using Current Limiting Bulb ...

[half_smith]

Hello, I've only used my Current Limiting bulb when first running an amp I've been working on. And it's never lit up (except if I shorted my probe to the chassis).

I was running an amp I just rebuilt the rectifier board on (BF Bandmaster), and the bulb was throbbing a little.
When I ran the amp hot (Volume 7+) the bulb was really glowing.

Is this expected?

Thought and advice please!

[martin manning]

This sounds like a parasitic oscillation problem, but what is your setup? Are you running into a dummy load? What wattage is the bulb? Do any of the controls affect the brightness?
It should be steady, and if its wattage is roughly equal to the amp output power, relatively dim. Here's a picture of a limiter with a 100W amp idling through 1x 60W, and 2x 60W: https://ampgarage.com/forum/viewtopic.p ... 10#p462410

[Stevem]

Since you where working on the rectifier board I need to ask if you still have at least minus 46 volts on pin 5 of either output tube?

[half_smith]

Since you where working on the rectifier board I need to ask if you still have at least minus 46 volts on pin 5 of either output tube?

good thinking. I adjusted the bias pot to make -50 (was -30), that really lowered the current and landed the plate dissipation on the 5881s at 15.5w (70% max is 16).

PD was over 29w (way over max for 5881's) they were probably red-plating and that's why the Current Limiting bulb was glowing, I'm guessing the bias was off and that's why the rectifier diode and fuse blew in the first place (I didn't change the cap or resistor on the amp, only diodes).
- but this Bandmaster has some Dumble mods and I wonder if it was setup to have insane PD?

[Stevem]

Well first off 5881s do not belong in a BF Bandmaster.

[wpaulvogel]

The light bulb limiter isn’t intended to be used while playing the amp or bias. It’s a start up protector for the fuse and indicates a heavy current draw that won’t allow enough current to blow the fuse. Once first start up is verified good with dim glow, and bias pot on lowest current setting, you’d remove it. Power on set bias and then if unsure, put the LBL back in and allow for a burn in. Playing the amp with the limiter in place is voltage starved and I see no reason. At this point the amp is confirmed good. The LBL needs a 100 watt or lower incandescent bulb. First start with switch will emit bright glow as heaters warm and draw less current, every switch on.

[half_smith]

Well first off 5881s do not belong in a BF Bandmaster.

It's a Bandmaster modded with Showman PT/OT and 4x 5881s, the owner loves the sound of the amp so I don't want to kill the mojo if it was intentionally set with insanely high PD...
I settled on 23w (the listed max PD for 5881) which is less than the 29w it came in as, but way above the recommended 70%

[half_smith]

The light bulb limiter isn’t intended to be used while playing the amp or bias. It’s a start up protector for the fuse and indicates a heavy current draw that won’t allow enough current to blow the fuse. Once first start up is verified good with dim glow, and bias pot on lowest current setting, you’d remove it. Power on set bias and then if unsure, put the LBL back in and allow for a burn in. Playing the amp with the limiter in place is voltage starved and I see no reason. At this point the amp is confirmed good. The LBL needs a 100 watt or lower incandescent bulb. First start with switch will emit bright glow as heaters warm and draw less current, every switch on.

Thanks for the reply, this is what I was wondering. I haven't ever used the Bulb beyond checking for runaway current, but I had a buddy in the shop and he was noodling while I adjusted bias, that's when I saw the bulb getting bright and stopped.