01-02-2012, 11:41 AM
Hi Dennis,
Agree with above posts. The reason you don't want to use a filter cap that is large is that it will increase the HV. Back in the 1920's and 30's the technology wasn't developed enough to make a cap that was more than a few mfd. That's why on these older set in the power supply circuit you'll find three or four small caps, 1.0 to 3.0mfd and a few chokes and the field coil to filter out all of the 60 cycle ripple. These are usually paper and foil wound around each other. Two pieces of foil with the paper as an insulator. It is the makeup of non polarize filter condenser. With this cap there is no + or -. Most of the time it's mounted in a metal can with tar to secure it in place.
Fast forward a few years and now we have electrolyte caps. These are in an aluminum tube or copper (Mershon) with some chemical inside and metal coil. Don't know how these work but they are polarize with the outside metal can being the - and the wire or terminal at the bottom being the + connection. For the most part these caps are 20mfd or less.
Things changed when small ac/dc sets with pm speakers came in to being. No more field coil on the speaker to use for filtering.That gets replaced by a resistor which doesn't have any ability to filter just couple the HV to the next large cap.These set use rather large filter caps in the order of 40 to 80mfd. We 'er talking the late 1940's.
So long story short the issue is the size of the filter. + 20 percent is OK and as a trick you can use is the input cap is the one that is going to have the most effect on the output voltage. That would be the one closest to the filament of the 80 tube. You can use this cap to set the HV level. The other filters won't have of ability. to raise the voltage.The others will won't have much effect on the voltage will increase the elimination of the 60cy hum.
I was working on a model 20 awhile back and put a 20 mfd caps for all three of the filters. The outcome was I nearly burned up the BC resistor because the HV was much higher and put more of a load across the resistor.
The early and late model 70's are pretty different as the late model as AVC and the earlier one doesn't. Haven't seen very many of the late models. As I type this my right is resting on a 370 (chairside version of the early 70) keep my elbow warm from the heat of the 80 and the 47 tubes.
Terry
ps
Pardon my rambling, I need to get a life.
Agree with above posts. The reason you don't want to use a filter cap that is large is that it will increase the HV. Back in the 1920's and 30's the technology wasn't developed enough to make a cap that was more than a few mfd. That's why on these older set in the power supply circuit you'll find three or four small caps, 1.0 to 3.0mfd and a few chokes and the field coil to filter out all of the 60 cycle ripple. These are usually paper and foil wound around each other. Two pieces of foil with the paper as an insulator. It is the makeup of non polarize filter condenser. With this cap there is no + or -. Most of the time it's mounted in a metal can with tar to secure it in place.
Fast forward a few years and now we have electrolyte caps. These are in an aluminum tube or copper (Mershon) with some chemical inside and metal coil. Don't know how these work but they are polarize with the outside metal can being the - and the wire or terminal at the bottom being the + connection. For the most part these caps are 20mfd or less.
Things changed when small ac/dc sets with pm speakers came in to being. No more field coil on the speaker to use for filtering.That gets replaced by a resistor which doesn't have any ability to filter just couple the HV to the next large cap.These set use rather large filter caps in the order of 40 to 80mfd. We 'er talking the late 1940's.
So long story short the issue is the size of the filter. + 20 percent is OK and as a trick you can use is the input cap is the one that is going to have the most effect on the output voltage. That would be the one closest to the filament of the 80 tube. You can use this cap to set the HV level. The other filters won't have of ability. to raise the voltage.The others will won't have much effect on the voltage will increase the elimination of the 60cy hum.
I was working on a model 20 awhile back and put a 20 mfd caps for all three of the filters. The outcome was I nearly burned up the BC resistor because the HV was much higher and put more of a load across the resistor.
The early and late model 70's are pretty different as the late model as AVC and the earlier one doesn't. Haven't seen very many of the late models. As I type this my right is resting on a 370 (chairside version of the early 70) keep my elbow warm from the heat of the 80 and the 47 tubes.
Terry
ps
Pardon my rambling, I need to get a life.
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
