[Penthode]

The 721TS has been running for about an hour this morning. I am told the electrolytics will not last long term. I am curious what evidence is there of that?

There is no sign of any heat or stress on the electrolytic capacitors after running about 14 hours in total. Any thoughts when they will fail?

The point of this exercise is to point out in a project such as this, that perhaps it is unwise to shotgun replace most of the electronic components. I am amazed reading here that even resistors that have marginally drifted out of tolerance are replaced. Paper capacitors indeed go leaky but even then, is it necessary to change all of them even if it makes no difference to circuit performance?

I have previously contested that the wholesale replacement of parts is unnecessary and will likely lead to considerable extra work and grief. It took me less than a day to get this set up and running.

The key here is to scrutinize the circuit in advance to identify components which will likely fail with catastrophic results eg up-in-flames, smoldering resistors or undo stress on the power transformer and rectifier. If concerned, you can always insert a slo=blow fuse in the B+ rail. More time should be spent paning before execution.

I contend the Sprague and Mallory branded electrolytics made from the late 1940's were very well manufactured and will operate satisfactorily and reliably even today. This is if and only if you reform the diectric properly and with a bit of care. In the last 50 years of repairing these old sets, I do not recall ever seeing a bad mica capacitor in an RCA set. I think more time spent understanding how the circuits work will result in less physical effort and result in a set that maintains more of its overall originality.