05-04-2019, 03:51 PM
Gary
For the most part changing out the 71A in the model 20 for another type of tube is problematic.
There are few output tubes that require 5v to power the filament. 112A were used in the old skool battery sets from the 20's so it was designed to have a low filament current consumption 5v @ 1/4A = 1 1/4watts. But in order for any tube to make a noticeable improvement the total need to be higher as the filament is the part that generates the electrons that flow inside the tube. In short the bigger the filament the more current the plate can draw. And conversely a small filament the less total current the tube can draw.
Now the 45 tube would seem to be a likely candidate but there are two issues. The 45 tube require 2.5v for the filament. So not a big problem you wire the two 2.5v filament in series so now you need 5v to power them perfect!! The the filament requirement for the 45 is 1 1/4 A. This poses a problem as the filament winding on the 20's is 5v@ 1/2amp so the 45's would draw more than twice as much @abt 3watts. The 45s require more grid bias. Think I remember Ron posting that he had swapped the power transformer from a newer set using 45s,24A, and 27. THis would give the proper voltages and current to support such a tube swap. The other 2.5v tubes (47, 2A5, 59, ect) are worst as they require more filament current.
Ron
Tnx for posting good read. Lots of cleaver bits throughout even a National Co. SW-3 sunken into a table top!
For the most part changing out the 71A in the model 20 for another type of tube is problematic.
There are few output tubes that require 5v to power the filament. 112A were used in the old skool battery sets from the 20's so it was designed to have a low filament current consumption 5v @ 1/4A = 1 1/4watts. But in order for any tube to make a noticeable improvement the total need to be higher as the filament is the part that generates the electrons that flow inside the tube. In short the bigger the filament the more current the plate can draw. And conversely a small filament the less total current the tube can draw.
Now the 45 tube would seem to be a likely candidate but there are two issues. The 45 tube require 2.5v for the filament. So not a big problem you wire the two 2.5v filament in series so now you need 5v to power them perfect!! The the filament requirement for the 45 is 1 1/4 A. This poses a problem as the filament winding on the 20's is 5v@ 1/2amp so the 45's would draw more than twice as much @abt 3watts. The 45s require more grid bias. Think I remember Ron posting that he had swapped the power transformer from a newer set using 45s,24A, and 27. THis would give the proper voltages and current to support such a tube swap. The other 2.5v tubes (47, 2A5, 59, ect) are worst as they require more filament current.
Ron
Tnx for posting good read. Lots of cleaver bits throughout even a National Co. SW-3 sunken into a table top!
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
