[Boatanchors] Check those filaments

[Ken Kaplan]

Just an fyi,

I am refurbishing an old B&W model 600 GDO. You know - replace the 
electrolytics and the selenium rectifier. I decided to be thorough and 
measure the transformer voltages as no voltages are on the schematic. My 
line is about 122v. This gave a secondary voltage of 165v and a filament 
voltage of 8.7v. Hmm - the 955 acorn tube is speced at 6.3v for the 
filament. Perhaps that was why it glowed so bright. I put it on the 
Variac and set the primary at 115v and got 8.3v. I had to the drop the 
primary down to 104.7v to get 6.3v. Assuming the transformer is not 
fubar (looks & smells ok), I suspect those old transformers were not 
quite precision devices.

I just stuck an 8 ohm resistor in series with the filament as the 
current is low. I don't think they are making 955s anymore. I guess the 
lesson is don't assume the filament voltage is right. In my case, it was 
2 volts high with the correct line voltage. Yes, I'll make a note of 
what I did and stick it inside the case for a future owner.

73 Ken kb7rgg