What is an Auxiliary power supply cap?

[DaMuffinMan]

George mentions this on the parts list for the SSE, Auxiliary power supply cap Used to improve power supply. Recommended for high end amps.

Can anyone give me a part # for this?

Parts List | Tubelab

 

[Lee from Chitown]

Physically it is the large, tall ,frequently silver cylinder you see on people’s amp.
When ordering one, you want a RUN cap, as opposed to a START UP cap.
What it does I will let someone more(which is much more) knowledgeable than me explain.

 

[Tubelab_com]

The aux cap is a capacitor added off board that complements the electrolytic that is already on the PC board. Electrolytic capacitors have characteristic shortcomings, but offer a lot of capacitance in a small package. We can get the best of both worlds by adding a different type of capacitor off board in parallel with the electrolytic.

The best choice is a polypropylene capacitor like the motor RUN caps found in air conditioners and other industrial uses. This is NOT a motor START cap, which is just another type of electrolytic that will fail if used in a tube amp. Some unscrupulous sellers do misrepresent start caps as suitable for use in tube amps, but if it is encased in plastic, stay away from it.

This is what I recently purchased for the big KT88 push pull amp being developed in the Universal Driver board thread, and can be seen between the output boards.

TEMCo 100 MFD uF Run Capacitor 440 vac Volts AC Motor HVAC 100 uf | eBay

If you don't have room for a cap that big, choose a similar cap of a lower uF value, but at least 370 VAC. I would use at least 10 uF, but bigger is better, up to about 100uF.

 

[colnago55]

I have used this clamp to mount them. It needs a little modification, but it works either to mount the motor run cap inside a 3 inch tall chassis or thru the top on a two inch chassis.
VR10A Cornell Dubilier Electronics (CDE) | Capacitors | DigiKey

You will need to drill an additional hole for it to fit the Temco capacitor.

 

[DaMuffinMan]

Ok thanks guys. I got the Temco one already. But I was confused and thought I need something else.

 

[DaMuffinMan]

Is this a good option for a power switch fuse. And c a n I install another on off switch up front? 06AK2D Delta Electronics | Connectors, Interconnects | DigiKey

 

[Altecboy]

using only the aux cap

Has anyone any experience using only the aux cap (motor run) without the onboard electrolytic? Say just a 40uF motor run, then a choke, then a 70uF motor run?

I have it burned into the back of my head that the difference in discharge rates between the motor run and a parallel electrolytic must smear something.

 

[Tubelab_com]

There was a builder here a few years ago who built his SSE without any electrolytics at all. It was obviously large:

I did finish this project. It sounds great. I used four 600uf asc caps for the cathode bypass capacitors and the power supply is all motor runs. The downside....the amp is 2 feet wide.

See this thread:

Electrolyticless Tubelab Simple SE

So, yes you can run the SSE without any of the electrolytics. I think that I would at least use a 100 uF or two for the second cap.....but that may just be my overkill engineering speaking. You can get the Temco 100 uF for cheap, so......how big do you want your amp to be?

Note: the 600uF ASC caps he mentioned were some Ebay surplus that popped up almost 10 years ago. They are no longer available.

 

[Altecboy]

very helpful, George. Thank you.

 

[Altecboy]

Using only the Aux Cap

A small experiment:

I use a pair of SSE's. One to drive the low/mids on my altec A7 clones, and the other to drive the HF horns.

The lower frequency SSE uses 40uF motor run for the first B+ cap and 3 parallel 70uF caps for the second B+ cap. Even with my high efficiency speakers there is no real hum even up close.

The higher frequency SSE has a 40uF motor run for the first B+ cap and a single 120uF electrolytic in parallel with a 70uF motor run for the second cap.

I measured the B+ at the second b+ cap on the higher frequency SSE to be 408VDC. The ripple was between 50-70mV. It jumped around a bit. Output hum and noise measured 1mV.

I yanked the electrolytic and ran the amp again.
I measured the B+ at the second b+ cap to be 410VDC. The ripple was between 60-80mV. Output hum and noise was 2mV.

I think it sounds better w/o the electrolytic and will leave it that way.

 

[Altecboy]

Further Experiment: DC-Link

Wima DC-Link high voltage polypropylene caps are available at Mouser. I bought a 75uF 600 Volt unit and used it to replace the 70uF motor run capacitor which follows the choke. There is no electrolytic in the b+ now. I think it sounds great and is worth the $40 I paid for it.