National SW-54 Thoughts

[Dubis7]

What's the purpose of the filament shunt resistor?

Mine keeps blowing. It was blown when I first got it. I swapped it originally, had no issues, had no issues still when I finished replacing a whole batch of components today, and then I swapped in a bunch of tubes from a non working parts set I picked up and suddenly my R15, which is listed as the "filament shunt" started blowing. It's also blowing out my dial light. Thankfully I have quite a few spares for both.

I'm taking a break before swapping it back again. My desoldering station needs a part and I had to order it, but my next test is going to be swapping those tubes back and replacing the resistor again. Could a dead tube cause that resistor and the dial light to blow on power up? I could very well have an open filament on one of the tubes.

[Electronic M]

Originally Posted by Dubis7

What's the purpose of the filament shunt resistor?

Mine keeps blowing. It was blown when I first got it. I swapped it originally, had no issues, had no issues still when I finished replacing a whole batch of components today, and then I swapped in a bunch of tubes from a non working parts set I picked up and suddenly my R15, which is listed as the "filament shunt" started blowing. It's also blowing out my dial light. Thankfully I have quite a few spares for both.

I'm taking a break before swapping it back again. My desoldering station needs a part and I had to order it, but my next test is going to be swapping those tubes back and replacing the resistor again. Could a dead tube cause that resistor and the dial light to blow on power up? I could very well have an open filament on one of the tubes.

The rectifier tube should a heater tap for the dial lamp that part of the rect heater that is in parallel with the dial lamp goes open the dial lamp would have too much power flow burn out then the resistor in parallel will cook. If any other tube goes open the whole heater string should go dark but otherwise be fine. For a different tube to cause the dial lamp to fail it's heater would have to short... though in some cases the dial lamp/tap/resistor also acts as a B+ fuse and a B+ short could also cause dial lamp failure.