[MinnesotaHam]

Hi all

I'm working on a model 60. I rebuilt the Bakelite blocks, now it's time for the condenser block. The original caps have a test rating of 1.2 and 2 kilovolts.

Have the yellow film caps with 630v rating proved acceptable replacements for rebuilding the condenser blocks?

[Ron Ramirez]

Hi

Yes.

The highest voltage any of those caps are going to see is around 250-275 volts on (20)C, the 6A7 plate bypass.

1200 to 2000 volts is, quite frankly, overkill for those caps. It made sense when the sets were being made...Philco was winding its own paper caps back then and striving for good quality without going too far overboard. But even with the over-rated caps Philco was making, they were still...paper caps, and had a fairly short lifespan. With today's better film caps, there is no reason to use anything rated higher than 630 WVDC.

[MinnesotaHam]

I saw no need for such an overkill either, so I had to ask. Thanks for the history of model 60 changes. It helped me to determine exactly which production run my set belongs to.
Now I'd like to track down the repair person who cut off the chassis "riser" that held the original filter caps. This was not necessary to fit in the paper box style replacement below the chassis

[gary rabbitt]

Hi Guys,

I had recapped a set not long ago, a Philco 38-3, and when it came to the plate caps I was advised that the 630v caps might work fine.

On the other hand someone else said that there may be a spike or transient voltage could occasionally occur, such as from changing the bandswitch or a loud crash of static that would raise that voltage over the 630v rating.

"Fast transients in the output stage can induce very high voltages on the plates of the output tubes. Those caps can be thought as snubbers to provide some protection from over voltage and they meant it when they used high voltage caps. Observing the startup voltage transient won't give you the story on them. Driving the audio stage into clipping will show just how high they can go. "

There was also talk about if Philco thought they needed a 1000v cap in there it must have been a good reason.

There was a pretty good discussion about this issue on 'the other' forum, you might find interesting:

http://antiqueradios.com/forums/viewtopi...&highlight=

I did replace the two plate caps with the 1000v rating since I had the set out.

So, maybe I didn't need them, or possibly those points were valid, I don't know. I didn't want to take a chance with something with a lower rating.

I just thought I'd throw that out to you all and see what you think.
Take care,
Gary.

[Chuck Schwark]

I've also thought that the older type wax-paper construction of these caps partially contributed to the higher voltage rating, due to needing a high value of capacitance.

Since the required capacitance was high (1uF and up) and a lot of paper dielectric was used, (paper is only 200 volts/mil breakdown), the thick paper by consequence, gave the cap a higher breakdown voltage, so that was the rating given in the Philco parts catalogues, even though the caps saw service in the 300-400 VDC ranges.

Chuck

[gary rabbitt]

Thanks Chuck for the explanation,
The ones I replaced though were only .003uf /1000v and weren't really a large size cap for the value.

I didn't have the equipment to make an actual test of the voltages present in a high spike situation, so I replaced them with the original rating as I had a couple in "stock".

Thanks,
Gary

[MinnesotaHam]

Good info Chuck. Thanks.