Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on August 06, 2018, 04:42:16 pm

Title: 12vac filament PT
Post by: dude on August 06, 2018, 04:42:16 pm

Not loaded this PT, 6.3-0-6.3 reads 7.3vac on each leg, can I assume that it's higher because of no load and with a load would it most likely be 6.3vac?


al

Title: Re: 12vac filament PT
Post by: shooter on August 06, 2018, 05:26:36 pm

Quote

can I assume

absolutely, but at some point ya gotta prove it  :icon_biggrin:

Title: Re: 12vac filament PT
Post by: PRR on August 07, 2018, 02:50:47 pm

Un-sag may be 5% on big iron, 20% on small iron.

So it will likely be closer to 6.3V when fully loaded.

I would just put a large tube on it. 7.3V does not make a 6.3V tube blow-up like flash powder; if you kept it up, it would reduce heater life from 10,000 hours to 1,000 hours. However everything sags, so it won't be 7.3V. If it comes to 6.0 to 6.6V, rock on.

(BTW: I just saw a Scott hifi amp with 12.6V heaters worked at 7V! It should be noted that they also worked near 0.25mA cathode current, far less than most designs. These amps still work 57 years later.)