Choice of caps

[TC Chris]

I'm getting ready to recap the old Detrola 4J, the little 4-tube TRF mini-cathedral set. It has a power transformer.

The schematic shows two 4 mfd, filter caps. In place I find a dual 10 mfd cap, probably a replacement because it's secured with a loop of string. I've got both 10s and 4s on hand. What's the best choice?

And the output amp cathode bypass is shown as 10 mfd. , 30 V. (that's off the cap, no clear votage n the schematic). Can I go with 25 mfd in that function? All the 10s I have are 450V, and I've read not to go that far afield in voltage ratings.

Chris Campbell

[electra225]

You can increase both the capacity and the voltage rating of the cathode by-pass cap. I would consult a tube manual to see what the max capacity of the filter cap after the rectifier should be. Somebody may have increased the capacity to cure a hum, or they may have used it because they had it on hand already. The filter after the choke you can increase considerably, in most cases, without concern. Zenith used to use the output filter cap for cathode bypass. The things would howl like a banshee, so all you could do was to increase the value of the cap, sometimes double what it should be. I wouldn't be concerned about using 450 volt filters with a power transformer. I wouldn't go over about 100 VDC for the cathode bypasss cap. 450 VDC is a tad overkill, although it probably wouldn't HURT anything.......

[TC Chris]

The 450 volt filters are what it calls for so I'll go with them. I'll choose the 25 mfd/50 volt bypass.

My guess is that the 10 mfd value is what some repair guy had on hand in a dual cap. What if i compromised: 4 mfd on input, 10mfd after the choke (speaker field)?

Chris Campbell

[electra225]

I'd say that should work just fine. Increasing the cap after the choke will decrease the chance for hum/squealing....