Need help identifying small value caps on Spectra Sonics cards

[Ego Tripper]

Hey everybody,
I recently picked up some old Spectra Sonics cards off e-B@y. I got a 110, a 110A, 2 complete 103s, 2 partial 103s, and 2 101s (the model I was after, due to it's use @ Stax). They all came in a Spectra Sonics 201 card holder (which made the shipping considerably more than it would have been otherwise):
http://twin-x.com/groupdiy/displayimage.php?pos=-4372

I plan on building mic pres based around the 101s, 110, & 110A. The 103s are mix-amps, so I'm gonna use the complete ones for some summing duties in a portable-mixer/small-rack-case into which all of these pre are going.

Obviously, with the age of these cards, a recap is in order. I was looking over the caps, and while the big ones are definitely electrolytics, I've never seen one quite like these smaller ones. Does anyone know what kind of cap these silver/clear ones are?
http://twin-x.com/groupdiy/displayimage.php?pos=-4373

A lot of the printing is worn off the caps on this 101 card, but some of the other caps like this read:

M​

5​

5%​

and:

M​

47​

5%​

...so obviously the middle line is the capacitance, but I don't know what decimal place they'd have been using, so if anyone has any ideas, please pass 'em along.

 

[abechap024]

The small ones look like polystyrene and probably don't need to be replaced.
also the 5 probably stands for 5pf
and 47 for 47pf

 

[Ego Tripper]

Thanks, I was thinking polystyrene, given the size & value, but I haven't been able to find any polystyrene caps this small. Obviously, If they are polystyrene then they don't need to be replaced, but I'd like to build another pair or two to go along with these. The only place I know of with a good selection of polystyrene caps is Surplus Sales of Nebraska, but their values seem to start at 51 pF and increase from there.
(http://www.surplussales.com/Capacitors/Poly-Unelco.html)

Does anyone know of a good place to get 5 pF & 47 pF polystyrenes?

By the way, the seller said that these were originally from Sunset Sound, then were at a studio in Santa Barbara in which Little Feat and Jackson Browne recorded. I don't know if this is true or not, as I picked them up mainly interested in adding some Stax flavor to my life. The seller was also selling a similar lot (sans 101s), a lot of maybe 8 A-20s, and some faders-- I assume to all be from the same console. If anyone has any info on what kind of board from which these modules may have been pulled, and/or what room in which it may have been used at Sunset Sound, please let me know.

 

[abechap024]

For the sake of keeping it strictly original aside, why not use c0g/npo?

 

[JohnRoberts]

another vote for 5pF and 47pF polystyrene...
and another vote to not replace them.

Polystyrene caps are probably good as long as you don't overheat them with an iron.

JR

 

[Ego Tripper]

Yeah, I'm definitely not planning on replacing these existing polystyrenes, but I do need to find some more of 'em for the other channels I'd like to build.

Oh, and there isn't a reason not to use NP0s (they spec out really well), other than the fact that I am looking for a lot of authenticity out of these. 

 

[plumsolly]

For polystyrenes check PartsConnexion, Michael Percy Audio, and Justradios. These can also be called "Styroflex".

Best,

Ben

 

[Ego Tripper]

I got home and started checking the exact values I'd need, looking at the values on the caps (the ones I can actually read) and the values on the schematics I have. In the process I noticed a couple things. First, there's also some caps of unknown dielectric visible on the card that I forgot to mention earlier. They're the ones in the solid silver wrap (one can be seen in the linked photo, directly to the left of the upper left-hand polystyrene cap). They almost look like non-polarized electrolytics, and the values look right (56 uF & 4.7 uF), but I'm hoping someone here can chime in here and tell me otherwise so I don't have to replace these caps.  

The second thing I noticed is that there are no 47 pF caps on the 101, just smaller, harder-to-find caps (The 47 pF caps were on the 103s). I checked the above recommended outlets (thanks plumsolly!) and while they have pretty good selections of polystyrene caps, none of them have any 5 pF polystyrenes. Then I checked over the schematic posted here by techardent, which supposedly has the exact values of Ardent's 101 cards.
http://ardentstudios.com/tech/spectrasonics/Spectra-Sonics-101-card.jpg

Since Ardent & Stax purchased sequential desks (twice), I figured Ardent's values would match Stax's pretty closely, so I've been planning on making my clone cards based on Ardent's component values, rather than the ones found in the more common schematic:
http://www.technicalaudio.com/pdf/SpectraSonics/SpectraSonics_101_amplifier_manual_schem.pdf

There are two schematics at the end of the linked .pdf, but the first seems to match Ardent's values the closest. In fact, the only significant difference between the two seems to be the lack of 5 pF caps! Instead, there appears to be a single 20 pF cap in one cap's place and nothing for the other. Does this seem like it could be right? It seems like it would require a different board layout, or at least a jumper (which would seem unusual here on a production item). I'm also unsure what the electronic result of this difference would be...

I also noticed a difference between my cards and the aforementioned schematics. All the schematics show a 220 pF cap in parallel with a resistor after the collector of the first transistor. On one of my 101s I have what looks like a 10 pF cap in parallel with that resistor and on the other other card I can't read the value, but it looks like it was two digits, not three. The unreadable one is the same type as the other polystyrenes, while the other has red ink, whereas the rest have black, so it seems possible, however unlikely, that they are merely unoriginal. I'm guessing that this is probably a situation where the 200 or so pF difference is negligible in this application??

Also, I didn't see any kind of gain-set feedback in the Ardent schematic. Does this seem weird to anyone?

 

[PRR]

> I didn't see any kind of gain-set feedback in the Ardent schematic.

 

[Ego Tripper]

Oh, I see how that works now. Thanks, PRR. Between the two different layouts and the unusual common-base design, my mind was pretty twisted 'round, but I thought I had it sorted out. Apparently not the feedback path...

So then the Ardent cards were set at 40 dB of gain, correct?

And it would seem then that the only difference is Ardent's lack of a cap between Q3's collector and the feedback path?