Sourcing/replacing obsolete tube caps?

[jdurango]

I'm restoring an old Stromberg Carlson AU42 tube amp. It has some pretty sweet oldschool domino caps that I plan to leave alone, but all the electrolytics I'd like to replace (loud LF buzz when amp is plugged into speakers and powered on). See schematics. The 50uf caps I had no trouble sourcing, but 0.1uf 400V, 0.05uf 400v, 0.01uf 600V are a different story. Can someone point me in the right direction for what to use?

I found these 0.1uf 500VAC axial film caps (safe to use?). I could put two in parallel for the 0.05uf ones, but still not sure what to do on that 0.01uf 600v. Let me know watcha'll think. Thanks!

 

[rackmonkey]

Not sure where you’re at geographically, but Mouser has plenty of axial film caps in the values you specified between 400 and 630 VDC (use .047uF to replace .05uF). Check those yellow film caps first though, before going to the trouble. They’re probably Aston polyester film/foil, rebranded as Stromberg. Desolder one and test. They may be fine. Replace all the electrolytics as a matter of course, especially those housed in paper.

 

[Philbo King]

I'm restoring an old Stromberg Carlson AU42 tube amp. It has some pretty sweet oldschool domino caps that I plan to leave alone, but all the electrolytics I'd like to replace (loud LF buzz when amp is plugged into speakers and powered on). See schematics. The 50uf caps I had no trouble sourcing, but 0.1uf 400V, 0.05uf 400v, 0.01uf 600V are a different story. Can someone point me in the right direction for what to use?

I found these 0.1uf 500VAC axial film caps (safe to use?). I could put two in parallel for the 0.05uf ones, but still not sure what to do on that 0.01uf 600v. Let me know watcha'll think. Thanks!

For high voltage caps less than 1 ufd, poly (AKA film) caps are your best bet. Varieties:
Polypropylene
Polyester
Polystyrene
Polycarbonate

General info on them:
http://www.learningaboutelectronics.com/Articles/Types-of-capacitors

 

[jdurango]

Not sure where you’re at geographically, but Mouser has plenty of axial film caps in the values you specified between 400 and 630 VDC (use .047uF to replace .05uF). Check those yellow film caps first though, before going to the trouble. They’re probably Aston polyester film/foil, rebranded as Stromberg. Desolder one and test. They may be fine. Replace all the electrolytics as a matter of course, especially those housed in paper.

My LCR meter is at the shop, all I have here is a Fluke 87v. One of the 0.05uF caps is measuring 0.2uF, one of the 0.1uF measures 0.4uF....so I'm guessing they're bad. I assumed they were an early electrolytic design? Paper and wax instead of aluminum can? The brown paper 50uF one I pulled out kept fluctuating between 70uF and 73uF on the Fluke. Wish I had a scope here to check it out, but pretty sure it's leaky. The domino mica caps should be fine eh?

 

[sodderboy]

The non-electrolytics should be fine, but they should be tested to make sure. I would not replace any non's if they measure properly.
When I saw "tube caps" I thought of the actual spring-held jammies. . .
Mike
PS: When you first power the replacements, variac-up over a minute-ish with a light bulb in series (search here if you don't know) with the AC input. I personally don't hit a new re-cap with full juice even if there is tube rectification.

 

[rackmonkey]

Agree that you should replace all but the dominos, given those results. The yellow ones are probably paper/wax, based on your tests.

 

[ccaudle]

One of the 0.05uF caps is measuring 0.2uF, one of the 0.1uF measures 0.4uF....so I'm guessing they're bad.

Capacitors generally do not increase capacitance when they go bad. Are you measuring in circuit or out of circuit? Any chance there could be other devices in parallel?
Do the capacitors have tolerances marked on them like modern caps do? Even modern caps are often marked -20/+80%, so it wouldn't be surprising if old caps had a pretty wide tolerance to start with. Not +300% wide, so something else is going on with the 0.1uF measuring 0.4uF.
Is it marked? Any chance someone else changed it for a larger value years ago?

I could put two in parallel for the 0.05uf ones

Two 0.1uF capacitors in parallel is 0.2uF. Two 0.1uF capacitors in series is 0.05uF, but as someone else pointed out, just use the modern standard value of 0.047uF.

still not sure what to do on that 0.01uf 600v

Film capacitors for tube circuits are not that difficult to find. The first place I checked has 630V film caps in stock:
https://www.tubesandmore.com/products/capacitors?filters=3109a3115Where are you looking that you can't find a 0.01uF cap with 600V or more rating?
I even checked Digikey, searching for 0.01uF/630V film cap has 113 different part numbers in stock. Not exactly a hard to find specialty item.

Are the domino caps (never seen one of those before, I don't generally work on antique gear) the ones marked 50MMF in the schematics? I guess that would be 50pF mica in modern notation?

pretty sure it's leaky

Leaky implies that the DC resistance value has decreased. A Fluke 87V is perfect for measuring resistance, just clip the leads on the cap and let it sit for a while. Make sure you don't hold the leads on with your hands, your skin resistance is too low to measure cap leakage accurately, you need clips. Make sure you wait for the reading to stabilize, trying to measure resistance of capacitors gives nonsensical results until the cap charges up to the measurement voltage (could take quite a while to stabilize with larger capacitance values).

As others pointed out definitely replace all the electrolytics, nothing wrong with replacing the film caps as well but they don't often go bad if they haven't been severely abused. This amp is a decade or two older than anything I've messed with, so maybe that era of caps had an eventual environmental susceptibility that shows up, like moisture absorption or something, so I'll defer to those that have actually rebuilt some equipment this old on how strongly it would be recommended to replace any old (1950's?) film caps.

 

[CJ]

Check for leakage on the coupling caps by using voltmeter, if leaky you will see a positive voltage on your grids or pots,

If trying to preserve antique look you can stuff the lyrics into the cardboard tubes of the old lytics.

 

[jdurango]

Thanks for all the replies. So the yellow caps labeled "STROMBERG", are these electrolytic? They are definitely paper and wax, and measured roughly 2x rated capacitance out of circuit (one leg desoldered). I've actually seen lots of caps fail with higher capacitance than rated....in fact that seems to be how most of them go IME. And I was finally able to find the correct caps on Digikey....just took a while to get acquainted with their search system and filters.

I'll check those yellow paper/wax caps again when I get my Agilent LCR meter, but I'm pretty sure they're toast. I checked some brand new Nichi and Panasonic caps with the same 87V, differents ranges (4.7uf, 100uf, 1000uf), all measured within +/-.

 

[k brown]

In the pic with two Stromberg caps, the top one is electrolytic, the bottom one is not.

 

[emrr]

I've seen PLENTY of bad Domino's, ones that measure fine. The other film caps may leak more DC the longer they're powered up, even if they measure fine. Watch for gain dropping or vanishing as leaking DC changes tube bias.

 

[Piedwagtail]

part of a series on his channel:
a nice combination of safety and zappy terminals - mind you, many must have shot an arm back with the Heathkit and Eico in the day.

R

 

[industrialarts]

FWIW, I get most of my capacitors from here - Capacitors, Resistors and Schematics for Tube Radios / Electronics

The MIEC electrolytics have proven to be very reliable for me. Have replaced hundreds at this point.