Oversized Supplemental Capacitor - Is it really needed?

I am happy to say that my Tubelab SSE is up and running using the triode configuration. Now as I tighten everything up I've started the ebay search for a supplemental capacitor and have found that it is very difficult to find the right kind. I see that most if not every Tubelab SSE build I've seen here on diyaudio is built using this supplemental capacitor. I am assuming it is fairly important... I just wish that it was easier to find.

I ended up ordering this one from ebay. I doubt I got the right kind but ordered it hastily and out of frustration (and it was under 10 bucks so whatever). It says it's "450V 100uF Motor Run" cap.

Can someone help me understand why I need this capacitor and where or how I might find one? Or did I order the right one? Do I need two?
 

Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
Do you mean the blue capacitor? The red capacitor? Another capacitor which is neither blue nor red? To which capacitor do you refer?

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The "red" capacitor is the electrolytic capacitor that is installed on the PC board. The "blue" capacitor is an optional capacitor added on many builds. It should be a metal film cap.

No capacitor is perfect. All capacitors have unintended internal resistance, inductance, dissipation factors and other imperfections. Electrolytic capacitors (especially large ones) have imperfections that restrict their ability to deliver a large current flow in a very short period of time. Many film caps do not have these imperfections, but may have others. The combination of an electrolytic and a film cap connected in parallel usually offer a much better overall quality than either by itself considering the overall cost.

The size of the film cap should be at least 20 uF and capable of handling 500 volts of DC. There is a continued improvement up to somewhere around 70 to 100 uF.

Most users report improved bass, and transient response when the cap is added.

One could purchase a specialized audio film cap in the 150 uF range and use it as the only power supply cap, but it would be extremely expensive. It turns out that a decent electrolytic on the board and a motor run cap, like those found in a central air conditioner unit, together works just as good, at a much lower cost.

Note, there are motor START caps. These are just electrolytics designed for AC use, and they should NOT be used here. These are used only for short periods of time and removed from the circuit as the motor spins up by mechanical contacts.

A motor RUN cap is a film cap designed for continuous use directly connected to the AC line voltage, and carrying the current for a motor winding, which is several amps continuously. They must also eat any surge that comes down the power lines. These MUST have very low internal losses or they will simply explode! A cap rated for 370 V AC will have no problem on 500+ V DC.

Or did I order the right one?

If it is truly a RUN cap, and Metallized Polypropylene Film as the ad copy says, it will work fine. There have been instances of cheap START caps being sold as run caps on Ebay. If you try it, keep an eye on it for the first few hours of use, if it gets warm, remove it.

There are several 35, 59 and 78 uF 370VAC and 440 VAC metal can caps on Ebay. Any one of those is probably a safer bet.

Do I need two?

No, one is enough. Two smaller caps in parallel is the same as one big one. IE a pair of 35uf 370 VAC caps for $8.95 each is the same as a single 70 uF 370 VAC cap. It is up to you as to which would be easier to mount.

I used a single 100 uF in my amps because they were available for $10 each at the time. I bought several.
 
Tubelab.com! Thanks so much for your reply. I was secretly hoping you would reply to this one. Your explanations are a great mix of being straight-to-the-point & in-depth.

Most users report improved bass, and transient response when the cap is added.

That's interesting.

If it is truly a RUN cap, and Metallized Polypropylene Film as the ad copy says, it will work fine. There have been instances of cheap START caps being sold as run caps on Ebay. If you try it, keep an eye on it for the first few hours of use, if it gets warm, remove it.

There are several 35, 59 and 78 uF 370VAC and 440 VAC metal can caps on Ebay. Any one of those is probably a safer bet.

I think I will follow your recommendation and try this Chinese cap while monitoring its temperature for a bit. I'll probably eventually get rid of it and grab a metal cap as you recommended for safety.
 
Hello,

The place to purchase locally if you live in a bigger than small town is the appliance parts store, the same place you would buy parts for your air conditioner or furnace. They also sell fan motors the item the run capacitor would normally attach to.

Also the run capacitors come with a AC voltage written on them. take the AC voltage and multiply it times 1.41 (root 2) for a working DC voltage.

DT
 
Last edited:

exeric

Member
2008-03-24 6:23 am
Here's a 100uf film cap I just ordered. EZP-E50107MTA Panasonic | Mouser
There seems to have been a minor engineering advance in film caps recently that's going under the radar. This cap is only about 1.5 x 2.5 x 2.5 inches, which is pretty small for a 500 volt rated 100uf film cap. Also only $21.

A question: it says it has 4 pins. I just ordered two for a project but how does one go about hooking up a 4 pin cap? I have no clue.
 

Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
It's either a 2 terminal capacitor with two extra unconnected pins that only provide mechanical support, OR it's a 2 terminal capacitor where terminalA is electrically connected to pins 1&2, while terminalB is electrically connected to pins 3&4.

Since you've already ordered the parts you can simply use a DC continuity tester to decide which is which. Allow 30 seconds for each "buzz" connection to stabilize, lest the capacitive charging/discharging fool you.