Use the same or next larger available capacitor value with at least the same voltage rating. I use capacitors rated at 160 volts, in AC/DC radios and 450 volts in all others.
Very old radios will have low values, sometimes only 1 mfd. That's all that was available in those days at reasonable cost. The radios used a full wave rectifier and large chokes for filtering. I would use 10mfd @ 450 volts in these radios.
Later radios had smaller chokes or resistors but still used full wave. With these radios 22 mfd will be ok.
Finally the AC/DC radios with only half wave and no choke, just a resistor. These radios use 30/50 or 40/60 mfd @ 150 volts. For AC/DC radios there are 33 mfd and 47 mfd @ 160 volts available.
Now to the real question. You can go higher in capacity as long as the peak current at the rectifier doesn't exceed the tube rating, usually up to 40 mfd. The 84 and 6X5 tubes are some of the first tubes to be damaged by high peak current.
The second capacitor, away from the tube, can be much larger in value as it is isolated from the rectifier by a choke or resistor. You could use 100 mfd but at some point no additional filtering will be gained, only size and expense will increase.
When repairing old radios there is no need to have all the values shown in the Antique Electronic Supply catalog. 10 & 22 mfd @ 450 volts are useful. For AC/DC radios 33 or 47 mfd @ 160 volts are handy. AC/DC/Battery radios use a large capacity at low voltage. Low voltage types can be purchased at local electronic parts stores.
I am sure there are have different thoughts on the subject. Maybe we will hear from others.
Norm
: Norm, how about a little philosophy on the sizing of power supply electolytic caps. Most radios are 110-60cycle (oops showing my age)-60 hz, run the same basic voltages (a,b+, etc.)yet seem to vary significantly in the value for filter caps. When replacing bad ones what flexibility do we have in cap value. Assumption is voltage ratings should be equal or higher. Thanks Dave G
For reliable and predictable results, Norm's approach works well. However, there are times when there is a desire to keep appearance of the radio components intact as much as possible, or to restore one that has had some previous work to look original; there are problems with space. Also too, manufacture of many of the older caps gave wide ranges of tollerences from as much as 20-40%(only occaisionaly more) under value, to sometimes 200% of rated value(or slightly more). Most production lines starting post war had better tollerences, but there are some radios of mine that were rated for a 10mfd cp, and a 6mfd worked just fine, and it was the only cap I had that I could fit into the old cap's body can.
On the other hand, there was one radio thatI replaced the filter caps with new ones that tested fine- of the same rating, and there was not adequate capacity for that specific circuit. Everything checked fine, I just added another small cap, and it worked fine.
Power supply filtering is one area that working on old radios is as much art as it is hard math. You can predict specific values for the RC circuits that are filtering, as well as with the LC circuits that are filtering the power supplies, but the art truly comes in with knowing what ranges you can get away with for a general type of radio. Sometimes you have to resort to "trial and error".
Keeping the points in mind that Norm raises with regard to "in-rush" current on the choke itself, and plate current limitations, the guidelines are pretty much a matter of what finally works, in as few tries and components as possible.
: Dave
: Use the same or next larger available capacitor value with at least the same voltage rating. I use capacitors rated at 160 volts, in AC/DC radios and 450 volts in all others.
: Very old radios will have low values, sometimes only 1 mfd. That's all that was available in those days at reasonable cost. The radios used a full wave rectifier and large chokes for filtering. I would use 10mfd @ 450 volts in these radios.
: Later radios had smaller chokes or resistors but still used full wave. With these radios 22 mfd will be ok.
: Finally the AC/DC radios with only half wave and no choke, just a resistor. These radios use 30/50 or 40/60 mfd @ 150 volts. For AC/DC radios there are 33 mfd and 47 mfd @ 160 volts available.
: Now to the real question. You can go higher in capacity as long as the peak current at the rectifier doesn't exceed the tube rating, usually up to 40 mfd. The 84 and 6X5 tubes are some of the first tubes to be damaged by high peak current.
: The second capacitor, away from the tube, can be much larger in value as it is isolated from the rectifier by a choke or resistor. You could use 100 mfd but at some point no additional filtering will be gained, only size and expense will increase.
: When repairing old radios there is no need to have all the values shown in the Antique Electronic Supply catalog. 10 & 22 mfd @ 450 volts are useful. For AC/DC radios 33 or 47 mfd @ 160 volts are handy. AC/DC/Battery radios use a large capacity at low voltage. Low voltage types can be purchased at local electronic parts stores.
: I am sure there are have different thoughts on the subject. Maybe we will hear from others.
: Norm
:
:
: : Norm, how about a little philosophy on the sizing of power supply electolytic caps. Most radios are 110-60cycle (oops showing my age)-60 hz, run the same basic voltages (a,b+, etc.)yet seem to vary significantly in the value for filter caps. When replacing bad ones what flexibility do we have in cap value. Assumption is voltage ratings should be equal or higher. Thanks Dave G