Good source for K40Y-9 630v caps

[labjr]

I've had a 500C that needs restoration for some time. I've actually been looking at the bottom of chassis for several years now and putting it off because someone messed with it before I got it.

I don't think I need to have a coupling caps debate since there are plenty of other threads to read. But I think I want to try some Russian PIO caps. I'm wonder which current sources are most reliable for the recommended K40Y-9 .068uf 630v caps?

Thanks

 

[Sam08861]

Might try reaching out to the below...

http://www.mcshanedesign.net/
jimmcshane@mcshanedesign.net

I got one of his kits that had these. (can't remember if they were the same value though)

Also, here, but no experience with the below...

http://tctubes.com/k40y-9-pio-capacitors.aspx

 

[notdigital]

That cap @ that voltage rarely comes up at auction. This guy has them in stock. I and others on Ak have purchased from him:

https://tubes-store.com/product_info.php?cPath=35_41_82&products_id=1275

See: Russian PIO cap source?

 

[labjr]

Thanks! Pretty sure I need that value for the 500c, correct?

 

[notdigital]

Thanks! Pretty sure I need that value for the 500c, correct?

Well, depends on how much you plan to do in terms of your restoration. Changing the original value of the coupling cap from .047 to .068 will require that you change the grid resistors to 220k from the original 330k. The original voltages for those caps were 630v and 250v (don't ask.) So, short ans is yes, if you're going to install .068 caps pair them with 220k grids and you're good. Voltage any where from 250v to 630 is fine, erring on the higher side if available.

The long answer will require a discussion of modern vs vintage power tubes, biasing schemes, timing constant, etc. Fortunately, there was a recent discussion of the subject:

Coupling Capacitors

 

[gadget73]

the two different voltages actually makes sense, but you don't have to keep it that way. The low voltage cap ran on the cathode, which never sees as much DC voltage. I'm sure it was a size and cost savings measure. Sherwood did it too and for the same reason.

just do not go lower than the original voltage in any spot.

 

[labjr]

Thanks! I emailed Jim McShane and received a quick reply! He recommends 200K resistors with .1uf PIO caps.

 

[larryderouin]

One of the reasons for the .068uf and 220K is that the R/C timing is the closest to that of the old .047uf and 330K pair. The .1uf and 200K was the old standard revised setting when cap sizes of .068uf and .082uf were not commonly available. There have been some new developments with regard to changing the values. The jist is that if the unit has either an IBAM or IBBA board for adjusting the bias to 70%-80% and keeping there, no changes are needed. See Coupling Capacitors

Actually the ORIGINAL COUPLING CAP Values/Voltages are .047uf/400V and .047uf/250v.